more successes

Next thing to do was cut something with multiple depths. Eventually I'd like to make some keychains for gifts and myself and family (well most of us) are Packers fans. So I grabbed this model off of Thingiverse and imported the STL into Fusion. I converted Mesh to BRep and scaled the model to fit the stock I was planning to use.

I set up the CAM for 4 operations (settings shown are the ones I converged on after 2 failures)

1. Adaptive clear the interior
• 1mm depth per pass
• 250mm/min feed
• 0.2mm stock left radially
2. 2D contour the G
• 0.5mm roughing depth
• 120mm/min feed
3. 2D contour the inner part of the outer ring
• 0.5mm roughing depth
• 120mm/min feed
4. 2D contour the outer part of the outer ring
• 0.5mm roughing depth
• 120mm/min feed

I used a 1/8" 2 flute end mill at 2500rpm.

The first attempt, I broke an end mill entering the first 2D contour. I initially add a 1mm depth per pass on the roughing and it dug in and broke right away. I measured after the cut and it was actually 1.5mm deep! Z must have slipped, and admittedly I had "0" set a bit low, in that the first move across the part cut a channel about 0.2mm deep.

Second attempt I dropped the roughing depth to 0.5mm and set zero on the Z axis by setting the end mill on a razor blade on top of the part. This left it slightly shallow but I wanted to be careful. This worked much better, but I finally did break and end mill on the final contour. This was caused by 2 factors. One, once again the end mill dug in, secondly the part was not perfectly flat. Thirdly, perhaps somewhat contributing, is that I didn't have the center of the cut aligned so the outer 2D contour left the stock and re-entered. I think the combo of unloading, reloading, and having too deep a cut did me in. The cut was 1.mm on that side and 0.8m on the other. You think you'd see that but I swear the stock was flat! Combo of using a diced-up part as a shim and getting swarf between the milling vise, shim and stock.

Anyways the Z digging was an obvious common culprit that needed to be addressed. When I have a problem I can't figure out I generally do one of two things. The first? charge ahead and just start doing stuff and generally screw up a lot and get frusturated. The other, rarely, walk away do something else and then find a eureka moment. Fortunately this was one of the rare times - I had my commute bicycle partially torn apart to fix something, so I worked on that awhile and eureka. I was thinking about the GRBL settings and recalled that the X and Y have 126 steps/mm or something like that and the Z has a mere 25 steps per mm because I'm using an 8mm lead screw with direct drive. So it only takes a few missed steps to make a notable impact in the Z whereas in the X and Y I can lose a few and never know it. So, what if I enabled microstepping? A 1/4 step would give me 100 steps/mm and would be the same order of magnitude as the other axes. Sure, it masks a problem, maybe, but it 'scales' the problem to be the same order of magnitude as the other axes.

Third try - success! Code ran to completion, no broken end mills, and the dimensions were good to within 0.1mm or so - again, 10 steps, but margin of error, that's 4 thou which is pretty damn good if you think about it, for a mini mill with printed parts!

I did get some wicked backlash in the Y axis - the screws came loose on the affixing print. I need to add some threadlocker.