With 0.1" per pass:
The following pics do not show it very clearly, but visually the top shapes look near-perfect to me, the bottom triangles also, while the two bottom diamonds still show a very slight sign of the problem, maybe a bit less than in the 0.25" depth per pass test.
Maybe the full depth triangles look a little better in mine this time? only a tiny bump… Otherwise, everything else looks nearly the same as last run to me. Put them on imgr this time, can’t seem to get direct link to the drive photos.
That pic:
makes me think it’s not mechanical, it’s so even and sharp in all four corners. Maybe let’s do the “declare the bit as being 61 degrees” test ?
91 you mean? Sure! updating files now.
Not much of a difference with 91 degrees specified, however for some reason it looks passable from the top down photo this time? maybe just because the lighting is a little more diffuse.
How about your Z steps/mm calibration? The corners rely on 3 axis motion. If your Z is lagging behind, you might see something like that.
That’s a great suggestion - I didn’t know that was even possible… I found this in the docs, and will attempt it with my cheap set of plastic dial calipers (it’s the best I’ve got).
Something tells me @neilferreri nailed it (as usual), so I’m looking forward to the results. I checked my setup and realized that while my X and Y are calibrated, my Z isn’t anymore (I messed with my GRBL settings a lot recently and may have forgotten to reset it), so who knows, I might be off by a little bit too.
I think your z belt may be too loose. After you set Z zero with the paper test, try jogging up a few inches and then back down to zero. Does the paper still feel the same? I had a bit of slop in my z axis which I discovered with a dial indicator, but you might be able to approximate that with the paper test.
-Dan
Is the tip of your V-bit come to a point? There are some made that have a flat tip…
Yeah, it’s a pretty fine point on this one (you can see it in the bottom of the flat-depth pieces). On the initial project where I ran into this problem, I was using a different 60 degree v-bit that I discovered had a much larger flat point due to bad design (or they skipped a sharpening step?) and I thought that was the issue, however it has persisted with a much finer point v-bit.
I’m working on that this morning. It seemed tight enough, but this morning I’m re-tightening it, checking Z carriage v-groove wheels, and then running calibration. We’ll see how it turns out.
The flat tip usually just causes individuals to set their zero too low, but you can still get a good carve when that’s taken into account. You’ll usually see bulging edges and rounded corners when the cut is too deep.
So, I tightened up the Z-belt a bit, re-adjusted eccentric nuts on the Z carriage, and then ran a calibration.
My measurements probably aren’t super accurate since I just have a cheap plastic dial caliper. I tried a few ways of mounting it, but ended up just making marks on the extrusions and measuring them afterwards. I zero’d it out all the way up - then moved 90mm down. it was nearly a whole mm off, about 90.9 - so I used one of the calculation spreadsheets floating around and adjusted $102 - then ran the same test we’ve been using.
Was really hoping for a big difference, but I didn’t see much of one upon close inspection. A is the first time we ran this test - B is after Z calibration.
I probably need to calibrate X/Y as well, but not sure if it will help.
I had not tried that yet - we went larger (91 degree) but didn’t go smaller. Here they are:
89 Degree:
88 Degree (sorry, the cut ran into a previous test cut I did on this piece of scrap)
Better? Hard to tell. Maybe 85 or 80? Just to see if something changes.
What bit (manufacturer, part #) are you using? Have you tried a different one?
I didn’t notice much of a difference at 88 or 89, but it’s definitely not any worse. I thought maybe I saw a little curve at first glance, but it’s not as noticeable in the pictures.
I already ran a triangle test at 85, 90, and 95 - and got the expected results… the weird corner problem persisted through all degrees. I can try it with these shapes though, willing to try anything at this point.
I’ve mostly been using a Whiteside 1550 (60 deg 1/2 inch), I’ve measured it and it seemed damn close to 60. I have tried 60 and 90 degree half inch Freud bits as well… (the Freud 60 is the one with the wide tip issue).
Just throwing this out there, but I doubt it is the cause…
Could it be a accel/decel issue? I had an old laser that would have wobbly lines in corners until I slowed the accel/decel parameters.
Like I said, I doubt this is it, but maybe worth a try?
I think your problem is a combination of:
Part 1: my thoughts about your cutter angle not being right. I grabbed a copy of CAMotics to simulate your gcode with stock removal instead of tool movement lines in space. Let’s see how it looks with a perfect 90 degree cutter:
Next, for illustration’s sake, let’s run the same program, but with a drastically wrongly-cut cutter (60 degrees actual, vs expected 90 degrees):
Part two - I don’t think you can perfectly correct in software for a wrongly-made cutter, at least using a V-carve pathway (hence why your adjustments don’t work). It’s a physical impossibility with angles. This part requires more of a mental exercise, because I can’t illustrate it with software, the way I can above.
Basically, let’s say you have a hilariously undercut cutter that’s maybe 5 degrees, instead of 90, and essentially looks like a needle. Without a 4th axis, and using only the movements that you’ve seen V-carve put out, is it even possible to cut nice 45 degree walls? I don’t think you can. No v-carve strategy can accomplish this. You end up doing a central plunge, maybe doing a lap near the bottom, then trying to run the tip up the corners at a 45 degree angle, but you’ll never clean up the walls between. You need to move to something like a stepover milling strategy (indeed, a 0 degree cutter is an endmill), which is kind of the antithesis of the V-carve technique.
I don’t think it’s possible to generate a v-carve pathway that perfectly produces your desired result, with your current v-carve bit. You can kind of fudge it, but the cost is the aberrations you’re seeing.
Sorry. I’d be happy to be proven wrong.
(Edit: a more concrete example of point two: holding a sharpened pencil perfectly upright, can you make a perfect inverted pyramid in clay with v-carve techniques? I can’t. The best I can do looks like the examples above.)
Edit again: I think there’s a philosophical/intent question here, too. The purpose of V-carving is to achieve a cut of a certain width at the surface, without having to do multiple passes (depth and everything else be damned). If you need it wider, you go deeper. Sometimes we can abuse this to get nice mathematically and mechanically clean results, but that’s the exception and not the rule. This thread is the rule.
For sake of completeness, a 95 degree cutter (problems become very apparent very quickly with overangle cutters - probably why we see so many underangle ones):
I agree with number one, but I don’t think number two really matters here. Regardless of the carve angle, a square should be square (hope that came out right).
