Axis scaling errors - is this just to be expected with C7 screws?

[GND]

I have a problem that first came to light when making some 150mm diameter gear wheels for a wooden clock on my CNC router. Essentially the gears weren't quite round - one axis seemed undersize and the other oversize - the difference being about 0.5mm, which was enough for the clock to bind after a couple of minutes. I've been using the router for about a year for lots of jobs, but this was the first task I’d done that relied on a decent level of accuracy.

Having analysed the problem, it looks like my X-axis works oversize by about 0.22%, and my Y-axis works undersize by about 0.13%. Doesn't sound like a lot for a wood router, but on a 150mm circle, it does work out at about a 0.5mm discrepancy. Surely we can do better than that? My machine is home-made of the gantry style, and uses C7 grade ballscrews directly driven by 4Nm steppers from Mach3 in the usual way. The screws appear to have an accuracy spec of at least 10x better than these errors suggest, especially over a short 150mm run. I have therefore been looking at all aspects of the machine I can think of, assuming there are other issues to be corrected.

First check was backlash, and it measures at no more than 0.04mm on either axis. The error I see seems to scale with cutting size anyway, so it really does appear to be an accuracy issue in the axis drive. Slowing the motors via Mach3 (speed or acceleration) has no effect at all. Swapping the axis drive electronics between the two motors has no effect - the issue remains with the axis, so my conclusion is that the issue is not in the electronics.

I did discover that the X and Y axes were slightly out of square, which meant that the circles were out of round, but the errors were "off axis" - since corrected. I also discovered about 0.1mm of runout on my Kress spindle - which isn't ideal, but surely wouldn't cause this issue. Just oversize width cuts.

I did feel that the Z-axis assembly is not as rigid as I might have liked. I can flex the router side to side along either axis by maybe 0.5mm without excessive finger pressure, so for a while I wondered if this was my issue. However to try and prove this, I cut simple 150mm diam circular slot in MDF with a clockwise cut and a second one with a counter clockwise cut, and the two circles were essentially identical in their distortion. I should explain further - the X and Y diameters are in error as described, but the "45 degree" diameters are slightly different to one another. This strongly suggested to me a rigidity issue, such that cut direction mattered. With the two cut directions I had therefore expected the errors to be mirror images if you like, but they weren't! I think this suggests it isn't a rigidity issue, but I'm ready to be shot down on that, as this feels like a strong contender to me.

However, I am wondering if the absolute error of a C7 ballscrew is such that you would expect to have to enter a measured rather than calculated "steps per mm" setting in Mach3? I suspect I could null out these errors by entering something other than the current 320 "steps per mm" entry I am using, as it seems you can use any number - doesn't have to be an integer. And you can use the inbuilt calibration button to help you do this along with precision measuring kit. But that seems like a cheat and potentially just covering the real issue, which I can't believe is simply a ballscrew error.

The forum's thoughts and comments would therefore be much appreciated.....

Cheers
Graeme

[GND]

PS. With the rigidity concern in mind, I did run a little "marker" cut - to put a 20mm long slot in a piece of MDF to act as a place marker. I then MDI for 100mm along an axis. Then run a second marker cut, and measure the distance between the two markers. This meant that the axis being measured was completely stationary during cutting. The measured error between markers was very much present. So again this suggested to me that the less than ideal rigidity was not my main issue here......

[magicniner]

Other than with maths how have you checked that your axis Step settings in Mach3 are correct?

[cropwell]

Hi Graeme,

My first thought was - Is there any set-up error. Has the problem always been there or has it crept in over time.

You make a great play on rigidity issues, but give us no info on the construction of the machine apart from C7 screws. Let's have some pictures of the machine and we might be able to suggest something.

I have a MD machine and it has rigidity issues (please no jokes here !). I found that the gantry rails (unsupported 16mm rods) were mounted on the twist (or had moved due to cutting forces). It took me a long time to realise why my squares came out as parallelograms, and you may have a similar problem.

Cheers,

Rob

[GND]

Thanks for the initial thoughts, gents - to cover them in order;

How did I check the Mach3 setup? I have used a plunger type DTI to check that a commanded movement of 1mm equates to 1mm of actual movement. It did, with no more than 0.03mm error each time - which wasn't cumulative, and so seems to be within typical C7 expectations. So 1mm might have come out as 1.03mm actual, and 2mm might have then been 1.99mm, if you see what I mean. These are therefore spot errors of perhaps 3%, but they average out over working distances. I also checked longer distances using digital verniers, and they showed the type of errors I have described in my initial posting. But I had difficulties getting repeatable readings using these, as a degree or two of skew made the numbers change by a lot!

In terms of the machine, I have some rather poor pictures - see below - which hopefully at least show you how each axis is made up. It is an unwieldy thing, with a too-high gantry and uses unsupported rails throughout. I could make many excuses for this - but the one I'm sticking to is that it wasn't designed nor made by me - I took it on as an unfinished project. And it wasn't originally supposed to be a router - it was a welding machine! Hence not ideal, but hopefully I can do something with it, which doesn't involve starting again....

As for how the errors have come in - I measured a circular item I made right after commissioning a year back, and it turned out to have the same error in it. So it was only when my application required accuracy, that I noticed this sub-millimetre error - it has always been in there.

Cheers
Graeme

[cropwell]

Hi Again,

If you have any twist in those gantry rails it will be magnified by your lanky gantry. If you clamp a large engineers square to the gantry side, you will see by running a DTI fixed to the Z, along the edge of the square whether there is any twist you could shim out.

Cheers,

Rob

[Neale]

I wonder if it would be possible to do a repeatability check over a longer distance? Set the DTI to zero against a fixed block (allowing for backlash), move off a fair distance, then move back. Repeat many times and see if there is any drift. I'm wondering if there are any lost steps due to pulse timing issues (dir/step pulse timing, in particular) or maybe even triggering on the wrong pulse edge which can give cumulative errors after significant numbers of reversals or start/stop motions. As you didn't build it yourself, then someone else might have just fiddled until it kind-of worked without really understanding what they were doing.

Interesting - mechanically-minded folk go for mechanical issues, electronics types go for the sparky bits! Anyway, that's a relatively quick one to try out with a bit of hand-written gcode.

[GND]

Hi Rob,

Many thanks for the suggestion - I think I may have checked and shimmed that already, but in a slightly different way. I cut two 6mm diameter holes in an MDF scrap board, along the X-axis. I then inserted two precision 6mm metal dowels in the holes, and set an engineer's square against them. I then fixed a DTI to the router bracket and ran it along the other edge of the square in the direction of the Y-axis to check for squareness. Initially it was out but some strategically placed shim washers got it to within 0.02mm over a 150mm run. This is what aligned my circular errors to the X & Y axes, such that one is oversize and the other undersize. The max and min diameters were previously off axis. Does this achieve the same result as you were suggesting? If not, then please feel free to clarify!

Hi Neale,

A good suggestion and one I had considered. However, since one axis runs over and one under, I assume missing steps can't be my issue? Surely they would be both undersize if I had missing steps? Also, I see the error with a single MDI move, hence the wrong edges setting is unlikely I think, as that only comes into play as you do reversals as you say. I have played with pulse widths and polarities anyway, but with no effect.

Worth adding that I was the one that set up the Mach3, so it is all my own work - it was just the mechanics that I took on. I take all the blame therefore for the electronics and software, but I'm currently pretty comfortable that it is OK, having seen the issues remain unchanged with a swap of the drive cables to the motors. But I am here to be proved otherwise!!

Any more bright ideas most welcome! Or I re-pose the initial question. Could it be C7 ballscrew accuracy, where I need to tune the steps accordingly?

Cheers
Graeme

[cropwell]

Hi Graeme,

I have that sneaky feeling that you need to check the whole geometry of the machine making sure the rails are set parallel and at 90 deg everywhere they should be, if you follow that rather contradictory sounding statement.

It doesn't sound like an electrics problem to me as it is consistent, so I understand. I doubt it is ball screw accuracy. You could do a 3,4,5 triangle and move x and y the 3 and 4 then measure the hypotenuse and with a bit of swapping x,y, 3,4 directions, you might work out which axis is inaccurate (proportionately) and then alter steps per to compensate.

Hope it is a hobby and not a source of income for you !

Cheers

Rob

[magicniner]

Could it be C7 ballscrew accuracy, where I need to tune the steps accordingly?

You should always start with a calculation that should give you correct movement per step, then accurately measure moves of various lengths in various places to see if you need to tweak your settings and if necessary adjust and re-check.

Assuming you don't have backlash or lost steps then if you cut a rectangle and it's corners are square then your machine is cutting square, if it's square but the width and/or length is off then it's axis movement calibration, just tweak your steps settings to give the correct moves and then check at several points along the axis travel that it's right.

Nobody Expoects The Spanish Inquisition ;-)

- Nick