ABS Machining question

[mikeoleary]

The part pictured is an ABS foosball man. I'll be going to a mold fabricator to make a mold for a part very similar to this (picture). The only problem is, when they take the ABS part out of the mold, sometimes there can be a small difference in the dimensions. The dimension that is critical is the big hole in the middle. It needs to be exactly .680. Plus or minus .002.

The part in the picture is one I bought from the worlds biggest foosball company, and they have screwed this up. . The center hole is .008 too big and the piece is no good. I really don't want to make that mistake, so I was talking with a friend and he suggested having the part molded with the center hole made a little small, and I finish the hole with a ream, or drill bit. I've been reading about machining ABS and it looks like there are specialty drill bits for plastic, but I'm more interested in using a ream. There are more size options, and would the hole be more precise?

So my question is, could I have them mold the piece with the middle hole .670 and use a finish ream to cut the hole to .680? Would that be too much material to remove with a ream, and would a regular ream (designed for cutting steel) work with the ABS?

 

[Tony Wells]

I believe you've reversed your dimensions. Leaving 0.010 to 0.15 is not unreasonable. ABS cuts easily, but is a but abrasive, and for production, I'd recommend a carbide tipped reamer. About the only difficulty I see is fixturing, which of course, can be done.

 

[mikeoleary]

Thanks Tony, I indeed had my dimensions backwards. Thanks for the advice.

 

[flutedchamber]

I spent two years as a setup mold mechanic for Wheaton Plastics. The oversize hole could be caused by poor temperature control. That can be the cooling of that part of the mold was not correct, or the plastic itself was too hot when injected. It could also be caused by incorrect cycle times of the mold.

As Tony said, mold it undersize and ream to the desired dimension. A nest to hold the piece for machining can be made of Devcon and your sample piece coated with release agent.

 

[mikeoleary]

A nest to hold the piece for machining can be made of Devcon and your sample piece coated with release agent.

That's a good idea.