Patent Publication Number: US-9427838-B2

Title: Head tool changer for use with deposition-based digital manufacturing systems

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to U.S. Provisional Patent Application No. 61/318,430, filed on Mar. 29, 2010, and entitled “HEAD TOOL CHANGER FOR USE WITH DEPOSITION-BASED DIGITAL MANUFACTURING SYSTEMS”, the disclosure of which is incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to deposition-based digital manufacturing systems for building three-dimensional (3D) models with layer-based additive techniques. In particular, the present invention relates to devices for loading multiple deposition heads to deposition-based digital manufacturing systems, such as extrusion-based digital manufacturing systems. 
     An extrusion-based digital manufacturing system (e.g., fused deposition modeling systems developed by Stratasys, Inc., Eden Prairie, Minn.) is used to build a 3D model from a digital representation of the 3D model in a layer-by-layer manner by extruding a flowable consumable modeling material. The modeling material is extruded through an extrusion tip carried by an extrusion head, and is deposited as a sequence of roads on a substrate in an x-y plane. The extruded modeling material fuses to previously deposited modeling material, and solidifies upon a drop in temperature. The position of the extrusion head relative to the substrate is then incremented along a z-axis (perpendicular to the x-y plane), and the process is then repeated to form a 3D model resembling the digital representation. 
     Movement of the extrusion head with respect to the substrate is performed under computer control, in accordance with build data that represents the 3D model. The build data is obtained by initially slicing the digital representation of the 3D model into multiple horizontally sliced layers. Then, for each sliced layer, the host computer generates a build path for depositing roads of modeling material to form the 3D model. 
     In fabricating 3D models by depositing layers of a modeling material, supporting layers or structures are typically built underneath overhanging portions or in cavities of objects under construction, which are not supported by the modeling material itself. A support structure may be built utilizing the same deposition techniques by which the modeling material is deposited. The host computer generates additional geometry acting as a support structure for the overhanging or free-space segments of the 3D model being formed. Consumable support material is then deposited from a second nozzle pursuant to the generated geometry during the build process. The support material adheres to the modeling material during fabrication, and is removable from the completed 3D model when the build process is complete. 
     SUMMARY 
     An aspect of the present disclosure is directed to a head tool changer for use with a deposition-based digital manufacturing system. The head tool changer includes a tooling unit configured to retain a deposition head of the system, and an actuator assembly operably mounted to the system, where at least a portion of the actuator assembly is configured to move along an axis. The head tool changer also includes a grip unit secured to the actuator assembly and configured to engage with tooling unit and to relay electrical power to the tooling unit, and a master unit operably mounted to a gantry of the system, where the master unit is configured to engage with the tooling unit and to relay electrical power to the tooling unit. 
     Another aspect of the present disclosure is directed to a head tool changer for use with a deposition-based digital manufacturing system, where the head tool changer includes a plurality of tooling units, each being configured to retain a deposition head of the system, and a plurality of actuator assemblies operably mounted to the system, where at least a portion of each of the plurality of actuator assemblies is configured to move along an axis. The head tool changer also includes a plurality of grip units secured to the plurality of actuator assemblies, where each grip unit is configured to engage with one of the tooling units, and a master unit operably mounted to a gantry of the system, where the tooling units are configured to interchangeably engage with the master unit. 
     Another aspect of the present disclosure is directed to a method for changing deposition heads in a deposition-based digital manufacturing system. The method includes providing a grip unit engaged with a tooling unit, where the tooling unit is secured to one of the deposition heads, relaying electrical power through the grip unit and the tooling unit to the secured deposition head, and engaging the tooling unit with a master unit that is operably mounted to a gantry of the system. The method also includes cutting the relay of the electrical power through the grip unit and the tooling unit, relaying electrical power through the master unit and the tooling unit to the secured deposition head while the tooling unit is engaged with the master unit, and disengaging the grip unit from the tooling unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of a deposition-based digital manufacturing system in use with a head tool changer of the present disclosure. 
         FIGS. 2A-2P  are schematic illustrations of a process for interchangeably loading deposition heads to a gantry of the digital manufacturing system with the use of the head tool changer. 
         FIG. 3  is a schematic illustration of the engagements between a grip unit, a tooling unit, and a master unit of the head tool changer. 
         FIG. 4  is a front perspective view of a tool rest assembly and actuator assemblies of the head tool changer. 
         FIG. 5  is a rear perspective view of the tool rest assembly and the actuator assemblies of the head tool changer. 
         FIG. 6  is a first side perspective view of one of the actuator assemblies and grip units retaining a tooling unit and a deposition head. 
         FIG. 7  is a second side perspective view of the actuator assembly and the grip unit retaining the tooling unit and the deposition head, which is taken from an opposing side from the view shown in  FIG. 6 . 
         FIG. 8  is a top perspective view of the tooling unit and the deposition head engaged with the master unit. 
         FIG. 9A  is a top perspective view of the grip unit. 
         FIG. 9B  is a bottom perspective view of the grip unit. 
         FIG. 10A  is a top perspective view of the tooling unit. 
         FIG. 10B  is a bottom perspective view of the tooling unit. 
         FIG. 11A  is a top perspective view of the master unit. 
         FIG. 11B  is a bottom perspective view of the master unit. 
         FIG. 12A  is a top perspective view of the grip unit, the tooling unit, and the master unit. 
         FIG. 12B  is a bottom perspective view of the grip unit, the tooling unit, and the master unit. 
         FIG. 13  is a rear perspective view of a portion of the tool rest assembly in use with a deposition head retained by the actuator assembly. 
         FIG. 14  is a schematic illustration of an alternative head tool changer of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is directed to a head tool changer that may be mounted to a direct digital manufacturing system, such as a deposition-based digital manufacturing system. The head tool changer is configured to interchangeably load multiple deposition heads to a gantry of the digital manufacturing system, where the multiple deposition heads may be used to build 3D models and support structures using a layer-based additive technique. As discussed below, this allows 3D models and support structures to be built with multiple materials, may reduce transition times when switching materials, and may allow operators to service and repair idle deposition heads while 3D model and support structures are being built. 
       FIG. 1  is a front view of system  10  in use with head tool changer  12 , where system  10  is a deposition-based digital manufacturing system and head tool changer  12  is an example of a suitable head tool changer of the present disclosure. Suitable deposition-based digital manufacturing systems for system  10  include extrusion-based systems and/or jetting systems, each of which may build 3D models and corresponding support structures using a layered-based additive technique. Suitable extrusion-based systems for system  10  include fused deposition modeling systems developed by Stratasys, Inc., Eden Prairie, Minn., such as those disclosed in Comb et al., U.S. Pat. No. 5,939,008; Swanson et al., U.S. Pat. Nos. 6,722,872 and 6,776,602; and Comb et al., U.S. Publication Nos. 2010/0100224 and 2010/0100222; and those commercially available under the trade designation “FORTUS” from Stratasys, Inc., Eden Prairie, Minn. 
     System  10  includes build chamber  14 , platform assembly  16 , and gantry  18 , and bays  19   a - 19   d , where build chamber  14  is an enclosed environment that contains platform assembly  16  and a portion of gantry  18 . During a build operation, build chamber  14  is desirably heated to reduce the rate at which the modeling and support materials solidify after being extruded and deposited. 
     Platform assembly  16  is a receiving platform on which a 3D model and corresponding support structure (not shown) are built, and desirably moves along a vertical z-axis based on signals provided from system controller  20 . Examples of suitable platforms for platform assembly  16  include those disclosed in Comb et al., U.S. Publication No. 2010/0100222. System controller  20  is one or more computer-operated controllers for operating system  10 , and may be located internally or externally to system  10 . 
     Gantry  18  is a guide rail system that is desirably configured to move a deposition head of multiple interchangeable deposition heads  22   a - 22   d  in a horizontal x-y plane within build chamber  14  based on signals provided from system controller  20 . The horizontal x-y plane is a plane defined by an x-axis and a y-axis, where the x-axis, the y-axis, and the z-axis are orthogonal to each other. In an alternative embodiment, platform assembly  16  may be configured to move along two axes within build chamber  14  (e.g., x-z plane or the y-z plane), and the loaded deposition head may be configured to move along a single horizontal axis (e.g., the x-axis or the y-axis). Other similar arrangements may also be used such that one or both of platform assembly  16  and the loaded deposition head are moveable relative to each other. 
     In the shown embodiment, gantry  18  is configured to retain a single deposition head. As such, head tool changer  12  may only load one of deposition heads  22   a - 22   d  to gantry  18  at any given time. Of course, when system  10  is not operating, all four deposition heads  22   a - 22   d  can be removed from gantry  18 . In alternative embodiments, gantry  18  may be configured to retain multiple deposition heads, such as disclosed in Swanson et al., U.S. patent application Ser. No. 12/180,140. 
     Suitable deposition heads for deposition heads  22   a - 22   d  may include a variety of different deposition-based devices, such as extrusion heads, jetting heads, and combinations thereof. Examples of suitable extrusion heads for each of deposition heads  22   a - 22   d  include those disclosed in LaBossiere, et al., U.S. Patent Application Publication Nos. 2007/0003656 and 2007/00228590; and Leavitt, U.S. Patent Application Publication No. 2009/0035405. Alternatively, deposition heads  22   a - 22   d  may each include one or more two-stage pump assemblies, such as those disclosed in Batchelder et al., U.S. Pat. No. 5,764,521; and Skubic et al., U.S. Patent Application Publication No. 2008/0213419. As discussed below, however, because head tool changer  12  allows deposition heads  22   a - 22   d  to be interchangeably loaded to gantry  18 , deposition heads  22   a - 22   d  each desirably only includes a single deposition line (e.g., a single extrusion line) rather than a pair of deposition lines that are toggled back and forth between active and non-active states. 
     Deposition heads  22   a - 22   d  desirably receive consumable materials (e.g., modeling and support materials) from one or more supply sources (not shown) loaded to bays  19 . In some embodiments, the consumable materials may be provided to system  10  as filaments. In these embodiments, suitable supply sources include spools and/or spooled containers, such as those disclosed in Swanson et al., U.S. Pat. No. 6,923,634; Comb et al., U.S. Pat. No. 7,122,246; and Taatjes et al, U.S. Publication Nos. 2010/0096489 and 2010/0096485. Deposition heads  22   a - 22   d  may each also receive consumable materials from two or more spools or spooled containers loaded into bays  19  to provide for a continuous operation, as disclosed in Swanson et al., U.S. Pat. No. 6,923,634. 
     In the shown embodiment, head tool changer  12  is secured to a top section of system  10 , and includes housing  24 , tool rest assembly  26 , and actuator assemblies  28   a - 28   d . Housing  24  is an exterior housing for protecting the components of head tool changer  12 . In one embodiment, housing  24  may encase tool rest assembly  26  and actuator assemblies  28   a - 28   d , and may include a service door (not shown) to allow an operator of system  10  to access components retained within housing  24  (e.g., deposition heads  22   a - 22   d ). Tool rest assembly  26  is a component that allows one or more of deposition heads  22   a - 22   d  to be initialized (e.g., warmed up and purged) prior to use. Actuator assemblies  28   a - 28   d  are extendable components that are configured to load deposition heads  22   a - 22   d  to gantry  18  in an interchangeable manner based on signals provided from head tool changer (HTC) controller  30 . 
     HTC controller  30  is also one or more computer-operated controllers for operating head tool changer  12 , and may be located internally or externally to system  10  and/or head tool changer  12 . In one embodiment, the functions of system controller  20  and HTC controller  30  may be combined into a common computer-operated controller that may be located internally or externally to system  10  and/or head tool changer  12 . Tool rest assembly  26  and actuator assemblies  28   a - 28   d  are discussed in more detail below. 
     The following discussion of head tool changer  12  illustrates the use of four interchangeable deposition heads (i.e., deposition heads  22   a - 22   d ). However, head tool changer  12  may be configured to load additional or fewer numbers of deposition heads to gantry  18 . Examples of suitable numbers of deposition heads for use with head tool changer  12  range from two to ten, with particularly suitable numbers ranging from three to six. 
       FIGS. 2A-2P and 3  are schematic illustrations of a process for interchangeably loading deposition heads  22   a - 22   d  to gantry  18  of system  10  with the use the head tool changer  12 .  FIGS. 4-13  subsequently illustrate suitable features of the components described in  FIGS. 2A-2P and 3 , pursuant to one embodiment of the present disclosure. As shown in  FIG. 2A , head tool assembly  12  also includes grip units  34   a - 34   d , tooling units  36   a - 36   d , and master unit  38 . Grip units  34   a - 34   d  are respectively secured to actuator assemblies  28   a - 28   d  and are configured to engage with and lock to tooling units  36   a - 36   d . Tooling units  36   a - 36   d  are respectively secured to deposition heads  22   a - 22   d  and are configured to engage with and lock to grip units  34   a - 34   d  and master unit  38 . Master unit  38  is secured to gantry  18  (e.g., in a carriage of gantry  18 ) and is configured to interchangeably engage tooling units  36   a - 36   d . Accordingly, gantry  18  is configured to move master unit  38  around in the horizontal x-y plane. 
     As discussed below, grip units  34   a - 34   d  are configured to relay electrical power and control signals from HTC controller  30  to tooling units  36   a - 36   d  when tooling units  36   a - 36   d  are respectively engaged with grip units  34   a - 34   d . Similarly, master unit  38  is configured to relay electrical power and control signals from system controller  20  to one of tooling units  36   a - 36   d  when the given tooling unit is engaged with master unit  38 . Tooling units  36   a - 36   d  are also configured to relay the received electrical power and control signals respectively to deposition heads  22   a - 22   d . As such, when tooling units  36   a - 36   d  are engaged with grip units  34   a - 34   d , deposition heads  22   a - 22   d  receive electrical power and control signals from HTC controller  30 . Alternatively, when one of tooling units  36   a - 36   d  is engaged with master unit  38 , the corresponding deposition head receives electrical power and control signals from system controller  20 . 
     In the example shown in  FIG. 2A , grip units  34   a - 34   d  are respectively engaged with and locked to tooling units  36   a - 36   d , thereby allowing deposition heads  22   a - 22   d  to be operably retained by actuator assemblies  28   a - 28   d . As such, deposition heads  22   a - 22   d  receive electrical power and control signals from HTC controller  30  via actuator assemblies  28   a - 28   d , grip units  34   a - 34   d , and tooling units  36   a - 36   d.    
     Actuator assemblies  28   a - 28   d  are each configured to retract and extend along the vertical z-axis between a raised position and one or more lowered positions. In particular, as shown in  FIG. 2A , actuator assemblies  28   a - 28   d  may extend to a resting position such that tooling plates  36   a - 36   d  rest on tool rest assembly  26 . As discussed above, tool rest assembly  26  is a component on which idle deposition heads  22   a - 22   d  may rest when not loaded to gantry  18 , and may be used for initializing (e.g., warming up and purging) one or more of deposition heads  22   a - 22   d  for use in system  10 . 
     Prior to building a 3D model or support structure, HTC controller  30  may initialize (e.g., warm up and purge) one or more of deposition heads  22   a - 22   d  for use in system  10 . For example, HTC controller  30  may direct head tool changer  12  may to initialize deposition head  22   a  at tool rest assembly  26 . When deposition head  22   a  is ready for use, system controller  20  may align gantry  18  such that master unit  38  is positioned below actuator assembly  28   a  in the horizontal x-y plane to receive deposition head  22   a , as shown in  FIG. 2A . 
     As shown in  FIG. 2B , HTC controller  30  may then retract actuator assemblies  28   a - 28   d  upward to their raised positions, as indicated by arrows  40 . This desirably positions depositions heads  22   a - 22   d  vertically higher than tool rest assembly  26 . As shown in  FIG. 2C , HTC controller  30  may then direct tool rest assembly  26  to slide along the x-axis in the direction of arrow  42  to avoid obstructing actuator assemblies  28   a - 28   d.    
     As shown in  FIG. 2D , HTC controller  30  may then extend actuator assembly  28   a  downward to an engagement position such that tooling unit  36   a  engages master unit  38 , as indicated by arrow  44 . At this point, tooling unit  36   a  may unlock from grip unit  34   a  and lock to master unit  38 . Additionally, control of deposition head  22   a  may transfer from head tool changer  12  and HTC controller  30  to system  10  and system controller  20 . In particular, deposition head  22   a  may receive electrical power and control signals from system controller  20  respectively via master unit  38  and tooling unit  36   a , and the electrical power and control signals relayed through actuator assembly  28   a , grip unit  34   a , and tooling unit  36   a  may be cut off. 
     As shown in  FIG. 2E , HTC controller  30  may then retract actuator assembly  28   a  to its raised position, as indicated by arrow  46 . This correspondingly disengages grip unit  34   a  from tooling unit  36   a , and raises grip unit  34   a  upward into head tool changer  12 . As shown in  FIG. 2F , HTC controller  30  may then direct tool rest assembly  26  to slide back along the x-axis to extend below actuator assemblies  28   a - 28   d , as indicated by arrow  48 . 
     As shown in  FIG. 2G , HTC controller  30  may then direct actuator assemblies  28   a - 28   d  to extend to their resting positions such that tooling units  36   b - 36   d  rest on tool rest assembly  26 , as indicated by arrows  50 . At this point, system controller  20  may also direct gantry  18  to move deposition head  22   a  (and tooling unit  36   a  and master unit  38 ) around in the horizontal x-y plane within build chamber  14  (shown in  FIG. 1 ), and may direct one or more feed mechanisms (not shown) to feed a consumable material through deposition head  22   a . The received consumable material is then deposited onto platform assembly  16  (shown in  FIG. 1 ) to build at least a portion of a 3D model or support structure using a layer-based additive technique. After each layer is complete, platform assembly  16  may be lowered by an increment along the z-axis to allow successive layers to be formed on top of the previously deposited layers. 
     While deposition head  22   a  is functioning as the active deposition head, HTC controller  30  may also direct one or more of deposition heads  22   b - 22   d  to be initialized for use in system  10 . This allows the initializations of the deposition heads  22   b - 22   d  to be performed at the same time as deposition head  22   a  is in use in system  10 . For example, HTC controller  30  may initialize deposition head  22   c  for operation after active deposition head  22   a  has completed its deposition steps. The timing sequence for initializing deposition head  22   c  desirably has deposition head  22   c  ready for use as soon as deposition head  22   a  completes its deposition steps. 
     In comparison, a deposition head that contains two deposition lines (e.g., extrusion lines), such as the deposition head disclosed in Leavitt, U.S. Patent Application Publication No. 2009/0035405, typically requires the non-active deposition line to be warmed up and purged between deposition steps. Otherwise, the non-active deposition line may interfere with the deposition from the active deposition line (e.g., material may potentially leak from the non-active deposition line). These the warm up and purge processes between the deposition steps, however, accumulate over the numerous layers used to build 3D models and support structures, This can account for a substantial portion of the overall build time. Initializing deposition heads  22   b - 22   d  in tandem with the operation of deposition head  22   a , however, effectively removes the delays incurred with warming up and purging non-active deposition lines, thereby substantially reducing the overall build time. 
     In addition, an operator of system  10  may inspect, repair, or otherwise perform work on deposition heads  22   b - 22   d  while deposition head  22   a  continues to build the 3D model or support structure. As such, in addition to initializing the non-active deposition heads (e.g., deposition heads  22   b - 22   d ) in tandem with the operation of the active deposition head (e.g., deposition head  22   a ), the non-active deposition heads may also be maintained while inactive, thereby reducing maintenance delays that may otherwise occur during operation. 
     As shown in  FIG. 2H , after deposition head  22   a  completes its deposition steps, system controller  20  may direct gantry  18  to position tooling unit  36   a  and master plate  38  below actuator assembly  28   a  in the horizontal x-y plane. HTC controller  30  may also retract actuator assemblies  28   a - 28   d  upward to their raised positions, as indicated by arrows  52 . As discussed above, this desirably positions depositions heads  22   b - 22   d  vertically higher than tool rest assembly  26 . As shown in  FIG. 2I , HTC controller  30  may then direct tool rest assembly  26  to slide along the x-axis in the direction of arrow  54  to again avoid obstructing actuator assemblies  28   a - 28   d.    
     As shown in  FIG. 2J , HTC controller  30  may then extend actuator assembly  28   a  downward to its engagement position, thereby allowing grip unit  34   a  to engage with and lock to tooling unit  36   a , as indicated by arrow  56 . At this point, tooling unit  36   a  may unlock from master unit  38  and lock to grip unit  34   a . Additionally, control of deposition head  22   a  may now transfer from system  10  and system controller  20  back to head tool changer  12  and HTC controller  30 . In particular, deposition head  22   a  may receive electrical power and control signals from HTC controller  30  via actuator assembly  28   a , grip unit  34   a , and tooling unit  36   a , and the electrical power and control signals relayed through master unit  38  and tooling unit  36   a  may be cut off. 
     As shown in  FIG. 2K , HTC controller  30  may then retract actuator assembly  28   a  to its raised position, as indicated by arrow  58 . This correspondingly disengages tooling unit  36   a  from master unit  38 , and raises grip unit  34   a , tooling unit  36   a , and deposition head  22   a  upward into head tool changer  12 . As shown in  FIG. 2L , system controller  20  may then direct gantry  18  to move master unit  38  to position it below the next actuator assembly to be used (e.g., actuator assembly  28   c ) in the horizontal x-y plane, as indicated by arrow  60 . 
     As shown in  FIG. 2M , HTC controller  30  may then extend actuator assembly  28   c  downward to an engagement position such that tooling unit  36   c  engages master unit  38 , as indicated by arrow  62 . At this point, tooling unit  36   c  may unlock from grip unit  34   c  and lock to master unit  38 . Additionally, control of deposition head  22   c  may now transfer from head tool changer  12  and HTC controller  30  to system  10  and system controller  20 . In particular, deposition head  22   c  may receive electrical power and control signals from system controller  20  respectively via master unit  38  and tooling unit  36   c , and the electrical power and control signals relayed through actuator assembly  28   c , grip unit  34   c , and tooling unit  36   c  may be cut off. 
     As shown in  FIG. 2N , HTC controller  30  may then retract actuator assembly  28   c  to its raised position, as indicated by arrow  64 . This correspondingly disengages tooling unit  36   c  from grip unit  34   a , and raises grip unit  34   c  upward into head tool changer  12 . As shown in  FIG. 2O , HTC controller  30  may then direct tool rest assembly  26  to slide back along the x-axis to extend below actuator assemblies  28   a - 28   d , as indicated by arrow  66 . 
     As shown in  FIG. 2P , HTC controller  30  may then direct actuator assemblies  28   a - 28   d  to extend downward to their resting positions such that tooling units  36   a ,  36   b , and  36   d  rest on tool rest assembly  26 , as indicated by arrows  68 . At this point, system controller  12  may also direct gantry  18  to move deposition head  22   c  (and tooling unit  36   c  and master unit  38 ) around in the horizontal x-y plane within build chamber  14  (shown in  FIG. 1 ). This allows deposition head  22   c  to deposit a consumable material to build an additional portion of a 3D model or support structure using the layer-based additive technique. 
     This process may then be repeated in a variety of patterns for building the 3D model and support structure with the materials from one or more of deposition heads  22   a - 22   d . As discussed above, initializing the idle deposition heads with head tool changer  12  in tandem with the operation of the active deposition head may substantially reduce the overall build time. Additionally, the interchangeability of deposition heads  22   a - 22   d  allows deposition heads  22   a - 22   d  to each include a single deposition line (e.g., a single extrusion line). This precludes the need for a second, non-active deposition line, which may otherwise interfere with the deposition from the active deposition line (e.g., material leakage). 
     Furthermore, the interchangeability of deposition heads  22   a - 22   d  allows different materials to be deposited from deposition heads  22   a - 22   d . This allows the 3D model and/or support structure to each be built with multiple materials having different physical, chemical, and/or aesthetic properties. For example, deposition head  22   a  may deposit an acrylonitrile-butadiene-styrene (ABS) modeling material that is black in color, deposition head  22   b  may deposit an ABS modeling material that is red in color, deposition head  22   c  may deposit an ABS-polycarbonate modeling material that is blue in color, and deposition head  22   d  may deposit a support material for building a corresponding support structure. Building 3D models from multiple materials may increase the functional and aesthetic properties of the given 3D models compared to a 3D model built from a single material. 
     Moreover, deposition heads  22   a - 22   d  may exhibit different build parameters. For example, one or more of deposition heads  22   a - 22   d  may be a jetting head while others are extrusion heads. Additionally, deposition heads  22   a - 22   d  may operate at different extrusion temperatures for use with different consumable materials and/or may have different extrusion tip sizes. These different parameters may be desirable in many applications and they increase the design ranges of 3D models and support structures that may be built with system  10 . 
       FIG. 3  is a schematic illustration of the engagements between grip unit  34   a , tooling unit  36   a , and master unit  38 , which corresponds to the examples shown in  FIGS. 2D and 2J , where actuator assembly  28   a  is extended downward to its engagement position. The following discussion of deposition head  22   a , grip unit  34   a , tooling unit  36   a , and master unit  38  may also apply to each additional actuator assembly of head tool changer  12  (e.g., actuator assemblies  28   b - 28   d ) in the same manner. 
     As shown in  FIG. 3 , when engaged together, grip unit  34   a  and tooling unit  36   a  define power line  70 , which is one or more conductive lines configured to receive electrical power from head tool changer  12  (via one or more external power lines), and to relay the electrical power to deposition head  22   a . Additionally, grip unit  34   a  and tooling unit  36   a  define signal line  72 , which is one or more data communication lines configured to receive control signals from HTC controller  30  (via one or more external signal lines), and to relay the control signals to deposition head  22   a.    
     Similarly, when engaged together, master unit  38  and tooling unit  36   a  define power line  74 , which is one or more conductive lines configured to receive electrical power from system  10  (via one or more external power lines), and to relay the electrical power to deposition head  22   a . Additionally, master unit  38  and tooling unit  36   a  define signal line  76 , which is one or more data communication lines configured to receive control signals from system controller  20  (via one or more external signal lines), and to relay the control signals to deposition head  22   a.    
     Deposition head  22   a  is secured to tooling unit  36   a , which allows deposition head  22   a  to receive electrical power from either power line  70  or power line  72 , and to receive control signals from either signal line  72  or signal line  74 , depending on whether head tool changer  12  or system  10  is selected as the controlling system. The transfer of which system controls deposition head  22   a  may be made when tooling unit  36   a  is engaged with grip unit  34   a  and with master unit  38 . 
     In one embodiment, the direction of the transfer of control may be determined based on the sequence of operation and the previous state of control. For example, while actuator assembly  28   a  is loading tooling unit  36   a  and deposition head  22   a  to gantry  18  (e.g., as shown in  FIG. 2D ), HTC controller  30  has initial control over deposition head  22   a . As such, when tooling unit  36   a  engages master unit  38 , control may transfer from HTC controller  30  to system controller  20 . Alternatively, while actuator assembly  28   a  is removing tooling unit  36   a  and deposition head  22   a  from gantry  18  (e.g., as shown in  FIG. 2J ), system controller  20  has initial control over deposition head  22   a . As such, when grip unit  34   a  engages tooling unit  36   a , control may transfer from system controller  20  to HTC controller  30 . System controller  20  and HTC controller  30  may also communicate with each other to initiate the transfers of control. 
     Providing electrical power and control signals to deposition head  22   a  while retained in head tool changer  12  is desirable for initializing deposition head  22   a  in tandem with the operation of another deposition head in gantry  18 . As discussed above, this can substantially reduce the overall build time. Otherwise, if the idle deposition heads only received electrical power and signal controls while loaded to gantry  18 , the non-active deposition heads would need to be loaded to gantry  18  before they could be initialized. This would effectively eliminate the benefits of head tool changer  12  for reducing overall build times. 
     As further shown in  FIG. 3 , the engagement between grip unit  34   a  and tooling unit  36   a  also defines locking mechanism  78 , which is a first mechanism for locking tooling unit  36   a  to grip unit  34   a . Locking mechanism  78  may function in a variety of manners, such as electromechanical or pressure-based (e.g., pneumatic or hydraulic) locks. Accordingly, when engaged together, grip unit  34   a  and tooling unit  36   a  define conduit  80 , which may be one or more power lines for supplying electrical power to locking mechanism  78  (for electromechanical locks) or one or more fluid lines for providing and expelling pressurized fluids to and from locking mechanism  78  (for pneumatic or hydraulic locks). 
     Similarly, the engagement between tooling unit  36   a  and master unit  38  defines locking mechanism  82 , which is a second mechanism for locking tooling unit  36   a  to master unit  38 . Locking mechanism  82  may also function in a variety of manners, such as electromechanical or pressure-based (e.g., pneumatic or hydraulic) locks. Accordingly, when engaged together, master unit  38  and tooling unit  36   a  define conduit  84 , which may be one or more power lines for supplying electrical power to locking mechanism  82  (for electromechanical locks) or one or more fluid lines for providing and expelling pressurized fluids to and from locking mechanism  82  (for pneumatic or hydraulic locks). 
     For example, while actuator assembly  28   a  loads tooling unit  36   a  and deposition head  22   a  to gantry  18  (e.g., as shown in  FIG. 2D ), tooling unit  36   a  is initially locked to grip unit  34   a  (i.e., locking mechanism  78  is activated). As such, when tooling unit  36   a  engages master unit  38 , in addition to transferring the control of deposition head  22   a  from HTC controller  30  to system controller  20 , locking mechanism  78  may deactivate and locking mechanism  82  may activate, thereby locking tooling unit  36   a  to master unit  38 . Alternatively, while actuator assembly  28   a  is removing tooling unit  36   a  and deposition head  22   a  from gantry  18  (e.g., as shown in  FIG. 2J ), tooling unit  36   a  is initially locked to master unit  38  (i.e., locking mechanism  82  is activated). As such, when grip unit  34   a  engages tooling unit  36   a , in addition to transferring the control of deposition head  22   a  from system controller  20  back to HTC controller  30 , locking mechanism  82  may deactivate and locking mechanism  78  may activate, thereby locking tooling unit  36   a  to grip unit  34   a . This locking arrangement provides an efficient manner for interchangeably retaining tooling unit  36   a  (and deposition head  22   a ) with either grip unit  34   a  or master unit  38 . 
     As discussed above,  FIGS. 4-13  illustrate suitable features of the components described in  FIGS. 2A-2P and 3 , pursuant to one embodiment of the present disclosure. For ease of discussion, the components of head tool changer  12  discussed in  FIGS. 4-13  are described with the same reference labels as those used for the components discussed in  FIGS. 2A-2P and 3 . 
       FIGS. 4 and 5  are expanded front and rear perspective views of tool rest assembly  26  and actuator assemblies  28   a - 28   d  of head tool changer  12 . As shown in  FIG. 4 , tool head changer  12  also includes cross plates  86  and  88 , and retention member  90 , each of which are desirably secured to a frame structure of head tool changer  12  (not shown). Tool rest assembly  26  may be secured to the frame structure with cross plate  86 , thereby positioning tool rest assembly  26  at a bottom front location of head tool changer  12  in the shown embodiment. Similarly, actuator assemblies  28   a - 28   d  may each be secured to the frame structure with cross plate  88 , thereby allowing actuator assemblies  28   a - 28   d  to be suspended over gantry  18  (shown in  FIG. 1 ) and tool rest assembly  26 . Retention member  90  is an additional plate configured to restrict lateral movement of actuator assemblies  28   a - 28   d  along the y-axis. 
     As further shown, tool rest assembly  26  includes tool rests  92   a - 92   d , air circulators  94   a - 94   d , and purge receptacle  96 , where air circulators  94   a - 94   d  may be secured to cross plate  86 . As discussed below, tool rests  92   a - 92   d  and purge receptacle  96  are desirably slidable relative to cross plate  86  to slide along the x-axis, as discussed above. 
     Actuator assemblies  28   a - 28   d  respectively include actuator arms  98   a - 98   d  and guide rails  100   a - 100   d , where the bottom ends of actuator arms  98   a - 98   d  are respectively secured to cross plate  88  with mounting brackets  102   a - 102   d . The top ends of actuator arms  98   a - 98   d  are respectively connected to the top ends of guide rails  100   a - 100   d , thereby allowing the retraction and extension of actuator arms  98   a - 98   d  to respectively move guide rails  100   a - 100   d  upward and downward between the raised position and one or more lowered positions (e.g., the resting and engagement positions). 
     As shown in  FIG. 5 , tool rest assembly  26  also includes slide track  104  and mounting plate  106 , where mounting plate  106  is configured to move back and forth along the x-axis on slide track  104 . HTC controller  30  may move mounting plate  106  with the use of a variety of drive mechanisms, such as pneumatic drives, hydraulic drives, and electrochemical motor drives. Tool rests  92   a - 92   d  are secured to mounting plate  106 , and purge receptacle  96  is desirably secured to tool rests  92   a - 92   d . This arrangement allows tool rests  92   a - 92   d  to slide along the x-axis, as discussed above, to avoid obstructing the lowering of actuator assemblies  28   a - 28   d  to their engagement positions. The use of tool rests  92   a - 92   d  and purge receptacle  96  for initializing deposition heads  22   a - 22   d  is further discussed below. 
     Actuator assemblies  28   a - 28   d  also respectively include sleeve brackets  108   a - 108   d , which are secured to cross plate  88 . Guide rails  100   a - 100   d  respectively extend through sleeve brackets  108   a - 108   d , thereby restricting the movement of guide rails  100   a - 100   d  to upward and downward directions along the vertical z-axis. 
     The following discussion in  FIGS. 6-13  is directed to deposition head  22   a , actuator assembly  28   a , grip unit  34   a , and tooling unit  36   a . However, the discussion may also apply to deposition heads  22   b - 22   d , actuator assemblies  28   b - 28   d , grip units  34   b - 34   d , and tooling units  36   b - 36   d  in the same manner.  FIGS. 6 and 7  are opposing side perspective views of grip unit  34   a  and tooling unit  36   a  operably connecting guide rail  100   a  and deposition head  22   a.    
     As shown in  FIG. 6 , grip unit  34   a  includes base component  110 , compensator  112 , leads  114   a  and  114   b , and couplings  116 . Base component  110  is the portion of grip unit  34   a  that engages with and is lockable to tooling unit  36   a  with locking mechanism  78  (shown in  FIG. 3 ). In the shown embodiment, base component  110  is fabricated from multiple sub-blocks that are secured together with fasteners. In an alternative embodiment, base component  110  may be fabricated as an integral block. Compensator  112  is secured to the top surface of base component  110 , and is the portion of grip unit  34   a  that is secured to the bottom end of guide rail  102   a . As discussed below, compensator  112  allows tooling unit  36   a  to float laterally and vertically when engaging tooling unit  36   a  with master unit  38 . 
     Leads  114   a  and  114   b  are electrical connections secured to base component  110 , and are configured to be connected to external cables (not shown) to receive electrical power and control signals from head tool changer  12  and HTC controller  30 , as discussed above for power line  70  and signal line  72  (shown in  FIG. 3 ). 
     Couplings  116  are gas couplings secured to base component  110 , and are configured to be connected to external fluid conduits (not shown) to receive and expel pressurized gases to operate locking mechanism  78 , as discussed above for conduit  80  (shown in  FIG. 3 ). Accordingly, in this embodiment, locking mechanism  78  may function as a pneumatic locking mechanism. In one embodiment, the external fluid conduits for supplying and recycling the pressurized gases may also extend along or within actuator arm  98   a  and guide rail  100   a  to connect with couplings  116 . 
     Tooling unit  36   a  includes base component  118 , which is secured to deposition head  22   a  and is the portion of tooling unit  36   a  that engages with and is lockable to base component  110  of grip unit  34   a  with locking mechanism  78 . In the shown embodiment, base component  118  is also fabricated from multiple sub-blocks that are secured together with fasteners. In an alternative embodiment, base component  118  may be fabricated as an integral block. 
     As further shown in  FIG. 6 , actuator assembly  28   a  also includes electrical connections  120  adjacent to mounting bracket  102   a , where electrical connections  120  are configured to receive electrical power and control signals from head tool changer  12  and HTC controller  30  via one or more external electrical cables (not shown). This allows HTC controller  30  to direct the operation of actuator assembly  28   a  for raising and lowering guide rail  100   a  along the vertical z-axis. In addition, one or more cables (not shown) may also extend along or within actuator arm  98   a  and guide rail  100   a , thereby relaying the electrical power and control signals from electrical connections  120  to leads  114   a  and  114   b  of grip unit  34   a.    
     In the shown embodiment, deposition head  22   a  includes control board  122 , drive mechanism  124 , thermal block  126 , and extrusion tip  128 , which may form a single extrusion line, such as a single extrusion line of the extrusion head described in Leavitt, U.S. Patent Application Publication No. 2009/0035405. Drive mechanism  124  may receive a filament of a consumable material from one or more supply sources retained in bays  19  (shown in  FIG. 1 ) through guide tube  130 . 
     As shown in  FIG. 7 , deposition head  22   a  also include bracket  132 . Bracket  132  is a frame component of deposition head  22   a  and may retain control board  122 , drive mechanism  124 , and thermal block  126 . Bracket  132  is also the portion of deposition head  22   a  that may be secured to base component  118  of tooling unit  36   a.    
       FIG. 8  is a front perspective view of tooling unit  36   a  and deposition head  22   a  engaged with master unit  38 . As shown, control board  122  of deposition head  22   a  includes power connection ports  134   a  and  134   b , and signal connection ports  135   a  and  135   b , which are configured to receive electrical power and control signals from tooling unit  36   a  via external cables (not shown). For example, power connection port  134   a  and signal connection port  135   a  are configured to receive electrical power and control signals from grip unit  34   a  and tooling unit  36   a , as discussed above for power line  70  and signal line  72  (shown in  FIG. 3 ). Correspondingly, power connection port  134   b  and signal connection port  135   b  are configured to receive electrical power and control signals from master unit  38  and tooling unit  36   a , as discussed above for power line  74  and signal line  76  (shown in  FIG. 3 ). 
     Tooling unit  36   a  also includes electrical contacts  136   a  and  136   b , lock ring  138 , guide holes  140 , and top surface  142 , where top surface  142  is the surface of base component  118  that engages with grip unit  34   a . Electrical contacts  136   a  and  136   b  are conductive contacts located at top surface  142 , and are configured to engage with reciprocating electrical contacts (not shown in  FIG. 8 ) located at a bottom surface of grip unit  34   a . This allows the electrical power and control signals that are received through leads  114   a  and  114   b  of grip unit  34   a  (shown in  FIGS. 6 and 7 ) to be relayed to tooling unit  36   a , as discussed above. 
     Lock ring  138  is a female portion of locking mechanism  78  (shown in  FIG. 3 ) disposed in base component  118  at top surface  142 . Lock ring  138  is configured to receive a reciprocating male portion of locking mechanism  78  retained by grip unit  34   a  for locking tooling unit  36   a  to grip unit  34   a.    
     Guide holes  140  are a pair of holes extending within base component  118  at top surface  142 , and are configured to receive guide pins (not shown in  FIG. 8 ) extending from the bottom surface of grip unit  34   a . This arrangement allows the guide pins to align with guide holes  140  when grip unit  34   a  and tooling unit  36   a  engage each other. 
     As further shown in  FIG. 8 , master unit  38  includes base component  144 , leads  146   a  and  146   b , and couplings  148 . Base component  144  is the portion of master unit  38  that engages with and is lockable to the bottom surface of tooling unit  36   a  with locking mechanism  82  (shown in  FIG. 3 ). Base component  144  is also the portion that may be secured to a carriage of gantry  18 , such as in an adjustable head mount as disclosed in Comb et al., U.S. Publication No. 2010/0100222. In the shown embodiment, base component  144  is also fabricated from multiple sub-blocks that are secured together with fasteners. In an alternative embodiment, base component  144  may be fabricated as an integral block. 
     Leads  146   a  and  146   b  are electrical connections secured to base component  144 , and are configured to be connected to external cables (not shown) to receive electrical power and control signals from system  10  and system controller  20 , as discussed above for power line  74  and signal line  76  (shown in  FIG. 3 ). Couplings  148  are gas couplings secured to base component  144 , and are configured to be connected to external fluid conduits (not shown) to receive and expel pressurized gases to operate locking mechanism  82 , as discussed above for conduit  84  (shown in  FIG. 3 ). Accordingly, in this embodiment, locking mechanism  82  may also function as a pneumatic locking mechanism. 
       FIGS. 9A and 9B  are respectively top and bottom perspective views of grip unit  34   a . As shown in  FIG. 9A , compensator  112  includes top surface  150 , springs  152 , and electrical ports  154 , and is configured to switch between an unlocked state and a locked state based on signals received through electrical ports  154 . Top surface  150  is the portion of compensator  112  that may be secured to the bottom end of guide rail  100   a  (shown in  FIGS. 4-7 ). While in the unlocked state, springs  152  allow grip unit  34   a  (and tooling unit  36   a  when engaged with grip unit  34   a ) to float laterally and vertically. This correspondingly provides grip unit  34   a  a small freedom of movement when actuator assembly  28   a  is aligning with master unit  38 . As such, when deposition head  22   a  is being loaded to gantry  18 , compensator  112  is desirably set to the unlocked state while grip unit  34   a  and tooling unit  36   a  align and engage with master unit  38 . When tooling unit  36   a  engages with master unit  38 , compensator  112  may then be locked to prevent further lateral or vertical floating. 
     As shown in  FIG. 9B , grip unit  34   a  further includes bottom surface  156 , electrical contacts  158   a  and  158   b , lock extension  160 , and guide pins  162 . Bottom surface  156  is the surface of grip unit  34   a  that may engage with top surface  142  of tooling unit  36   a  (shown in  FIG. 8 ). Electrical contacts  158  and  158   b  are conductive contacts located at bottom surface  156 , and are configured to engage with electrical contacts  136   a  and  136   b  (shown in  FIG. 8 ) of tooling unit  36   a.    
     Lock extension  160  is a male portion of locking mechanism  78  (shown in  FIG. 3 ) extending from bottom surface  156 , and is configured to extend into lock ring  138  of tooling unit  36   a  for locking grip unit  34   a  to tooling unit  36   a . In the shown embodiment, lock extension  160  includes a plurality of plugs that are capable of expanding outward and contracting inward from lock extension  160  based on the pressure within lock extension  160 . As a result, lock extension  160  may be secured to lock ring  138  by introducing pressurized gas through couplings  116 , which cause the plugs to expand outward to physically trap lock extension  160  in lock ring  138 . Guide pins  162  are a pair of pins extending downward from bottom surface  156 , and are configured to engage guide holes  140  of tooling unit  36   a , as discussed above. 
       FIGS. 10A and 10B  are respectively top and bottom perspective views of tooling unit  36   a . As shown in  FIG. 10A , tooling unit  36   a  also includes lateral surface  164 , power connection ports  166   a  and  166   b , and signal connection ports  168   a  and  168   b . Lateral surface  164  is the surface of tooling unit  36   a  that may be secured to bracket  132  of deposition head  22   a  to secure deposition head  22   a  to tooling unit  36   a . Power connection ports  166   a  and  166   b , and signal connection ports  168   a  and  168   b  are configured to relay electrical power and control signals from tooling unit  36   a  respectively to power connection ports  134   a  and  134   b  and signal connection ports  135   a  and  135   b  of deposition head  22   a , via external cables (not shown). As discussed above, this arrangement allows deposition head  22   a  to receive electrical power and control signals from power line  70  and signal line  72  (shown in  FIG. 3 ) that are relayed through grip unit  34   a  and tooling unit  36   a , and from power line  74  and signal line  76  (shown in  FIG. 3 ) that are relayed through master unit  38  and tooling unit  36   a.    
     As shown in  FIG. 10B , tooling unit  36   a  further includes bottom surface  170 , electrical contacts  172   a  and  172   b , lock ring  174 , guide holes  176 , and mating guides  178 . Bottom surface  170  is the surface of tooling unit  36   a  that may engage with master unit  38 . Electrical contacts  172   a  and  172   b  are conductive contacts secured to base component  118 , and are configured to engage with reciprocating electrical contacts of master unit  38  (not shown in  FIG. 10B ). 
     Lock ring  174  is a female portion of locking mechanism  82  (shown in  FIG. 3 ) disposed in bottom surface  170 , and is configured to receive a lock extension of master unit  38  (not shown in  FIG. 10B ). Guide holes  176  are a pair of holes extending within bottom surface  170  and are configured to receive guide pins of master unit  38  (not shown in  FIG. 10B ). Mating guides  178  are a plurality of guides configured to receive domes (not shown in  FIG. 10B ) of master unit  38 . As discussed below, the engagement between the domes and mating guides  178  provide a precision mating mechanism for tooling unit  36   a  and master unit  38 . 
       FIGS. 11A and 11B  are respectively top and bottom perspective views of master unit  38 . As shown, master unit  38  also includes top surface  180 , electrical contacts  182   a  and  182   b , lock extension  184 , guide pins  186 , and domes  188 . Top surface  180  is the surface of master unit  38  that may engage with bottom surface  170  of tooling unit  36   a . Electrical contacts  182   a  and  182   b  are conductive contacts located at top surface  180 , are configured to engage with electrical contacts  172   a  and  172   b  of tooling unit  36   a . This allows the electrical power and control signals that are received through leads  146   a  and  146   b  to be relayed to tooling unit  36   a , as discussed above. 
     Lock extension  184  is a male portion of locking mechanism  82  (shown in  FIG. 3 ) extending from top surface  180 , and may function in the same manner as lock extension  160  (shown in  FIG. 9B ). Accordingly, lock extension  184  is configured to extend into lock ring  174  of tooling unit  36   a , thereby allowing master unit  38  to lock to tooling unit  36   a  based on the pressurized gases received via couplings  148 . Guide pins  186  are a pair of pins extending upward from top surface  180 , and are configured to engage guide holes  176  located bottom surface  170  of tooling unit  36   a . This arrangement allows guide pins  186  to align with guide holes  176  when tooling unit  36   a  and master unit  38  engage each other. 
     Domes  188  are a plurality of topographical features (e.g., half spheres) extending above the plane of top surface  180  and are configured to engage with mating guides  178  to provide a precision mating mechanism. In the shown embodiment in which master unit  38  includes three domes  188  and tooling unit  36   a  includes three mating guides  178 , this precision mating provides six degrees of restraint. This restraint defines a rigid body that resists lateral movement of tooling unit  36   a  relative to master unit  38  in the horizontal x-y plane when tooling unit  36   a  is locked to master unit  38 . This is desirable to allow gantry  18  to prevent tooling unit  36   a  and deposition head  22   a  from moving laterally relative to master unit  38  during operation in system  10 . 
       FIGS. 12A and 12B  are respectively top and bottom exploded perspective views of grip unit  34   a , tooling unit  36   a , and master unit  38 , which illustrate their respective engagements with each other. For example, when grip unit  34   a  is lowered onto tooling unit  36   a  to retract tooling unit  36   a  from master unit  38  (e.g., as shown in  FIG. 2J ), guide pins  162  may enter guide holes  140  to align grip unit  34   a  with tooling unit  36   a , and lock extension  160  may enter lock ring  138 . The lateral freedom attained with compensator  112  in the unlocked state reduces the risk of damage to grip unit  34   a  and tooling unit  36   a  during the alignment. 
     When grip unit  36   a  engages tooling unit  36   a , electrical contacts  158   a  and  158   b  of grip unit  34   a  engage with electrical contacts  136   a  and  136   b  of tooling unit  36   a , thereby allowing electrical power and control signals to be relayed from leads  114   a  and  114   b  of grip unit  34   a  to electrical contacts  166   a  and  168   a  of tooling unit  36   a . Head tool changer  12  may then introduce pressurized gas into couplings  116  to engage lock extension  160  within lock ring  138  to lock tooling unit  36   a  to grip unit  34   a . Additionally, compensator  112  may be locked to restrict lateral movement. 
     System  10  may also release the pressurized gas from couplings  148  to unlock lock extension  184  from lock ring  174 , thereby unlocking tooling unit  36   a  from master unit  38 . Additionally, transfer of electrical power and signal control of deposition head  22   a  to HTC controller  30  may also occur after electrical contacts  158   a  and  158   b  engage with electrical contacts  136   a  and  136   b . Actuator assembly  28   a  may then raise grip unit  34   a  to disengage tooling unit  36   a  from master unit  38 , as discussed above. 
     Alternatively, when grip unit  34   a  and tooling unit  36   a  are lowered onto master unit  38  (e.g., as shown in  FIG. 2D ), guide pins  186  may enter guide holes  176  to align tooling unit  36   a  with master unit  38 , and lock extension  184  may enter lock ring  174 . The lateral freedom attained with compensator  112  in the unlocked state also reduces the risk of damage to tooling unit  36   a  and master unit  38  during this alignment. 
     When tooling unit  36   a  engages master unit  38 , electrical contacts  172   a  and  172   b  of tool unit  36   a  engage with electrical contacts  182   a  and  182   b  of master unit  38 , thereby allowing electrical power and control signals to be relayed from leads  144   a  and  144   b  of master unit  38  to electrical contacts  166   b  and  168   b  of tooling unit  36   a . Head tool changer  12  may then introduce pressurized gas into couplings  148  to engage lock extension  184  within lock ring  174  to lock tooling unit  36   a  to master unit  38 . Additionally, compensator  112  may be locked to restrict lateral movement. 
     Head tool changer  12  may also release the pressurized gas from couplings  116  to unlock lock extension  160  from lock ring  138 , thereby unlocking grip unit  34   a  from tooling unit  36   a . Additionally, transfer of electrical power and signal control of deposition head  22   a  to system controller  20  may also occur after electrical contacts  172   a  and  172   b  engage with electrical contacts  182   a  and  182   b . Actuator assembly  28   a  may then raise grip unit  34   a  disengage grip unit from tooling unit  36   a , as discussed above. 
       FIG. 13  is a rear perspective view of a portion of tool rest assembly  26  in use with deposition head  22   a  retained by actuator assembly  28   a , where deposition head  22   a  may be retained at tool rest  92   a  when not loaded to gantry  18 , as discussed above in  FIG. 2P . As shown in  FIG. 13 , tool rest  92   a  includes tool mount  190 , purge trap  192 , and purge line  194 . Tool mount  190  extends above purge trap  192  and includes top surface  196  and guide pins  198 . Top surface  196  is the surface of tool rest  92   a  that may engage with bottom surface  170  of tooling unit  36   a . Furthermore, guide pins  198  are configured to engage with guide holes  176  of tooling unit  36   a , thereby aligning deposition head  22   a  into purge trap  192 . 
     When tooling unit  36   a  rests on top surface  196  of tool mount  190 , a portion of thermal block  126  and extrusion tip  128  extend into purge trap  192 . Purge trap  192  is a secondary trap for collecting any excess purge material that does not travel into purge line  194 . Purge line  194  interconnects purge trap  192  and purge receptacle  96  (shown in  FIGS. 4 and 5 ) for directing purge material to purge receptacle  96 . In one embodiment, purge receptacle  96  may be connected to a vacuum line (not shown) to actively draw purge materials through purge line  194  into purge receptacle  96 . In an alternative embodiment, purge receptacle  96  may be omitted and purge line  194  may be directly connected to a vacuum line. 
     During an initialization process to warm up and purge deposition head  22   a , deposition head  22   a  desirably rests on tool rest  92   a . HTC controller  30  (shown in  FIG. 1 ) may then power up deposition head  22   a  and heat up thermal block  126  to an operating temperature. HTC controller  30  may the direct drive mechanism  124  to feed successive portions of a consumable material to thermal block  126 . The consumable material may then melt in thermal block  126  and deposit from extrusion tip  128  for a preset purge period. 
     During the purge operation, air circulator  94   a  also desirably directs cooling air to drive mechanism  124  and/or the entrance of thermal block  126  to prevent the consumable material from melting at the entrance of thermal block  126 . Gantry  18  (shown in  FIG. 1 ) may also include an additional cooling manifold (not shown) for use with a deposition head in system  10 . However, air circulators  94   a - 94   d  allow the non-active deposition heads  22   a - 22   d  to also receive cooling air to prevent premature melting of the consumable materials during purge operations. 
       FIG. 14  is a schematic illustration of head tool changer  212 , which is an example of an alternative head tool changer of the present disclosure for use with system  10 . Head tool changer  212  functions in a similar manner as head tool changer  12 , and the corresponding reference labels are increased by “ 200 ”. As shown, head tool changer  212  includes a single actuator assembly  228  and a single grip unit  234  rather than multiple actuator assemblies and grip units. Accordingly, HTC controller  30  is further configured to slide tool rest assembly  226  along the x-axis to position tooling units  236   a - 236   d  below grip unit  234  in the horizontal x-y plane, thereby allowing actuator assembly  226  and grip unit  234  to selectively retain tooling units  236   a - 236   d  and deposition heads  22   a - 22   d  in an interchangeable manner for loading and uploading to and from master unit  238 . 
     In one embodiment, the tool rests of actuator assembly  226  may each provide electrical power and control signals to deposition heads  22   a - 22   d  while tooling units  236   a - 236   d  rest on the tool rests of actuator assembly  226 . In this embodiment, HTC control  230  may also be connected to the tool rests of actuator assembly  226  to relay electrical power and control signals to tooling units  236   a - 236   d  and deposition heads  22   a - 22   d . This arrangement allows one or more deposition heads  22   a - 22   d  to be warmed up while disengaged from actuator assembly  228 . Examples of suitable electrical connections include those discussed above for master units  38 ,  138 , and  238 , thereby allowing deposition heads  22   a - 22   d  to be powered and controlled from connections located below tooling units  236   a - 236   d.    
     Accordingly, during operation, tool rest assembly  226  may slide along the x-axis to position one of tooling units  236   a - 236   d  below grip unit  234 , and actuator assembly  228  may then extend downward to engage and lock grip unit  234  to the given tooling unit. Actuator assembly  228  may then retract to its raised position, tool rest assembly  226  may then slide out of the way (e.g., as shown above in  FIG. 2C ), and actuator assembly  228  may then extend downward to its engagement position to engage the given tooling unit with master unit  238 , as discussed above. This process discussed above in  FIGS. 2A-2P  may then be applied to selectively load tooling units  236   a - 236   d  and deposition heads  22   a - 22   d  to master unit  238  and gantry  18  in an interchangeable manner. 
     In an alternative embodiment, actuator assembly  228  may also be movable along the x-axis to selectively position grip unit  234  over tooling units  236   a - 236   d . Accordingly, the tool head changers of the present disclosure (e.g., head tool changers  12  and  212 ) may include at least one actuator assembly and at least one grip unit for loading deposition heads to gantry  18  of system  10 . 
     Furthermore, in one embodiment, deposition heads may be supplied to head tool changers  12  and  212  in magazines, turrets, and other similar carrier units. For example, tool rest assemblies  26  and  226  may each be loadable and unloadable to and from head tool changers  12  and  212  This arrangement allows different deposition heads to be supplied to head tool changer  212 . When each supply of deposition heads is provided to head tool changer  212  or  212 , the given system may then perform a calibration routine to align grip units  34   a - 34   d  and  234  with the corresponding tooling units  36   a - 36   d  and  236   a - 236   d . The supplied deposition heads may then undergo initializations and may be loaded to gantry  18  in an interchangeable manner, as discussed above. This increases the versatility of the head tool changers in providing multiple deposition heads to system  10 . 
     Although the present disclosure has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the disclosure.