Calibration question


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My calibration still isn’t as close as I know it can be so I created a file today for some testing and calibration. This is a busy piece but served its purpose. Here’s what I cut with a 1/4" two-flute downcut spiral bit running 18k rpm and 125 ipm. I checked the bit - it is 0.250" on the nose. If I really work at it I can make it be 0.2495" at the very tip of the bit but for all practical purposes, and for this test, it's a 1/4" bit. The depth of cut is 0.1875" and it is cut with one pass leaving 0.005" on the side walls for a final clean up cut to get the fuzz off. The depth needed to be just deep enough to allow my dial calipers in for good measurement.


I started on the bottom left, ‘Before’, and made my first cut. You can see it was out +0.005” on Y and -0.010” on X. I changed the settings in Mach4 and then ran the top left. Ignore the diagonals, those are to ensure I didn’t get too close to the screws. The circled dimensions are exactly as they’re supposed to be. I then ran the top right and it’s exact on Y and out +0.005” on X. The middle right was next and you can see it is exact on Y but out -0.002” on X. Bottom right is exact on X and +0.005” on Y (I wrote it wrong on the piece). The circle is very close, as well.

However, all of the inside circles and squares are 0.015” to 0.025” undersize and I don’t get that. The outside squares, rectangles, and circles are close enough to be acceptable. I think the variance is due to the material which is a sort of soft Purebond plywood from Home Depot.

So how do I get the inside circles to cut accurately? This makes it very difficult when I’m doing inlays and inserts, fitting dowels or bolts in holes, etc. I can’t change settings now or the other measurements will be off and I don’t want that. Is it something not set correctly in Fusion 360? Something in Mach4? It's not mechanical; the machine is very rigid and tight and if it was mechanical the squares, rectangles, and outside circles would be off, as well.

I know using hardwood or acrylic would be better but until I get this a lot closer I don't want to waste good materials. Because the X and Y are very close now I need to figure out what's going on with the inside dimensions. I can slow the feed rate but I don't really think that's the issue. And going from conventional to climb (or vice versa) won't amount to the 0.020" and greater errors I'm seeing. And if the X and Y are as close as they are I don't think any error in the R&P factors in on the inside dimensions.

I have Forstner bits of the size holes I cut and I can't get those bits into the holes. I can accurately measure the Forstner bits and most are about 0.010" undersize but even at that they won't fit into the holes I'm boring, so that tells me my inside measurements aren't too far off.


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- Have you tested the machine for backlash?

- Was this problem happening before?

- Have you calibrated the steps per mm with accurate measuring tools? 

- Can you test again the same cuts but comparing conventional to climb milling?

- Have you checked tramming and squaring? (these rarely affect dimensions, but might help)

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Until my cheapo HF magnetic base broke I got about 0.0015" backlash on Y and 0.001" on X. I used MDI and went in 0.10" increments back to zero and then to 0.750" in one large step and back to zero. I saw 0.003" one time but the magnetic base was starting to slip right before it broke. The first few readings seemed repeatable. Nate is sending me some new spindles and belts so I'll get those on soon and test it again. In the meantime I need to either buy another magnetic base or go into DIY mode, likely the latter.

I suppose the problem has been there.  When I calibrated it before I probably didn't get it close enough.

Yes, the steps per inch have been tweaked now to the best I can get it.

I modified the file for the final finish cut of 0.005" and changed to conventional cutting.  It is far better and much closer, though the inside dimensions are still off relative to the outside dimensions.

About 6 months ago I installed a tramming plate and got the spindle perpendicular to the axes and spoilboard. 

All good suggestions and thank you for asking.  It helps me to verify I've done what I can.  I've now recalibrated the machine, generated new toolpaths, and flipped the board over to make new test cuts.  They're still off but a whole lot closer than they were.  I also added a backlash compensation into Mach4 though I'm not certain that feature really works.

Tonight or tomorrow I'll cut some test pieces with Walnut and Maple to see if the inserts fit any better than before.


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On ‎8‎/‎13‎/‎2018 at 5:05 PM, difalkner said:

I also added a backlash compensation into Mach4 though I'm not certain that feature really works.

That's what I was going to suggest. It works, mostly, but be careful with the settings, climb will give you more backlash/chatter but lets you run higher RPMs/Feeds and is easier on the machine, so I'm guessing the compensation settings would be different for conventional.

On ‎8‎/‎13‎/‎2018 at 5:05 PM, difalkner said:

I've now recalibrated the machine, generated new toolpaths, and flipped the board over to make new test cuts.  They're still off but a whole lot closer than they were. 

That's good to know, good luck with the next test cuts

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