Re: End Mill Advice for ABS

Matt Goodman

Thanks to everyone (on and off-list) for the advice on removing details from steam era freight car models with a mill. Especially the advice to use a non-center cutting end mill. While my intention was to cut from the side, I can see the potential for doing more damage with a center cutting mill.

I searched my usual haunt for these types of items (McMaster Carr) and could find only center-cutting bur end mills - at least in the smaller diameters I was looking for. I may pick one of those up anyway, but will also pick up the Dremel cutter that Dave suggested. 

Dennis brought up an issue that I’ve been thinking about - how to prevent vibration and how hold the car body. Thanks, Dennis for also providing the solution - I was considering something along the same lines, though I may - instead of tape - use a second fixture to hold the carbody hard against internal fixture described. 

Thanks Ben, for the pointers to the earlier articles on kit bashing hoppers.

With the cutter question answered, I can re-assess whether the rest of the project makes sense and move ahead.

Matt Goodman
Columbus Ohio US

On Apr 20, 2019, at 11:08 AM, Dennis Storzek <destorzek@...> wrote:

On Sat, Apr 20, 2019 at 07:24 AM, Todd Horton wrote:
A reverse helix cutter will push the material down instead of trying to lift it.  I think Harvey Tool Company has these.  They sell very small tools.  I’ve used a .004 diameter cutter from them in the past making medical parts.     Todd Horton 
This is true... but the cutting forces are still liable to make the plastic part bounce, and gouge the part with the end of the cutter on the upstroke of the bounce.

The hardest thing with machining on one piece body shells is holding the darned things. You can't squeeze them in a vise without crushing them. Trying to clamp an open hopper in a vise with the top chords against one jaw and the hoppers against the other is just asking to have the sides bow, either up or down, doesn't matter, the plane of the side you are trying to match is no longer a flat surface.

Best strategy is to build a fixture that holds a block that fits into the body and provides clearance for the other side; this is how our pad printing fixtures work, directly supporting the side being printed. The fixture can have notches machined into it to clear internal features of the plastic body shell, which can then be held in place with double faced tape. Use the strongest grip tape you can get; isopropyl alcohol will normally dissolve the adhesive without hurting the plastic to free the part.

When machining plastic, always use new, sharp cutters.Cutters that have been used on metal will not cut as good and are more likely to melt the plastic.

Dennis Storzek

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