Inconsistant cutting errors.

[SOSx]

Hi Ken,

I'll take some pics of it in a bit.

[Surfcnc]

Back to basics, are my assumptions correct here (from previous posts)

1. 1.8 degree 200 step per revolution stepper motors
2. 20 tooth module 1 pinions
3. 10 microstep setting on you MSD556 driver

My answer is 38.831 steps per mm ??

All that stuff about tuning things off was just to reduce other possible sources of error.
I'm a massive fan boy of reductionist problem solving.
I run my machine with the wireless on all the time.

So what gives with the steps, have I got something wrong.

Regards
Ross

[Surfcnc]

8 microstep = 25.465 ?

Ross

[viz]

Originally Posted by Surfcnc

Back to basics, are my assumptions correct here (from previous posts)

1. 1.8 degree 200 step per revolution stepper motors
2. 20 tooth module 1 pinions
3. 10 microstep setting on you MSD556 driver

My answer is 38.831 steps per mm ??

All that stuff about tuning things off was just to reduce other possible sources of error.
I'm a massive fan boy of reductionist problem solving.
I run my machine with the wireless on all the time.

So what gives with the steps, have I got something wrong.

Regards
Ross

I got 31.831 not 38.831

Another one with 8 microstep is ok.

[SOSx]

Hi Ross - 1.8 degree 200 step per revolution indeed. But running on dp20 rack and pinions (20T indeed).

[SOSx]

Oh yeah sorry, yes we are set to 10 micro steps on the MSD556 drive.

[SOSx]

we made that by the maths to be 25.06337

[Surfcnc]

Also thinking out loud...

The mechanical slop in the z axis may be from errors in the Z slide grinding.
It may be something as simple as being tight at one part of the z axis stroke and loose at another part of the stroke.

The check is to run the verniers over the width of the slide in several places to see if it has any significant variation in width.
Maybe tighten the V roller eccentrics while the Z slide is at the bottom of the stroke (where it does the majority of the cutting).

Ross

[Surfcnc]

Typo, thanks viz

Ross

[SOSx]

That does make sense. The rail grinding on the z slide is a little better than on on the x and y but still by no means exact. I had thought they were certainly contributing once again as we had tightened the concentric bushes as much as we could with the slide up.

[rischoof]

When I see this cutting pattern, it's clearance some where. If you take your router in your hands, and you put some force on it, you must be able to move it.
put a magnet holder and a dial indicator to the different axis and figure out where you clearance is.
springs should be any problem to get, if you buy a longer spring, make it shorter. if you can not find the wright strength, mount 2, check my building log, I explain with pictures how to create the right spring. Yes I am spoiled, there's one street in the middle of Taipei with 4, yes 4 specialized sping shops, and a shop where I can buy teflon next door, but even across the supper marked in my naberhood is a shop run by 3 ladys were I can by the proper springs it's open to 22.00 in the evening.

that rail, I can see from 16.000 km away that's not straight at all
I was able to make a rail within 0.03 mm (not inch!) flatness. check my building log to see the measurement device I used.
When I should have this kind of cutting quality, I should not be able to sleep.

[SOSx]

At the moment were just working out how to re-fix / re-drill the x racks and rail so that it sits both square to the rail from end to end and not tilted down its length. We actually just
tried shimming with a few washers on the outer edge to see if it corrected the slight tilt but it didn't seem to have much effect. The problem there is the bolts that hold it on actually go through the angle section right on the curved part of the angle and due to this have not been drilled strait (it looks like they were drilled from the bottom though).

[SOSx]

Rik - in fairness Ive not been sleeping properly due to this b****y machine. And yes there is definatly some play in the z axis but we have been having a hard time tracking it down. As for the rails we know they are not good but at the same time they should not be causing errors in our calculation of steps per mm and this is my biggest concern right now. We can work on the slop but unless we can get it running different cut/travel distances correctly its absolutely no good to us replacing the rails.

We have ordered springs this morning so they should be here Monday or Tuesday. You are lucky to have supply shops like that locally. There are only a few specialist spring suppliers nationally over here. When I get a chance this evening i'll have perusal of your thread.

[Surfcnc]

Keep up the good work.

OK try 31.831 steps per mm. Sorry about the typo adding to the chaos.
We use Gerald's gear steps calculator, it is an excel spreadsheet to calculate the steps per mm.
It is so powerful it can make dumb guys like me look smart

Ross

[bradm]

I think you've proven that you aren't loosing steps, so I wouldn't spend money on encoders - you can read more opinions on encoders in general here in the forum.

Try Ross' long calibration method, and see how that works out. He seems to be working on a good theory, which is that pinion wobble is making it impossible to get accurate measurements for calibration at short distances. A 3mm error over a 1m calibration set is a more accurate result that a .5mm error over 150mm calibration set.

[SOSx]

Hi brad the last calculations / test measurements were over 980mm in both x and y as
our shorter measurements were not giving us good results on larger measurements even though they were less than +-0.1 repeatedly. So we moved to long measuments after borrowing some 1m long vernier calipers. We were able to get the machine repeatedly to cut the 980mm it to within +-0.1mm. so the results matched the program. So after running these test and setting it accordingly so that x and y correlate over this distance we decided to run a couple large panel test cuts of 600x600mm squares.
what we got was basically 601 in the x and 598 in y? It just doesn't make sense.

[Surfcnc]

Apologies the numbers are wrong that I gave you for your DP20 rack and pinions.Went down the wrong path there not paying proper attention and specified the steps for module one rack and 20 tooth pinions.. but you already know this
Ross

[SOSx]

No problem Ross. Yeah had twigged onto the mistake.

[smreish]

Mach 3 system has a variable that might affect that.
In the General Config page you will see:
Motion mode
(constant or exact)
Distance Mode
(absolute or incremental)

Try toggling into exact and absolute mode and try your cutting profile. Did you see a change from the original results?

[SOSx]

Will try that tomorrow once we've got the racks back on properly. Once again its not proving to be an easy task. Thank sean

[KenC]

If the exact stop mode test fail, What I'll do is dismantle the Z-axis -> inspect & measure the parts -> re-assemble it the correct way.

I suspect the plate V isn't parallel enough. tight at top loose at bottom or vice verse. This can't be fix by turning the eccentric bush...

[Gerald D]

On the Z-axis, all the rollers must be on the same flat plane, and the slide plate itself must be flat.

Then the eccentric bushes must set the roller pressure quite tight (a lot of "springy" preload). If the holes for the bushes were drilled incorrectly (too far apart) in the spider, then the holes on one side need to be welded up amd re-drilled about 2mm closer to the other side.

[SOSx]

Hi Gerald,

I think we'll be looking/dismantling the z Slide tomorrow now as we have only just finished getting the racks fixed back on strait this afternoon. They are certainly much better now anyway. Hopefully that is another issue with the machine ticked off now.



After a couple of attempts at re drilling through the rack and rail/angle section we decided that we would just try the tape on its own as this way we would not have bolts not drilled and tapped perfectly square twisting the rack. As you have suggested previously Gerald this is actually quite adequate on its own with out the bolts. In fact we have actually just used a good quality automotive tape that we use for quite a few things and to be honest unless you cut it off it is not moving on its own. We did make sure we prepped and cleaned both the rack and bottom of the angle section very thoroughly with both acetone and IPA before applying it then clamped down its length to give a decent amount of pressure to the join.

Sean (or anybody else) - Any chance you can explain the difference between the setting you have noted? They are not ones we have used or altered previously. And I really have feeling that the distance traveled / steps per mm issues we are having could be a software or calibration issue.

[Red_boards]

Forgive me stating what you may have already done, but have you tried the automatic steps calculation from Mach 3? Put a pen in the spindle and move the gantry 2m a few times, measure the actual distance moved and let Mach calculate steps (or you could fit your fancy meter at the end of the 2m stroke to get an even more accurate measure). I did a few rectangles 2mx1.5m for precision (repeatability) purpose.

Remember, you may be able to calculate to 3 or 5 decimal places, but the decimals are only accurate to the number of places you did your measures to. Wikipedia significant figures entry quote:

For multiplication and division, the result should have as many significant figures as the measured number with the smallest number of significant figures.

For addition and subtraction, the result should have as many decimal places as the measured number with the smallest number of decimal places (for example, 100.0 + 1.111 = 101.1).

When performing a calculation, do not follow these guidelines for intermediate results; keep as many digits as is practical until the end of calculation to avoid rounding errors

Ithink this means that, if you can measure 2000.0 (150.0)mm, then you can have confidence in xx.xxx (xx.xx) steps.

Only tape holding my racks on for a year now. No signs of weakness, even though the motors only run over the racks once a week.

[SOSx]

Hi Red - Thanks for replying but we have been using the calibration setting and taking measurements over 980mm with a good quality set of engineering vernier calipers.
I am hoping that having the racks straitened/squared properly will help with this incremental error but haven't had a chance to really check it properly as we are working on the z slide at the moment.

Gerald - The holes for the V rollers (W3) are drilled at a spacing of 138mm and the Z slide measures roughly 99.3mm across from point to point. There are captive bolts welded on the back of the slide. On the Drawings it says the spacing should be approximately 132mm depending on v rollers and z slide.

Sean - we had a look at the Mach 3 manual regarding those settings and they appear to only to refer to the post processing reference/origin points. Ie one references the origin for all cordinates from 0,0 the other takes the origin as the previous point. Our post processing is set up for all points taken from origin.

[SOSx]

Gerald dont worry just found your post regarding this subject http://mechmate.com/forums/showthread.php?t=878