Z offset for probe

[Michael M]

I just set up the probe on my mill and I've done a couple of bosses/bores and things seem to work well for X and Y coordinates. I was reading in the manual on probing and notice that on the "single surface" probing you can probe in Z. The only thing is I can't figure out how that does any good.

I have a 325 Tree Journeyman with CAT40 tools and I have a dedicated Z Ref tool for use with the TT1. That works very well, but you sure can't have the TT1 and the probe hooked up at the same time. Since the TT1 seems to depend on the tools being electrically conductive it doesn't seem likely that I can get the probe tip to pass the current to complete the circuit, so I wouldn't be able to set the probe length offset to the Z Ref tool using the TT1.

The manual doesn't address this. What I'm thinking is to load the Z Ref tool and touch off the table or top of the vise, record the Z dimension, and then probe that same spot and see what the Z dimension is with the probe. I think that should give me a Z Ref offset just as with manually measuring tools that I can put in the tool library.

What I'm hoping is that if I then set part Z zero with the probe the control will take that and apply it to all the other tools so that I don't have to pick a "real" tool and set the Z zero with it. If I've got the probe in there and I'm using it to set X zero and Y zero, changing tools to set Z zero seems like a waste of time since it means I'm back to the "feeler gauge between the tool and part" method instead of using the probe that reads in tenths.

Is that likely to work?

cheers,
Michael

[MarcL]

You can indeed use your probe to set Z zero. As you note, that is the simple and sensible way to do it when you are already using the probe for X and Y.

You do need to measure the probe's height offset, compared with the same reference tool that all other height offsets are measured.

You can do this on the TT1. You unplug the TT1 and plug in the probe, then use the normal F3/Auto Measure procedure in the Offset Library. You are right that the TT1 cannot detect the probe, as it has a ruby ball. However, the probe will trigger itself when it touches the TT1, and the control doesn't know the difference. So you plug in the probe instead of the TT1.

Just be sure to plug the TT1 back in before you go to measure any other tools!

If you want to avoid the cord swapping and the potential costly mistakes that go with it, I sell a splitter box and extension which gives you two probe sockets within easy reach in front of the machine. Leave the TT1 plugged into one socket all the time. Plug and unplug the probe in the second socket as needed. See www.cncsnw.com/products.htm.

[Michael M]

Marc, is there any issue with the probe and the TT1 plungers both moving at the same time throwing off the probe measurement, or is the spring in the TT1 stiff enough that it doesn't move a significant amount by the time the probe compresses enough to trigger?

I'd thought about a splitter box - I found that you can't just leave the probe plugged in and sitting beside the machine without getting a "remove the probe from the spindle" message popping up.

I'd hoped it might be possible to just wire a couple of receptacles in parallel and leave the TT1 and probe both plugged in all the time. I'm always concerned that those little pins are going to get bent as I fiddle trying to get the plug aligned. Centroid should consider doing a cable that works in either device so you can leave the cable plugged into the control all the time, and just change it at the TT1 or probe instead (if that is possible).

I'll consider your splitter at some time when I'm flush with cash, but this has been a year of a LOT of unexpected tool expense (like a new Miller Syncrowave 250 last month when my old Miller Gold Star died in the middle of a project) so swapping the TT1 and probe is something I can live with for now.

cheers,
Michael

[MarcL]

The spring force in the TT1 is a lot stronger than in the probe, so I expect the TT1 would not move at all.

It is possible that you would see a very slight error, because:
1) when you measure a normal tool on the TT1 it overtravels by a tiny amount between the time that the tool tip touches the TT1, and the time that the PLC input message gets to the controller and the DSP stops the motion; and
2) when you measure the probe against the TT1, the TT1 probably does not compress at all. The probe compresses a certain amount before tripping, but that would be the same amount as when you later use the probe to set the Z axis part zero. I.e. the effective length of the probe in both contexts is just a little shorter than its actual length.

Because of the different trip methods, the normal tools will appear a little shorter than they really are.

Assuming that you have your slow probing rate (Parameter 15) at just 1 or 2 in/min, I expect the discrepancy will be under 0.001". If you find it is significant enough to measure, you should be able to compensate by subtracting that amount from the measured height offset value for the probe.

[Michael M]

Thanks, I'll give that a try and see what I can find out. If the TT1 is a lot stiffer than the probe it should deflect very little, though it will deflect some. But I probably shouldn't be too concerned about a tenth here or there.

cheers,
Michael