Thursday, July 26, 2012

Pilot Truck Frame Fabrication and Assembly

 The above drawing from Alibre CAD program illustrates the pilot truck overall assembly, bottom view.  The dark grey section is the center of the truck frame that spaces the bearing mounts.  Those mounts have the keepers bolted on the bottom.  The bearings are the parts with round-like flanges.  The flanges are both inside and outside the bearing mounts.  The axle passes through the bearings and into the center of the wheels.

The top of the pilot truck assembly is shown above from the Alibre CAD program.  Two radial arms extend back from the bearing mounts to the pivot point.  The two arms are joined as they approach the pivot.  One arm set is attached to the truck frame just above the axle.  The other set is attached to the frame at the bottom of the bearing mounts.  The center spacer has a slightly curved slot to accommodate the pilot truck equalizer bar.  That bar will be held at the center line of the locomotive and a rounded portion will extend down into the slot.   
 Pictured above is the entire center frame of the pilot truck less the bearings, wheels, axle and keepers.  The green ABS material forms the bottom raft that was attached to the 3D printer and provides spacer supports for black ABS material that forms the truck frame.  In general, the print came out good with some ripples along the pivot arms, but was otherwise very good on top.
This view shows the slightly curved slot for the equalization lever post.  The radius of that curve centers on the pivot hole at the left end of the pivot arms.  This kind of feature development is quite easy using the Alibre software.
 After removal of the large amount of raft and support structure, some green strips of support material still adhered to the truck frame.  This same result occurs on virtually all parts built on the 3D printer.  the strips must be mechanically removed either with a model knife or Dremel sander wheel.
 The joined sets of pivot arms came out per the design files and need only clean-up of the green ABS material residue.
 Two sets of keepers were fabricated as shown above on the raft with almost no support material.
The keepers were easily removed.
The bottoms of the keepers had green ABS support material residue as shown on the bottoms above.  The tops were clear.
The bottoms were cleaned up using the Dremel sanding wheel.  Two are clean, two still have residue in the photo above.  Note the small holes that mark where the 0-82 screws will pass through.  Although the holes were designed to clear, a clearance drill will be required to compensate for the tendency of the printer to close up holes slightly.

Clean up of the pilot truck frame was initially done using the Dremel sanding wheel and small round cutter bit and followed up with the model knife.
 The bottom of the pilot is shown above after clean up.
The last step is hand tapping of the 0-82 screw threads into the bearing mount frames.  The soft ABS plastic taps very readily by hand.
 The photo above shows the trial fit of the keepers on the ends of the bearing frame posts.
 Above is a bottom view of the pilot truck assembly with the wheels, axle and keepers in place.
 The top photo above shows the assembled pilot truck.
finally, the pilot truck is attached to the pivot using a 3-48 screw and washers.  The pivot frame between two locomotive side frames just forward of the front drivers is tapped to accept the bolt.  The pilot wheels are centered in the upward jog of the front locomotive frame.  The front frame is still not glued in place in the photo above.  It requires some modifications to the front platform to make it square.  The drop down section of the frame between the pilot wheels and front driver will mount the cylinder assembly after it is built.  Also, the pilot equalization bar pivot will be located between the frame members where the cylinders mount.

Monday, July 9, 2012

Pilot Truck Axle and Bearing Fabrication

 The pilot truck axle is a rod about 1/4" in diameter and 2.75" long.  Such a high aspect ratio part of relatively small diameter would be a relatively high risk to make.  It would need to stand vertically and might distort or topple over during fabrication due to the drag of the hot ABS filament being deposited by the 3D printer head.  To help mitigate the risk, a feature was added to the top end that caused the Axon build software to add support material to hold up the overhang of the part.  The overhangs faced each other of two identical axles thinking that the Axon software would build a support frame between the two axles.  Instead the software built a support frame that essentially surrounded each axle and at the top a bridge was thrown across to support the overhangs.  This works as shown above.  It took seven hours to print the parts due in large part to the warm up time for the two ABS materials when the heads switched at each of the 200+ layers.
The support materials were easily removed mainly by hand.  The two axle rods had very little green ABS support material except at the bottom end where the rods were anchored to the raft.  The bridge between the two overhang features at the top took a bit more force, but still came away easily.
The above photo shows the two axles that were built, one with the top end towards the camera and the other bottom first.  The top has the overhang feature which will be removed and the bottom has the green strips of ABS raft material that need removing.  The top overhang feature snapped off nearly clean.
 The Dremel tool sanding wheel was used to remove the residue green ABS and smooth out the region where the overhang feature was snapped off.  The axles took very little time to clean up.  The above photo shows that the axle and wheel fit as built, neither requiring any adjustments.
 The pilot wheels are 1.44" in diameter and 0.26" thick and came out well during fabrication as reported in a previous blog.  The above photo shows the arrangement of axle and pilot truck wheels.
The pilot truck as in the main drivers have bearings that surround the axle.  An assembly of four was created and printed.  Unfortunately, the 3D printer experienced a separation of the green raft and support part way the printing process which resulted in two bearings being destroyed.  The other two however came out very well.
The two good bearings are shown above in the orientation used for printing.  The axle hole in the middle has some support material in place to hold the top of the part during fabrication.
The two good bearings cleaned up with very little work using a model knife to cut away most the material and the Dremel sanding wheel to remove the green strips at the bottom.
The bearings also came out well on dimensions and slipped on the axle with free rotation as built.  The above photo shows the axles in their approximate position on the axle.  The pilot truck frame will slide on the slots of the bearings after assembly.
 The above photo shows the approximate position of the pilot truck wheels under the front frame.  The open section of frame to the left of the pilot wheels will mount the cylinder assembly.
Above photo shows a bit more of the front frame section from drivers to pilot wheels.

Wednesday, July 4, 2012

Fabricating Pilot Truck Wheels

The 3D printer was fired up again to build a set of pilot truck wheels.  Two sets were fabricated on a common raft as shown above.  Printing time was 3hrs 1 minute.  They came out good with no lifting or deformation of the wheel flanges.  The printer had not been in use for a couple of months and there was little effort required to set up the new print.  The platform was cleaned to remove dust that had settled on it and the degree of level for the extruder movement around the platform was checked.  It took less than five minutes to check the printer before the printing was started.  No problems occurred during the print.  The wheels were black ABS while the raft is green ABS.
After printing the wheels were removed from the green raft leaving the usual residue of strips on the back.  Initially strips spanning the distance across the wheel were removed with the model knife shown above.  The opening for the axle was cleared with the knife also.  The remaining green strips were tightly adhered to the wheel. 
The green strips on the flat portions of the wheel around the rim and hub were removed using the small sanding wheel with the Dremel tool.  After that the remaining green support material in the concave portion of the wheel were left as they could not be reached by the sanding wheel.
 The concave portions were cleared of green support material using a small round Dremel cutter tool shown above.  At this point the back of the wheels were free of all support material residue.  The grinding and sanding process resulted in a grey shade due to residue dust.
The front of the wheels are as printed while the backs (grey shaded) required removal of green support material residue using a knife and Dremel sanding and small cutting tools.  The wheels are now ready to use.