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How to machine Acetal

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AdrianR14/06/2019 17:31:59
272 forum posts
20 photos

Hi

I am trying to turn down the diameter of a 12mm Acetal rod to 8mm. I have tried a CCMT insert which does not seem to want to cut it. Sort of half cuts and deforms it.

Does it want to run fast or slow?

If I used HSS what is the best shape of tool to use?.

Should I use lubrication?

Any other hints?

Adrian

Andrew Johnston14/06/2019 17:35:43
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4855 forum posts
544 photos

Try a CCGT insert, cut dry and the secret to machining plastic is slowish speed and high feedrates. In theory you can whizz through plastic, but the big problem is it melts - equals a right mess.

Andrew

Roger Woollett14/06/2019 17:50:20
109 forum posts
3 photos

I have machined Acetal from time to time. I use a HSS - keep it sharp with plenty of rake. I usually use quite high speeds - the same as I would use for brass. This gives a nice finish.

ega14/06/2019 18:09:17
1265 forum posts
108 photos

See advice at:

https://www.directplastics.co.uk/about_plastics/post/all-about-delrin-as-an-engineering-plastic

Boiler Bri14/06/2019 18:54:45
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806 forum posts
285 photos

We use tips made for cutting aluminium. High speed and high feed rate with air cooling.

Bri

Howard Lewis14/06/2019 19:35:14
2337 forum posts
2 photos

Adrian,

I would be inclined to use a HSS knife tool, or one with a small nose radius.

Carbide tips are not really sharp, or intended for plastics.

The nose radius will prevent the stress raiser of a sharp corner.

Guessing at what you are making, may I suggest turning down the minimum length, for the spigot, and leaving the rest at the maximum required size? No point in making extra swarf for the sake of it!

Howard

Douglas Johnston14/06/2019 20:00:11
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611 forum posts
32 photos

The sharp polished carbide tips as sold for aluminium are what I use and they work a treat giving a very nice finish. They don't seem fussy about speed or feed rates. In fact I now use these inserts for most of my cutting from plastic to aluminium to mild steel and stainless steel, they are just superb tips especially on smaller lightweight lathes.

Doug

Samsaranda14/06/2019 20:05:20
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785 forum posts
5 photos

I have had variable results with carbide on plastic, I now use HSS but make sure it’s sharp, I always cut at a reasonably slow speed, too much speed generates heat which you don’t want.

Dave W

Nick Hulme14/06/2019 20:10:57
703 forum posts
37 photos

I use polished the TC tooling specified for Aluminium when machining plastic, it works very well.

Martin Whittle14/06/2019 20:57:30
81 forum posts
9 photos

The CCGT (which is ground to a polished sharp edge) should work very well indeed on delrin, which is also a joy to machine (if the tool is sharp)..

The CCMT does not have a sharp edge at all! It is suitable for taking a deep cut on steel at high cutting rates, where the material at the cutting edge is raised to temperatures where it becomes soft (even close to red heat at the cutting edge). It is totally unsuited to shallow cuts on steel, or on plastics

Hope this helps - I have been through this; some CCMT may be good for shallow cuts on steel, some is not, but I recommend CCGT for plastic.

Martin

Martin Whittle14/06/2019 21:06:59
81 forum posts
9 photos

PS

I do use CCMT on steels for heavier use, but use CCGT for more gentle uses on all materials. The sharp tips on CCGT are more easily broken by the shocks from intermittent cutting loads on non-round work . I have generally been very pleased by cheap tooling from China

Martin

John Reese14/06/2019 21:34:39
782 forum posts

I use HSS. Lots of rake and polished top and face.

Mick B114/06/2019 22:25:21
1187 forum posts
66 photos
Posted by John Reese on 14/06/2019 21:34:39:

I use HSS. Lots of rake and polished top and face.

+1, though a moderate rake as for mild steel works fine too, sharp and stoned fine. So long as you're slicing away the material rather than deforming it you can get a nice silky finish. Speed as for brass, but it's not especially critical.

Gary Wooding14/06/2019 22:52:47
582 forum posts
138 photos

I use a sharp tangential tool, medium to high speed. But the long swarf creates a birds nest if you're not careful. For the roughing cuts I move the saddle in bursts to let the swarf escape during each pause. The final, fine, cut is done in one movement so as to get a nice, even, finish.

Sam Stones15/06/2019 01:24:38
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647 forum posts
254 photos

Adrian, I sent you a PM.

Sam smiley

AdrianR15/06/2019 07:52:41
272 forum posts
20 photos

Thanks for all the replies, I will give the HSS a try. I am trying to replace the broken oil pump/lift in my mill. Which in the parts list is called a Muddler. Which a dictionary tells me is a drink stirring stick.

img_20190613_135239.jpg

not done it yet15/06/2019 08:43:52
3364 forum posts
11 photos

Probably the easiest material I have turned.

A sharp tool, I use HSS for this. Reasonably slow speed and watch out for heat - it can burst into flames apparently, except they are invisible!

I have never tried to cut that length in small diameter. It likely needs small cuts and a following steady as it will easily deform, particularly with heavy cuts and when warmed up. Definitely keep the pressure axially, not radially.

AdrianR16/06/2019 10:18:40
272 forum posts
20 photos

Well it turned out OK in the end, I made a couple of mistakes which ruined the finish in places, but I am happy with it.

I rounded the part that broke and the camera angle makes it look a lot longer,

dscf0961.jpg

Mick B116/06/2019 10:59:03
1187 forum posts
66 photos

How did you cut the helix? Looks very coarse for a ME type lathe.

AdrianR16/06/2019 11:57:30
272 forum posts
20 photos

Yes it is coarse, it is 19mm pitch. I cut it on my ML2, I luckily had enough change gears to give me a 1:6 ratio i.e. lead screw 6 times faster than spindle, which gave me a pitch of 3/4". As I was overdriving the lead screw, I drove it from the lead screw by hand cranking the hand wheel.

I used a narrow parting tool with a lot of rake, and made multiple passes with the left right offset supplied by the cross slide. The swarf was not completely cut free on by the tool, so I had to use a Stanley knife resting against the parting tool to occasionally gut the swarf away.

Unfortunately my flexible drive is still in my old workshop down south, If I had that I would have tried using a router/milling cutter with the drive mounted on the cross slide.

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