Engraving/machining acrylic

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been trying to engrave some clear acrylic to make an led sign for a friend, but its not been going as well as id like.

first tried a 1mm end mill, 22000rpm, 0.5mm DOC and 500mm/m. acrylic just melted and stick to the cutter and produced a poor finish.

I then tried a 4mm spot drill, 0.25mm DOC, 22000rpm and 300mm/m. started off very well with a perfect finish then after a while the acrylic melted and stick to the spot drill and gave a crap finish.

I then tried an 8mm spot drill, 0.25mm DOC, 22000rpm 300mm/m. went ok for about 30 seconds then acrylic melted and stick to the spot drill.

so decided 4mm spot drill was giving best results and played around with a few different feed rates, I can get good results but then every time after a few mins of cutting the acrylic melts and sticks to the end and causes a shit finish.

anyone got any tips?

I use V-bits to engrave, but when I am milling acrylic I am using single flute bits. Feed rate depends on the type and diameter of the mill bit and the DOC. My current crappy DC motor I have as spindle is running at 9000 RPM and the feed rate for engraving with V-bits is normally around 300mm/min. Milling with 2, 3 and 4mm end mills I normally run with 500-900mm/min feed rates.

So my advise for you is to use single flute end mills and V-bits for engraving and reduce the spindle speed by 50% and keep everything as it is. If you still experience melting then reduce the spindle speed even more and find an optimal speed when melting stops. Once you found that you can increase feed rate and spindle speed.

Remember that the 0.25mm DOC is more like shaving than cutting material, the chips are very thin and can not transport away the heat very efficiently. Use a vacuum cleaner to keep the surface clean to avoid milling the already milled material and making even more tinier chips. If you manage to keep the track of the bit clean then you don't need cooling and the edges will be fine. CA good dust/chip extraction is more important than cooling. Blowing air may work, but I found that at least for me, vacuum is much better. It is also more pleasant to use than blowing plastic chips and dust around the room and all over the machine.

What I also experienced problems with is that the protective sheet some times is of a material which melts considerably easier than the acrylic it is protecting, and that is a real problem. You can't run the machine and adjust to the protective sheet, because that will cause problems with milling the acrylic. In those cases I remove the protective sheet before I start milling, because once the protective sheet starts melting it will destroy the work piece as well and will continue melting, taking the acrylic with it and may also destroy your bits.

In my experience, acrylic is a lot of trial and error. What one person is saying is working for him is not always the same that will work for you, unless we would use the same plastic and every other parameter would be the same. There are different material qualities, so just because it is clear acrylic, it is not that all clear acrylics are the same. Especially when you run out of material and need more, you may need to adjust some of the parameters again.

Good luck.