Machining Acetal/Delrin guides

[PMOrthotics]

hello all

we currently mill acetyl products with an axyz millenium as 3d shape by flipping the work piece etc. Using 2 flute carbide cutters of ball nose 10mm on raster 1.2mm step over. Max cut depth is around 25mm, but average across piece is around 16mm. The finish is good/excellent. The product is all similar but different sizes/surface geometry (slight variations).

Newbie question really...Should we be undertaking this work in one cut/pass or should multiple passes be used. We require reasonable turnaround but do not want to damage our kit? Any experienced guys on here cutting this all day every day to offer some guidance?

Any help or pointers appreciated!

[DMF_TomB]

Acetal machines easy and usually gives a good finish.
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only thing i can say is plastics are not as rigid as metal. i have made fixtures from plastic before only to find cutting forces can easily bend part out of position.
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clamping plastics in a vise or a chuck it is easy to distort their unclamped shape or they are curled in vise and after machining part is not parallel and or straight / flat.
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sometimes using smaller diameter cutters and less depth of cut and feed may take longer but produce a more accurate part.
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how a plastic part is held can have a big influence on speed it can be machined.

[PMOrthotics]

Many thanks for your reply.

we took a look today at how our fixture could be bettered. Currently we are using a picture frame of aluminium (milled from sheet) with a 5mm lip all round internal diameter of 500mm. 12mm bolts * 4 hold the plastic from side through jig. The plastic (30mm) bows easily and visibly and when we turnover is not held by anything other than bolts. So as a result of your advise and then us taking a look, we are modifying the design to incorporate 8 clamps to hold against the lip from top instead. The sound when milling on occasion would be wrong, although the parts were of a fine tolerance as they are low tolerance. Placing straight onto the vac bed gives a better result that we can hopefully imitate with the modified fixture.

As for the cut depths are these acceptable if the finish is fine or are there rules to follow that i am unaware of? The chips are fine, although i want to gain the optimum cutter life without destroying productivity?

Thanks for the suggestion, as i say very helpful.