Abstract:
An apparatus for making three-dimensional physical objects of predetermined shape by sequentially depositing multiple layers of solidifying material on a base member in a desired pattern, the apparatus comprising: 
     a movable dispensing head ( 112 ) having a flow passage ( 124 ) therein in flow communication at one end thereof with a dispensing outlet configured in the form of a tip with a discharge orifice ( 126 ) of predetermined size therein to dispense material in a fluid state; 
     a supply of flexible strand of thermoplastic resin material that solidifies by cooling to a solidification temperature; heated relatively rapidly to a temperature just above its solidification temperature and will solidify due to a drop in temperature upon being dispensed onto said base member. 
     means for supplying said material in a fluid state through said flow passage to said dispensing outlet; 
     a delivery surface disposed in working proximity to said dispensing outlet of said dispensing head; 
     means for moving said dispensing head and said base member relative to each other in three dimensions in a predetermined sequence and pattern with respect to said dispensing outlet; and 
     means for metering the discharge of said material in a fluid state from said discharge orifice onto said base member as said dispensing head and base member are moved relative to each other so as to thus form a three-dimensional object, characterised in that: 
     material is provided to said movable dispensing head in the form of a continuous flexible strand; 
     said means for supplying said material in a fluid state comprises a heater ( 128 ) on said dispensing head proximate said flow passage that heats said flexible strand to a predetermined temperature above the solidification temperature of said material; and 
     said means for metering comprises a material advance mechanism operatively associated with said flexible strand which controllably advances said flexible strand towards said flow passage at an advance rate controlled in relation to said mechanism means, to thereby regulate the flow rate of said material in a fluid stream from said discharge orifice in relation to the relative movement of said discharge head and said base member; 
     said material advance mechanism comprises one or more roller pairs that grip said flexible strand therebetween and a speed-controlled driver motor ( 142 ) that supplies rotational power to said roller pairs.

Description:
CROSS-REFERENCES 
       [0001]    This application is related to United States provisional application No. 62/026,435, filed Jul. 18, 2014, entitled “3-DIMENSIONAL PRINTER”, naming Chris Padgett and David Padgett as the inventors. The contents of the provisional application are incorporated herein by reference in their entirety, and the benefit of the filing date of the provisional application is hereby claimed for all purposes that are legally served by such claim for the benefit of the filing 
     
    
     date. 
     BACKGROUND 
       [0002]    An apparatus and method for fabricating three-dimensional objects using additive process modeling techniques is described and, more particularly, an apparatus and method incorporating unique linear motion systems and control for relative movement of the components of the apparatus in three dimensions. 
         [0003]    The field of and additive modeling and manufacturing systems is commonly known as three-dimensional (“3D”) printing. Additive machines, such as 3D printers, make three-dimensional models by incremental deposition of a modeling material on a surface, usually in planar layers, based upon design data provided from a computer aided design (CAD) system. A mathematical description forming the CAD model of a 3D object to be created is split into multiple layers. For each layer, a host computer, or controller, generates a path for depositing the material to form the 3D object. The layers are individually applied and shaped to produce the final part. 
         [0004]    An additive machine comprises a print head including an extruder for dispensing heated flowable modeling material from a nozzle onto a surface of a build platform. The controller controls movement of the print head in a horizontal x, y plane, the build platform in a vertical z-direction, and the feeding of modeling material into the print head. The modeling material is thus deposited at a desired flow rate layer-by-layer in areas defined from the CAD model as the print head and the surface are moved relative to each other in three dimensions by an x-y-z gantry system. Movement of the print head is performed under computer control, in accordance with build data from a host computer. The result is a 3D object that resembles the CAD model. The modeling material thermally solidifies after it is deposited, and the finished 3D model is removed from the surface. 
         [0005]    A drive system for the print head should minimize backlash. The extruder frequently changes directions when stopping and restarting the flow of molten plastic. Any slop in the drivetrain will result in poor performance at the start and stop points. Precise control of the start and stop function within the extruder is essential for producing parts with complex geometries or parts printed as a final assembly with moving parts within it. 
         [0006]    In a conventional print head, a Bowden extruder has an extruder motor assembly mounted separately from the moving print head. A feed gear located in the motor assembly grips a filament of feed material. A somewhat flexible, low friction PTFE tube joins the output of the feed gear to the print head. The feed material filament is fed through the tube in a sliding motion as it is melted and extruded at the print head. The tube must be stiff enough to counteract the pushing force the extruder exerts on the filament. However, backlash is high due to several factors, including the gap between the filament and tube ID, the filament acts as a compression spring, slop in the tube end retention within the extruder and print head assembly, and axial tube distortion. There are limitations for a high axial stiffness, off-axis flexible, yet low-friction tubing material. The approach also places an upper limit on print speed due to filament compression and relaxation effects, which results in excess material buildup during deceleration of the print head and thinning out of material during acceleration. 
         [0007]    For the foregoing reasons, there is a need to improve the relative three-axes movement of a print head of an apparatus for fabricating three-dimensional objects. 
       SUMMARY 
       [0008]    An apparatus for making three-dimensional physical objects of predetermined shape by sequentially depositing multiple layers of solidifying material on a base member in a desired pattern, the apparatus comprising:
       a movable dispensing head ( 112 ) having a flow passage ( 124 ) therein in flow communication at one end thereof with a dispensing outlet configured in the form of a tip with a discharge orifice ( 126 ) of predetermined size therein to dispense material in a fluid state;   a supply of flexible strand of thermoplastic resin material that solidifies by cooling to a solidification temperature; heated relatively rapidly to a temperature just above its solidification temperature and will solidify due to a drop in temperature upon being dispensed onto said base member;   means for supplying said material in a fluid state through said flow passage to said dispensing outlet;   a delivery surface disposed in working proximity to said dispensing outlet of said dispensing head;   means for moving said dispensing head and said base member relative to each other in three dimensions in a predetermined sequence and pattern with respect to said dispensing outlet; and   means for metering the discharge of said material in a fluid state from said discharge orifice onto said base member as said dispensing head and base member are moved relative to each other so as to thus form a three-dimensional object, characterised in that:   material is provided to said movable dispensing head in the form of a continuous flexible strand;   said means for supplying said material in a fluid state comprises a heater ( 128 ) on said dispensing head proximate said flow passage that heats said flexible strand to a predetermined temperature above the solidification temperature of said material; and   said means for metering comprises a material advance mechanism operatively associated with said flexible strand which controllably advances said flexible strand towards said flow passage at an advance rate controlled in relation to said mechanism means, to thereby regulate the flow rate of said material in a fluid stream from said discharge orifice in relation to the relative movement of said discharge head and said base member;       
 
         [0018]    said material advance mechanism comprises one or more roller pairs that grip said flexible strand therebetween and a speed-controlled driver motor ( 142 ) that supplies rotational power to said roller pairs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    For a more complete understanding of the apparatus and method, reference should now be had to the embodiments shown in the accompanying drawings and described below. In the drawings: 
           [0020]      FIG. 1  is a top perspective view of an embodiment of an apparatus for fabricating three-dimensional objects. 
           [0021]      FIG. 1A  is a top exploded perspective view of the apparatus for fabricating three-dimensional objects as shown in  FIG. 1 . 
           [0022]      FIG. 2  is a top plan view of the apparatus for fabricating three-dimensional objects as shown in  FIG. 1 . 
           [0023]      FIG. 3  is a bottom plan view of the apparatus for fabricating three-dimensional objects as shown in  FIG. 1 . 
           [0024]      FIG. 4  is a front elevation view of the apparatus for fabricating three-dimensional objects as shown in  FIG. 1 . 
           [0025]      FIG. 5  is a rear elevation view of the apparatus for fabricating three-dimensional objects as shown in  FIG. 1 . 
           [0026]      FIG. 6  is a left side elevation view of the apparatus for fabricating three-dimensional objects as shown in  FIG. 1 . 
           [0027]      FIG. 7  is right side elevation view of the apparatus for fabricating three-dimensional objects as shown in  FIG. 1 . 
           [0028]      FIG. 8  is a perspective view of an embodiment of a print carriage for use with an apparatus for fabricating three-dimensional objects as shown in  FIG. 1 . 
           [0029]      FIG. 9  is an exploded perspective view of the print carriage as shown in  FIG. 1 . 
           [0030]      FIG. 10  is a front perspective view of the bearing assembly for use with the print carriage as shown in  FIG. 10 . 
           [0031]      FIG. 11  is a rear perspective view of the bearing assembly for use with the print carriage as shown in  FIG. 10 . 
           [0032]      FIG. 13  is a front perspective view of the bearing assembly as shown in  FIG. 10  on a guide rail. 
           [0033]      FIG. 14  is a transverse cross-section of the bearing assembly on a guide rail as taken along line  14 - 14  of  FIG. 13 . 
           [0034]      FIG. 15  is a transverse cross-section of the bearing assembly on a guide rail as taken along line  15 - 15  of  FIG. 13 . 
           [0035]      FIG. 16  is a front perspective view of a bearing assembly for use with a gantry. 
           [0036]      FIG. 17  is an exploded perspective view of the bearing assembly as shown in  FIG. 16 . 
           [0037]      FIG. 18  is a perspective view of the bearing assembly as shown in  FIG. 16  on a guide rail. 
           [0038]      FIG. 19  is a perspective view of an embodiment of a drive mechanism for use with an apparatus for fabricating three-dimensional objects as shown in  FIG. 1 . 
           [0039]      FIG. 20  is a perspective view of a longitudinal cross-section of a pulley for use with a drive mechanism as shown in  FIG. 19 . 
           [0040]      FIG. 21  is a longitudinal cross-section view of the pulley for use with a drive mechanism as shown in  FIG. 20 . 
           [0041]      FIG. 22  is a schematic view of a cable drive system for use with an apparatus for fabricating three-dimensional objects as shown in  FIG. 1 . 
           [0042]      FIG. 23  is an exploded perspective view of an embodiment of an end bracket and idler pulleys in a drive mechanism for use with an apparatus for fabricating three-dimensional objects as shown in  FIG. 1 . 
           [0043]      FIG. 24  is an exploded perspective view of an embodiment of a print platform assembly for use with an apparatus for fabricating three-dimensional objects as shown in  FIG. 1 . 
           [0044]      FIG. 25  is an elevation view of a pulley for a lead screw assembly for use in the print platform assembly as shown in  FIG. 24 . 
           [0045]      FIG. 26  is a schematic view of a cable drive system for use with a print platform assembly as shown in  FIG. 24 . 
           [0046]      FIG. 27  is a perspective view of an embodiment of a material delivery assembly for use with an apparatus for fabricating three-dimensional objects as shown in  FIG. 1 . 
           [0047]      FIG. 28  is a perspective view of an embodiment of a print cartridge assembly for use with an apparatus for fabricating three-dimensional objects as shown in  FIG. 1 . 
           [0048]      FIG. 28A  is an exploded perspective view of a print cartridge assembly as shown in  FIG. 28 . 
           [0049]      FIG. 29  is a perspective view of a portion of the print cartridge assembly as shown in  FIG. 28 . 
           [0050]      FIG. 30  is a perspective view of a portion of the print cartridge assembly as shown in  FIG. 29 . 
           [0051]      FIG. 31  is a longitudinal cross-section view of a portion of the print cartridge assembly as shown in  FIG. 28 . 
           [0052]      FIG. 32  is another longitudinal cross-section view of a portion of the print cartridge assembly as shown in  FIG. 31 . 
           [0053]      FIG. 33  is a transverse cross-section view of a portion of the print cartridge assembly as shown in  FIG. 28 . 
       
    
    
     DESCRIPTION 
       [0054]    Certain terminology is used herein for convenience only and is not to be taken as a limiting. For example, words such as “upper,” “lower,” “left,” “right,” “horizontal,” “vertical,” “upward,” “downward,” “top” and “bottom” merely describe the configurations shown in the FIGs. Indeed, the components may be oriented in any direction and the terminology, therefore, should be understood as encompassing such variations unless specified otherwise. The words “interior” and “exterior” refer to directions toward and away from, respectively, the geometric center of the core and designated parts thereof. The terminology includes the words specifically mentioned above, derivatives thereof and words of similar import. 
         [0055]    Referring now to  FIGS. 1-7 , wherein like reference numerals designate corresponding or similar elements throughout the several views, an apparatus for fabricating three-dimensional objects using additive process modeling techniques is shown and generally designated at  50 . The apparatus  50  comprises a frame assembly  60 , a gantry assembly  80  including a print carriage  90 , a print platform assembly  150 , and a modeling material delivery system  180 . The three-dimensional fabricating apparatus  50  builds three-dimensional objects by dispensing modeling material from the print carriage  90  onto a surface of the print platform assembly  150 . The print carriage  90  is configured to receive strands of modeling material from the modeling material delivery system  180 . A controller commands movement and operation of the print carriage  90  such that layers of modeling material are dispensed onto the surface of the print platform assembly  150 . The print carriage  90  and the surface of the print platform assembly  150  are moved in three-dimensions relative to one another in a pattern determined by a control signal from the controller. In one embodiment, the gantry assembly  80  moves the print carriage  90  in an x,y plane as the print platform assembly  150  moves the surface in a z-direction perpendicular to the x,y plane. 
         [0056]    The frame assembly  60  includes four lower structural members  62  connected at their ends to form a lower frame portion  63  defining a square footprint. Lower ends of vertical structural members  64  are connected to, and extend upwardly from, the corners of the lower frame portion  63 . Upper ends of the vertical support members  64  are connected to and support four upper structural members  66  connected at their ends to form an upper frame portion  67 . The structural members  62 ,  64 ,  66  can be an aluminum extrusion “T-slot” profile or similar structure made from other suitable materials. 
         [0057]    The gantry assembly  80  comprises a guide member  82  disposed between two opposed upper structural members  66 . The guide member  82  is a structural member and, as with the other structural members  62 ,  64 ,  66 , can be an aluminum extrusion “T-slot” profile or similar structure made from other suitable materials. The ends of the guide member  82  can be fixed to the structural members  66 , or the guide member  82  can be movable relative to the frame assembly  60  such as in a 2-axis (x, y) motion arrangement as described below. The print carriage  90  rides on the guide member  82 . 
         [0058]    Referring now to  FIGS. 8-15 , the print carriage  90  comprises a front plate  92 , a spaced parallel rear plate  94 , and bearings  96  disposed between the plates. In the embodiment shown, a circular bearing  96  having an axis of rotation perpendicular to the plane of the plates  92 ,  94  is positioned between each of the four corners of the plates  92 ,  94 . The bearings  96  are mounted on standoffs  98  which, in one embodiment, may be integrated into the plates  92 ,  94 . The standoffs  98  are configured to position the bearings  96  midway between the plates  92 ,  94 . The front plate  92 , rear plate  94 , and bearings  96  are held together by threaded fasteners  100  that span the plates  92 ,  94 , passing through the centers of the bearings  96  and the standoffs  98 . 
         [0059]    The bearings  96  are enclosed in covers formed from a low-friction plastic, such as Delrin or HDPE, but other suitable materials may be used. The outer edges of the covers have an approximately 45° chamfer  102 . The chamfered outer edges  102  of the bearings  96  rollingly engage the grooved lip of the guide member  82  of the gantry assembly  80 . This engagement generates a self-centering effect that keeps the print carriage  90  aligned on the guide member  82  in a manner that provides precise positioning of the print carriage  90  along the guide member  82 . The size of the bearings  96  relative to the guide member  82  need not be as shown in the FIGs. It is understood that smaller or larger bearings may be used, and the size of the bearing covers may be adjusted accordingly such that the bearings  96  correctly engage the guide member  82 . 
         [0060]    One or more of the bearings  96  may be mounted on an eccentric cam ( FIG. 12A ) that can rotate independently of the plates  92 ,  94 . Rotating the eccentric cam varies the gap between the upper bearings and the lower bearings and can thus be used to adjust the engagement of the bearings  96  on the guide member  82 . When the associated fasteners  100  are tightened, the eccentric cam resists rotation and holds the adjustment ensuring that the print carriage assembly  90  maintains the correct position. It is important to note that all bearings are captured in double shear so that no bending loads are generated in the assembly. 
         [0061]    In one embodiment, the print carriage assembly  90  includes at least one bearing  104  positioned on the rear plate  94  such that the axis of rotation of the bearing  104  is parallel to the plates  92 ,  94  and perpendicular to the axis of rotation of the first set of bearings  96 . The bearing  104  engages the side channel of the guide member  82  in a manner identical to the other bearings  96 . The bearing  104  may be mounted in a bracket  106  that is laterally adjustable with respect to the rear plate  94 . The space between the bearing  104  and the plates  92 ,  94  may thus be adjusted using threaded fasteners  108  for establishing a lateral preload applied to the print carriage assembly  90 . This is desirable to minimize play in the bearings  96 . 
         [0062]    A roller carriage  110  is attached to each end of the guide member  82  of the gantry assembly  80 . As shown in  FIGS. 16 and 17 , each roller carriage comprises a front plate  111 , a spaced parallel rear plate  112 , and circular bearings  114  disposed between the plates. In the embodiment shown, the bearings  114  have an axis of rotation perpendicular to the plane of the plates  111 ,  112  and are positioned between each of the four corners of the plates  111 ,  112 . The bearings  114  are mounted on standoffs  115  which, in one embodiment, may be integrated into the plates  111 ,  112 . The standoffs  115  are configured to position the bearings  114  midway between the plates  111 ,  112 . The front plate  111 , rear plate  112 , and bearings  114  are held together by threaded fasteners (not shown) that span the plates  111 ,  112 , passing through the centers of the bearings  114  and the standoffs  115 . The bearings  114  are enclosed in covers formed from a low-friction plastic, such as Delrin or HDPE, but other suitable materials may be used. The outer edges  116  of the covers have an approximately 45° chamfer. Each roller carriage  110  rides an opposed parallel upper structural member  66  of the upper frame  67  ( FIG. 18 ). The chamfered outer edges  116  of the bearings  114  rollingly engage the grooved lip of the upper structural members  66  of the upper frame portion  67 . As shown in  FIGS. 16 and 17 , a bracket  117  is secured to the front plate  111  of the roller carriage  110 . The bracket is configured such that each roller carriage  110  may be mounted to ends of the guide member  82 . The upper structural members  66  function as linear motion guides for the guide member  82 . The bracket carries a front pulley  118  and a rear pulley  119 , as will be described below. 
         [0063]    A drive mechanism  120  is attached to the front end of each of the linear motion guides  66  ( FIG. 1 ). Referring to  FIGS. 19-21 , the drive mechanism  120  comprises a stepper motor  122 , although it is understood that a closed-loop servomotor is also suitable, as is conventional. The motor  122  is suspended from a hanger  125  secured to the end of each linear motion guide  66 . A cylindrical pulley  124  is mounted via a central axial passage  128  of the pulley  124  onto the output shaft  123  of each motor  122 . The pulley  124  is configured for securely retaining a drive cable  126 . 
         [0064]    As shown in  FIGS. 20 and 21 , the drive cable  126  is wound onto the pulley  124  in a manner such that there are two separate portions of wrapped cable  126 , one at the top of the pulley  124  and one at the bottom of the pulley. The drive cable  126  between the wrapped portions passes down a longitudinal hole  130  radially spaced from the central axial passage  128  of the pulley  124 . The cable  126  passes outwardly to the periphery of the pulley  124  via angled passages  131  extending from the vertical hole  130 . The cable  126  portion at the top of the pulley  124  is wrapped in a direction opposite to the cable  126  portion at the bottom of the pulley  124 . Thus, as the pulley  124  rotates, one portion of the drive cable  126  pays off and the other portion of the cable pays onto the pulley  124  so that the total amount of cable  126  on the pulley  124  stays the same. This arrangement prevents slip between the drive cable  126  and the pulley  124 . The total length of cable  126  wrapped around the pulley  124  is greater than the amount of cable needed for the guide member  82  to travel the full swept area of the apparatus. The diameter and length of the pulleys  124  are configured to accommodate this length. There are no grooves in the face of the pulley to guide the cable on and off such that the drive cable  126  is self-tailing. A suitable drive cable is formed from a high-tensile strength material, such as braided Kevlar which has very low static and dynamic stretch making the braided Kevlar an ideal application for timing belts that are commonly used in 3D printers. 
         [0065]    A schematic of the drive cable route is shown in  FIG. 22 . Two separate cables  132 ,  134  are used. Each cable  132 ,  134  is wound around one drive pulley  124  and the ends of the cable  132 ,  134  are attached to the print carriage  90  diagonally opposite from one another. The drive cables  132 ,  134  are routed through an arrangement of idler pulleys  140  at the rear end of the linear motion guides  66 . The drive cables  132 ,  134  are also routed through the front and rear pulleys  118 ,  119  on each roller carriage  110 . All of the idler pulleys  140  and roller carriage pulleys  118 ,  119  are in the same horizontal plane as the drive pulleys  124 . In one embodiment, the horizontal plane is at the mid-plane of the linear motion guides  66 . In this arrangement, both motors  122  are held stationary, which reduces moving mass. The reaction forces on the pulleys at the ends of the linear motion guides  66  are balanced during all movements, which ensures there is no tendency of the guide member  82  to tilt or cock with respect to the linear motion guides  66 . 
         [0066]    The rear idler pulleys  140  are shown in  FIG. 23 . The drive cable  126  tension may be adjusted using a selected one of the rear idler pulleys  140  which is mounted in a rear elongated slot of a bracket  144  with a threaded fastener  142  applying clamping force to the stack standoffs and idler pulley  140 . The bolt  142  can be loosened, the position of the idler pulley  140  longitudinally adjusted to achieve the correct cable tension, and then the bolt  142  re-tightened. To adjust the “square” of the X rail with respect to the Y rails, the relative tension in the drive cables can be adjusted so that they are not equal. This method squares the frame assembly  60 , which is important to ensure the tool path exactly follows the intended path. 
         [0067]    Referring to  FIGS. 24-26 , the print platform assembly  150  comprises a platen  152 , which functions as a work surface, and three lead screws  154  for moving the platen  152  vertically along two smooth guide rods  156 . The platen  152  is a single plate of ⅛″ thick steel, although other thicknesses and other materials, such as metals, wood, or any structural material including fiberglass or carbon fiber can also be used. In one embodiment, the lead screws  154  are standard ACME threaded rod with a shoulder  155  on one end ( FIG. 25 ). The shoulder  155  diameter matches the inside diameter of a radial bearing  158 , for example, 6 mm. The top of the shoulder  155  is knurled or otherwise made to have an interference fit with the radial bearing  158  so that the bearing can be press-fit onto the lead screw  154 . The bearing  158  fits into a socket  161  in a lower structural member  62  of the lower frame portion  63 . A pulley  160  is installed on the shoulder  155  below the bearing  158 . The tops of the lead screws are free so as not to over-constrain the system and cause binding. It is understood that other types of lead screws may be used, such as multi-start lead or ball screws. 
         [0068]    A stepper motor or servo motor  162  is mounted to a lower structural member  62  of the lower frame portion  63  for driving a toothed timing belt  164  that engages the pulley  160  on each of the lead screws  154 . In this arrangement, all three lead screws  154  are connected and turn together relative to the nuts  166  with the motor  162  drive shaft. The motor  162  can selectively drive the pulleys  160  in either a clockwise or a counterclockwise direction. The timing belt  164  synchronizes the movement of the pulleys  160 , which effects synchronous rotation of the lead screws  154 . As the lead screws  154  rotate, the lead nuts  166  move either up or down, causing the platen  152  to be either raised or lowered relative to the frame assembly  60 , depending upon the direction of the pulley rotation. 
         [0069]    The belt  164  is tensioned by adjusting the distance between the motor  162  and the lead screws  154 . An idler pulley  165  may be added to the system as needed in order to achieve sufficient tension to the engagement between the belt  154  and the pulleys  160 . In order to level the platen  152 , the pulley can be loosened so that a lead screw  154  can be manually turned independently of the belt  164  and pulleys  160 . Once the platen  152  is level, the pulley  160  can be tightened again so that it rotates with the lead screw  154  and provides a very stable and accurately positioned surface. 
         [0070]    The lead screws  154  are positioned approximately equidistant from each other around the perimeter of the platen  152 . However, there is flexibility in the position of the lead screws  154  thereby providing flexibility in locating other components of the 3D apparatus  50 . For example, adjustability is built into the system to allow the base of the lead screws  154  to be positioned precisely under the corresponding point at which they are connected into the platen  152 . It is understood that the number of lead screws may vary, and the lead screws may be positioned as needed for work surfaces of different sizes or geometries. 
         [0071]    Engagement of the lead screws  154  with the platen  152  is via at least one threaded member built into, or attached to, the platen  152 . In one embodiment, a nut  166  corresponding to the thread type and pitch of the lead screws  154  is fastened onto the surface of the platen  152 . Specifically, the nuts  166  are press-fit into a carrier that is then attached to the platen  152 . The nuts ride up and down on the lead screws  154  and precisely control the position of the platen  152  relative to the rotational motion of the lead screws  154 . It is understood that the platen  152  could have threads created within it or other means may be used. 
         [0072]    The pair of linear guide rods  156  are mounted firmly to lower structural members  62  on opposite sides of the lower frame portion  63  in parallel to the lead screws  154 . Each guide rod  156  rides a linear recirculating ball bearing or a self-lubricating bushing fixed in the platen  152 . The platen  152  is mounted to be firmly constrained with respect to lateral translation during vertical movement, but there is some small amount of angular compliance. This is preferred because the guide rods  156  exclusively position the platen  152  in the horizontal plane and are not subject to bending loads as in other vertical axis designs. Because there are no bending loads, the platen  152  is supported evenly by the lead screws  154 . 
         [0073]    The platen  152  and gantry assembly  80  are configured to move in three-dimensional space as defined by an (x, y, z) Cartesian coordinate system. Specifically, the drive system directly manipulates the gantry assembly  80  and print carriage  90  to move in the (x, y) plane based on the build data. The print platform assembly  150  is configured to move the platen  152  vertically along the lead screws  154  and guide rods  156  orthogonal to an (x, y) plane defined by the gantry assembly  80 . In other embodiments, the gantry assembly  80  can be configured to translate in the vertical direction while the platen  152  remains stationary with respect to the frame assembly  60 . 
         [0074]    The print platform assembly  150  has an upper print surface  170  comprising a layer of about ⅛″ standard mirror glass or other type of glass, such as standard plate glass, tempered glass, borosilicate glass, as well as plastics, metals or other suitable materials depending on the application. The print surface  170  comprises a substrate defining a planar workspace upon which three-dimensional objects are produced. The print surface  170  is stacked on top of a thin aluminum sheet  171 , typically about 0.040″ thick. Other materials and thicknesses can also be used in other applications. A multi-zone heater (not shown) is attached to the bottom surface of the aluminum sheet. The heater may be a kapton film or silicone mat resistance heater. The multi-zone heater is designed so that the entire surface of the glass is at a uniform temperature. Multiple zones are needed because the heat loss is greater around the edges vs. the center, so in order to have the same temperature more power must be fed to the zones heating the areas closer to the perimeter of the print bed. In the preferred embodiment, a ratio of 2:1 power distribution is used to achieve even heating. Note that tailored temperature profiles are feasible for special applications. Optionally, insulation may be added below the heater to limit heat loss. 
         [0075]    The print surface  170  is mounted on an array of small compression springs  172 , or other compliance members, disposed at each corner of the print surface  170 , and optionally, one at the center. The springs  172  are attached to the platen  152  by mechanical means. The corners of the print surface  170  are captured in small clips  174  that fit over a top edge and on each corner of the print surface  170 . A bolt  176  passes through each corner clip  174  and threads into the platen  152  via a PEM insert installed in the bottom of the platen, a threaded hole, a captured nut, or other means. This arrangement allows the height of the print surface  170  at each corner to be adjusted by simply turning the bolt  176 . To level the print surface  170  with respect to the print head or other tool, the print head is positioned over each corner of the print surface  170  and the gap between the tip of the print head and the print surface  170  is adjusted so that it is the same at each corner. Typically this may be done with a feeler gauge, the thickness of which is selected to achieve the correct gap. 
         [0076]    The modeling material delivery system  180  includes a flexible filament feed. The modeling material may, for example, be supplied in solid form as a flexible filament wound on a supply reel spool to deliver a continuous strand of modeling material from a supply source, such as a reel, or a series of filament segments of modeling material, to the print carriage  90 , such as through a flexible feed tube  184 . Regardless of the form in which the feed stock material is supplied to the dispensing head, the material supply pump must be controllable so that the dispensing rate of the material can be controlled accurately to form the three-dimensional object. A conventional feed system is shown in  FIG. 27  in combination with a Bowden extruder, including a print head having an inlet for receiving the filamentous modeling material and an outlet nozzle for dispensing the modeling material onto the platform in a flowable state. The nozzle outlet will typically be heated so as to deposit the modeling material at a predetermined temperature. 
         [0077]    An embodiment of a remotely-driven, low-lash print head for thermoplastic extrusion is shown in  FIGS. 28-33  and generally designated at  190 . In this embodiment, the bowden tube  184  is replaced by a flexible torsional drive cable  192  extending between an extruder motor  194  and a print head  196 . The driven end of the torsion cable  192  is attached directly to the extruder motor output shaft. The drive end of the torsion cable  192  is fitted into a gear reduction assembly  198  with a greater than a 10:1 ratio. In the embodiment shown, the gear reduction is a worm gear  200 . Alternatively, a planetary gear reduction may be used. Depending on the orientation of the torsion cable  192 , the worm gear  200  may be beneficial because it also rotates the power output by 90°. The torsion cable  192  is located on the input side of the gear reduction assembly  198 . Because torsion cables typically have a large amount of angular distortion when they reverse directions, placing the torsion cable on the input side of the gear reduction assembly  198  cuts the angular distortion by the ratio of the gearbox. This significantly improves performance of the system and minimizes backlash effects. 
         [0078]    A filament feed gear  202  is mounted on the print carriage  90  above the print head  190  and adjacent to the output shaft of the gear reduction assembly  198 . The feed gear  202  is operatively connected in the gear train for turning the gear, which provides a driving force for feeding filament strands of modeling material through and driving the filament  206 . A pinch wheel assembly  204  is used to retain the filament feed  206  against the feed gear  202 . The pinch wheel assembly  204  comprises a pivoting toggle  208  which is spring-biased against the feed gear  202  such that the combination is configured to engage the filament  206 . The toggle  208  is movable between a first closed position for capturing the filament  206  between the feed gear  202  and the pinch wheel  204  and a second open position. The feed gear  202  and the drive wheel  204  are driven by the gear reduction assembly  198 , which is powered by the feed motor. When the toggle  208  is in the first closed position, rotation of the feed gear  202  continuously feeds filament strand  206  into a liquefier block  210 . While traveling through liquefier block  210 , the liquefier block  210  melts the filament strand  206  to a desired extrusion viscosity. The un-melted portion of the filament strand  206  acts as a plunger that forces the melted print material to extrude out of the nozzle  212  as liquefied material. This allows the print head  190  to dispense print material at a desired flow rate generally based on the rotation rate of the motor. 
         [0079]    During delivery of print material, the temperature of the extruded print material is modulated within the extruded volume in close proximity to the print head  190 . In order to achieve optimal performance (e.g. feature tolerances, geometry such as overhangs, etc) it is desirable to extrude the material in liquid form well above its melting point, such that the temperature of the deposited material, and the temperature of the surface on which it is deposited, are elevated enough to achieve good bonding between the layers. The deposited material is rapidly cooled to slightly above its glass transition temperature (Tg) in order to “set” the material and prevent distortion. Simultaneously, extracting too much heat is avoided so that the print material does not drop below its Tg and undergo contraction. Precisely controlling the rate of cooling is a critical aspect of high-speed printing, because the heat must be removed from the material in a shorter amount of time. A 3D printer relying on ambient cooling to accomplish heat removal does not work at faster print speeds. 
         [0080]      FIGS. 28-33  show an apparatus comprising a small axial fan or centrifugal blower  220  mounted to the print head  196 . A centrifugal blower is smaller in size and delivers a higher output pressure, which allows for reduction in air duct cross-sectional area and a smaller output orifice. The blower  220  pulls in ambient air for cooling purposes. The blower  220  is electrically connected to the on-board controller  68 . The controller  68  is designed so that the blower  220  may be controlled at partial power settings, typically through a form of PWM (pulse width modulation), which can be set through the microcontroller. The fan power setting can also be controlled within a print file with a specific command, (e.g., M106 SXXX, where XXX is the PWM setting). A duct  222  is mounted on the outlet of the blower  220 . The duct  222  directs the exhaust air of the blower  220  down towards the print head  196 . The shape and outlet of the duct  222  are designed to direct the air down and across the axis of the print head  196  as the air exits the duct  222  at and below the nozzle. This ensures even coverage of the cooling air on both sides of the print head  196  even though the duct is located off-center relative to the print head. In a dual print head setup, a blower and duct may be provided for each print head to ensure adequate and even cooling of the deposited material from both print heads. 
         [0081]    A method for thermal control comprises computing the time for each layer based on the toolpath for that layer when the solid model is prepared for printing (aka “sliced”). Processing software has configurable limits on what the allowable minimum layer time is, and these are adjustable on a per-material basis. If the layer time falls below a certain limit, the software calculates what level of cooling is required, and writes the appropriate command into the outputted print file. For each print material, settings are configured for minimum layer time, maximum allowable fan speed, minimum allowable fan speed (to prevent the fan from stalling at extremely low power levels), and optionally if the fan is configured to run constantly and at what power setting. These settings are determined experimentally by the manufacturer, user, or material supplier. As such, this is an “open loop” system in that the actual temperature of the deposited plastic is not measured. 
         [0082]    A second method yields improved cooling results and takes into account the geometry of the part. For instance, parts with thin walls, such as vases, need less cooling for a given layer time, because the surface area to volume ratio is quite high. In contrast, parts with overhangs need additional cooling in order to produce optimal results because the overhanging sections are inclined to curl upwards. In this method, the software takes the part geometry into account when calculating the optimal fan setting, and adjusts the resulting value accordingly. 
         [0083]    The 3D printer has many advantages, including a two-axis linear motion system which is highly scalable for larger or smaller applications while maintaining the core design and the associated cost, speed-of-motion and precision. In particular, the parallel linear motion guides may be positioned an arbitrary distance apart. The guide member is mounted to the carriage rollers and spans the gap between the linear motion guides. By adjusting the spacing between, and the length of, the linear motion guides, the swept area may be adjusted in both of the X and Y directions. In an ideal form, the front and rear plates, standoffs, and eccentric cams are designed to be 3D printed. This saves cost and reduces weight and allows part count to be reduced by combining separate parts. The plates can also be modified to provide attachments for other hardware, such as a print head or other tool. High performance of the 3D printer derives from high stiffness of the guide member and carriage subsystem, stationary motor drives, smooth motion, high movement speeds in excess of 1 meters per second, and low moving mass to minimize inertial effects and backlash within the system. 
         [0084]    Although the apparatus and method for fabricating three-dimensional objects has been shown and described in considerable detail with respect to only a few exemplary embodiments thereof, it should be understood by those skilled in the art that we do not intend to limit the apparatus and method to the embodiments since various modifications, omissions and additions may be made to the disclosed embodiments without materially departing from the novel teachings and advantages, particularly in light of the foregoing teachings. For example, the apparatus and method may also have application in general purpose machine control. Accordingly, we intend to cover all such modifications, omission, additions and equivalents as may be included within the spirit and scope of the apparatus as defined by the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.