Continued Builds of Back Frame Parts

    The first run of the back frame sides resulted in distortion particularly where the frames fit into the pockets of the main side frames.  Although the frames were filed down and smoothed somewhat, they were too loose in the pockets and would have made alignment more difficult.  Consequently a second run was made as will be shown later.

    One of the parts that go on the back frames is the rear truck spring mount.  The mount cantilevers the spring mount outward from the back frame so the springs will be located over the trailing truck journals.  Also the mount part attaches the rear of the spring saddle with a vertical pivot point.  The above photo shows the second attempt at build immediately after removal from the 3D printer.  The raft and support structure penetrated through the part in places that should not have the need for support material.  The part design may be at fault.

     The top part is the first attempt.  During the run the printer stopped building support structure half way through the build due to excess spool pressure of the PLA supply spool.  That material is tightly wound on the spool and retains a tight circular shape that exerts significant spring pressure on the printer feed mechanism.  It is suspected that the spool spring pressure may need to be relieved by unwinding and rewinding the spool to reduce the pressure.  The machine feed failed several times during this and subsequent runs.  The bottom is a second attempt.  This time the support material continued OK, however, it was located through the main vertical walls of the part.  This is probably a result of poor design of the part and will need further investigation.  Perhaps the wall thickness was too little.

    A second run of the long back frame sides was done using a sacrificial internal frame.  The internal frame resembles a staircase.  The internal frame members are held with small extensions at the ends to the side frames so that the internal framework can be cut away after the front and back spacers are glued in place.  This approach also holds the frames in good alignment for the gluing process.  The photo shows the discontinued support material that exemplified the difficulties with the printer feed of the clear PLA from a partially used spool.  

    Another part being developed is the trailing truck centering spring assembly.  This consists of a housing and two cone shaped ends plus an internal spring that presses the cone shaped parts outward.  The trailing truck will have two screws that connect to the center of the inside of the cones providing spring pressure to center the trailing truck.  This part will mount on the bottom of the rear firebox mount.  The part above is the second trial and is not quite correct yet.  The cones should not pass through the housings freely, but should provide a stop keeping the cones from coming out of the housing under spring pressure.

    After removal of the support material the rear frames with the sacrificial internal frame is shown above.  The rear and front spacers have been inserted temporarily to check fit.  The fit into the main frame pockets is much tighter and now appear correct.  Also, both spacers fit well.  The spacers will be glued in place then the internal framework removed.  After other parts are properly located and attached, the back frame assembly will be glued in place in the pockets.  The trailing truck will mount under this portion of the chassis framing and the firebox will be located above it.