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Plastic for machining threads in particular

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Nige07/06/2018 10:42:39
370 forum posts
65 photos

I want to practice thread cutting on the new lathe and looking for recommendation for a plastic that will cut ok. I have heard 'Delrin' mentioned in connection with this sort of thing but happy to use anything really, the cheaper the better. It's practice in machine setup and technique which is the purpose and the first thread I want to cut is about 1 inch OD and 12 tpi. Going for plastic as I believe it's a cheaper alternative to metals 😊

Journeyman07/06/2018 11:07:11
576 forum posts
85 photos

Nige, I don't think there is that much difference in price it all depends where you get it and the delivery charge. For example: 2m of 1" 6082T6 aluminium = £20.05 from Aluminium Warehouse whereas 2m of 25mm  black acetal from Metals 4 you = £25.38.

I might be inclined to go for aluminium to practice on, you can always turn the threads off and make something useful! I don't think you will find the actual threading easier in acetal/delrin without sharp tool it will pull and tear giving a rough thread. Aluminium threads easily or even practice on free-cutting steel.


Edited By Journeyman on 07/06/2018 11:10:31

Clive Foster07/06/2018 11:11:33
1600 forum posts
45 photos

Delrin threads nicely. Need a good sharp tool and probably best to support the end with a centre.

However cheapest way to begin is with paper or, better, thin card wrapped round a suitable circular support rod and pencil in the tool post. You know you've got the knack of handling half-nut, thread dial et al once all the lines go directly over each other.

When it comes to threading technique the best method for beginners (and everyone else!) is the Zero-to-Zero / Zero-2-Zero technique. Written up a couple or three times here and elsewhere on the internet so a search should find it.

The big advantage is that you set-up both cross and topside for the correct "book" infeed before you start so that both dials read zero when you have finished. So if you don't get the right result you know exactly what you have done and have a sorting chance of figuring out exactly what went wrong. Usual issue with hand ground or partial profile threading tools is cutting the thread too shallow because the tool is more pointed than it should be. Easy to add bit more cut without getting lost then correcting the zero setting for the next one once you have it right. After that its like shelling peas.

Zero-to-Zero / Zero-2-Zero has the topside at an angle so that the tool cut is primarily on one flank which generally makes it easier to get a good finish. Any angle a bit smaller than the half angle of the thread works just fine as the lathe effectively calculates the required infeed for you. I use 25° for everything. Slightly smaller angle gives a shaving cut on the trailing edge of the cut thread helping to give a good finish and ensuring that the thread profile exactly replicates that of the tool.


Speedy Builder507/06/2018 12:32:19
1667 forum posts
113 photos

The only thing I would say is that crashing into plastic on a new machine is less harsh than any metal. Stick with Delrin, but check your tool geometry is set for plastic.

Ps Neil - are we allowed to say DELRIN - Its just another adopted trade name!

Ron Colvin07/06/2018 12:44:39
37 forum posts
1 photos

Machinable wax would be another alternative, once you have turned it all into shavings, just collect them up, cast them into a new bar, and start again.

Bazyle07/06/2018 12:51:27
4391 forum posts
184 photos

I think plastic conduit pipe has been suggested before and is available cheap in skips.

Jon Gibbs07/06/2018 15:01:42
738 forum posts


Once you've tried a few bits of plastic though I don't think there's any substitute for cutting steel if that's your intended end-product. Plastic is too forgiving and especially if you've got an old or worn machine like mine the thread depthing, springing and crest burs will be entirely different with steel than plastic.

Moving on to some leaded EN1A would be my suggested path PDQ.

Also, although you can get by without for the very basic basics, buy yourself some thread measuring wires and Martin Cleeve's book "Screwcutting in the Lathe". You can buy both for ~£15 total from Arc.



not done it yet07/06/2018 16:53:30
2643 forum posts
11 photos

The technique is the same for 1/4” as for 1”.

Practise on small diameter bolts or scrap rod pieces is my advice. Far, far cheaper, but will need a supporting centre, to afford more rigidity to the workpiece..

Of, course, for smaller thread forms, there are specific 90 degree feed type cutters, rather than the historical ‘cut on one side’ cutting technique.

John Reese07/06/2018 17:38:39
669 forum posts

I would practice on scraps if PVC pipe.

Nige07/06/2018 19:01:56
370 forum posts
65 photos

PVC pipe have a quite a bit of Thanks guys

JasonB07/06/2018 19:04:47
14867 forum posts
1492 photos

Just be careful if cutting 12tpi, you may end up with a spring!

Trevor Crossman 107/06/2018 19:10:53
125 forum posts
15 photos

Nige, as has been said earlier plastics are not necessarily cheaper than metals, indeed many are very expensive and can be tricky to machine well, PEEK for example. If you try and practice with whatever cheap stuff you can find you may be uncertain of its specification and its workability . I would suggest that Aluminium alloy 2011 bar would be a much better practice material because it is cheap, easy to obtain, cuts cleanly and easily with short chips and gives an acceptable finish. If you are set on plastic then perhaps this link will give you some guidance .

For what it is worth, I have had several ""dig ins "" ripping work out of a chuck on plastics but none with metals.


Jon07/06/2018 19:15:44
952 forum posts
45 photos

Cant get over the price of plastics lately, thought aluminium was bad the last 6 years.
Above links way to dear vastly inflated prices for model engineers. Just first one came across for aluminium round 1" comes in at £14.40 delivered, there will be cheaper

Used to by 3m lengths of 1" round 6082 for £20 until a few years back 1 1/4" for £28 x 3M long collected though.

Practice on anything you can find even potatoe! Think im joking i am not its just a material and techniques all the same.
Idea being soft materials it wont hurt in the event of crashes. Just be methodical and will learn quick.

If going the full hog recommend full profile tips that leave a full thread form but dont skimp on the shop branded asian ones. If using the AG55 or 60 tips that just cuts a V.

Presume you mean the diy way by using compound at an angle and feeding in, that uses 90 degree cutters the same as the professional way.

Andrew Johnston07/06/2018 19:28:08
4449 forum posts
516 photos

Personally I wouldn't mess with plastics; they have their own problems when machining. I'd go straight for EN1A or similar. You can always do a few practice runs with the tool close but not cutting. Then a couple with the tool just touching, to make sure you're following the same path each time. Then you've just got to go for it. You can't put half a cut on and there's no point in pussyfooting around. smile


Muzzer07/06/2018 20:28:16
2904 forum posts
448 photos

If you're determined to avoid metal for your "first cut", you could always buy a box of candles and let rip with those. Self lubricating and impossible to break a tool on. Make a bit of a mess mind but you could catch the cuttings or hoover them up afterwards.


richardandtracy07/06/2018 21:36:02
938 forum posts
10 photos

Murray's idea of candles is the one I always use. Particularly when considering a new design of pen.

In my pen making, almost exclusively on plastics, the deflection of plastics is a real pain, so much so that it's a pleasure to start on metals. I usually have to use a steel mandrel inside the pen bits to ensure that they don't deflect away from the cutting tool. The acrylic/polyester alloys I often use behave very much like Delrin, and while it threads well, it often needs a mandrel. Trying to get a good thread seems to throw out the rules for metal, and is often not a good indicator of feeds and speeds. And some things cut when you think they shouldn't (like blunt carbide inserts) and some don't when you think they should (sometimes sharp HSS doesn't work). I can find neither rhyme nor reason to it and just go with whatever works on the day.

Weight for weight, some small quantity exotic plastic pen blanks end up at £20,000 a ton or more.



XD 35108/06/2018 10:30:27
1174 forum posts
83 photos

I remember reading an article somewhere about using the plasic caps from milk containers and melting them in a pot of hot wax this gave a machinable soft plastic / wax mix that the author used to make patterns for castings .

Might be worth a try and by using a soft drink can as a mold you would get a nice ingot to play with , the bonus is the swarf can be re melted to make a new ingot . I haven't tried this but just a thought and thinking about it maybe the PLA type filament used for 3D printing might work as well ?

Ian S C08/06/2018 12:01:34
7224 forum posts
227 photos

The first thread that I cut on a lathe was on a bit of cast iron from the small scrap bin I had at the time, and the tool was made from a bit of an old square file (about 1/2" sq), that was a success, so I got some 1/4" HSS, and ground up a 55*, and a 60* thread tool, and I'm still using them 25 years later.

For practice even a bit of rebar will do, and if you get a good thread on that, you'r doing ok because the metal won't help you, but a good sharp tool(HSS) will , although the modern modelers lathe I feel often isn't slow enough.

Ian S C

Bazyle08/06/2018 13:30:11
4391 forum posts
184 photos
Posted by XD 351 on 08/06/2018 10:30:27:

I remember reading an article somewhere about using the plasic caps from milk containers and melting them in a pot of hot wax this gave a machinable soft plastic / wax mix that the author used to make patterns for castings .

Not quite. You use LDPE which is what plastic bags are made from and they have high surface area so melt more easily. Bottle tops are MDPE which has a slightly higher melting point.

Vic08/06/2018 13:47:22
1968 forum posts
10 photos

I wanted a lathe for some time and what prompted me to buy one aside from the special offer on the price at Axminster was the need to cut an odd thread. Well odd to me anyway. The very first job for my new Lathe was to make a new motor pulley as small as possible though to reduce the speed. The lowest speed on the machine was far too fast for a new lathe owner who’d never machine cut a thread before. Second job was to machine the part and cut the thread. I was well chuffed with my first ever thread, trouble was it wouldn’t fit the other component. I wracked my brains and checked and double checked the thread chart and the gear train and discovered that the gear train printed on a label on the lathe was wrong for the thread I was cutting. What were the chances of that! I reset the gear train using the correct settings in the printed lathe manual and the part turned out fine. I later on even made the same component out of 99% pure Titanium and that turned (pun intended) ok as well. The morale of this story, for me at least, is sometimes it really isn’t your fault and someone else has made a mistake!

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