Lookin’ good! My spring is quite soft, and I think it does help, but I’m not certain. I think the lower pressure on the piece leads to more stability in the blade. Lower pressure does mean more passes though, but I’ve found pushing the speed doesn’t hurt to a point.
Regarding the spreadsheet, I suspect it would be hard to translate to Google Sheets or Open Office as the macros do some of the work. I’m willing to try it out in imperial and use Excel. I think the best way forward for general use with that would be to implement it again with Python - not a small undertaking unless you’re jeffeb3.
My two hopes for cutting thick card (no so into foam board, no DollarTree here in UK, and it’s not so cheap) are:
Offset
Last night I managed to increase the offset a few mm by moving the blade in the current arrangement. It didn’t seem to help. I might end up trying to re-model the design to allow for adjustable offset. Would involve possibly splitting the bottom assembly, mounting the lower part in a groove and indexing with a screw. The problem here is that with increased offset, your minimum cut radius is going to increase.
Blade shape
The standard blade leaves a triangular protrusion. This means with deeper cuts there is quite a lot of blade length in the material asking to turn and snag. When I’m next in the shop, I plan to grind down the back edge of a standard blade to see if a much steeper blade angle (like an Exacto or steeper) will help. I’ll be wearing safety glasses when I try it out 