Gordon the engine is looking good and great tutorial. I would like to add what I have learned about cutting circles in styrene over the years. I start by drilling a small hole clear though the the sheet at the center. Just big enough for the cutters point to fit tight in the hole and go down into the cutting pad when cutting I make light cuts going around over and over keeping the cut grove uniformed then I turn the work over and cut from the other side if you can get a good cut on both sides you can break it apart with your fingers or finish with the cutting tool. I used a set of dividers (compass) for 30 years it more like scoring or scraping not cutting. The past 10-15 years I’ve been using the PAT.P cutter, and if you learn how to use this tool it dose a great job.
Drill a small hole (small drill bit and a pin-vice). #67-65 good for .030, ..040, ..060. Smaller drill for the thinner sheets.
Use a sharp blade.
Light cuts round and round over and over.
Hold cutter with your thumb and first two fingers at the top of the tool twist and reset and twist and reset over and over until you finish the cut (light at first when you get a good grove going you can add pressure.)
Gordon Birrell
https://www.flickr.com/photos/gordonbirrell/
