Adjusting Tolerances of wheels, axles and bearings

    Tolerances between axles, bearings and wheels need adjusting.  Shown above are several iterations of each shown alongside the MIDS chassis.  Essentially three iterations of wheels and axles have been done and five trials of the bearings.  Alternate materials such as PLA and ABS have been tried and two Z layer thicknesses.  Almost all changes resulted in changes in key interface dimensions.  

    Shown above are the three most recent trials of black ABS axles.  On the left is one made with 0.125mm Z layer while the other two were made with 0.25mm Z thickness.   All three were built on top of a 0.25" thick green ABS support layer above the thin raft section to improve ease of removal.  The support layers are run in accordion folds which reduces the contact with the part.  The base raft comes out almost solid which increases adhesion greatly making removal very difficult.
    The middle axle is slightly undersized and the right hand axle has adjusted diameter which gets the part very close to the desired scale size, 1/2" per foot of a 10" prototype diameter (1/24th scale).  Indents on the ends provide a ready means to key the drivers to 90 degrees as done for steam engine drivers.  This axle is used for Front, Intermediate and Back drivers.
    A slightly larger 12" prototype diameter axle is used for the Main drivers, so another set of iterations to achieve correct scale size will likely be required for that axle and probably the wheels and bearings as well.   The Main drivers also have a much larger more complex side rod shaft structure to fit the side rods, main rod and eccentric rod.

    The original bearings built were PLA shown on the assembled driver and axle set at the top of the picture above.    These were difficult to clean up so a pair of ABS bearings were built, at photo middle.  Those bearings fit OK on the PLA axle but are a bit undersized for the design axle diameter.   Consequently a third set was built with adjusted axle hole diameter as shown embedded in the green ABS supports on top of the thin solid layer ABS raft base layer.  

    Also, the width of the bearing slot was a bit too liberal providing too much looseness to the frame slots.  Measurements confirmed that the bearing slot is too narrow while the frame slot is correct.  The new bearings still remain to be removed from the support material, cleaned up and checked for proper fit with both the final axle and frame. 

    The wheels also have undergone adjustment.  Above right is a PLA wheel, the green middle being ABS and the one at left black ABS.  The PLA wheel came out good, but the approach was to use black ABS.  Once the black ABS material arrived, the left wheel, an intermediate driver, was made using the dimension adjustments of the green Front or Back driver above.  The difference between them is the larger counterweight of the intermediate driver.
    Both the green ABS and black ABS wheels were built on thick ABS supports reducing contact with the ABS solid raft.  This approach greatly reduces contact adhesion permitting relatively easy removal from the raft-support structure.  The photo of the bearings above shows one such structure.  The bearings are not directly in contact with the raft but ride instead on several layers of accordion fold support structure that is much easier to separate from the part.  This approach has proven superior to using a PLA raft and support approach.
    Adjustments will continue of the above parts until good appropriate fits are achieved for axles, wheels and bearings with the frame slots.  These are the key moving parts and must fit well to permit good operation of wheels with the side rods and bearings with the equalization scheme.
    A conclusion reached after using the 3D printer is that accuracy to design dimensions is not of the order achieved using metal machining and requires small adjustments to correct for the build software (Axon) conversion process.  The repeatability is good so that once adjustments are incorporated, multiple parts may be built and achieve comparable accuracy.
    Code 250 rail, ties and suitable spikes have been ordered to permit build of short sections of track for use in checking rolling operation of drivers and other parts.  In the end a suitable length of track section will be built and finished with ballast and appropriate coloring for display of the finished locomotive and tender.