This past week I have been trying unsuccessfully to engrave some plastic that I had purchased at my local Lowe's store. This engraving work was intended for the plexi-glass clock project that I started a couple of weeks ago.
I could not understand why the engraving tests that I had done at varying speeds continually produced parts that were worthless. And I am being kind when I use the word "worthless". This was especially confusing to me as I had made some beautiful engraved lit angel displays for a couple of my sisters this last Christmas.
This photo shows how badly the engravings are now turning out. I spoke with my good friend Steve Hamer from the QC Co-Lab maker space in Davenport Iowa and asked him what kind of acrylic plastic he was using as it worked wonderfully for my angel displays. What we found out was that the plastic is indeed acrylic but it is "cast" acrylic. Before I had found this out I had already purchased a nice sized piece of poly-carbonate plastic. I thought surely this would be harder than the Lowe's plastic which by the way is a rolled acrylic not cast.
That blob on the end of the bit is what happens when you try and engrave poly-carbonate plastic as well. Just like the rolled acrylic the engraving bit melts the plastic. At least it does on my CNC machine anyway.
The answer to the problem here is that the CNC machine has to run a lot slower to do engraving in these softer materials. Something like 800 rpm on the spindle and 10 inches a minute travel. I can run the travel speed this slow but it is far to slow for the spindle speed on my machine. I would be lucky to go half that speed with the DeWalt trim router that I am using even with a speed control. So I will have to track down some cast acrylic as I know this will engrave properly at the speeds that I am able to run at.
The good news about all of this is that I now have a nice supply of the acrylic and poly-carbonate plastic on hand that will not go to waste. These sheets I know cut very well using a plastic cutting bit in the CNC. I will use them for plastic gears, parts, etc that would be great for toys or any other gadget that I would normally use wood on. I hope all you guys reading this post and who are just starting out with your new CNC machine have learned something here today. I know I have.