Hello. I built my Primo almost a year ago. I got it dialed in enough for what I needed, but now I want to get it dialed in to where it really should be. I cut a 6" square and it was short ~1/16". I cut a 4" square and it was short ~1/32". I cut a 2" square and it was short ~1/64" (which I could probably live with). Since the distance I am short is not the same for each size square I am wondering if I should adjust my steps per mm.
Any thoughts or advice is greatly appreciated.
Matt
Maybe, but probably not. There are many variables when chasing tolerance. Especially when actually milling material. I have a few posts on it. I got down to around 0.5mm cutting a soft material.
Best to concentrate on either the pin drawing tests or dropping a mini drill bit on a large machinist scale which is my preferred method. Linear X/Y axis tests will tell you if you need to adjust steps. But a repeatable true square box X will most likely will expose the problem.
When you get the “unloaded” machine travel error to a level you can no longer measure < 0.1mm you should hit your target. If you still have too much play when cutting you will need to locate and correct all the flex in the machine. Again you can read about my journey if you like.
Sounds like there is more than one thing going on.
I would start with measuring if an inside cut is the same dimension as an outside cut.
That is to say, if you cut a 2" square hole and a 2" square piece (with appropriately rounded corners) does the part fit inside the hole? Tightly?
I found that describing my 1/8" mill as 3mm is what I need to do in CAM. The 3.175mm results in tabs that don’t fit in slots. Described as 3mm it’s a tight fit, but it fits.
Once you have that, then you should find that the error for n 8" piece is double the error for a 4", and you can adjust steps/mm.
Try it and see what happens. Any experiment is good because you will learm something.
As mentioned before, I got my no load error down very small. I ended up measuring with string pots to get beyond limitations of a visual check. I also got my machine super tight and no runout in the router. My idea was to adjust the machine until I could cut a perfect circle. I cut about 50 125mm circles while adjusting the machine. The error, buldges in the circles, was usually on the diagonals. It was very hard to get to 0.5mm error but under 0.75mm was fairly consistent. Best I could tell, the error was due to flex in the rails, sort of a twist in the core. My machine is 24" square. I was cutting 1/2 inch plastic sheet which is fairly dense with 3 passes I think and I experimented with finishing passes. I would guess material density will greatly affect the results. I also cut some round aluminum parts and the error stayed around 0.75mm.
Good luck.