Z axis error woes

[HankMcSpank]

I'm worn out here...& ma brain is well fogged, so thought I'd throw this one open to the 'MYCNCUK Massive'!

Basically, I've got Z errors creeping in ....but I don't know where they are creeping in from!

The CNC software I use - CNC-USB has a way of 'auto Z probing' & storing the points, so what I did was leveled my bed (thanks for the endmill Jonathan!) then mounted some copper board on it - I'm pretty sure the board is flat (both because I levelled the bed, but after mounting the copper, I also did a shallow test cut from Y0->Y10 & it was perfect.

So I then then did auto Z probing at 0.2mm spacing along the Y0->Y10 line ...by the end of the 10mm probing the error recorded between 1st & last probe was 0.3875mm!! (see the list below.....that'll be the last Z probe recorded (3.30) minus the first probe recorded (2.92125)) ...this is a massive amount of difference over such a short distance of 10mm ...and like I say, in parallel with this 'probed line' I'd already done a shallow test cut & the cut was perfect (in other words the bed is very level along this 10mm line)

I am literally tearing my hair out here......the CNC machine I bought touts itself on repeatability (http://www.youtube.com/watch?v=6HZnKCrpCkI ...2m25s) ...so it might not be my machine (or maybe they just lie!) - it may be my stepper board? (an easy CNC 3 axis)...I'm just a bit puzzled where to start & if there's any diagnostic/test anyone can think of it'd be most welcome!

Here are the Z probe points along the 10mm line Y0->Y10 (I chose 0.2mm spacing to ensure there were a lot of Z moves up/down/up/down etc), every line introduces a little bit of cumulative error by the looks of it....

X0.00000 Y0.00000 Z-2.92125
X0.00000 Y0.20250 Z-2.91750
X0.00000 Y0.40500 Z-2.92875
X0.00000 Y0.60750 Z-2.93625
X0.00000 Y0.81000 Z-2.94375
X0.00000 Y1.01250 Z-2.95125
X0.00000 Y1.21500 Z-2.95875
X0.00000 Y1.41750 Z-2.96625
X0.00000 Y1.62000 Z-2.97375
X0.00000 Y1.82250 Z-2.98125
X0.00000 Y2.02501 Z-2.98875
X0.00000 Y2.22751 Z-3.00000
X0.00000 Y2.43001 Z-3.00375
X0.00000 Y2.63251 Z-3.01500
X0.00000 Y2.83501 Z-3.02250
X0.00000 Y3.03751 Z-3.03000
X0.00000 Y3.24001 Z-3.03750
X0.00000 Y3.44251 Z-3.04500
X0.00000 Y3.64501 Z-3.05250
X0.00000 Y3.84751 Z-3.06000
X0.00000 Y4.05001 Z-3.07125
X0.00000 Y4.25251 Z-3.07875
X0.00000 Y4.45501 Z-3.08625
X0.00000 Y4.65751 Z-3.09375
X0.00000 Y4.86001 Z-3.10125
X0.00000 Y5.06251 Z-3.10875
X0.00000 Y5.26501 Z-3.12000
X0.00000 Y5.46751 Z-3.12375
X0.00000 Y5.67001 Z-3.13500
X0.00000 Y5.87252 Z-3.13875
X0.00000 Y6.07502 Z-3.14625
X0.00000 Y6.27752 Z-3.15750
X0.00000 Y6.48002 Z-3.16500
X0.00000 Y6.68252 Z-3.17250
X0.00000 Y6.88502 Z-3.18000
X0.00000 Y7.08752 Z-3.18750
X0.00000 Y7.29002 Z-3.19500
X0.00000 Y7.49252 Z-3.20250
X0.00000 Y7.69502 Z-3.21000
X0.00000 Y7.89752 Z-3.21750
X0.00000 Y8.10002 Z-3.22125
X0.00000 Y8.30252 Z-3.22875
X0.00000 Y8.50502 Z-3.24000
X0.00000 Y8.70752 Z-3.24750
X0.00000 Y8.91002 Z-3.25500
X0.00000 Y9.11252 Z-3.26250
X0.00000 Y9.31502 Z-3.27000
X0.00000 Y9.51752 Z-3.27750
X0.00000 Y9.72002 Z-3.28500
X0.00000 Y9.92253 Z-3.29250
X0.00000 Y10.12503 Z-3.30000

[i2i]

it could be noise making the z step randomly. Try the same moves and put your finger on the z axis ballscrew/motor and see if it's stepping now and then.

[HankMcSpank]

Thanks for the idea...I will try this tomorrow (finished for the night now!)

One thing I have just done, is changed the Z axis to 1/8th stepping (and calibrated the machine - again!) the error is down to 0.1mm (ie between Y0 & Y10 ...in oter words a 10mm line) - which actually might be correct because i put a V cutter in the collet, lowered it to just touch the copper at Y0 then slowly moved towards Y10 & the line disappeared, so this 0.1mm disrepancy might be true deflection in the copper board itself...I'll havea better idea tomorrow when I can do a proper cut!

[Jonathan]

it could be noise making the z step randomly

I just plotted the point list:



That doesn't look very random! In fact that's an excellent linear correlation (R²=0.9996) so it looks like an alignment problem to me. If you hadn't said that the bed was surfaced I would say that it is the gantry that's not level and if you hadn't said that the cut with a V-cutter looked fine I'd say it was the bed/PCB that's not level as a straight line like that implies one or the other is inclined.

One way to test if it's a cumulative error, for instance missing a couple of steps for every down move (seems unlikely but you never know), is to just do two points. One at Y0 and one at Y10 and see if the error is a 50th of what you just got from measuring 50 points.

This is why I sent you the 1mm endmill - with that the bed can be a lot further from level as the width of the cut does not depend on the height. Only problem is 1mm it a bit useless for anything but the simplest PCBs. I bought them on eBay as the seller said they were 0.5mm, but they weren't hence I've got no use for them.

[JAZZCNC]

Hank The error is too consistant to be noise issue.! . . .It's repeating 0.0075.

I presume you have done the obvious checking binding, slipping couplings etc.!

This sounds very much like pulse timing issue.? basicly it's not completeing the full step before it starts the next step.!

Before doing anything else I would check simple things like wire connections, make sure the wire polarity of the motor phase's are correct regards drive connections IE: A+ B+ A- B- from motors go to same on drives. They will still work ok if switched and it can cause silly issue's like this with some setups.

If the USB Software as a similar Dir/step pulse timing setting that mach does then I'd try increasing this time.!
Back off the velocity/Acclereation and see if it still happens, you could just be over tuned.?

Are 100% sure the motor is Man enough for the Job.? Some times if the motor is on the edge regards holding Torque they can "spring" steps while going slow or near stop, again this would do this kind of repetative error.!

Can you try swapping the Axis to see if it moves or goes away.!

If you have one I would even try swapping the USB cable, I have had silly things like this with dodgy PP cables. I had a small noise issue with my smooth stepper and one of the tricks or work rounds was to remove the shielding at one end.!! . . . .Basicly the shielding at each end was causing ground loop problems. I know it sound like a long shot but worth a shot.?

hope you find it quick I know how these things can drive you crazy . . . . . . . Keeps us informed.!

[JAZZCNC]

That doesn't look very random! In fact that's an excellent linear correlation (R²=0.9996) so it looks like an alignment problem to me. If you hadn't said that the bed was surfaced I would say that it is the gantry that's not level and if you hadn't said that the cut with a V-cutter looked fine I'd say it was the bed/PCB that's not level as a straight line like that implies one or the other is inclined.

Yes Machine setup was my first thought Jonathan but the leveling of the bed rules this out.! Not sure the pcb would be so consistant increasing thickness.?

Thats why I turned to pulse timing issue.? This would be repeatable to this level.!

I feel for hank because things like this can drive you crazy.!!

[Jonathan]

Yes Machine setup was my first thought Jonathan but the leveling of the bed rules this out.! Not sure the pcb would be so consistant increasing thickness.?

Yep, the PCB itself has good thickness tolerance so I was more thinking the PCB itself might not be stuck down flat, but as we both said... that makes no sense! Maybe the gantry slipped at one end? Again unlikely.

I doubt it's the USB cable as interference or interruption in the USB signal would cause all sorts of problems, not a nice linear error. Similarly if the motor is connected wrong it'll either break, not spin at all or spin the wrong way.

Out of the causes you suggested I think the acceleration setting is the most likely cause as that could well induce a consistent error. Testing on a different axis, or even probing a grid so we can plot a plane is also a good plan.

Try running the probing routine exactly as before, but twice in a row ... probe from Y0 to Y10, then start again at Y0. If the two point lists are virtually identical you know it's not a motor or motor tuning problem as those problems would make the second probing routine output different to the first.

I feel for hank because things like this can drive you crazy.!!

Yep.

[JAZZCNC]

Similarly if the motor is connected wrong it'll either break, not spin at all or spin the wrong way.

No it will work fine but as you say spin in reverse, changing the pin hi/lo setting is what most folks do to get round this. But wrongly wired polarity of motor/drives is known to cause this kind of error, it comes up every so often on Mach Yahoo forum.! . . .Yes I know this is USB but still motor/drive issue so still possible.? Certainly worth checking as it's 5min job.!

[HankMcSpank]

Guys, I've just picked up your posts (been out all day), will take a trip back into my workshop with a clear head (won't last long though!), I actually have high hopes that this will be sorted tonight, a couple of things I did late on last night was ....

1. Changed my Z resolution (from full step to 1/8 step...my Z 'pitch' is 1.5mm so steps per mm has gone from 133222.3 to 1066.6666)...at least this way i'm figuring, if I'm missing stepos then the error won't be so huge!

2. I calibrated my Z axis (not even sure whether this could affect the outcome, bt it was out.

Re the pcb kmaterial....clearly I've leveled the bed, but the copper board is double sided sticky taped down to some 5mm (flat) acrylic spoiler material...this in turn is clamped to the bed...so there is the possibility for an unplane copper work surface error...it shouldn't amount to what I was seeing which was 0.35mm over just 10mm (also, the fact that I took my V cutter down to 'just' scratch the copper & then tyraversed 10mm to Y10 ....the line disappeared half way along (since there's no ascent/descent going on here, then this must be board level error)

The CNC software itself can't be ruled out...this auto Z probing is a *very* recent feature in CNC-USB software.

Anyway, lets see what this evening brings!

Thanks for your support though....sometimes you just get a little close to the wood & can't see the trees blah blah.

[Swarfing]

Hank

This is definitely a problem with the rails not being at parallel heights? Now i would check that the supports are equal, bearing cases are equal etc. I had similar problem because one of the liner bearing holders was not from the same batch. The linear line you are seeing is because opposite corners are at different heights.