Homemade CNC - Plexiglas melts whereas Perspex cuts fine?

[HankMcSpank]

I have an industrial estate near me, where there are always skips containing pieces of perspex & plexiglass ....weird shaped offcuts, but nevertheless ideal for me to learn the art of CNC on my homemade CNC

i'm particularly interested in 3mm white sheet, so I thought I'd won the lottery when I found heaps of white engths of 3mm Plexiglas in one particular skip! (& I mean *loads*).

Alas, though this excitement turned out to be premature - no matter what I try, when I come to mill it on my CNC, the darned stuff melts & clogs up my cutting bit ...soon rendering the bit ineffective. Now I've tried slow speeds (800RPM), thru 8,000RPM to no avail - it just melts & clogs the cutter. I'm of the opinion that this particular stuff just isn't meant to be milled (possibly just for laser cutting?)

On the other hand, all the Perspex I've found has cut wonderfully on my machine.

This begs the question.... why?

I's always thought Perspex & Plexiglas were more or less one in the same thing - apparently not. The Plexiglass, has a 'softer' feel to it (flexes more etc)...whereas the Perspex I have seems quite stiff by comparison.

Now it may well be that I was just unlucky & maybe the Plexiglas I found wasn't meant to be milled (I'm sure Plexiglas have more than one product with different composition/structure depending on the end useage).

Has anyone on here got experience of cutting Plexiglas with a CNC machine that can advise? (differences, tips etc)

edit: I found some 'markings' on one piece...it says 05070 ...which apparently translates into White Extruded. A bit of Googling suggests extruded *is* much softer that the other variant cast (which is likely what my perspex is made out of) .....tips seem to be a high feed rate yet slow cutting RPM....I'll give it a bash.

[LeeWay]

You are using brand names really, like lexan.
Lots of folks make polycarbonate, acrylics and cast acrylics, PetG etc.
Poly C, Cast acrylic, petG all machine very well with little fuss.
Standard acrylics like found at Big Box stores is not so nice. It's kinda hard a brittle where the others are a bit softer. They are also less prone to cracking.

I use two flute straight cutters in polycarbonate all the time feeding from 80 IPM to 150 depending on the shape with a PC router.
No coolant or lube.
You might try using some soapy water, heavy on the soap, on the bit to see if that helps out.

I do have some 1/8" white acrylic that I got for a project, but have yet to cut any of it.
I have cut a few small acrylic boxes with the router but it wasn't the cheap acrylics for those stores, it came from a supplier and I think it was cast.

[Signmaker]

What you've found is a skip load of Plexi XT which is the extruded acrylic you thought it might be.

We use XT quite a lot... what you've found on Google sounds half right. Higher feed rates are best but we keep the RPM's up high- something like 20,000.

The MOST important thing is to use a very sharp single flute cutter. XT is very temperamental about tooling- if your cutters aren't sharp they drag and pull which gives you an awful cut edge.

[Geof]

As has been pointed out; Plexiglas = Perspex = Acrylic = PMMA = Polymethylmethacrylate, which is the correct chemical name for it and explains why trade names are used.

Cast PMMA is pretty much pure polymethylmethacrylate and it machines like a charm using, when possible, tools with zero top rake and a very sharp cutting edge. Brand new HSS cutters are sometimes best. Speeds can be what you would use for brass or even aluminum.

Exruded PMMA has stuff added to it (I think) and it does not machine nicely. It can tend to melt easily and gum up on the cutter. Also it can be very prone to stress cracking and crazing on the machined surfaces.

[HankMcSpank]

Originally Posted by Signmaker What you've found is a skip load of Plexi XT which is the extruded acrylic you thought it might be. We use XT quite a lot... what you've found on Google sounds half right. Higher feed rates are best but we keep the RPM's up high- something like 20,000. The MOST important thing is to use a very sharp single flute cutter. XT is very temperamental about tooling- if your cutters aren't sharp they drag and pull which gives you an awful cut edge.

Thanks (to you all)...I think where I've come unstuck, is that I've previously been solely machining cast acrylic - & my word it machines lovely. I've then come across this 'treasure trove' of Plexiglass & naively used the same feed rates & RPM (my spindle only goes up to 8,000RPM) ....almost instantly this Plexiglass XT (05070 variant) melts & clogs my cutter (particularly bad when my CNC to drills holes 3-5mm dia .....the drill bits clog more or less instantly)....this little bit of research/questioning suggests I really need to crank my feed rate up.

For example for the cast I've previously been machining I only use a feed rate of 40-50mm per minute (on account my homemade CNC hasn't that much grunt)...since this extruded acrylic appears much softer, I'm going to double/triple this with this pesky extruded & see if the acrylic still melts on the tool.

I think I have a new single flute cutter somewhere - I'll give that a go too.

[HankMcSpank]

ok, just popped some more plexiglass onto my machine...this time Black - it milled perfectly. It transpires that this black Plexiglass is 'Plexiglass GS' ..which i've googled & surprise, surprise it's 'cast' acrylic too.

I've not had time to revisit the original Extruded Plexiglass XT, but I'm now beginning to think that cast acrylic is the only one I'm likely to have success with on my particular machine!

[skottc]

Originally Posted by HankMcSpank ok, just popped some more plexiglass onto my machine...this time Black - it milled perfectly. It transpires that this black Plexiglass is 'Plexiglass GS' ..which i've googled & surprise, surprise it's 'cast' acrylic too. I've not had time to revisit the original Extruded Plexiglass XT, but I'm now beginning to think that cast acrylic is the only one I'm likely to have success with on my particular machine!

I agree with Signmaker, make sure you are using a sharp tool. Not something you have been using on wood. We machine all the brands of acrylic mentioned all day, every day with no problems. You want to run as high a rpm as possible, conventional direction and blast air at the point of cutting. Here are some manufacturers manuals.

Acrylic Fabrication Manuals

[Valisk]

Thanks for posting skottc - that information has been a great help!