Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: mgandy02 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.

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

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.

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

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.

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.

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.

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.

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.

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.

simpson36,
I never tried putting a some long value into the steps per unit, but you may want to try and then use the xmll reader to see what value is stored in the xml and if it takes it.
What does the manual say?
RICH

simpson36,
I never tried putting a some long value into the steps per unit, but you may want to try and then use the xmll reader to see what value is stored in the xml and if it takes it.
What does the manual say?
RICH
It appears Mach saves 6 only decimal places in the XML.
Regards,
Ray L.

I'm about to pull my hair out. I reinstalled mach and did my driver test. Everything shows good. The auto function that sets the steps per inch states my new value should be 1818.896532. Ok. So in my motor tuning on the X axis the steps are set to 1818.896532, max speed is 120, Step uS is 0 and dir is 0. I open the jog screen and set the cycle jog step to 1.0. Next I zero the caliper mounted to my X axis. I make 4 moves in the + dir on the X axis and come out with the following: 0.995, 2.014, 3.034,and 4.055. I zeroed it again and made 4 more moves in the  dir and had these values: 0.995, 2.012, 3.031, and 4.047. I have checked everything I know. Could this be a drive problem (203V) or a motor problem (KL34H2120428A). My pc has 2.5 GHz pentiium and 2 GB of memory. The only software that has been installed on it is Windows XP and all of the updates and Mach3.

no it is not your drivers. post picks of your X and Y gearing setup. post info about all the gears, belts and the rack. include the part numbers and manufacture info. dont repost your earlier parts post. look on the belt and post the part number. teeth per ft on the rack, pinion size and count etc,etc,etc,. we will help you get it figured out. a number like 1818.896532 is so bad .
bill

Ray  I didn't mean to argue  the way I saw it, the 0.2 is the pitch per tooth  thats how these read in my catalogues. A 40 tooth gear revolving once would give 8 units of movement.
Perhaps mgandy02 can photo the set up and show us. Tell us what the movement is on this rack. i.e. how far does the table travel for one revolution on movement of the final drive.

I hooked up my o'scope to the step and common on the 203V and ran it. Should I be getting 25KHz. I am only getting 3.3KHz and 5.5V at 120 ipm which is the max on the motor tuning. I will post pics of my setup later today. Thanks for the help so far.

I sent you a personal message.
Brett

Thanks for the help Brett.
After we did a couple of moves and set my steps per to a good round number I was able to fine tune it without the auto function. After getting it +/ 0.001 I did as he suggested and got my backlash comp. set. It is running pretty good now and accurate enough for me and wood work. It won't have to go on ebay after all.
Again thanks for the help.

Hi Brett.. What Magic you send on the PM.. Kindly share with us!!!