Finish on soft plastics

[Dean jeffery]

Been struggling with the finish on plastics but only soft stuff like nylon and HDPE the left parts are acrylic and are totally fine, the right is nylon and it's shite.

Anybody got any speeds and feeds for me to try I run the acrylic at 24k 1750 feed, nylon I've tried anything from 800-1750 and the results appear very similar.

Cutter is one from cutwel tools 2mm single flute and specially for soft plastics, DOC is 1mm

https://uploads.tapatalk-cdn.com/20170505/e79803f8baf1a9c9be6a29033aeb3194.jpg

[m_c]

Are you climb or conventional milling?
Do you have good chip clearance?

[Dean jeffery]

Tried both climb and conv with very little difference, I just use a dyson and dust boot.
Chip clearance is not perfect but I've also fully cleaned parts down and done 2 opps to try and get better finish.

[magicniner]

I use single flute cutters from 1mm to 6mm with compressed air to clear chips out of the cut, it also cools the work and the cutter, if you use a big hypodermic with the point ground off as a nozzle it doesn't take much air and you can get the nozzle pretty close to the action,

- Nick

[Dean jeffery]

I will rig up my air blast on the next batch of parts and see if this helps matters, customers ain't said nothing but I don't like doing shity work [emoji106]

[Dean jeffery]

Think I found the issue same place I get the HDPE from same cutter same speeds and feeds but better finish, only the very bottoms got recut chips.

Easy to see from the pic not so easy on the eye, so my best guess is different batch of material or a crap batch before.

https://uploads.tapatalk-cdn.com/20170506/c5dbdfdb98b65d7571443381c83d1d05.jpg

[A_Camera]

Why are you using 2mm cutter for this? Generally i think 4 or 6mm 2 flute would be better. You could also increase the feed rate with a larger cutter. It is also a good idea to make a final fine cut at full depth.

[Dean jeffery]

More parts per sheet, some small parts are made from that same sheet, less tools to stock, less tool changers.

But I could get away with a 3mm cutter on that part without an extra tool change, I've done a finish pass on some 10mm acrylic with the same 2mm cutter to give a clean cut in the past.

Only really any good doing full clean up passes on new cutters or soft material other wise you still get cut lines.

[m_c]

That latest photo looks more like a flex/resonance/chatter problem, which is much harder to address.
First option would be a dial indicator mounted on the cutting area, with the needle against the spindle/cutter, and see if there is much movement when you pull/push the cutter.

[Dean jeffery]

The photos do make things look worse than appear, the cut lines can't be felt with the nail when run on the edge.

Every time I've cut that material I've only had 2 decent results one been above, that's why I posted up I was convinced the speeds and feeds was the reason.

But I'm now leading towards the materials spec is way off, the customer sends the material I just cut it.

It's cheap from direct plastic and tbh if not personally buy from there, nothing comes new and never as protective film on.

Cheers for the help and suggestions though guys.

[magicniner]

I'm not sure in what way is a plastic likely to be "off spec" that would cause a surface finish like that, it looks more like a tooling/machine issue.

[Dean jeffery]

Well same as anything that needs multiple materials to make a product, add to much of this and not enough of that even down to shelf life.

Been sent that Plastic from the same place 5 times and 3 of which was crap, the first time used it was like the pic then the next 3 times crap and now decent again.

Same machine same same tool, same speeds and feeds.

[magicniner]

It's plastic causing the poor finish then, no doubt about it! ;-)

[Dean jeffery]

Yeah I've convinced myself the plastics the issue, but the customer is not willing to use another supplier because it's cheap.

[magicniner]

The cause of the finish defect has to be a mechanical one, if it's the plastic then it is somehow deflecting the cutter or machine from the programmed path.

[Dean jeffery]

Few weeks ago I made this from 6063 DOC was .5 1.5mm 3 cutter all I had [emoji23]

https://uploads.tapatalk-cdn.com/20170507/e831611972322db559c3d9a6220a4d51.jpg

[magicniner]

6063 makes nice chips which clear easily, you're comparing apples to oranges and that won't help you to work out the source of the problem with soft plastics.

[Dean jeffery]

Point here was if machines out or tool deflection, then alloy is harder than the plastic I was cutting [emoji106]

[magicniner]

Point here was if machines out or tool deflection, then alloy is harder than the plastic I was cutting [emoji106]


6063 makes nice chips which clear easily.

If you don't understand that, your only option is to refuse the job unless he buys materials you like. :D

[Dean jeffery]

2mm thick black nylon seems to be hard to find, unless from direct plastics.

Customers are happy with a shit finish I'm not, and wanted to improve the quality of product.

[m_c]

What we're trying to highlight, is that that finish is down to cutter/work piece deflection. The fact it's only noticeable in certain materials, doesn't make it a problem with the material.
If you zoom in on that last pic, the problem is still there, it's just not as obvious.

Different cutters and materials will create different cutting forces and resonances, so what can be barely noticeable with one material and cutter combination, can become glaringly obvious by just changing either the material or cutter. And if you have any flex/backlash in the system, any issue can get amplified.

[magicniner]

The material is so soft that it's what you're doing with it and not the material causing the problem, have you tried disposable HSS 2 flute endmills (Carbide isn't as sharp unless you grind it razor sharp yourself and then it's a crap-shoot on anything but plastic so you have to reserve the tools for plastics), with lots of air from several directions to shift the chips and adequately cool the tool and material in all cutting directions?
Another alternative is the above plus a shallower cut using a ramped profile path.
Something as simple as ambient temperature differences can change what happens when you're cutting temperature sensitive materials and your basic set up isn't adequately controlling all the required environmental variables.
This stuff quite literally is Rocket Science ;-)

- Nick

[Dean jeffery]

These are the cutters I buy for cutting plastics, not tried or even thought about HSS tbh.


http://www.cutwel.co.uk/routing/router-bits/single-flute-solid-carbide-upcut-spiral-o-flute-soft-plastic-lmt-onsrud-63-850-series

Limited to 1 nozzle for air blast but can get another from work, will try that next time.

Cheers for the help and suggestions [emoji106]

[magicniner]

If the surface defect is imperceptible to the touch have you considered flame polishing?
I use a waft of the old blowtorch flame on clear acrylic surfaces to improve finish and render machining marks almost invisible.

[Clive S]

Not that I have had experience with nylon but I did read somewhere that some people but the nylon in a fridge or freezer to cool it then machine it. Just saying:whistle:

[magicniner]

Not that I have had experience with nylon but I did read somewhere that some people but the nylon in a fridge or freezer to cool it then machine it. Just saying:whistle:

The colder it is the better it machines as it's harder when cold and chips shear off and clear the work better, if you have the extraction to safely do it alcohol mist with good air flow can be a boon for keeping heat sensitive materials cool

[Dean jeffery]

I bought a buffer to polish clear and solid coloured acrylics when needed, can't use the buffer on nylon though I tried [emoji23]

Yeah I've red that cool the plastic before cutting it

[A_Camera]

Not that I have had experience with nylon but I did read somewhere that some people but the nylon in a fridge or freezer to cool it then machine it. Just saying:whistle:

That's a very good idea. The only issue is that it will shrink in the freezer and expand when it is at room temperature, so if accuracy is important at room temperature then that must be taken into consideration.

[A_Camera]

The photos do make things look worse than appear, the cut lines can't be felt with the nail when run on the edge.

I see shadows along those lines and that means the surface is not even. If it can be felt with your nails or not is another thing and depends on your own personal finger sensitivity.

[A_Camera]

More parts per sheet, some small parts are made from that same sheet, less tools to stock, less tool changers.

But I could get away with a 3mm cutter on that part without an extra tool change, I've done a finish pass on some 10mm acrylic with the same 2mm cutter to give a clean cut in the past.

Yes, more parts but also greater risk for tool deflection if you are using 2mm cutters.


Only really any good doing full clean up passes on new cutters or soft material other wise you still get cut lines.

But we are discussing soft material here... aren't we?

Also, the full pass cleaning I normally make is just shaving off about 0.1mm. That gives very good results, even if the cutter is not brand new.

[Dean jeffery]

I'm going to order a 3mm cutter that's the max size without tool change, then see how it pans out.

But the main problem was with nylon and that's only 2mm thick.

Yes we are talking soft materials here and taking .1 off in plastic with a dull cutter is not the technique one should use.

When cutting soft plastics the cutter needs to be razor sharp.

[A_Camera]

I'm going to order a 3mm cutter that's the max size without tool change, then see how it pans out.

But the main problem was with nylon and that's only 2mm thick.

Yes we are talking soft materials here and taking .1 off in plastic with a dull cutter is not the technique one should use.

When cutting soft plastics the cutter needs to be razor sharp.

Yes, of course razor sharp cutter is is the best and I did not mean using a dull cutter. OK, the definition of "dull" is difficult, but my definition is if I cannot feel the edges then it is dull and won't usi that cutter any more, no matter if it is soft/hard/thin or thick material which I want to cut.

Another thing I am doing and which also reduces deflection and vibration is that I try to use cutters with considerably larger shafts than the cutter itself. I found it especially important for cutters below 3mm diameter. My spindle is ER11 so I am using 6mm shaft as much as possible. Perhaps it is just my imagination, but I think it improves the quality.

When it comes to 2mm nylon, I really don't know. Never tried that thin, but I think the biggest challenge is the fixture. How do you do it? Do you have a vacuum table? Personally I'd use some sort of glue or double sided tape, because I don't have a vacuum table, and I think it is important that the sheet is fixed and prevented from deflection, flexing or vibrating. So it is not just the cutter, even material flexing can cause the same issues.

[Dean jeffery]

Now that's a good point the cutter shank diameter, I will make sure I try and get at lest 6mm.

I use DS tape for everything I cut and if needed also add bolts, bolts added when cutting carbon or alloy.

I still only use tape when machining .3-.4 carbon but then I only use .6-.8 dia cutters.

The DS tape works really well and can be difficult to remove parts once machined, no that's not saying they don't move that's why bolts was added to thicker carbon and alloy.