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Idea for Beginners - experiment with plastic

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Neil Wyatt14/07/2015 11:58:53
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Last night I needed to make a couple of thick plastic rings. The obvious route was to trepan them out of thick sheet, and after wondering how best to attach them to a faceplate, I realised that mounting them on an M12 screw and gripping the nut would be much simpler.

But that's not my point - I used a simple 1/8" HSS d-bit style boring bar as a cutter and as the swarf flew off in an endless ribbon. I've also recently made washers, a nut and bolt (to repair a metal detector), spacers and a few other turned parts from plastic. It occurred to me that perhaps using plastics might be a good idea for beginners who are experimenting with new techniques.

It's common enough for plastic to be used when trying out CNC programmes in industry to protect valuable cutters from damage.

Using plastics beginners could get the hang of screw cutting, boring and even parting off with a lot less worry about depth of cut, feeds and speeds or the consequences of a mistake. The only caveats are the need for cutters to be sharp and not to get careless.

The same approach could be used for milling.

Does anyone have any further thoughts on 'practice materials' for beginners, or even an idea for a beginners' project largely made using machined plastics?

JasonB14/07/2015 13:16:52
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Posted by Neil Wyatt on 14/07/2015 11:58:53:

Using plastics beginners could get the hang of screw cutting, boring and even parting off with a lot less worry about depth of cut, feeds and speeds or the consequences of a mistake. The only caveats are the need for cutters to be sharp and not to get careless.

How much would they really learn? Stick a parting tool into the toolpost and happily part off a bit of 1" dia plastic. Now put a bit of 1" dia steel in the lathe and they would soon learn that they need to consider tool mounting, speeds and feeds, etc and the only way to do that is on like materials as they would not get the same feedback from plastic

Michael Cox 114/07/2015 13:27:33
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Using plastics for machining practice is a good idea. My first tries at single point screw cutting were carried out on 20 mm plastic (PVC) conduit just to discover the potential pitfalls. This size fits nicely in the spindle bore of the minilathe.

The big difference between plastic and most metals is the low modulus. This means that thin sections bend much more than the same thickness of most metals so the protrusion from the chuck must be much less or else the material moves away from, or worse still over, the cutting tool. Plastic also do not like the use of normal steadies as these tend to score or scratch the surface. Steadies with ball bearings on the ends of the arms are OK.

Acetal (Delrin) is one of the easiest plastics to machine in my experience but PVC and nylons also machine well. Thin sections of polyethylene and poly propylene bend very easily making them difficult to turn accurately. Plastic kitchen chopping boards (these are mostly polyethylene) are a good source of sheet plastic for milling practice.

Sharp tools are essential and water is a good lubricant / coolant. Blunt tools and high speed will cause some plastics to melt rather than cut.

Mike

Capstan Speaking14/07/2015 14:17:54
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If there is a risk to the machine or user then maybe but plastic is more expensive than mild steel by volume.

richardandtracy14/07/2015 16:16:26
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I agree it's much safer to try on plastics first. With the pen making I do, I get lots of interesting colour swarf.

Regards,

Richard

Boiler Bri14/07/2015 21:17:52
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If it is only plastic why not use double sided tape to stick it to the face plate. I also seem to think that there used to be a special product for fixing things to face plates??

I can supply the tape.

Bri

richardandtracy15/07/2015 15:43:09
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Usually use carpet tape on my cnc router when machining plastic sheet. Prevents flex from the clamps too.

Regards,

Richard

Tim Stevens15/07/2015 18:23:05
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Thinking of further extensions of the idea, could you learn about casting by making a chocolate teapot ... ?

Cheers, Tim

Neil Wyatt15/07/2015 20:50:19
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Posted by Tim Stevens on 15/07/2015 18:23:05:

Thinking of further extensions of the idea, could you learn about casting by making a chocolate teapot ... ?

Cheers, Tim

Probably.

Yum.

Peter Tucker16/07/2015 20:17:26
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Thinking of further extensions of the idea, could you learn about casting by making a chocolate teapot ... ?

Cheers, Tim

Yes; but who would want chocolate tea?

Peter.

Bazyle16/07/2015 22:40:55
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Anyone tried melting down milk bottle tops to make a solid block? I seem to have and endless supply of them.

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