Cutting SMALL plastic part

[woodturner]

Need a 'lil help please. I need to cut some prototype parts from plastic (Delrin, I think) blocks (approx 2" and 4" x 1-5/8" thk). The finished parts are approx. 3/4" x 1/2" x 3/4 deep. I plan to machine it on it's edge so I can work my way down the block, then cut it off later.

Wondering if I should use a router bit or end mill (as I did when I made one on the manual mill). Either way, I was wondering about speeds and feeds. How slow can I go with such small movements and still hold a decent tolerance? Are there machine settings I should change/adjust for this operation? I plan to screw or clamp the block to a fixture that is clamped to the table. Machine is an alpha with spindles.

Thanks in advance.

[Brady Watson]

How slow do you anticipate needing to go? Acetal should machine easily with either an end mill or straight 2-flute router bit and give a good finish. You would only need to go slower than the default SB system is set up for if you need to use a very small bit to get details.

Posting a pic of a typical part would be helpful.

-B

[woodturner]

I don't have a picture I could post. I could best describe the part as a small wedge shape with a couple of steps on the flat side and curved ends. The angled area and curved ends are why I said I plan to machine it as if it was standing on one end like an extrusion.

Needing to get to 3/4" deep, I figured on making a pass at "some" depth and stepping down until I get to the final depth. Kind of the opposite of a "stereo lith" machine. On the mill, I was turning a 1/4" end mill at 1100 rpm and cranking by hand.

I am more familiar with cutting cabinet parts on the bot and when I've pocketed small holes I get "so so" results. When I envision that carriage moving around in this odd shape at any kind of normal move speed (4 or 5 ips) I foresee problems with the bit shaping a good part. I was thinking of reducing rpm to 1000 and move speed to 1/2" per second or less. Some of the moves are 1/8" and a straight run is less than 3/4". Again more like a milling machine than a router. I would go back to the mill but the machine time would kill me making a few dozen pieces.

Thanks again for your input.

[Brady Watson]

The biggest issue is how you are going to hold the part down to the spoilboard. Cutting it is the easy part. If you can draw this part in CAD and post up a screenshot, or email it to me, I'd be happy to make suggestions.

-B

[2talltary]

Delrin (http://www.iplasticsupply.com/materials/acetal-delrin-celcon-basf-pom-pomalux-sheet-rod) is the best machinable plastic out there. Try the highest speed possible. Make sure piece is flat and adequate clamps should be used. Band saw or circular saw will work great. If you are cutting blocks with straight edges, go with a circular saw. If drilling, use high speed steel drill and make sure chips are removed so drill won't clog.

[bleeth]

If I have this right you are going to
slice your material and turn it on its side for machining.
If so you do need to ensure a good holddown. I would space my parts out along the slice far enough so I could have a hold between each part as well as the ends. You could make a two sided clamp that resembled a dentil molding shape or just screw it at the ends and in-between. I would go around 1-1.5ips and set up my passes around a 1/4" deep if I were using a 1/4" bit and leave a skin at the bottom for my last pass. I wouldnt use a tolerance as that often results in a small part jumping when the last bit is cut. Added insurance of double sided tape wouldn't hurt.

If the toolpath keeps your tool inside of the outer edge of the part you could clamp it between a couple of pieces of plywood or set up your plywood and run your toolpath without your delrin in it to give you a shaped clamp.