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machining acetal

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David Brown 919/06/2020 14:04:32
75 forum posts
4 photos

I have a 60 mm round acetal rod that I need to machine accurately to a rectanglular shape to fit inside some alumium rectangular tube one inch x one and a half inch (25.4 mm x 38.1 mm) wall thickness 1/8 th inch (3.1 mm.)

I have a mini mill without Dro and a portable (Aldi) band saw.

What us the best way to machine this? How to set it up so I can machine it accurately?

Douglas Johnston19/06/2020 14:23:07
767 forum posts
34 photos

Any sharp endmill will cut acetal without a problem. It is lovely stuff to cut and gives a very nice surface finish. Just play around with speed and feed to get the best result.


Howard Lewis19/06/2020 14:32:41
6024 forum posts
14 photos

I would feel inclined to turn the Acetal to length, or at least so that the ends are at right angles to the axis.

The tools need to be sharp, or they will rub and melt the material, similarly, if in doubt use a slow speed.and fine feed. The swarf and the finish will tell you whether you have got it right or wrong.

Then grip in the vice on the mill, using a Vee block, to hold the round Acetal vertical.. You can then mill along each side using the X and Y axis obtain the required size. (depths of cut are below )

The depth of each cut will depend upon how much (length ) grip you have for the raw material. If in doubt, err on the side of caution. Over ambition could equal "scrapper"

If the vice is too small to do this, than lay the Acetal horizontal in the Vice, and mill one side, removing half the material required to bring from raw size to finished size, in one plane. Say 0.681" or 17.3 mm

Rotate the Acetal in the vice, and using a square, set it up with the new flat face vertical, and repeat the original milling operation., again, removing half the difference between the 60 mm and the other dimension of the Aluminium tube. Say 0.431" or 10.95 mm.

Repeat the first set up and mill operation again, so that you should then have two sides the required distance apart, and parallel.

Repeat the second set up and mill operation, and you should have your round Acetal with a good fit in the rectangular Aluminium tube.



Martin W19/06/2020 14:45:53
916 forum posts
30 photos


As Douglas has said with sharp tools it is great to work with either in the lathe or milling. Have used it quite frequently in the past and with a little care the finish is superb. When turning don't allow the swarf to gather round the tool as this can cause problems with the swarf getting caught on the tool, this really depends on the tool, depth of cut and speed.


old mart19/06/2020 14:48:05
3721 forum posts
233 photos

It might be best to forget the saw and do it all in the mill. A new sharp end mill, or slot drill, the biggest you have would be best. With the bar in the milling vise, sitting on parallels, mill the first flat until it is just about 58mm wide in several stages. Then turn the work round so it rests on the milled flat and mill it to 22.3mm thick. Do the third face until the last of the original outside of the bar is gone and the corners are square. Turn it round for the last side and mill it to 35mm. You should have a rectangular section 22.3mm by 35mm. I don't know what acetal is like to machine, somebody here will know.

Lee Rogers19/06/2020 16:29:51
166 forum posts

If your turning it watch out for the long strings of swarf, It doesn't break easily and wraparounds can be nasty.

Andrew Johnston19/06/2020 16:41:20
6575 forum posts
701 photos

Acetal machines beautifully; use low speeds and high feedrates or it'll melt. What do you mean by accurate, 1mm, 0,1, 0.01mm?


not done it yet19/06/2020 19:30:39
6736 forum posts
20 photos

Rather simpler than as per Howard, I would be sticking it in the vise horizontal and machine a flat. Turned 90 degrees in the vise allows another flat to be cut. With two flats at 90 degrees it should then be easy enough to form a rectangle and then machine that to the required size. Only parallels needed, I would have thought? Might be wrong, of course...

Mick B120/06/2020 11:37:06
2161 forum posts
119 photos
Posted by Andrew Johnston on 19/06/2020 16:41:20:

Acetal machines beautifully; use low speeds and high feedrates or it'll melt. What do you mean by accurate, 1mm, 0,1, 0.01mm?


If the tool's sharp it hardly matters. I slot-drill a 1/4" square slot across a ~ 2" diameter acetal cap now and again, and find it pays to use a fine feed for easy deburring.

Martin Connelly20/06/2020 12:46:41
2123 forum posts
222 photos

I needed to square off a piece of 4" diameter nylon bar stock so I set it up for fly cutting and videoed it for an example on machining plastic. It is similar to acetal for machining purposes.

The tool is a 6mm button insert held in a fly cutter. It was set out as far as I could go and still have 2 screws holding it so it is a little over 100mm diameter. This left a little to be removed from the front and back edges but this is an area that will be machined off later so not a problem. RPM is 140, depth of cut is 0.5mm and feed rate is 50mm/min (0.357mm chip load).

The photo below is the finished face, the ruler is there for something to focus on for the autofocus.


Martin C

Sam Longley 120/06/2020 13:24:16
939 forum posts
34 photos

As a beginner--This might be a stupid question

--But- (Not knowing how long the piece is) would it not be just as accurate to put the piece sideways in a 4 jaw chuck in a lathe & turn a flat Then turn it over & do the opp face which would be easy to do accurately then repeat for the other 2 faces. Would it come out square & parallel etc? Easy & quick to set up?

Assuming of course the OP has a lathe sad

Or is that totally wrong


Edited By Sam Longley 1 on 20/06/2020 13:27:55

Howard Lewis20/06/2020 14:35:41
6024 forum posts
14 photos


A long as you are accurate, that would be a method. But if the flat is not to cover the full length of the material, it would need to mounted off centre by some distance, so that the flat is produced either at the outer end, or for a short distance from the centre .

One of the students that I mentored was ex Royal navy, and one of his test pieces was a large hexagonal nut, produced in a lathe!

I have just squared up a piece of sawn rectangular bar by turning the faces in the 4 jaw. But the object was to cover the entire face, which meant that the cut was interrupted, four times a rev, until the tool came closer to the centre.


Martin Connelly20/06/2020 15:13:51
2123 forum posts
222 photos

There was no mention of a lathe in the original post.

There is another issue with using the lathe and that is the slow speed near the centre. You don't want to go too fast machining plastic but equally you don't want to go too slow either as it is not going to cut well unless your tool is razor sharp. The elasticity of plastics has an effect as well. Anyone who has drilled plastic will have experience of the hole closing on the drill behind the cutting edge. The change in speed trying to face it flat is going to change how much the plastic distorts under the cutting forces and so how much it bounces back. As you approach the centre line there is a risk the cutting process changes enough to make the finished face not flat and have a poor finish.

Not as much of a problem with a CNC lathe that can maintain a constant surface speed or when facing a part with a hole in the centre of the face.

Martin C

Michael Gilligan20/06/2020 19:09:30
20091 forum posts
1042 photos
Posted by Martin Connelly on 20/06/2020 12:46:41:

I needed to square off a piece of 4" diameter nylon bar stock so I set it up for fly cutting and videoed it for an example on machining plastic. It is similar to acetal for machining purposes.

The tool is a 6mm button insert held in a fly cutter. […]


That’s mighty impressive, Martin

Quite astonishing for an insert on soft material.


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