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Author Topic: calibration problems need help please!  (Read 13155 times)

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Re: calibration problems need help please!
« Reply #10 on: April 04, 2006, 06:48:46 AM »
Hi Brian

Sorry I think I confused things a bit. The basic problem I have found is this :-
I eliminate backlash (g0 x-10)
Move to start position (G0 x0) set DRO to zero
Run calibrate axis, enter 50mm, enter value, e.g. 48.97mm
Remove backlash (G0 x-10)
move to start (G0 X0) ser DRO to zero
then move to x50 (g0 x50)
Read Dro Value - this value (even after about 6 calibrations) does not equal 50mm
If I manually tune the motor values I can get the table to move exactly 50mm as read on the DRO.

Has anyone else experienced this. Brian, I don't mean to have a go but the calibration doesn't work on my system when compared to the DRO. The values are always fractionally out. However, it is possible to achieve the exact values through tweaking. I wondered whether some where there was a rounding being performed too early in the routine.


Re: calibration problems need help please!
« Reply #11 on: April 04, 2006, 07:01:59 AM »
There is no rounding but I could put in a few moves to take out lash... It could be a checkbox that tells you it is going to remove the lash. I am thinking about changing how that works as well.. I think it would be better to have the user press a key to zero the axis, move the axis a known distance and the computer would keep track of how far it thinks it should have moved. This would make it so you wouldn't have to deal with the axis moving without you :(

That is just an idea and I have very little time to write the code but if it will helppeople I will make the time.
Thank you
Fixing problems one post at a time ;)

Re: calibration problems need help please!
« Reply #12 on: April 04, 2006, 11:27:55 AM »
Uh, this may sound stupid but, what is the "calibrate tool".  I thought that was my tape measure and my micrometer.  Is there some kind of wizard?  Where?

Graham, thanks for the reply- I'll double check my 7.3 but I think it's pretty darn close.  FYI- by playing around with the steps per unit manually I have observed the following

G1 X1.000 - The X-axis actually mics out at around .978 - using the same Steps per setting I'll type
G1 X5.000 - The axis axis will  move to 5.25 .

DAMN this is weird?  It makes no sense to me that it would be under @ 1.0 and over @ 5.0- WTH?? 

I start out being too low on Steps per unit and somewhere in the 5" of Travel I "catch up" and overtake and end up having "gained steps".  If I was just gaining or just losing I would understand but I am doing both in the same moevement.  How?, Why? Help!

Thanks, Sid   ???
Re: calibration problems need help please!
« Reply #13 on: April 04, 2006, 11:40:35 AM »

If you go to the settings page there is a button bottom left is marked "Axis Callibration" - You tell the machine how far to move, it moves, then it asks you to tell it how far you measured that it moved. It then calculates the steps per unit required.

Are you aware of backlash? You need to make sure that this is taken out before you perform you're measurement. I.e. before you do you're measurment move, you should move in the same direction as you plan to measure. If you need any help just give me a shout.


Re: calibration problems need help please!
« Reply #14 on: April 04, 2006, 11:56:07 AM »

AWESOME info- Thanks!  How do I take out the backlash before I calibrate.  Well, I know where to enter the settings but if the machine is not calibrated how do I know how much backlash to compensate for?

Thanks in advance for you assistance!
Re: calibration problems need help please!
« Reply #15 on: April 04, 2006, 12:06:03 PM »

You need to move in the same direction, an amount greater than the backlash before you start your actual measurment move. Lets try an example as it is probably easier to see. Lets look at the X axis which has say 0.5mm of backlash.

Initial postion is X27 - random number I came up with !
Now move in the -Ve direction i.e. G0 x-10
We now need a +ve move to remove the backlash i.e. G0 x0
The backlash is now removed
Now start the measurment move e.g. G0 x50 (+ve move)

Make a note of the measurment - you now need to go back to G0 x-10
then G0 X0 (remove backlash again)
Now change the step settings (you don't need to remove the backlash as you already have!)
repeat over and over until your happy.

The important thing is that where you change direction you need to do two moves, first to remove backlash, second the actual measurment.

Any more probs just shout


Re: calibration problems need help please!
« Reply #16 on: April 04, 2006, 12:54:27 PM »
 ;D  Dave Dude!

You are awesome!  That is an excellent explanation.  I'm gonna try that right now and I'll let you know how it turns out!


Re: calibration problems need help please!
« Reply #17 on: April 04, 2006, 05:00:17 PM »
OK, here's the poop...

Brian's calibration hoo ha on the settings page seems to work pretty damn good for me (Thank you Brian!).  Now moving in the +ve (x +) direction everything is spot on.  Things get funny after a move in the -ve (x-).  Instead of moving to -10.00, it moves to -10.02 (i.e. it didn't move enough.  Yet, when I have it goto 0.00 it is spot on.

I think that I have a backlash issue?  I am stuck about how to fix because I don't think I understand enough about backlash and how to compensate for it. 

Thanks for the patience and enlightenment!

Re: calibration problems need help please!
« Reply #18 on: April 04, 2006, 05:19:43 PM »
No probs sid

Are you making sure that you take out the backlash before the negative move? In this case you would need to go to a positive value then move to zero to remove the backlash and then move to your final negative value.

To use backlash compensation what you need to do is this :-
First remove backlash - so G0 x-10, G0 x0
Set your indicator to zero.
Now move to a new position e.g. G0 X50
And move back to the original position i.e. G0 X0
Read the value off the dial indicator - This is your backlash value.
Go into the Config, backlash menu and enter the value you just found for the relevant axis in the NATIVE units of the machine.
Click the enable backlash button - then you must restart MACH before the compensation starts to work.

In theory if the steps per unit is correct and the backlash you should be able to move back in forth in any combination of positive and negative moves and always return to the same position. Of course this is only theory and things like the leadscrew pitch not being perfectly consistent can lead to small errors.

Let us know how you get on.


Re: calibration problems need help please!
« Reply #19 on: April 04, 2006, 05:43:13 PM »

Just realised that I should realy explain backlash - apologises if you know this already.

A screw and a nut can never be perfectly matched, i.e the male portion can not be a 'perfect' fit in the female portion. If they were you would have a press fit and you wouldn't be able to turn the screw. As a result there must be some clearance between the nut and the screw. As a result, when you turn the screw in one direction it pushes the nut along, but when you reverse the direction the screw has to turn a certain amount before it actually takes up the slack. The amount that the screw has to turn before the nut starts to move is the backlash.

Obviously this rotation would be very difficult to measure. So the easy way to do it is to measure how far Mach thinks it's moving the table, and compare this to the actual movement WHEN CHANGING direction. This will give the backlash which Mach converts into the number of steps to give the desired rotation to remove the backlash. When you get backlash setup, whenever you change direction, Mach will add an extra motor move to take up the slack, before the real move starts.