Trailing Truck Assembly

    The journal bottoms for the trailing truck were printed and came out well as shown above.

    The small amount of support was readily removed and the bottoms are ready to glue in place

     The journal bottoms are shown along with the side frames.  

    A bottom view of the frame shows the remaining support strips on the bottom of the journal bottom covers.    

    After cleaning up the frames and tapping the screw holes the trailing truck frame is ready for assembly.  By using screws the hold the two side frames together the truck can be disassembled in future if need be.  The frames are held together with 2-56 screws one at front and another at the rear.  The frame parts are tapped so nuts are not used.  The large part at bottom center is not part of the truck, but is the pivot assembly that will later be assembled as part of the back frame.  The truck pivot is also a 2-56 screw so the pivot assembly was also tapped.  Tapping is done by hand and for the small size screws is very easy to do.

    The fully assembled truck is shown above along with the pivot assembly that will be assembled into the back frame of the locomotive.

    The temporarily assembled back frame of the locomotive was slid into the back of the main frame and the trailing truck pivot screw installed to hold the parts in their respective location.

    A close up of the trailing truck pivot attachment is shown above.  The truck rides quite close to the back frame.  Other parts will be assembled on the back frame to provide truck spring and equalization coupling and truck centering.

Trailing Truck Frame Parts Build

    Improvement of the engineer side frame was achieved by orientation the part on the 3D printer build platform.  The fireman side frame was oriented in approximately the same manner as the engineer frame previously built with the results above.  Very little lifting occurred and largely away from the part, consequently the part appears to have little if any distortion.

    After removal from the printer the part was examined in a bit more detail and moderate lift was found between the raft and support portion, but not between the part and support.  It appears that little or no part distortion resulted.

    A close up of the front end near the pivot of the frame shows a modest amount of separation between the green raft layer at the bottom and the green support for the part above the raft.

    After removal from the support material the part looks intact and free from distortion.

    Detail of the journal internal is good so the axle should rotate freely.

    Frame halves will be bolted together at front and rear for form the overall truck frame.

    

     The axle is shown in position above.  The bottoms of the two journals are in build and will enclose the axle bearing area.  To install or remove the wheel-axle assembly the truck frame will need to be disassembled by removing the two bolts.

    The tops of the frame and journals have support residue strips that will need to be removed.  This will be done using the Dremel tool with a sanding wheel attachment as was done for the drivers and frame parts.  The truck wheels were cleaned of support residue and turned round on the Sherline 4400 lathe.  

    The back frame section was held together with rubber bands and slipped into the grooves in the main frame for a temporary fit check.  The trailing truck frame was also temporarily assembled using rubber bands and slipped into the truck pivot.  A bolt will be used to retain the truck in the pivot to permit removal from time to time as needed.    

    Many other parts will go on the rear frame to incorporate the rear truck springs and equalization scheme which connects to the back driver springs with rods and levers.

    The front frame portion is not attached above so the locomotive will be about 20% longer than in the photo above.  The front frame portion hold the cylinders and front deck, pilot, etc.  At this point all moving parts such as wheels, springs and equalization rods and levers are in place except for those associated with the pilot and trailing trucks.  Much fit checking will be necessary as the back frame section is assembled to make sure that the trailing truck will fit and move correctly.

Finishing Trailing Truck Wheels

    With the arrival of the Sherline model 4400 mini-lathe an effort was undertaken to us it to remove the adhered support material and round up the wheel diameter;

    After removal of the wheel from the raft and support, strips of support material remain adhered to the wheels.  During clean up of the locomotive drivers the excess support material was removed using a Dremel tool with a sanding wheel.  Although the sanding process works it requires great care on the part of the operator to avoid over-sanding and creating uneven surfaces.  The mini-lathe can be used to simply turn the part and remove the excess.

     The wheels were chucked up in the lathe three-jaw chuck in preparation to turning off the green support strips.

    The two trailing truck wheels above are shown after turning off the support material on the back using the lathe.

    Since the trailing truck wheels (and locomotive wheels also) were built using a series of short straight edges for the tire, the mini-lathe was used to fabricate a holding tool for the trailing truck wheels so the rim could be turned to a round shape instead of the series of short flats.  The above photo shows a drilling operation into the center of a piece of round Delrin stock being turned into the tool.  The drill is the correct size for a 8-32 tapping operation to be done later.

    After drilling a short section was cut off to make a washer used to clamp the wheel.  The above photo shows a portion of the tool turned down to the correct diameter to fit into the axle hole in the wheels.

    The above photo shows one of the trailing truck wheels mounted on the tool during a fit check. 

    The above photo shows the short flats that formed the wheel rim.  Apparently the Axon software used to convert the 3D CAD design into g-code for the 3D printer transformed the round surface into the series of flats.

    After fit checking the holding tool it was cut-off on the lathe to a length that will keep the wheel close to the chuck on the lathe.

    The above photo shows the overall group of parts that make up a tool mounted wheel to chuck into the lathe when turning the rim.

    The assembled tool above is ready for tightening to clamp the wheel in place.  Once the tool is mounted in the latch the wheels can be mounted and turned one after the other.

     The tool above is now tightened ready for turning in the latch.

     The wheel mounted on the tool is shown chucked up in the lathe ready for turning.

    The turned wheel above is finished turning round.  Note the strings of ABS plastic material removed from the wheel.  The wheel is now round and will roll smoothly on the track.  Should the rounding procedure on the lathe be used in future, the wheels should be made a bit oversize and the lathe then used to turn to final dimension.

Control of Part Warpage

    The fireman side of the trailing truck was set up in the 3D printer aligned a bit more angled than the engineer side previously built.  The photo above shows significant separation of the support material holding up the frame above the raft at the bottom.  Because of the separation, the frame was significantly warped.  This was noticed part way through the 3D printer run and the machine was stopped before completing the part.

    The separation is believed to occur due to the extensive shrinkage of the just applied ABS material to the cooler material below.  The shrinkage applies force to the structure in a manner similar to a bowstring.  When a number of layers accumulate their total force, the support structure separates due to the pulling forces above.  The bottom most layers are quite strongly adhered to the build platform and remain fairly rigid.

    After removal of the support material the bend is quite apparent.  The center of the part is the bottom portion of the journal where the wheel bearing is located.  The vertical rib of the "I" beam sections to the left and right of the journal was just beginning, it too will be printed bent due to the curvature of the support material.

    The partially complete engineer frame at left is considerably more bent than the completed engineer side truck frame at right.  The main reason for the bending is that the support structure consists of long straight ribs in parallel which collectively apply a common bending force leading to the eventual separation.

    The Axon conversion software supplied with the printer permits a two material object to be built, with or without support.  The design consists of three files, one for each material and a third with both combined to provide alignment data.  The above design 3D illustration is the first attempt to use this method where the support will be a series of small tubes attached to the bottom of the part without support in this case.  The green tubes will act as supports for the grey part.  The part shown is the bottom closure of the trailing truck journal which will be glued in place.  The tubes will be cut or pulled away after printing.  The tube material will be green ABS while the dark grey part will be black ABS.  The intersection of the two materials is defined as well as the flat bottom of the green tubes that will attach to the raft.  The three files will be built from this one common file by deleting the green from the black part and deleting the black from the green support tubes to form the two material files.  The design file is where the part and support are aligned.  This method has not been used yet, but promises to allow custom designed supports which may eliminate the combined stress buildup and resultant support separation.

Second Attempt Building Trailing Truck Frame Half - Engineer Side

    After a five hour 38 minute 3D printer run the engineer side trailing truck frame was completed.  The part appears to be near perfect.  For this run the part was rotated in "Z" axis on the build platform so the raft and supports were much shorter segments as shown above.  The shorter segments did not lift except slightly at the extreme left of the part in the photo.  Despite lifting at the left, the top of the part was level and no distortion was apparent.

    After removal of the green ABS support material, the drawbar at the pivot end was design shape and expected thickness.   The part at left is the 2nd run and the 1st run is at right.  The pivot section is thinner than it should be and appears slightly twisted.

    The upper part is the first run and the pivot end is bent downwards too much.  The second run part in the foreground is properly aligned and full size.

   The first run part in the foreground has a heavy coating of support material that does not come off readily.  The lower end of the pivot part is thinner than design.  The second run part in the background is design shape.  The pilot hole for jointing the two frame halves is better defined and circular on the second run part.

    The balance of both parts is very nearly the same with less distortion and twist of the front end of the frame of the second build near the pivot region at left.