CNC router and plastic forms

[MMM Jewellery]

Hi
I am playing with making complex forms and punches with a cnc router, and wondered if anyone on here has experience with this? General tips welcome, but also, in particular I am having problems with the plastic (Delrin, I think) forming a fibrous mass attached to the form where the angles are not steep, rather than cutting cleanly. See photo attached for my current project - a star form, the material is visible as very white stuff in the bottom of the form. ps_bond - do you have any experience here?
Thanks Matt
p.s. here is my first attempt at making a tea spoon form: https://m.facebook.com/story.php?story_fbid=406087037979984&id=100057360741593

[MMM Jewellery]

This is the latest project outcome- a domed star. You can see the poor cutting on the punch in particular. I suspect the answer will be feed rate and drill speed adjustments.

[josef1]

I think you will need to experiment with feeds and speeds and stepdown .Do you make a finishing pass ?

[alastairduncan]

I think that what is probably happening is that the tool tip is heating up and the plastic is sticking to it. This also can happen when machining aluminium, the chips that come off can become welded to the tool.

Smaller tools with small tips such as the V cutters and ball nosed cutters are more prone to this. Clearing the chips quickly and easily so that the cutter tip remains cooler than the material being machined is the objective. It's difficult to make small cutters with different cutting angles so you are left with speed of rotation of the cutter feed rates as well as using some sort of coolant/lubricant.

What is the size of the star and what thickness of material are you using? Very interesting.

[MMM Jewellery]

Hi- thanks for your help. I do a rough cut and then a finishing cut. I have only a generic 3018 cnc machine so no option to cool really. I think I'll try a slower spindle speed. Possibly invest in a lower flute cutter too, but I'm trying to keep it cheap :) The form is 20mm by 10mm deep, and metal thickness is 0.5mm copper, probably going for 0.3mm sterling. I might need to cut a smaller punch to give the metal more room.

[MMM Jewellery]

Thanks- I dropped my spindle speed from 10,000 to 5,000 and used my larger bits which have two flutes rather than 4 and it was better but not perfect- but a quick sand cleaned it up nice tho. The larger bit wouldn't work so well for the form shape as it wouldn't get inside the tight details. I'll maybe try an even slower spindle in future.
Thanks Matt

[alastairduncan]

It can be a bit of a black art to find the sweet spot for a particular cutter and that can be different when the cutter is new and really sharp and after it has been used for a while.

On a related note: I've seen a video on YouTube of someone pressing 1.6mm steel using 3d printed PLA. He used a hydraulic press. And I've also seen someone using a vice instead of a hydraulic press. He had a multipart mould.

https://youtu.be/fxzqkhmcRlY

https://youtu.be/zAYfomutINQ

[ps_bond]

I've seen that one before... I'd be inclined to use a filled epoxy (and a less brittle filament), but it's impressive.

I did write a chunk at the weekend and then managed to not post it. Oops. Short version was have you tried a single flute cutter, feeds & speeds are critical and if you can rig some cooling air to the cutter it should help both chill & remove the chips, hopefully before they stick to the cutter...

My own CNC dev is languishing a bit at the moment - I've got the spindle rigged, but haven't had time to do any more than the Hello World plot.

[MMM Jewellery]

Thanks- I've ordered some 1 flute cutters now, so will play some more once they arrive. I used the form to press out 7 pieces so far- 0.5 mm copper and sterling, and 5 pieces of 0.3 mm sterling- it's held up really well, a little bit of damage from sharp edges as I learnt how to use them but the later pieces worked out great. I suspect they'd last for a good 100 pieces if I was that way inclined. I now feel compelled to actually use the pieces I've stamped to make a final product, although all I want to do is experiment :) Once done I'll post some photos here and elsewhere. Those videos are cool- I have to buy a 3d printer now.......but need to learn to use fusion360 better, and maybe sell something to fund my tool habit.

[ps_bond]

https://3d.formlabs.com//rs/060-UIG-504/images/WP-EN-How-to-Do-Rapid-Tooling-for-Sheet-Metal-Forming-With-3D-Printed-Dies.pdf - no idea if that link will work if you've not signed up for it.
While that's printed press forming parts, that uses their glass-filled resin which is considerably gutsier than any of the usual filaments (or most of the resins for that matter).

[alastairduncan]

It is available to the ordinary punter. An interesting article, this is an article about the same part which comes with a video.

https://hackaday.com/2020/03/10/forming-sheet-metal-parts-with-3d-printed-dies/

Design of the tooling is all important. There are a few objects I want to make and was heading down the tin or lead press moulds that I've used before but I could be off down a different rabbit hole.

[MMM Jewellery]

Very interesting- its a nice 3d printer, but I think at $4K+ a bit out of my price range for now.....Here are some progress shots of my project, The first one is a set of earrings, waiting for some stones to arrive to finish them off; and a second experiment, where I used the laser attachment on the cnc to burn away marker pen resist, and then electroetched a design that complements the design of the form. Pressing it out then gave the final shape- I think they might become a pendant. I need to practice my electroetching a bit more, not totally happy with the result. On the other hand the form and punch show no real sign of degradation.

1327513276

Cheers Matt

[ps_bond]

I think the CNC plasma cutter & CNC mill he uses will set you back considerably more... I'm surprised that tough resin handled as much abuse as that, let alone the normal one.

[alastairduncan]

He has done a follow up video that discusses the mould failure. He allowed for the thickness of the metal but the 3d software had an adaptive mode switched on which automatically adjusted the other part of the mould so there was no clearance hence the excessive pressures. Solving this issue allowed for the pressing of the parts without failure.

[Devanet]

Hi

Cnc machining of plastics like Delrin is best with a single flute profile, we run at 20,000 rpm stepover if contoured 0.1 mm, stepdown 0.1mm, feed rate from 1000 to 3000 mm/min. We use special tools for this type of work.


Regarding cutting of sterling silver etc we have a 60 watt MOPA laser to do that, we can cut stainless, gold,silver,brass etc. settings are critical but very accurate detailed cuts are possible with no burring. Email me if you need help Dmw@devanetbelts.co.uk

[MMM Jewellery]

Thanks- I suspect your machines would be very fun to play with! My specs aren't so high but I've got some single flute cutters to play with now. Just need some time to get into a new project and try them out.

[alastairduncan]

I came across this clip today which you may find interesting.
https://www.instagram.com/tv/CYv2zDuFMF-

[ps_bond]

I do like his work. Met him a couple of times.
I wonder if that's lead or pewter being used for the form?

[alastairduncan]

Given the health and safety concerns of lead these days I would think it's pewter(tin). I've only ever used lead but that was years ago. It's fully recyclable, the down side is that it can muck up your silver if there's any residue when annealing. It needs to be well cleaned first. There was a pot of muric acid(HCl) where I used to work which was kept outside on the fire escape, the item was dunked in there to remove the lead residue before rinsing and annealing.