Machining Acetal

[jon38r80]

Has anyone here had experience machining Acetal? I have only played at it before with limited success but the parts I wish to make now need a bit better tretment than fly by seat of well worn trousers.

The parts are to be turned on a lathe and then have a slt machined in them. my question is realy what sort of speeds give a reasonable to good finish, are they the sort of speeds used for soft metals or fast say like stainless steel and what sort of tooling inserts are best. I imagine the nice shiny ones like those for Aluminium would be best.

I have got a bit of cheek asking this here cos I haven't done my usual digging on the internet yet to see what advice is out there.

[Deleted]

Hi Jon

I'm no expert but have machined most plastics now, generally, I use HSS bits and a slow speed. You don't want a fast speed as the plastic is likely to melt. Coolant is probably best when drilling, I don't use any for turning or milling, just drilling.


I'm sure a real expert will be around to correct me soon...

Pete

[springcrocus]

In the factory, we used to machine acetal at high speed using polished carbide tips with the smallest radius - 0.4mm back then. A couple of the jobs were rollers for conveyor systems and were 30mm dia by about 200mmm long. We used normal suds as coolant and never had a problem with melting or poor finish. The greatest hazard was the well-known birds-nest wrapping itself around the workpiece, unavoidable because interrupting the cut left a witness on the material.

Other materials such as nylon or nylatron (graphited nylon) needed much more care but acetal is quite rigid.

Regards, Steve

[92220]

Hi Jon. When I was an apprentice, and spent a couple of years in the development workshop, I used to machine a lot of acetal and nylon, and other plastics. In those days we used HSS and VERY sharp, with probably around a 10 degree top rake and a very tiny corner radius, though top-rake may even have been slightly greater than 10 degrees. Don't have a large corner radius on the tip as this will not give a decent finish as it will put too much load on the material, which will bend away from the tool during cutting. Speed was around the same as leaded mild steel (EN1a), though it wasn't that critical. I've used high and low speeds, just like for the mild steel. There were various specs of acetal and I would guess there are even more today. As far as speed goes, I would suggest you try both high and low speeds, starting with low and increasing as necessary, with newly sharpened HSS tools, with a deep top-rake, to see what sort of finish you get on YOUR material. Light cuts will usually give better finishes that heavy cuts too, but that also depends on the grade of acetal. Basically, the softer it is, the slower and shallower cuts, will give better finishes. The harder it is, the reverse can be the case.

Bob.
Edit:-  Forgot to say: Unlike Steve, we never used suds when machining any plastics, so that is something you will have to try for yourself. It may well help when using a higher speed, but you probably won't be working at industrial speeds anyway.
           
          Bob.

[rrmrd66]

Hi Jon

This is possibly just what you want?

www.theplasticshop.co.uk/plastic_technical_data_sheets/engineering_plastics_machining_guide.pdf

Cheers

Malcolm

[jon38r80]

Thank you gentlemen for your advice from experience and that guide. Just what I wanted.

Using hand ground HSS cutters isn't really an option for me as I don't have a very good set of stones on my cheap grinder and previous experience of grinding tools myself has not been good. That was why I was thinking polished inserts for Aluminium would be my best be the way Steve has suggested.
Cooling with suds isn't for me either as I don't have that facility on my lathe or mill. Just use a basting brush and oil when needed. Cooling with compressed air also blowing the chips away is a distinct possibility. The Acetal I have is pretty hard and is supposed to chip rather than make coils when turned. I shall have to see.

As Bob said, I think it will be suck it and see on the speeds but it's good to know what others have found to work

[jon38r80]

Managed to post that twice!

[jon38r80]

Had a go at turning the blanks to size today.
I seems Acetal machines nicely at high speed. turning to diameter I( got a realy nice smooth finish. the facing is smoothto the touch but concentric marks on it.
I used a DCGT070204 insert , perhaps a finer radius on the cutting tip might give a better finish but I am not unhappy anyway.
I did get the usual birds nest but fouind if you use a realy fast feed rate as well as fast rotation the birds nest breaks up into long chips which go everywhere. Ill put up with the birds nest as its less mess. Just need the ball nose cutter and 11mm drills to turn up in the post so I can finish a couple of these to see if they actualy work.

[JonL]

Sorry to be late to the party, but if you use a hacksaw or similar to cut a groove lengthways in the piece you are machining ALMOST to the depth you intend to cut to, then instead of a mile long piece of swarf you get one streamer per revolution at a lot less messy length of 1C. A bit safer than the usual candy floss.

[Roger]

Razor sharp tools with a radius give a really good finish. I use the polished inserts sold as being for Aluminium, and they work really well.

[92220]

Hi Roger.

That's worth knowing, for future reference. Not having ever used carbide cutters for aluminium, I have no experience with them. On a similar track (no pun intended!): are carbide endmills, and slot drills, listed on the various suppliers sites as being for aluminium, sharper edged than standard carbide cutters?

Bob.

[uuu]

I've been using an Alu-power cutter that John the Pump gave me. John was using it for plastic. It's single point, and has an aggressive helix angle, so it clears the chips away much better than a standard one. The recommended feeds and speeds are quick- 1800mm/min at 15000rpm for a 4mm cutter. It's just lovely to use.

Data sheet

Wilf

[Roger]

Mar 3, 2021 16:49:14 GMT 92220 said:

Hi Roger.

That's worth knowing, for future reference. Not having ever used carbide cutters for aluminium, I have no experience with them. On a similar track (no pun intended!): are carbide endmills, and slot drills, listed on the various suppliers sites as being for aluminium, sharper edged than standard carbide cutters?

Bob.

Hi Bob,
Yes, generally speaking they're sharper and hence less rugged. You will probably also find that the helix angle is greater so that the top rake is steeper. You can use these on Steel too, just take it gently so as to avoid chipping the edge. They are great for taking tiny cuts and getting a really good finish.

[92220]

Hi Roger.

Thanks for that info. I'll get some to try to see how I get on with them.

Bob.

[jon38r80]

The DCGT inserts that I used are the polished ones for Aluminium and on Acetal they do give a lovely smooth finish. The ones I used have a 0.4 mm radius, I wondered if a smaller radius would give a better finish still. The ones I have are for Aluminium and Stainless steel. Haven’t tried them on steel but they give a really good finish on Ali.
Acetal is quite hard with a sort of soapy feel to it which is what I wanted as it’s for a pressure plate facing that is to allow slip.
I got it from Direct Plastics who cut the rod into slices which saved me a lot of hassle. They gave rely good advice on which plastic to use too. 60mm diameter Acetal is quite reasonably priced compared to the stuff Roger was machining. Over the past few years I have bought quite a lot of different plastics from them, mostly PTFE. I have no connection with them other than as a happy repeat customer.