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calibration problems need help please!
« on: March 23, 2006, 09:20:43 PM »
hi everyone
i'm having very good luck using mach 3 however i'm have a major issue with calibration
I am using a 5' X 12' router table.  It is 3 axis and uses 3 geoko drives with 1.8 stepper motors

My problem is... when i use settings -> calibrate, I enter 2 units (2"), take the measurement and enter that.

Then the machine is perfectly accurate at 2" however is more than 1/2" off at 120"

So, then i enter calibrate 120" take the measurement and enter that.

Now the machine is perfectly accurate at 120" however is more than 1/4" off at 2"

Therefore,... I cannot use this program  (I think i might not have all settings properly adusted)

I need the machine to be accurate to 1/64... is this possible?

Thanks in advance
So you See, I just can't win!

Offline Graham Waterworth

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Re: calibration problems need help please!
« Reply #1 on: March 24, 2006, 02:35:04 AM »
how many decimal places are you measuring to?

The longer the axis movement the more accurate your measurement needs to be to allow for rounding errors.

What is the pitch of your leadscrew/ballscrew?

How many steps per rev of the motor.

What is the ratio of the motor to the screw?

With this you can work out what the steps per 1" should be.

With well adjusted equipment and good screws you could get the accuracy down to .001"

Graham.

Never give up, time will pass, the cloud will clear, the sun will shine once more.
Re: calibration problems need help please!
« Reply #2 on: March 24, 2006, 08:39:02 AM »
i'm measureing to 1/64
not sure of pitch because the machine uses serveral pulleys then a row of teath down the side and a gear on the pulley
this is why i'm trying to use settings->calibrate
200 steps per rev (gecko drive) 1.8 motors

I can get the machine to run 120" perfectly every time but is inaccurate at 2"!

this problem is happening on x and y axis

Offline Graham Waterworth

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Re: calibration problems need help please!
« Reply #3 on: March 24, 2006, 10:38:09 AM »
are the pulleys toothed type or v belts?

if you count the number of teeth on each gear and on each pulley we can work out the ratios and set mach3.

if the pulleys are v belt type we need to know the exact diameter.  a rule measurement is not good enough.

Graham.
Never give up, time will pass, the cloud will clear, the sun will shine once more.

Offline Graham Waterworth

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Re: calibration problems need help please!
« Reply #4 on: March 24, 2006, 10:55:38 AM »
To use the calibrate method you need to have a block or tool shank of a known diameter, e.g. 1.000" dia end mill shank.

You can then use this as a setting block.

Clamp a block (any block NOT the known size one) to the machine axis, it must be set square.

Gently push the table up to it.

Zero out.

Move the table so that you can insert the known size block and push the table up to it so it is clamped by the table and the other block.

Don't move the table so hard it moves the clamped block.

Then calibrate.

This method should be better than a ruler.

Graham.
Never give up, time will pass, the cloud will clear, the sun will shine once more.
Re: calibration problems need help please!
« Reply #5 on: March 25, 2006, 09:29:57 AM »
thanks graham
i will try that this weekend and get back to you with the results
Re: calibration problems need help please!
« Reply #6 on: April 04, 2006, 01:39:39 AM »
Hey Graham,

I am having a similar problem and Since Yeager didn't repost his results I was wondering if you (or anyone else) might have any additional ideas about what might cause this scenario?

Here's what I have going...I am making my table larger and have decided to use a belt drive.  Specifically 'L' type synchro timing belt (reinforced with steel strands).  here's a link http://www.mcmaster.com/nav/enter.asp?pagenum=978

Single 18 tooth pulley at one end of table- other end of table has another 18 tooth and a 20 tooth.  Turning this shaft so the 2) 18tooth pulleys make a full rotation results in the axis moving 7.30 inches.

The 20 Tooth pulley is connected to the motor (with another small 14" belt).  The Motor has a 10 Tooth pulley. See attached cartoon for a (hopefully) better explanation.

Motor has 400 steps per rev.

I calculate 109.5 steps per unit.  So when I try G1 X1.00 it's all good (i.e. the axis does measure out to be 1 inch with a micrometer). 
If I G1 X5.00 and put a tape measure on it, it is at 5.25.
If I G1 X30.00 and put a tape measure on it, it is at 33.00.

It seems like I need to tweak my Steps per.  I try doing that and (like Yeager) get it accurate @ 30" put then the 1" cal is not correct.

Any ideas what gives?  The axis SHOULD travel in a linear (measurement speaking) way right.  What would cause this phenomenon?  Are my Belts cursed?  Do you think this could be a bad cable connection?  Any help is GREATLY appreciated!

Thanks,
Sid

Offline Graham Waterworth

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Re: calibration problems need help please!
« Reply #7 on: April 04, 2006, 04:50:16 AM »
How accurate is your 7.30" measurement?

Graham.
Never give up, time will pass, the cloud will clear, the sun will shine once more.
Re: calibration problems need help please!
« Reply #8 on: April 04, 2006, 05:37:13 AM »
Hello

Just noticed this thread - I am very wary of the calibration tool. I have a DRO on my machine so used this to do various tests. I used the calibration tool to try and set the correct values over various distances - from 10mm upto my full table travle (only 200mm). After calibrating I repeated the same tests (all this was done with backlash turned off so I compensated manually). Every time I used the calibration wizard I found the values were slightly out - e.g. a move of 50mm gave a physical move of 49.97mm. Every time I used the calibration tool it was slightly out. I then went to motor tuning and decided to 'hand tune' the values. These had been calculated to about 6 or 7 decimal places. I found that if you reduce the value down to about 3 decimal places, and play with the last two decimals then you can get really good results. (I use about 500 steps per mm)

This may be a rounding problem in the software - perhaps Art can shed some more light.

Cheers

Dave
Re: calibration problems need help please!
« Reply #9 on: April 04, 2006, 06:36:59 AM »
The math could be setup to round the values but in the end the wizard worked for setting up the axis :) The Calibrate tool is one that I wrote in VB so if you would like to have it round I can make that happen.
Fixing problems one post at a time ;)

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