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

[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

[Jonathan]

I think you are asking a lot of a machine with unsupported rails on all axes. Maybe put some better rails on your Christmas list!

Yet another way to check the XY axes are perpendicular is do put some MDF on the bed and drill holes in the corners of a rectangle, then push drill bits in the holes and measure the diagonals with a digital caliper. Size the rectangle to match your biggest caliper.

You should also try measurements on different parts of the bed - e.g opposite corners. This will tell you more about the source of the error.

To make sure it's not something electrical (I doubt it too), you could try zeroing the machine at a known point on the dial indicator, then run some random g-code:

http://www.mycncuk.com/threads/5843-...g-motor-tuning

If anything will cause it to loose position, that should do it.

Edit:
A note on ballscrew accuracy - C7 is up to 0.05mm over 300mm. This doesn't mean the error is distributed over the 300mm, e.g. a screw with a 0.04mm error over 40mm and 0.01mm error over the remaining 260mm would be classed and sold as C7. This sort of error (though hopefully not as extreme as my example) is more likely than an error over a broader distance, as if that were the case the manufacturer could compensate for it similarly to how you might do so in software. That's why I'd be wary of entering anything other than the calculated value for step/mm, as it will compensate only for the portion you measure, on average. You need to be very confident of the measurement accuracy to do this.

[GND]

Hi Rob,

The geometry has certainly needed some adjustments to be made, so more checks are probably not a bad idea. Recently I have been focussing totally on tweaking the squareness of the X and Y axes, but I must confess I haven't done a whole lot over the Z axis beyond proving that the table is level over the working area to a reasonable margin of error. Something else to add to the job list!

That said, I've convinced myself that this isn't the cause of my measurable axis errors currently. I was initially thinking the out of squareness was the only cause of my non-circular circles - but that just "squared up" the axis errors I am now seeing....

Why isn't this more straightforward?!

Cheers
Graeme

[GND]

Hi Nick,

Many thanks for your thoughts - certainly something along those lines would be a good plan I think.

I guess I was just sceptical that this tweaking of settings should be required for a C7 screw. Maybe what I have are out of spec - or indeed maybe they aren't actually C7 screws? The guy I acquired the machine from just remembered that they were from Marchant Dice and that they were "not precision ones". Marchant Dice currently only go down to C7, so I have just emailed them to see if they have ever supplied lower spec screws like C10. If that was the case then I have an explanation, and the steps per mm tweaking could be a legitimate way forwards.

We'll see what Marchant Dice have to say....

Cheers
Graeme

[GND]

Hi Jonathan,

Many thanks for your comments. As soon as I took on the machine it seemed obvious that the long supported rails under the table for the X-axis in particular were far from ideal. The gantry wallows from side to side mid-travel if you wobble it, especially due to it being too tall and with significant mass on top with the Z-axis motor. It came with NEMA-32 motors - since swapped for NEMA-23 - so it was even worse then!

So new rails sound like a really good move - I presume for the X-axis at least? Do you think I could get away with the other axes as they are for now? I do have a plan to shorten the Z-axis travel by about 100mm - thereby shortening its rails and overhang - whilst still retaining a 150mm Z-zxis movement, which I think is fine for my needs. It also loses 50mm off the gantry height, which must be a plus!

All that said, I am still mystified by the result of my MDI tests. Cut a mark - MDI for 100mm - cut another mark. And the distance as measured with a digital caliper between the marks is wrong by the percentages I have been quoting (+0.22%, -0.13%). In fact the error is clearly visible using a steel ruler! I felt the rigidity was not a factor in that test, hence I'd isolated the error to the screws. Maybe my simplistic understanding of it all is getting in the way here....!

So what kind of effects would I expect to see currently, based on your experience? Would there be dimensional errors like I have described with these unsupported rails? Reasonably repeatable ones? And can you think of a way I could run a test that proved it was a rigidity issue? I thought my CW and CCW circles did that, but perhaps not? It would be amazing to have a more definitive test that just showed up that the machine was not rigid enough, and that the errors were the direct result. I'd be ready to believe that ahead of the ballscrew errors - especially based on your comments at the end of your post.

Many thanks for the pointer to the test G-code spreadsheet. That looks like a great test - I'll give it a go at the weekend and see what happens....!

Cheers
Graeme

[GND]

Thanks for your take on this, JAZZ - I certainly don't have a practical way to measure over a significant length of the screw travel to the accuracy I would need! I'd have to base it on short segments, which it sounds would be far from ideal.

I just heard from Marchant Dice and they confirm the ballscrews I have should be no worse then C7. So to me it just feels like my issues are being caused by something other than screw pitch errors. Prime target must be lack of rigidity. But how to prove this....?

Cheers
Graeme

[JAZZCNC]

That's why I'd be wary of entering anything other than the calculated value for step/mm, as it will compensate only for the portion you measure, on average. You need to be very confident of the measurement accuracy to do this.

I don't exactly agree with this but at same time I do.?
Completely agree that not good idea to use over short distance but if done over the full length or large portion of screw then can set the machine up very accurately, much more than working to fixed calculated values which don't account for screw or other factors like belts/pulleys.
However to do this requires very accurate measuring device done over long distance.

I set machines up using Glass linear scales to verify movement and find that if using timing belts/pulleys this is only way to very accurately set machine up as the calculated values often don't get it spot on. Even when direct mounted there can be some error but to much lesser degree.

So case of Yes and No to me also how accurate or Anal you want to get about it.??


However in this case I'm 99% sure the issue is compulation of mechanical and design weakness.