Abstract:
An image processing apparatus ( 10 ) comprises an imaging drum ( 300 ) for holding print media ( 32 ) and donor material ( 36 ) in registration on the imaging drum ( 300 ). A print head ( 500 ), driven by a lead screw ( 250 ), moves along a line parallel to a longitudinal axis (X) of the imaging drum ( 300 ) as the imaging drum ( 300 ) rotates. A lead screw assembly ( 90 ) is secured in place in a scanning frame by magnetic attraction, with one magnet disposed to constrain axial motion by holding the lead screw to a fixed point and the other magnet disposed to secure the lead screw assembly ( 90 ) in place and allow rotational motion. Magnetic attraction allows the removal and replacement of the complete lead screw assembly ( 90 ) without tools.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to co-pending application Ser. No. 09/080,841 filed on May 18, 1998, entitled MAGNETICALLY HELD MOTOR STOP; co-pending application Ser. No. 09/144,390 filed on Aug. 31, 1998, entitled METHOD OF CONTROLLING A PRINTHEAD MOVEMENT BASED ON A SCREW PITCH TO MINIMIZE SWATH-TO-SWATH ERROR IN AN IMAGE PROCESSING APPARATUS; and co-pending application Ser. No. 08/795,171 filed on Feb. 4, 1997, entitled A METHOD AND APPARATUS FOR MAGNETICALLY PRELOADING A BALL BEARING ASSEMBLY. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the mechanical configuration of a lead screw and its stepper motor in an image processing apparatus. 
     BACKGROUND OF THE INVENTION 
     Pre-press color proofing is a procedure that is used by the printing industry for creating representative images of printed material, without the high cost and time that is required to actually produce printing plates and set up a high-speed, high-volume, printing press to produce a single example of an intended image. These intended images may require several corrections and may need to be reproduced several times to satisfy the requirements of customers, resulting in a large loss of profits. By utilizing pre-press color proofing, time and money can be saved. 
     One such commercially available image processing apparatus, which is depicted in U.S. Pat. No. 5,268,708, is an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring colorant from a sheet of donor material to thermal print media by applying a sufficient amount of thermal energy to the donor material to form an intended image. This image processing apparatus is comprised generally of a material supply assembly or carousel, a lathe bed scanning subsystem (which includes a lathe bed scanning frame, a translation drive, a translation stage member, a print-head, and a vacuum imaging drum), and thermal print media and donor material exit transports. 
     The operation of the image processing apparatus of U.S. Pat. No. 5,268,708 comprises metering a length of the thermal print media (in roll form) from the material assembly or carousel. The thermal print media is then measured and cut into sheet form of the required length, transported to the vacuum imaging drum, registered, wrapped around and secured onto the vacuum imaging drum. Next a length of donor material (in roll form) is also metered out of the material supply assembly or carousel, measured and cut into sheet form of the required length. It is then transported to and wrapped around the vacuum imaging drum, such that it is superposed in the desired registration with respect to the thermal print media (which has already been secured to the vacuum imaging drum). 
     After the donor material is secured to the periphery of the vacuum imaging drum, the scanning subsystem or write engine provides the scanning function. This is accomplished by retaining the thermal print media and the donor material on the spinning vacuum imaging drum while it is rotated past the print head that will expose the thermal print media. The translation drive then traverses the print head and translation stage member axially along the vacuum imaging drum, in coordinated motion with the rotating vacuum imaging drum. These movements combine to produce the intended image on the thermal print media. 
     The lathe bed scanning frame provides the structure to support the vacuum imaging drum and its rotational drive. The translation drive with the translation stage member and print head are supported by two translation bearing rods that are substantially straight along their longitudinal axis and are positioned parallel to the vacuum imaging drum and a lead screw. Consequently, they are parallel to each other therein forming a plane, along with the vacuum imaging drum and lead screw. The translation bearing rods are, in turn, supported by the outside walls of the lathe bed scanning frame of the lathe bed scanning subsystem or write engine. The translation bearing rods are positioned and aligned there between, for permitting low friction movement of the translation stage member and the translation drive. The translation bearing rods are sufficiently rigid for this application, so as not to sag or distort between the mounting points at their ends. They are arranged to be as exactly parallel as is possible with the axis of the vacuum imaging drum. The front translation bearing rod is arranged to locate the axis of the print head precisely on the axis of the vacuum imaging drum with the axis of the print head located perpendicular, vertical, and horizontal to the axis of the vacuum imaging drum. The translation stage member front bearing is arranged to form an inverted “V” and provides only that constraint to the translation stage member. The translation stage member with the print head mounted on the translation stage member, is held in place by its own weight. The rear translation bearing rod locates the translation stage member with respect to rotation of the translation stage member about the axis of the front translation bearing rod. 
     In U.S. Pat. No. 5,268,708, the translation stage member and print head are attached to a rotatable lead screw (having a threaded shaft) by a drive nut and coupling. The coupling is arranged to accommodate misalignment of the drive nut and lead screw so that only rotational forces and forces parallel to the lead screw are imparted to the translation stage member by the lead screw and drive nut. The lead screw rests between two sides of a lathe bed scanning frame of the lathe bed scanning subsystem or write engine, where it is supported by deep groove radial bearings. At the drive end the lead screw continues through the deep groove radial bearing, through a pair of spring retainers, that are separated and loaded by a compression spring to provide axial loading, and to a DC servo drive motor and encoder. The DC servo drive motor induces rotation to the lead screw moving the translation stage member and print head along the threaded shaft as the lead screw is rotated. The lateral directional movement of the print head is controlled by switching the direction of rotation of the DC servo drive motor and thus the lead screw. 
     Although the presently known and utilized image processing apparatus is satisfactory, it is not without drawbacks. In order to achieve the positioning accuracy for high-resolution imaging at 1800 dots per inch or greater, the apparatus described above utilizes a lead screw having a very fine thread pitch. Approaches to this problem disclosed in co-pending application Ser. No. 09/144,390 filed on Aug. 31, 1998 allow a coarser lead screw pitch to be used. 
     It can be appreciated that a significant amount of design work is required to maintain synchronization and dot addressability in an imaging apparatus where a print head, possibly having a variable number of light sources, is moving linearly along a high-speed rotating imaging drum. To achieve the necessary timing for this imaging task, a specific lead screw thread pitch is selected for the imaging resolution that is required. Co-pending application Ser. No. 09/144,390 filed on Aug. 31, 1998 discloses a method and example for calculating lead screw pitch for an apparatus imaging at 2540 dots per inch. 
     It would be advantageous to be able to readily change the resolution of an imaging apparatus to suit different requirements of end-customers who use such equipment. For example, there are significant advantages for an image processing apparatus that could operate at both 2540 dots per inch and at 2400 dots per inch. A preferred solution for meeting this requirement is to enable each resolution using a different lead screw pitch. 
     It will be appreciated that changing the lead screw in a high-resolution imaging apparatus presents considerable problems. Conventional solutions would require a significant amount of disassembly to loosen the lead screw from mounting, fastening, and support hardware at each end and to install the alternate lead screw in its place. Service costs for lead screw replacement at an end-customer site would limit the market value of such a solution. End-customers would be likely to reject conventional solutions for lead screw replacement as troublesome, costly, time-consuming, and error-prone. 
     Lead screw replacement conventionally requires tools and involves well-trained personnel to make necessary adjustments so that synchronization timing can be maintained. Patents that disclose methods for lead screw replacement include U.S. Pat. No. 4,628,171, which discloses a mechanical-feed boring machine tool with interchangeable lead screws, where different lead screw pitches are needed to change the threading pitch achieved by this machine. Conventional hand tools and detailed disassembly procedures are required to substitute another lead screw having a different thread pitch with this approach. 
     The apparatus disclosed in U.S. Pat. No. 5,771,059 employs a magnet integrally attached to the lead screw that allows one end of the lead screw to be removed from its position in the scanning frame. Also, the apparatus disclosed in co-pending application Ser. No. 08/795,171 uses a magnetically loaded radial bearing integrated with the lead screw shaft that allows the opposite end of the lead screw to be securely held in position within a frame, while at the same time providing a bearing to allow rotational movement. However, none of the arrangements noted above show or suggest a structure or method which permits the removal and the re-seating of a complete lead-screw assembly without requiring tools. 
     SUMMARY OF THE INVENTION 
     The present invention provides for an apparatus which overcomes the drawbacks noted above. Briefly summarized, according to one aspect of the present invention, the invention resides in an imaging processing apparatus of the lathe-bed scanning type, where a print head is secured to a translation stage member. A lead screw provides linear movement of the translation stage member. The assembly comprises the lead screw and its attached motor and support hardware which form a removable, modular assembly that is held in place by magnetic attraction, and can be removed from and re-seated in a scanning frame without tools. 
     An object of the present invention is to provide for a lead screw assembly that installs in the scanning frame as a single unit and is self-seating, fitting into place and secured in the proper position without mechanical fasteners. 
     It is an advantage of the present invention that it enables an image processing apparatus to be operable with any one of a set of lead screws, where each lead screw can have a different thread pitch or other characteristics. 
     It is a further advantage of the present invention that it allows installation or removal of a lead screw assembly in an image processing apparatus without tools and without mechanical adjustments for precision alignment. 
     It is noted that the present invention could be used in other applications, including imaging applications that are not limited to imaging using dye transfer. It is recognized that the present invention is pertinent to various types of laser, heat, or radiation-induced transfer involving colorants such as inks, dyes, or pigments. The present invention could also be employed in other types of devices where it is useful to be able to remove a lead screw and its associated components without tools. 
     The present invention relates to a writing assembly having a removable self-seating lead screw. The writing assembly comprises a supporting frame; a lead screw which defines a linear direction of movement for a writing element; and an attraction assembly for permitting an insertion of the lead screw to an operating position on the supporting frame. The lead screw is held in the operating position while being permitted to rotate about a longitudinal axis of the lead screw. The attraction assembly permits a manual removal of the lead screw from the operating position on the supporting frame. 
     The present invention further relates to a writing assembly having a removable lead screw which comprises a frame member for supporting the lead screw; first attraction means attached to a first end of the lead screw; second attraction means on the frame member for attracting the first attraction means to removably hold the first end of the lead screw on the frame member when the lead screw is in an operating position on the frame member; a magnetically loaded radial bearing mounted on a second end of the lead screw which permits a rotation of the lead screw when the lead screw is in the operating position; and a receiving means on the frame member for removably holding the radial bearing therein when the lead screw is in the operating position, the lead screw being manually removable at the first and second ends from the frame member. 
     The present invention further relates to a lead screw assembly for an image capture device which comprises a lead screw which defines a linear direction of movement for a writing element; a first attraction member on a first end of the lead screw which cooperates with a second attraction member on a frame of the image capture device to rotatably and removably hold the first end of the lead screw on the frame; and a bearing member provided on a second end of the lead screw which cooperates with a receiving member on the frame to rotatably and removably hold the second end of the lead screw on the frame. 
     The present invention further relates to a method of removably mounting a lead screw assembly of an image capture device. The method comprises the steps of: providing a first attraction member on a first end of a lead screw, with the lead screw defining a linear direction of movement for a writing assembly of the image capture device; providing a bearing member on a second end of the lead screw; and attaching the lead screw to a frame member of the image capture device. The first attraction member cooperates with a second attraction member on the frame and the bearing member cooperates with a receiving member on the frame to removably hold the lead screw to the frame at the first and second ends. 
     The present invention further relates to an image processing apparatus which comprises a writing assembly mounted on a supporting member so as to be adjacent to an imaging member; a removable lead screw assembly which provides a linear movement to the writing assembly relative to the imaging member, with the lead screw assembly comprising a lead screw and a drive motor which rotates the lead screw; and an attraction assembly which holds the lead screw assembly in an operating position on the support member in a manner which permits a removal of the lead screw assembly as a unit from the operating position on said supporting member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view in vertical cross section of an image processing apparatus of the present invention; 
     FIG. 2 is a perspective view of a lathe-bed scanning subsystem or write engine of the present invention, as viewed from the rear of the image processing apparatus; 
     FIG. 3 is a top view in horizontal cross-section, partially in phantom, of the lead screw of the present invention; 
     FIG. 4 is a perspective view showing components on a motordriven side of the lead screw in a preferred embodiment of the present invention; 
     FIG. 5 shows an exploded view of the assembly of components on the motor-driven side of the lead screw in the embodiment shown in FIG. 4; 
     FIG. 6 shows an opening provided in a side panel of a scanning frame for placement of the lead screw assembly; and 
     FIGS. 7 a  and  7   b  illustrate an exploded view showing the lead screw assembly as it is installed or removed, relative to the print head and to the main chassis of the lathe-bed scanning subsystem. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, wherein the like reference numerals represent identical or corresponding parts throughout the several views, FIG. 1 illustrates an example of an image processing apparatus  10  relevant to the present invention. Image processing apparatus  10  includes an image processor housing  12  which provides a protective cover. A movable, hinged image processor door  14  is attached to a front portion of image processor housing  12  permitting access to sheet material trays, such as lower sheet material tray  50   a  and upper sheet material tray  50   b , that are positioned in an interior portion of image processor housing  12 , for supporting print media  32  thereon. Only one of sheet material trays  50   a ,  50   b  will dispense print media  32  out of its sheet material tray to create an intended image thereon; the alternate sheet material tray  50   a ,  50   b  either holds an alternative type of print media  32  or functions as a back up sheet material tray. In this regard, lower sheet material tray  50   a  includes lower media lift cam  52   a  for lifting lower sheet material tray  50   a  and ultimately print media  32 , upwardly toward rotatable, lower media roller  54   a  and toward a second rotatable, upper media roller  54   b  which, when both are rotated, permit print media  32  to be pulled upwardly towards a movable media guide  56 . Upper sheet material tray  50   b  includes upper media lift cam  52   b  for lifting upper sheet material tray  50   b  and ultimately print media  32  towards upper media roller  54   b  which directs it towards media guide  56 . 
     Media guide  56  directs print media  32  under a pair of media guide rollers  58  which engage print media  32  for assisting upper media roller  54   b  in directing it onto a media staging tray  60 . Media guide  56  is attached and hinged to a lathe bed scanning frame  202  at one end, and is uninhibited at its other end for permitting multiple positioning of media guide  56 . Media guide  56  then rotates its uninhibited end downwardly, as illustrated in the position shown, and the direction of rotation of upper media roller  54   b  is reversed for moving print media  32  resting on media staging tray  60  under the pair of media guide rollers  58 , upwardly through an entrance passageway  204  and around a rotatable vacuum imaging drum  300 . 
     A roll  30  of colorant donor roll material  34  is connected to a media carousel  100  in a lower portion of image processor housing  12 . Four rolls of roll media  30  are used, but only one is shown for clarity. Each roll media  30  includes a donor roll material  34  of a different color, typically black, yellow, magenta and cyan. These donor roll materials  34  are ultimately cut into donor sheet materials  36  and passed to vacuum imaging drum  300  for forming the medium from which colorant imbedded therein is passed to print media  32  resting thereon. In this regard, a media drive mechanism  110  is attached to each roll  30  of donor roll material  34 , and includes three media drive rollers  112  through which the donor roll material  34  of interest is metered upwardly into media knife assembly  120 . After the donor roll material  34  reaches a predetermined position, media drive rollers  112  cease driving the donor roll material  34  and two media knife blades  122  positioned at a bottom portion of media knife assembly  120  cut the donor roll material  34  into donor materials  36 . Lower media roller  54   a  and upper media roller  54   b  along with media guide  56  then pass a donor sheet material  36  onto media staging tray  60  and ultimately to vacuum imaging drum  300 ; and in registration with print media  32  using the same process as described above for passing print media  32  onto vacuum imaging drum  300 . The donor sheet material  36  now rests atop print media  32  with a narrow space between the two created by microbeads imbedded in the surface of print media  32 . 
     A laser assembly  400  includes a quantity of laser diodes  402  in its interior. Lasers diodes  402  are connected via fiber optic cables  404  to distribution block  406  and ultimately to a print head  500 . Print head  500  directs energy received from laser diodes  402  causing the donor sheet material  36  to pass the desired color across the gap to print media  32 . Print head  500  is attached to a lead screw  250  (FIG. 2) via a lead screw drive nut  254  and drive coupling (not shown), for permitting movement axially along a longitudinal axis of vacuum imaging drum  300  for transferring the data to create the intended image onto print media  32 . 
     For writing, vacuum imaging drum  300  rotates at a constant velocity, and print head  500  begins at one end of print media  32  and traverses the entire length of the print media  32  for completing the transfer process for the particular donor sheet material  36  resting on print media  32 . After print head  500  has completed the transfer process, for the particular donor sheet material  36  resting on print media  32 , the donor sheet material  36  is then removed from vacuum imaging drum  300  and transferred out of image processor housing  12  via a skive or ejection chute  16 . Donor sheet material  36  eventually comes to rest in a waste bin  18  for removal by the user. The above described process is then repeated for the other three rolls of roll media  30  of donor roll materials  34 . 
     Referring to FIG. 2, there is illustrated a perspective view of a lathe bed scanning subsystem  200  of image processing apparatus  10 , including vacuum imaging drum  300 , print head  500  and lead screw  250  assembled in lathe bed scanning frame  202 . Vacuum imaging drum  300  is mounted for rotation about an axis X in lathe bed scanning frame  202 . Print head  500  is movable with respect to vacuum imaging drum  300 , and is arranged to direct a beam of light to donor sheet material  36 . As an example, the beam of light from print head  500  for each laser diode  402  (not shown in FIG. 2) can be individually modulated by modulated electronic signals from image processing apparatus  10 , which are representative of the shape and color of the original image; so that the color on the donor sheet material  36  is heated to cause volatilization only in those areas in which its presence is required on the print media  32  to reconstruct the shape and color of the original image. 
     Print head  500  is mounted on movable translation stage member  220  which, in turn, is supported for low friction slidable movement on translation bearing rods  206  and  208 . Translation bearing rods  206  and  208  are arranged as parallel as possible with axis X of vacuum imaging drum  300 . A longitudinal axis of print head  500  is perpendicular to the axis X of vacuum imaging drum  300 . Front translation bearing rod  208  locates translation stage member  220  in vertical and horizontal directions with respect to axis X of vacuum imaging drum  300 . Rear translation bearing rod  206  locates translation stage member  220  with respect to rotation of translation stage member  220  about front translation bearing rod  208 , so that there is no over-constraint condition of translation stage member  220  which might cause it to bind, chatter, or otherwise impart undesirable vibration or jitters to print head  500  during the generation of an intended image. 
     As shown in FIG. 3, lead screw  250  is attached to a linear drive motor  258  on its drive end and to lathe bed scanning frame  202  by means of radial bearing  272 . Lead screw drive nut  254  includes grooves in its hollowed-out center portion  270  for mating with threads of threaded shaft  252  of lead screw  250 , for permitting lead screw drive nut  254  to move axially along threaded shaft  252  as threaded shaft  252  is rotated by linear drive motor  258 . Lead screw drive nut  254  is integrally attached to print head  500  through a lead screw coupling and translation stage member  220  at its periphery so that as threaded shaft  252  is rotated by linear drive motor  258 , lead screw drive nut  254  moves axially along threaded shaft  252 , which in turn moves translation stage member  220  and ultimately print head  500  axially along vacuum imaging drum  300 . 
     As best illustrated in FIG. 3, and as described in U.S. Pat. No. 5,771,059, an annular-shaped axial load magnet  260   a  is integrally attached to a driven end of threaded shaft  252 , and is in a spaced apart relationship with another annular-shaped axial load magnet  260   b  attached to lathe bed scanning frame  202 . Axial load magnets  260   a  and  260   b  are preferably made of rare-earth materials such as neodymium-iron-boron. A generally circular-shaped boss part  262  of threaded shaft  252  rests in a hollowed-out portion of annular-shaped axial load magnet  260   a , and includes a generally V-shaped surface at the end for receiving a ball bearing  264 . A circular-shaped insert  266  is placed in a hollowed-out portion of the other annular-shaped axial load magnet  260   b , and includes a shaped surface on one end for receiving ball bearing  264 , and a flat surface at its other end for receiving end cap  268 . End cap  268  is placed over annular-shaped axial load magnet  260   b  and attached to lathe bed scanning frame  202  for protectively covering annular-shaped axial load magnet  260   b  and providing an axial stop for lead screw  250 . Circular shaped insert  266  is preferably made of material such as Rulon J or Delrin AF, both well known in the art. 
     Lead screw  250  operates as follows. Linear drive motor  258  is energized and imparts rotation to lead screw  250  about axis  301 , as indicated by the arrow  1000 , causing lead screw drive nut  254  to move axially along threaded shaft  252 . Annular-shaped axial load magnets  260   a  and  260   b  are magnetically attracted to each other which prevents axial movement of lead screw  250 . Ball bearing  264 , however, permits rotation of lead screw  250  while maintaining the positional relationship of annular-shaped axial load magnets  260   a ,  260   b , i.e., slightly spaced apart, which prevents mechanical friction between them while obviously permitting threaded shaft  252  to rotate. 
     Print head  500  travels in a path along vacuum imaging drum  300 , while being moved at a speed synchronous with the rotation of vacuum imaging drum  300  and proportional to a width of a writing swath. The pattern that print head  500  transfers to print media  32  along vacuum imaging drum  300  is a helix. 
     FIGS. 4 and 5 show components at the drive end of lead screw  250 . Radial bearing  272  which is a magnetically loaded radial bearing is mounted on threaded shaft  252  (FIG.  5 ). Linear drive motor  258  is a stepper motor in the preferred embodiment of this invention. As shown in FIG. 5, a shaft  258   a  of linear drive motor  258  attaches to threaded shaft  252  of lead screw  250  by means of a collet  284 , secured by a nut collet  286 . Motor  258  mounts to a rotational motor stop or frame  292 , which provides a rotational stop that constrains movement of motor  258  as its shaft rotates. A stop button  290  attached to rotational motor stop  292  is magnetically attracted to a stop magnet  294  which is installed inside lathe bed scanning frame  202  (at the position shown in FIG.  4 ). 
     The components illustrated in FIG.5 make up a lead screw assembly  90 . Lead screw assembly  90  is removable as a unit from its position in lathe bed scanning frame  202 . 
     FIG. 6 shows an aperture  86  in a motor support member  88  of lathe bed scanning frame  202 , with lead screw assembly  90  removed. In an operating position, the motor end (with motor  258 ) of lead screw assembly  90  is held in place in motor support member  88  by magnetization of radial bearing  272 . The opposite end of lead screw assembly  90  is held in place by attraction of axial load magnets  260   a  and  260   b  as shown in FIG.  3 . With this arrangement, magnetic attraction at both ends fixes the axis of threaded shaft  252  into position with respect to lathe bed scanning frame  202 . Then, to prevent rotation of lead screw assembly  90  as threaded shaft  252  rotates, rotational motor stop  292  is provided, and held in position by magnetic attraction at stop button  290 . 
     An access slot  86   a  of aperture  86  is sized to be slightly larger than a diameter of threaded shaft  252 , to permit the removal of lead screw assembly  90  only after the opposite end of lead screw assembly  90  is pulled away a slight distance from axial load magnets  260   a  and  260   b , so that radial bearing  272  and other components on the motor end of lead screw assembly  90  can clear the access slot. A circular inner portion  86   b  of aperture  86  is sized so that radial bearing  272  fits snugly into motor support member  88 , held by magnetic attraction of radial bearing  272  to motor support member  88 . Attraction of axial load magnets  260   a  and  260   b  at the opposite end of threaded shaft  252  hold lead screw assembly  90  at the correct position so that lead screw assembly  90  can be removed and re-seated in the same position each time. 
     FIGS. 7 a  and  7   b  show how lead screw  250  or lead screw assembly  90  including lead screw  250  are removed from lathe bed scanning frame  202 . First, translation stage member  220  and print head  500  (not shown in FIGS. 7 a  and  7   b ) must be disconnected from lead screw  250 . In the preferred embodiment of this invention, two screws (not shown) must be removed to unfasten translation stage member  220  from lead screw  250 . In the preferred embodiment of this invention, a modular electrical connector (not shown) must also be disconnected from linear drive motor  258 . 
     To free lead screw  250  or lead screw assembly  90  including lead screw  250  from its magnetic attraction points, rotational motor stop  292  is first pivoted up from attraction at stop magnet  294 . Next, lead screw  250  or lead screw assembly  90  including lead screw  250  are pulled away from axial load magnet  260   b . Lead screw  250  or lead screw assembly  90  including lead screw  250  can then be pulled out horizontally from its normal operating position (to the right, as viewed in FIG. 7 a ), so that the diameter of threaded shaft  252  clears access slot  86   a  in aperture  86 , allowing removal of lead screw assembly  90 . 
     Insertion of an alternate lead screw  250  or lead screw assembly  90  including lead screw  250  is the reversal of the above procedure. Once lead screw  250  is fed through access slot  86   a  in aperture  86 , axial load magnets  260   a  and  260   b  attract the end of lead screw assembly  90  against lathe bed scanning frame  202 . Then, rotational motor stop  292  is pivoted into place, and held securely at stop magnet  294 . Finally, any needed electrical connections can be made to linear drive motor  258  and translation stage member  220  can be reinstalled. 
     The invention has been described with reference to the preferred embodiment thereof. However, it will be appreciated and understood that variations and modifications can be effected within the spirit and scope of the invention as described herein above and as defined in the appended claims, by a person of ordinary skill in the art without departing from the scope of the invention. For example, the overall configuration and arrangement of the slot and circular inner portion for the aperture can be altered without changing the scope of the invention.