How to machine Acetal

[AdrianRParticipant @adrianr18614]

[AdrianRParticipant @adrianr18614]

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

[Anonymous]

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 WoollettParticipant @rogerwoollett53105]

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.

[egaParticipant @ega]

See advice at:

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

[Boiler BriParticipant @boilerbri]

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

Bri

[Howard LewisParticipant @howardlewis46836]

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 JohnstonParticipant @douglasjohnston98463]

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

[SamsarandaParticipant @samsaranda]

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 HulmeParticipant @nickhulme30114]

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

[Martin WhittleParticipant @martinwhittle67411]

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 WhittleParticipant @martinwhittle67411]

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 ReeseParticipant @johnreese12848]

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

[Mick B1Participant @mickb1]

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 WoodingParticipant @garywooding25363]

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 StonesParticipant @samstones42903]

Adrian, I sent you a PM.

Sam

[AdrianRParticipant @adrianr18614]

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.

[not done it yetParticipant @notdoneityet]

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.

[AdrianRParticipant @adrianr18614]

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,

[Mick B1Participant @mickb1]

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

[AdrianRParticipant @adrianr18614]

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.

[Mick B1Participant @mickb1]

Smart work.

Presumably it's not just rake you'd need when cutting, but a helluva lotta trailing side clearance to avoid rubbing on the helix angle.

I meant to ask: would have been easier to find a way to re-use the business end of the existing stirrer by joining to a new stem in some way?

 

Edited By Mick B1 on 16/06/2019 19:25:44

[Ian Johnson 1Participant @ianjohnson1]

Posted by AdrianR on 16/06/2019 10:18:40:

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,

That turned out nice. Looks like a very aggressive square thread! And there is always more than one way to get the job done.

I tend to use tips with a good positive top rake for all my general machining these days, and apart from form tools I try to avoid grinding HSS, it's just too messy!!!. My favourite tips are these DCMT 070204 with the ground/polished finish, they will machine anything, brass, aluminium, steel etc. and are very good on plastic especially Acetal.

Ian

[Martin Hamilton 1Participant @martinhamilton1]

Posted by Ian Johnson 1 on 16/06/2019 21:58:42:

Posted by AdrianR on 16/06/2019 10:18:40:

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,

That turned out nice. Looks like a very aggressive square thread! And there is always more than one way to get the job done.

I tend to use tips with a good positive top rake for all my general machining these days, and apart from form tools I try to avoid grinding HSS, it's just too messy!!!. My favourite tips are these DCMT 070204 with the ground/polished finish, they will machine anything, brass, aluminium, steel etc. and are very good on plastic especially Acetal.

Ian

Ian where did you get those particular dcmt inserts from if you dont mind me asking, thank you.

[Ian Johnson 1Participant @ianjohnson1]

Hi Martin I get those tips from JB Cutting Tools. They usually have a stall or two at classic bike shows (Stafford), steam fairs or model engineering shows. I think you can order on-line too although I haven't tried that yet. I don't know the technical specification for the tips, but if you ask Jenny she will give you the same ones, very good quality. About £3.50 each Silver or Gold I think they are the same spec, gold is prettier though!

Ian

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