Melting acrylic

[Townhill]

I'm new to this forum and to CNC in general and wondering if anyone can give me some advice on machining acrylic. I'm using a dremel motor and carbide bits to mill designs into the sheets and then edge lighting the designs with LED's. I cut several designs succesfully, but then for some reason started having trouble with the plastic melting and forming a ball around the bit. The material hasn't changed, I've tried slowing the speed on the Dremel and changing the feed rate, but am still having the problem. I'm wondering if I'm using the wrong bit or the wrong material? I am a complete newbie and I'm going through a lot of bits trying to figure out how to do this. I would appreciate any advice anyone might have.

[Geof]

Originally Posted by Townhill ......The material hasn't changed,.....

When you say the material hasn't changed you mean you are still dealing with acrylic - but - did the problem start with a new sheet of acrylic?

You can get cast acrylic and extruded acrylic both in sheets, rods, tubes and both look very similar; they do not machine in the same manner. Many times extruded acrylic will behave just as you describe; melting and sticking to the tool.

[sdantonio]

It may help to use an upspiral bit, the slowest rotary speed you can use while still having the torque to swing the bit, slow feed rate and small depth of cut increments (most of these you have already tried I think). And, if you can do it without killing the dremel or electrocuting yourself a coolant like cool soapy water may help too. Not a huge amount of soap, just enough to slip things up a bit.

[Townhill]

Thanks for the input. Yes, the problem did start with a new sheet. It looks like the same material, but it might be extruded instead of cast. I found a piece that I know is cast (it says cell-cast on the paper surface) and I'm going to give that a shot. I'm also going to slow the rotary speed down as far as I can without stalling the bit. I may try a little (very little) soapy water, just need to make sure I'm well grounded first .

I really like working with acrylic and I'm determined to figure out how to do it properly.

Thanks again, Bob

[Geof]

Originally Posted by Townhill Thanks for the input. Yes, the problem did start with a new sheet. It looks like the same material, but it might be extruded instead of cast. I found a piece that I know is cast (it says cell-cast on the paper surface) and I'm going to give that a shot. I'm also going to slow the rotary speed down as far as I can without stalling the bit. I may try a little (very little) soapy water, just need to make sure I'm well grounded first . I really like working with acrylic and I'm determined to figure out how to do it properly. Thanks again, Bob

Have a look at the edges and also view the surface obliquely under a bright light. Cell cast is generally polished and is very flat; extruded is smooth but sometimes has undulations in the surface.

If you are finishing edges just rub them with a bar of soap and for cutouts trace your path if you know where it will be with soap. Paraffin (candle) wax also works.

[chopshop]

Originally Posted by Townhill I'm new to this forum and to CNC in general and wondering if anyone can give me some advice on machining acrylic. I'm using a dremel motor and carbide bits to mill designs into the sheets and then edge lighting the designs with LED's. I cut several designs succesfully, but then for some reason started having trouble with the plastic melting and forming a ball around the bit. The material hasn't changed, I've tried slowing the speed on the Dremel and changing the feed rate, but am still having the problem. I'm wondering if I'm using the wrong bit or the wrong material? I am a complete newbie and I'm going through a lot of bits trying to figure out how to do this. I would appreciate any advice anyone might have.

try using some coolant on the acrylic...potentially water or water soluble oil and water....maybe it will keep the cutter clean.

[mxpro32]

I've never tried the soap method for finishing acrylic edges, but I can see how it would work. another good way to make the edges of machined acrylic look really nice is to flame polish them with a butane torch, but be careful not to burn it or bubbles will form. just enough heat to make it gloss over works well.

[aifactory]

1. Use cast acrylic. Extruded will melt on your tools, cast will cut clean. With sharp cutters, you won't need coolant.

2. If you need to flame polish, use a hydrogen & oxygen torch. Butane will take forever, acetylene will leave the edges black. H&O burns hot and clean. However... flaming will put stress on the edges that are heated, this will cause "crazing" (small crackly lines in the acrylic). It may not happen immediately but crazing will happen in time or quickly if cleaned with the wrong cleaner such as alcohol or ammonia.

3. The best way to polish the edges would be to use a buffing wheel and compound. This will give you the best looking edges and virtually eliminate the possibility of crazing.