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Any recommendations for a newb regarding tools for calibrating the steps per unit settings on a simple 3 axis CNC?

I've experimented with digital callipers, a dial indicator, but I'm getting the best accuracy just using a good quality 36" stainless steel rule.

Seems to me I could be doing something better but I'm not sure what to try next.

My work envelope is 60" x 24" x 6" and an accurate tool to measure and calibrate the longest axis is what I'm looking for.



I'm also wondering if anyone has experience with glass scale encoders - Bob Garrish maybe?

Seems like they would really increase the precision of my machine if I can find ones that are affordable to me and can hook into Mach4 via Ethernet Smooth Stepper.


Any advice would be greatly appreciated

Leo

There was a huge line of threads in CNCZone about six or seven years ago using glass scale encoders to turn step driven CNC into a full feedback system. Someone even was marketing a kit. I can't find it now, and it seems to have gone into Internet oblivion but I don't know why.

Thanks rlrhett, I'll try searching there.


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Anyone have any experience with these Chinese DRO scale systems?

http://www.ebay.com/itm/MILL-LATHE-MACH ... Sw3ydVn0CM


Seems like they couldn't be hooked up as CNC encoders, but could be significantly better than my ruler method.


Still wading through the piles of glass encoder threads on CNC zone...

Leo

What kind of problems are you having with accuracy?

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Good question.

I tried calibrating with a dial indicator. My first project after that was cutting out a fretboard mask from vinyl using a drag knife and the X axis was off by almost a 1/4" over the length of the fretboard.

I figured I need to calibrate over a much larger movement so I re-calibrated with machinist square lined up against the gantry and accross a 36" stainless steel rule held in place with magnets. The next fretboard mask test cut fit perfectly well, but I don't feel confident in my calibration methods at all.

That's actually not too bad a method. Can you tell us a bit more about your machine, specifically how you're getting your motion? i.e. do you have a ballscrew? leadscrew? R&P?

I have run of the mill mid-grade ballscrews on mine and calibrated to the theoretical values and it's working out "good enough" I'm able to hold within a few thou over most of the machine.

Another good way is to cut squares and measure them.

I think for larger machines like you have, the glass scales would be quite expensive and wouldn't really buy you any practical accuracy improvement. i.e. it will certainly be more accurate, but will you be able to tell the difference?

If you have 6" calipers, cut a 5.5" square and measure it using the calipers. Cutter diameter will play a role there but it will definitely give you some more information.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Thanks Andy I do have 6" callipers and I'll give your square cutting test a try.

I have ballscrews on all 3 axes with servos. This was by far the cheapest machine I could find with ballscrews, servos, and an ATC and I have no reason to think the ballscrews are high quality.

I've never tried to do the math for what the theoretical steps per unit of my hardware should be. Finding the specs for the ballscrews and steps on the servo encoders might be a challenge but maybe I should give that a try.

I've only ever done the commanded movement vs. measured movement method.


This is only the second CNC router I've used and it's the fist one I've had to assemble myself with no instructions, so I've got loads of learning to do.

If you've got ballscrews and servos, there's little need to add scales to your machine. Ballscrews are typically 5mm, 10mm or sometimes 25mm for these types of machines. You can probably turn the screw one rev by hand with the machine off to figure out what you have rather easily. Somewhere in your control software is the steps per unit and with that info you can figure out the math.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

The longest and most accurate steel rule you have should do the trick for working out the distance in the X and Y. Keep up with refining the calibration and you should get it dialled in. On my machine it was a meter long one.

I had tried with a really accurate dial indicator over 50mm and got my machine to repeat to within .001mm - .003mm according to the indicator, but over a long distance it wasn't what I was hoping for. The long steel rule ended up being a better method, and the sanity test with the dial indicator showed the same results as previously. So I guess that on such a short distance in this use, they aren't the best option.

For the Z I used a digital hight gauge that would measure 8".

When it came to the squareness of the machine, I used a jig that had a micrometer head mounted into a tram gauge. Could measure the diagonals and squareness to .01mm, which means .005mm out. Got that one spot on with a 500mm square. So close enough for anything I'll ever need to worry about.

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Allen R. McFarlen
Barron River Guitars & Ukuleles
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Cairns, Australia

Leo, see if anything in this document helps. Not mach but should be similar. You didn't mention if you have belts/pulleys but they list a formula for that.
http://support.technocnc.com/Scale_factors.htm

I could go on for a long time about calibration, a really long time, but I'll throw in some quickies here. It's really nice to have basic ideas on some of these numbers when you start. Like Andy said, ballscrews come in a number of common pitches and they're likely to be metric(the ones on my Fadal were 8mm and 10mm). Encoders are almost always powers of 2, so you're going to be expecting 1024/2048/4096/8192 counts per rev on most commodity encoders.

Order of operations (assuming you tried to build it square and flat in the first place):
-Rough scale X and Y until they both move the same distance for the same input (ie: 16" on X = 16" on Y...actually doesn't matter if it's really 16", though!)
-Rough level the table (stick a tool in spindle and adjust the table until it's the same height everywhere, or machine the table at constant Z height, or both)
-Tram the Z axis (stick a bent something in the spindle, make sure that something stays the same height over the table everywhere when you spin it)
-Rough scale Z (I like to jog and measure with a ruler, then fine tune by cutting a slot or hole of specific depth)
-Check square by drilling four holes in a big square on the table and measuring diagonals, adjust the machine until it's actually square or use software compensation
-Start over from step 1 until your square is square, on size, and a 2" deep hole is a 2" deep hole.

Tools that are just great to have for all these processes:
-A very solid 36" steel ruler. A very sold one meter ruler would be even better, but they don't sell 'em at Princess Auto for $5!
-A long level, for a big machine
-Some sort of tram gauge (a stiff bent wire works well enough...)
-1" Gauge Block
-Decent 6" steel ruler
-6" Vernier/Dial/Digital Caliper
-A dial indicator and a dial caliper (Shars)
-A 1-2-3 block

Tools that are SUPER to have for some of these:
-A tool setting gauge (Shars)
-A decent square (Carpenter's square < Machinist's square < Big Machnist's square < Granite machinist's square)
-A really good short ruler (I have a 6" ruler from Fowler that's graduated in half-millimeters on one side and decimal inches down to 0.01" lines on the other. And it's got a gold coating.)
-A linear scale with DRO (get a one axis lathe DRO from Shars, use it as a super ruler). You can now measure backlash and screw error!
-A really good level (Carpenter's level < Machinist's level < Precision Level < Two axis digital master level I'm so happy I had an excuse to get from McMaster-Carr)
-A real tram gauge (Shars)

If you're calibrating a linear scaling value, like steps or encoder counts per inch, then the longest rigid calibration scale you can use is generally best. A 4' long ruler with 1/16" lines on it is as good as a 1' ruler with 1/64" lines on it: they'll both measure an error to better than 1.5%. But the 4' ruler is better because the other errors inherent in measurement (deflection, user reading error, tool location, list goes on forever) are mitigated more over the longer distance. A nice solid 3/32-1/8" thick 3' steel ruler is one of the greatest rough calibration tools there is for scaling -and- flatness.

Get your axes scaled to eachother, that's step 1, then you can cut a square. It actually doesn't matter if your ruler is really 3' long, so long as both axes read the same when they move the length of the ruler. If you can make a square square, you just need to multiply both axes movement by the same number to make a square on size square. Rough level the table against a tool in the spindle (either jack it to level, or use a tool in the spindle to surface the whole thing at a constant Z height). With a roughly level table, you can tram the spindle with whatever's around (I have a two-indicator tram gauge now, but a piece of bent wire will get you dang close with a little patience).

After all this fun, you can then check out how badly things go out of whack due to mechanical errors. Set a 10lb weight on your gantry with the spindle in the middle of motion and see how much it moves down. Set a 20lb weight in the middle of your table and check the same. Do backlash tests. Do backlash tests at rapid speed. Cut a circle and see how circular it is (can be mechanical and/or control in this case).

OK, maybe that went on a bit longer than I was expecting. I'm really into calibration.

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Thanks very much for all of your responses!

I've got lots to try out and I really appreciate the detailed guidance.

Bob your reply really puts things into perspective for me and gives me a taste of how far things can be taken with calibration issues and tools.

This forum is fantastic!

Happy New Year!

Leo