How do people "home" or "touch off" Z axis consistently?

Hi all,

I'm just experimenting (and learning!) with my CNC machine. Eventually I'll have working homing switches on X and Y but I'm not planning on having a switch on Z.

Currently I'm driving the 'bit' to the bottom left of the material and manually clicking "home" in linuxCNC for each axis and also making sure that the G54 values are all '0'.

Then I click the 'run' button and off it goes. This is working fine (in the main) for experimenting but I'm finding that it's difficult to get a consistent Z position? I've fitted a 'dust shoe' now so can't really see what it's doing and I found last night (after it had been running for an hour) that it was just cutting fresh air!

I wondered if there's a recognised way of getting a consistent Z position? Do people lower the Z on to a bit of wood of a known height? Or do people take the Z to a set switch and then somehow 'offset' it from that position minus the depth of their wood or... ???

I'm probably approaching this all wrong but if anyone could explain this to me (preferably - explain it to me like I'm 5) then it would be very much appreciated.

Cheers

uptown47 wrote: I wondered if there's a recognised way of getting a consistent Z position? Do people lower the Z on to a bit of wood of a known height? Or do people take the Z to a set switch and then somehow 'offset' it from that position minus the depth of their wood or... ???

Yes and yes, on both counts.
With no home switches, most tend to use a piece of paper and slowly lower Z till it touches the paper so it does not move freely any more and home Z there. The paper is generally regarded as 0.1mm thick so that would be the best you can get for repeatability, but i would not bet on it. 0.2 or 0.3 is a margin of error.

Thanks Tommy. I'll give the paper trick a go! I never even thought of that!!

I'm going to eventually fit X and Y homing but I'm struggling to see how I'll ever get Z homing fitted. I'm sure it's possible but will need some serious thought.

For now a bit of paper will be more than adequate for my needs I think.

Thanks again

John

I built a touch plate that has a button on it which starts the probing process.

I just set it on top of the stock, jog the cutter to within 1/4 inch of the plate and press the button. As soon as the bit touches the plate it records the Z height minus the thickness of the touch-plate.

JohnnyCNC wrote: I built a touch plate that has a button on it which starts the probing process.

I just set it on top of the stock, jog the cutter to within 1/4 inch of the plate and press the button. As soon as the bit touches the plate it records the Z height minus the thickness of the touch-plate.

Hi Johnny

Thanks for the info. I'm intrigued by this. Can you give me more details on how it works? How does the 'bit' know it's made contact with the plate? And also how does it automatically enter the Z height into the software?

Thanks

I don't think that "home" is ever the right way to set the coordinate system...

Much better to push the machine all the way to the end-stops when powered down, then home, then do all your other touch-off with touch-off.

Better still to fit limit switches.

"Home" is to tell the system where the axes are in their travel limits.
"Touch off" is how you set the part origin and tool lengths.