The middle chassis side frames are a bit over 8" long. The first effort to build a pair was made using ABS for the parts with PLA raft and no supports. The frames have three wheel bearing slots. From left to right they are for the intermediate driver, main driver and back driver. The front driver slot is part of the front frame structure. The prototype locomotive frames are essentially one piece encompassing the front and middle sections of the model, however, those would have been too large for the 3D printer size capacity of 9" or 10" depending on orientation. Consequently the chassis was divided into two parts so they halves would fit within the limits of the printer. This section was chosen to permit fit checks with the wheels and bearing proof parts previously built.
As can be seen in the above photo, the frames warped substantially. The view shows the bottoms of the frames where the small holes for machine screws were included. Those holes are primarily to locate the positions for drill and tap to accept small 0-80 machine screws that hold the frame keepers in place. The frame keepers are bars that hold the bearing blocks in place. On the prototype the keepers provide considerable stiffening of the frame in the long dimension as well. In the background are the four wheel and axle sets glued during a prior effort.
After building the ABS frame sides, the printer build file was configured to build PLA frames on ABS raft with ABS supports. As the first couple of layers of PLA were applied it became apparent that the ABS was coming loose and strands were separating from the base layer. The printing was stopped at the point shown in the above photo. The loose strands of green ABS are clearly visible. Those in turn caused the PLA layers to be highly distorted and dislocated.
Shown above is a photo of the overall print pattern for the two side frame members on the printer build bed. The loose ABS raft strands are clearly visible. This photo was taken just prior to removal from the machine.
The above and below photos show a close up view of the loose strands and heavily distorted black PLA print layers above the strands. It is clear that the resulting PLA print would have very poor and essentially useless.
After examining the results of the PLA attempt, a decision to retry using the finest Z layer dimension was made. The above print attempt was made using 0.25mm Z layers. The next photo shows that result in the upper part of the photo and a reprint using 0.125mm Z layers in the next attempt in the lower part of the photo.
The finer Z layers resulted in a more secure ABS raft structure, however, the raft lifted slightly from the build platform at the right side during the printing process resulting in warped frame pieces. Also, the right hand portion of the bottoms (bottoms shown in the photo) were badly printed and would require considerable effort to repair, probably impractical. Consequently, the admonition to avoid using ABS for runs longer than 100mm is clearly born out in both cases.
The main conclusion to be drawn are that it will not be feasible to build the frames in sections so large as 8". The frame will now be broken up into portions of about 4" which will then be glued together to form the overall main frame some 15+" long. Another conclusion reached is that the frames can be either ABS or PLA depending on how well the smaller parts print out. One issue is to determine a good interface between sections that will help align them and provide a strong joint without the need to use external reinforcement that would be non-prototypical.
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