Diamond Insert Turning Acrylic Rod in my small cnc lathe. Includes cuts with carbide insert

Diamond turning of acrylic.

I have not used Diamond inserts before, which will probably be obvious to folks that have! Changing the lathe gearing to reach 4000 rpm, using 40ø acrylic, I try a few options. 40ø at 4000 rpm is still only giving a surface speed of 500m/min and the recommended would be 1000m/min. I tried to get 80ø but it was just too expensive.

[johngamal]

Great video as always.
The cutter should be sharp, with aggressive rake angle, lots of clearance, and as smooth as possible.
You should also generally avoid tools with excessive radii (like round inserts) – this can cause rubbing if the feed is too low and then the surface finish will be cloudy. It is recommended to go with a 0.060″ nose rad, since it’s a nice balance between lighter cutting pressure and a good surface finish on turned diameters.

[stonecraft]

I would love to see some MCD Diamond insert. Seen some El cheapos VCMT on Aliexpress for 120€ per piece. I only have a mill but I will probably try those as a flycutter.

[TheKnacklersWorkshop]

Hello Nigel,
An interesting video and please don't tell anyone but until watching this video I did not know that there were lathe diamond inserts... but I do now...
Take care
Paul, ,

[Phantom-mkkp]

Years ago went for an interview for a company that made CNC lathes for optical lennses. The type they fit inside your cornea after cataract procedure. The item produced went from lathe to eye with zero polishing

[lockbuilder]

Just the thought of an inssert/tool touching the chuck at ANY rpm, let alone 4K rpm gives me the heebie Serious pucker factor!

[qualified_monkey]

Keep in mind two things: first is that diamond shows huge difference when you make use out of it in terms of greatly increasing cutting speed. Second thing is most of diamond tools come with large nose radius. You were surprised that spring passes are taking a lot of material. That is because you shouldn't be taking less than 2/3 tool nose radius depth of cut PER SIDE. Doing shallow passes with such a large nose radius especially on not very rigid small machine causes tool to rub and push material away instead of cutting which is leading to deflection, that leaves worse surface finish. You ideally always want to bury entire nose radius or at least 2/3. For this size of lathe I would go for polished uncoated high rake angle carbide inserts with 0.4 nose radius that you would use for aluminium.

[robbiestevens]

Great video 😁 Its nice to do some testing and evaluating now and again, especially when suppliers are constantly improving and moving the goalposts. You may have had a little push off due to turning inside to out, as the tip is cutting on the flank aswell as the radius. Turning outside to in may well reduce that effect 👍

[WarGrade]

Nigel I wonder how a lubricant would effect the finish? say water, WD40 & oil.
what about a finish pass on the others too?

[Arnthorg]

maybe you could try it with a thick walled acrylic pipe?

[GaryForgingOn]

That was an interesting experiment. Don't they usually get those clear with heat? I wonder if you could use friction heat to get it clearer?

[SirFlibbertyJibbit]

I've never really understood how to keep acrylic clear. Is it just having a large nose radius, small DoC and slow feed/rev, that is just rubs the surface

[ragnarhairybreeks]

Hi Nigel,

It’s so very interesting why the diamond top cuts better than polished carbide on the plastic, and you did tell me the diamond tip didn’t feel especially sharp. Something is going on at the tip on a microscopic level.

Well that’s an obvious observation isn’t it? :-)

Cheers

Alistair

[andrejsgelins]

Ihave not seen them in real life personaly, but for optics (like alumiium, copper and similar plastics) special ?monocrystalic? diamond is used. In books it apeers yelowish in color ad is patialy transparent

[olfoundryman]

Yep, comparing the results of the cheap against the expensive diamond insert - its very much a case of you get what you pay for!... Martin

[matthewdom]

Very cool. I wonder how they do in soft metals. Are you going to try them on some aluminum or brass or copper?

[lockbuilder]

Turning those acrylics gets messy don't it?! Does that stuff stink when you cut it?

[ipadize]

im not an expert but iirc ideally you would have a lathe with hydraulic ways, an air bearing spindle and linear motors if you want to achieve real mirror finishes.
Oh and vacuum the chips directly from the tool