Large Scale Central

Aftermarket & Scratch Trucks

I’m starting this thread as a place for people to document methods of mounting both trucks and couplers to existing and scratch built rolling stock. A lot of the same considerations apply for truck and coupler mounting, ie. maintaining the proper heights and alignments between coupler, truck, body and track.

I would appreciate people adding pointers to existing threads on scratch building trucks. Questions, answers, .stl files, photos, etc. all welcome!

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I’m making this 1st post the “holder of the links”. I’ll edit, add, remove as appropriate, so always check back here for new items.

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To get started, here is an (as yet untested) body bolster modeled after an MTH Pullman part. The intent is a part that can be attached to the bottom of a scratch car and mount an MTH (or aftermarket) truck
at the appropriate height.

The bottom of an MTH Pullman car, showing the body bolster:

The blueprint for a similar model:

The Cura picture:

The actual .stl file:
MTHPullman-Part.stl (15.9 KB)

If the post is printed upright with filament it will have low shear strength. I print the bolster with a hole to accommodate a bit of 6mm ABS rod.

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Yes, this is true. I rushed that out yesterday without any refinements to illustrate what I was attempting. I’m considering the possible use of brass rod or threaded brass insert.

You also have to pay attention to how you create the part and slice it.

Here the part was designed with the stud placed on top of the pedestal.
printed upright:


Note how the base of the stud is placed on top of the pedestal skin - WEAK

printed on its side:


Stud wall attached to pedestal wall, better, but still WEAK

Here the stud was made the length of the pedestal and the exposed stud and embedded into the pedestal:
printed upright:


Same as the first test, attached to skin of pedestal - WEAK

But here we rotate the embedded part on its side:


Note how the pedestal and stud are printed as one solid part.

Finally, increase the wall count to 8:

I added a fillet to the base of the stud, should be even stronger. Think of 3d print strength the same way you think of wood grain.

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So by making the stud bolt hole slightly smaller (down to 0.060" from 0.100") the slice has an additional strand (from 6 to 7) of filament in it - even stronger.

This smaller size if fine. I usually under size bolt holes that I intend to tap, then ream them out to the proper tap size with a bit in a hand chuck. You can’t get accurate (ie. to tap size) holes with FDM anyways.

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Nice insights on 3D printing for sturdiness!

And if you want to go even further drill that tap hole deeper into the pedestal base, the bolt will add yet more shear strength! And be sure to drill with a hand chuck … power drills tend to overheat the material and melt it instead of removing it cleanly.

Here’s my final .stl file. It was built in FreeCAD stud up. Rotate 90deg to its side and then slice. Using the slider you can examine the layers yourself.
MTHPullman2d-Fusion.stl (472.2 KB)

Nicely done Illustrated description of FDM structural considerations! I’ll stick with my method of using a separate post. My bolsters tend to incorporate the coupler mount as well which would make it far more difficult to print on end. Here is one for a Thrall well car I’m working on.

So I assume you are using something like:
https://smile.amazon.com/uxcell-800pcs-Spacers-Washers-Height/dp/B07FKK7R14/

Nope. Just plain solid 6mm ABS rod. Drills very nice. No melting with my variable speed drill. No need to be perfectly centered to just hold a washer. Self tapping screws work very well. Another benefit is since some trucks require longer posts, I can decide on the trucks last and cut to appropriate length. I also share many of my designs so saves me from publishing parts with different length posts. :grinning:

I’m quite enjoying this post and understanding the 3D printing process a bit more with the pics posted.

Thanks Steve and Dan

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They work well if you have the proper sized hole and take your time. MTH products are notorious (in my mind at least) for haves broken parts associated with tap screws. This is a motor block mount from one of my MTH GS-4 Daylight locos:
brokenstud

Ordered the part, waited a few weeks, then decided to print it myself:

Drilled and tapped the holes, works great!

It’s a rare moment when the rooster enjoys anything. :wink:

Lucky I have no MTH. The ABS is less brittle/more forgiving. I bought the ABS rod back in 2017. Before that, I believe I was using Acetal rod which worked as well. I assume you are printing in ABS?

I use PLA, PLA+, ABS, ASA and others. Depends on my mood and the project. Using ABS right now for truck and bolster parts. Might switch to ASA:
https://en.wikipedia.org/wiki/Acrylonitrile_styrene_acrylate

I use ASA for some things. In fact, on topic, I print these USAT truck to Aristo loco adapters in ASA (in the orientation shown). ASA is good strong stuff and sands pretty well also.

I’ve only experimented with it so far - have 1 roll of white. I find that white shows details much better when experimenting. Didn’t do enough to get completely dialed in. Once I do I’ll get a roll of black and do my trucks with it. Read several comments recently to the effect that once you master ASA you’ll
never bother with ABS again.