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Author Topic: Is this backlash or missed steps????  (Read 7167 times)

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Is this backlash or missed steps????
« on: June 08, 2009, 07:51:19 AM »
I have had my router going for about a week now and have cut a couple of simple shapes with it. This morning I tried to cut out something a little more complex. Not a very good idea. I will describe what is happening first. Doing a simple move to check my steps works perfect, If i tell it to go 1" it goes 1.00". Great. Next cut out a square or circle again perfect ( or good enough for wood ). Then cut out something that is going to take a while, Not great. I wrote the macro that it describes in the config guide that follows:
F400
G20 G90
M98 P1234 L10
M30
O1234
G1 X20
G1 X0
M99

After the 10 moves the X axis will come up 0.011 short from zero, So I thought to double it. After 20 moves of 20" it comes up 0.022 short of zero and it doubles again after 40 moves. I have a moving gantry type (4'X8') with the 1200oz motors and the 72vDC supply from keling driven by 203Vs on both the x and Y axis. THe kernal speed is 35000. I have also tried the backlash function but that doesn't seem to work either. The same is happening with the Y axis also. The only difference is that is starts out as being 0.015 over with 10 moves of 20" and doubles with 20 moves then doubles again with 40. I have R&P on the X and Y axis with a set of timing pulleys to gear down. At first I was running @ 400 ipm then went down to 50, the same result in both cases.
I someone could help of have any ideas I would appreciate it.

Offline RICH

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Re: Is this backlash or missed steps????
« Reply #1 on: June 08, 2009, 09:17:52 AM »
What happens if you tell the axis to move 10", 20", 40" based on your steps per unit compared to a standard for
those dimensions? So if a small error in steps per unit at 1" existed then it would be maginified 10x at 10".
If the steps per unit is correct then it would appear to be backlash. Based on your trials that would be
0.011 / 10= 0.0011" provided you took the backlash out before you did the G1 x20 move.
RICH

Offline jimpinder

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Re: Is this backlash or missed steps????
« Reply #2 on: June 09, 2009, 03:29:15 AM »
Backlash does not repeat - by that I mean that doing it 10 times or 100 times makes no difference, the backlash is always the same.

It is the amount the axis "dwells" at either end of a move, as it changes direction. If you set off from zero, and move 1 inch, that will be accurate (if you moved to zero in the same direction. If you now move back to zero, the axis will now come up short, becasue of the backlash the axis had, whilst reversing.

However, if you now move again to the 1 position, this will be accurate again (because the axis did not start from zero, and backlash could be taken up before the axis moved) - so backlash only counts once, at either end of the cut.

You cannot be loosing steps, becasue you say that one of your axis moves too far. This must then be steps per unit.
You must CALCULATE the number of steps per unit - not measure.
If your motors are steppers, they need 200 pulses per rev.If your drivers have microsteps, then your must multiply by that - e.g. Gecko is 10 - i,e, 2000 steps. Your gearing, through the timing pulleys, must be taken into account - I have 3 to 1 on my lathe - i.e. 6000, so you come to the final drive.

I do not understand what your final drive is, on an 8' x 4' gantry, but this is simple enough - how far does it move for one turn of the drive spindle from your last timing pulley. From that you must work out the number of pulses per inch - or per mm.

Be careful with your final drive, is it imperial or metric - a lot of stuff these days comes in either, and is wrongly marketed as being imperial when it is metric and vica versa.The conversion is 25.4mm to one inch - and the .4 makes all the difference

You say your carriage moves exactly one inch, when you move it one inch. Make sure you are taking any backlash out by - moving right - stop - set up measuring device - move right again - stop - measure. If this is exact, then move back one step and see how much short this is on the return. That is your backlash.
You can enter this in the backlash table under Config and turn on the backlash compensation.

The only other thing we have had on these pages is that some of the electronics (breakout boards particularly) which inverted signals, were sometimes prone to miss the first pulses, which means that things came up short, bu I wouldn't bother with that till you have checked everyting else.

Sorry I have gone on a bit, you probably know most of this anyway.






Not me driving the engine - I'm better looking.
Re: Is this backlash or missed steps????
« Reply #3 on: June 09, 2009, 05:35:20 PM »
I have rack and pinion on both the X and Y axis. I am using the 1200 oz motors from keling with the 72v 20A power supply. They X axis is geared 4.875:1 and the Y axis is geared 5.143:1 using timing belts. Boths have 40 tooth 2.0 pitch diameter pinion gears on racks. Can you tell me how to calculate this. I have just been using the set axis function in Mach. Thanks
Re: Is this backlash or missed steps????
« Reply #4 on: June 09, 2009, 06:51:52 PM »
I have rack and pinion on both the X and Y axis. I am using the 1200 oz motors from keling with the 72v 20A power supply. They X axis is geared 4.875:1 and the Y axis is geared 5.143:1 using timing belts. Boths have 40 tooth 2.0 pitch diameter pinion gears on racks. Can you tell me how to calculate this. I have just been using the set axis function in Mach. Thanks

*If* I'm understanding your setup correctly:

The motor is 200 steps/rev, 4.875:1 belt reduction on X, 5.143:1 belt reduction on Y.  Both will move 2" for one rev of the pinion.  So, the steps/rev is:

X Axis: 200 * 4.875 * 1/2 = 487.5 steps/inch
Y axis: 200 * 5.143 * 1/2 = 514.3 steps/inch

If your stepper drivers do microstepping, multiply these by the number of microsteps.  So, if using Geckos, which have 10 microsteps, your steps/inch would then be 4875 and 5143.

Regards,
Ray L.
Regards,
Ray L.

Offline jimpinder

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Re: Is this backlash or missed steps????
« Reply #5 on: June 10, 2009, 04:07:11 AM »
I am a bit at odds with Ray - this might be because we see your set up differently.

I see it as the motor drives the gearing. (why you didn't use simple numbers for this I don't know - I use 3 to 1 timing belts) the gearing drives a 40 tooth pinion which sits on a rack of 0.2 pitch. You do not say whether this is mm or inches, but it does not matter.

With this set up, every turn of the 40 tooth gear means 8 units of movement.

Both axis -  200 (motor steps per rev) x 10 (microsteps - 10 for gecko - alter this to whatever you are using) = 2000.
Pinion on rack - divide by 8 = 250 steps per unit.
Multiply this by 4.875 for X axis = 1218.75 steps per unit.
Multiply this by 5.143 for Y axis = 1285.75 steps per unit.

This needs amending for the microstep function - may be to 8 or 16 whichever your drives use but I think it is accurate.You will notice, of course, that the steps per unit are not whole numbers, and this is becasue of your odd gearing, It doesn't make much difference in the scheme of things, becasue Mach calculates to 12 places of decimal - but for working these things out a simple 5 to 1 would have been easier, or even a 4 to 1 to offset the pinion. Are these numbers anywhere near what you have been using.

Not me driving the engine - I'm better looking.
Re: Is this backlash or missed steps????
« Reply #6 on: June 10, 2009, 04:17:36 PM »
I am a bit at odds with Ray - this might be because we see your set up differently.

I see it as the motor drives the gearing. (why you didn't use simple numbers for this I don't know - I use 3 to 1 timing belts) the gearing drives a 40 tooth pinion which sits on a rack of 0.2 pitch. You do not say whether this is mm or inches, but it does not matter.

With this set up, every turn of the 40 tooth gear means 8 units of movement.

Both axis -  200 (motor steps per rev) x 10 (microsteps - 10 for gecko - alter this to whatever you are using) = 2000.
Pinion on rack - divide by 8 = 250 steps per unit.
Multiply this by 4.875 for X axis = 1218.75 steps per unit.
Multiply this by 5.143 for Y axis = 1285.75 steps per unit.

This needs amending for the microstep function - may be to 8 or 16 whichever your drives use but I think it is accurate.You will notice, of course, that the steps per unit are not whole numbers, and this is becasue of your odd gearing, It doesn't make much difference in the scheme of things, becasue Mach calculates to 12 places of decimal - but for working these things out a simple 5 to 1 would have been easier, or even a 4 to 1 to offset the pinion. Are these numbers anywhere near what you have been using.



Where do you get 0.2 pitch from?  Actually, I think it's even worse.... He describes the gearing as "40 tooth 2.0 pitch diameter pinion gears on racks".  If I'm interpreting that right, these are 40 tooth gears with a pitch diameter of 2".  The # of teeth is really irrelevent here.  What matters is that one rotation of the gear will move the rack 2" * PI or 6.283185307179586476925286766559", where my calculation assumed it would be 2".  So, I now get:

X Axis: 200 * 4.875 * 1/6.14 = 155.1760695145979523746616692882 steps/inch
Y axis: 200 * 5.143 * 1/6.14 = 163.70677446432354237187383900497 steps/inch

Not pretty numbers!

Regards,
Ray L.
Regards,
Ray L.
Re: Is this backlash or missed steps????
« Reply #7 on: June 10, 2009, 04:38:01 PM »
I am still a noobie so forgive my ignorance but why would those numbers not be pretty. If i multiply those by the microstepping of the geckos i would get 1551.76 for X and 1637.70 for Y.
Re: Is this backlash or missed steps????
« Reply #8 on: June 10, 2009, 04:51:50 PM »
I am still a noobie so forgive my ignorance but why would those numbers not be pretty. If i multiply those by the microstepping of the geckos i would get 1551.76 for X and 1637.70 for Y.

Yes, if you're willing to ignore all those other digits, which will lead to position errors on long moves (many steps).  On a small machine, or short moves, it won't matter.

Regards,
Ray L.
Regards,
Ray L.

Offline simpson36

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Re: Is this backlash or missed steps????
« Reply #9 on: June 11, 2009, 06:34:17 AM »
I get about 1/28" error over the 8 foot table using only 2 decimal places for the steps per inch vs 10

Mach can probably accept (and actually use) more than twp decimal places in the steps.

I'm in agreement with using simpler ratios, but when that's not possible, can't you just enter enough places until you get to the accuracy you need.