How to calibrate unit - i.e. ensure that x0 and y0 are always the same

[omegasea21]

I have built a very large machine, that uses coordinates to locate certain objects. It is important that that machine is accurate when locating the coordinates.

If one of the axis skips a little, the coordinates are off.

I would like to create a simple macro to run every few hundred lines to reset the coordinates.

The machine does not have limit switches. Any ideas how do this?

I was thinking of purposely sending the unit past 0,0 something like -1, -1 (the machine will skip a little) and then reset the x,y DRO to 0,0.

Any better ideas?

Thank you!  

[Tweakie.CNC]

I was thinking of purposely sending the unit past 0,0 something like -1, -1 (the machine will skip a little) and then reset the x,y DRO to 0,0.

I don't think this is to be recommended unless the current to your steppers is small enough to ensure that no damage could be caused by hitting the buffers (so to speak).
Surely a better way would be to fit home switches / sensors and use the built in Mach routines for homing.

I suppose I have to ask the question - Why are your axis losing position in the first place ?.

Tweakie.

[omegasea21]

Hello, thanks for the feedback. The machine very large. 10x10 and occasionally the machine "skips", one of my biggest concerns with the machine has been repeatability.

Guess I will have to get some limit/home switches, what is the best way to go about do this? are there are other recommended hardware for positioning/homing?

Thanks

[omegasea21]

What about using this proximit sensor? http://cgi.ebay.com/CNC-Proximity-Limit-Detector-stepper-servo-motor-/330343504056?pt=LH_DefaultDomain_0

[Tweakie.CNC]

I don't use sensors on my CNC but I think that the fully enclosed, short distance type would be better ( similar to this http://cgi.ebay.co.uk/CONTRINEX-DW-AD-703-M8-3MM-INDUCTIVE-PROX-SENSOR-NEW-/260465431469?cmd=ViewItem&pt=UK_BOI_Industrial_Automation_Control_ET&hash=item3ca4f313ad )
There are many manufacturers of this type and you should be able to find a couple at a good price.

If it were me I would investigate your problem with skipped steps and resolve that issue as a priority. My machine (although small in comparison) regularly runs GCode programs in excess of 340,000 lines for 8 hours at a time with no lost steps or loss of position and I think this is true for all regular Mach users. The machine size is not really relevant as long as the axis are free without binding or tight spots etc.

Tweakie.

[ASC]

What kind of power supply, motor and drivers are you using?  If the machine always skips at a certain spot in the travel I'd suspect a mechanical issue.  If its truly random it is more likely noise or a power supply issue.  Make sure your motors are getting enough current!