I went through quite a bit of frustration and tinkering to get my machine sorted out. It’s hard to know just how much tightness is right on each of the wheels, backlash nuts etc. In the end I learned I had everything too loose, as I had a few bolts come out and my Y axis get our of square. An evening patiently adjusting and resetting things, and learning about how not to crash the machine in to the ends of the axis (which can throw things out of sync if you have anything loose already) was well worth it.
In the end I grabbed a piece of scrap MDF that was pretty large and I cut a pattern in it with circles inside squares. Screen capture attached. Each of the squares was the exact size it should have been and the circles were perfectly round. Took me quite a while to get to that point, but most of that was lack of knowledge and understanding (and lack of locktite - which I still need). Also, learning to avoid crashing the axis helped a lot.
I apologize, I added various things to it and it’s got a few confusion distractions. It’s a 400 mm x 800mm (happened to be roughly the offcut I had and it covered most of the bed on the Y axis) and the squares are 400mm, 200mm, 100mm and 50mm with matching circles inside them. Make sure you tell it to do a “tool inside boundry” when cutting, and then you can use the end of your tape pushed up against the cut line for one end of your measurement to check it, and read off the far end. My squares were bang on and I could see the circles were properly round. Definitely could not have done this within the first two weeks though…
I learned that GRBL, the software running on the Longboard, sets your “real machine home” 0,0,0 to whatever the position of the router is at power on. If you have limit switches you can “re-home” the machine, but the Longmill doesn’t come with these by default (@chrismakesstuff you really should consider offering some 3D printed mounts and simple end-stops as an add-on).
If you don’t set your default “real” home position to a known position, the liklihood of crashing you axis due to sending the machine somewhere at the end of a carve (at least if you’re using Fusion 360) is high. Reading up, I understand the it’s fairly standard in the business to send your machine to top Z height and the distant rear corner at the end of a job. Not a hard and fast rule, but you’ll notice it in photos and videos now that I’ve mentioned it. I started making sure my machine was in that position at shut down and I added a macro to go “Z-Up and return to real home” and I do that every time before shut down and also when I need the router out of the way. Ever since, I’ve had a lot fewer problems.
I’ve been cutting precision 19mm, 20mm and 25mm dog holes for machine metal dogs and they are coming out dead accurate (using the circular 2D operation). I talked a bit more about my settings in another post.