Struggling with lathe tool auto touch off with tt not at centerline

[suntravel]

I think Parameter 279 and 280 are not used, so they could be used for the position of the center from probe stylus.

Even with a normal disc tip commonly used for a lathe the shape is actually a rectangle like on the 4 point or square Renishaw tool setting arms.

https://www.renishaw.com/en/high-precis ... arms--6098

Uwe

[Muzzer]

I've had another fiddle about with the probing functions this evening. I haven't got a full understanding of what's happening yet but I'm getting there.

When touching off in the Z+ direction, the measured offset is consistently out by the width of the probe tip. So for instance, when my tool offset is reported as being -38.30mm, I need to add 9.525mm to it (it's a 3/8" square tip), resulting in -28.80mm. That value makes sense because that example was the difference between a RH (towards the headstock) external tool and a LH (away from the headstock) external tool. These have a 20mm square body with a ~4.5mm offset to the tip. The touched off faces on these 2 tools are around 29mm apart. So yes, the offset needs to have the probe tip width removed from the offset.

As for tools that need to be touched off in the X+ direction, I seem to get a strange offset value that I can't figure out. I end up needing to add 24.7mm (in diameter units) to the offset in order to get the correct DRO readings. So an auto touched off offset of 21.563mm has to be corrected to -3.137mm. There's no obvious combination of dimensions I can see that would result in this value.

I also played with the probe dimensions that are stored in parameters #281 (probe tip diameter) and #282 (probe tip length in Z). Changing these has no effect on either of the Z+ or X+ auto touch off moves. It's probably not surprising that the Z+ and X+ offsets are wrong when they aren't involved in the calculation. As Marc pointed out, they should do so.

I could play a bit more with things like the "X dia" (X ref?) setting to see if that plays a part in the results but it seems unlikely. I'd like to understand where the 24.7mm value comes from - is it somehow hard wired in or is it pulled in from somewhere / can I change it somehow? Certainly, it's clear that the Z+ and X+ touch off calcs need some attention and I can't see how they could have been tested previously.

[cncsnw]

The X Reference (ambiguously labeled "X Diam" on the screen) should be the diameter distance (2x distance from spindle centerline) to the X+ face of your tool probe.

You could determine the value by turning an OD with your chosen Reference Tool; setting the X axis position under Part Setup to the measured diameter, with Tool 0, in all WCS; then backing off and observing the X position on the DRO when that same tool just trips the probe.

[Muzzer]

Well I've spent some more time investigating what's going on. I can't make any sense of the numbers that are generated when touching off away from the spindle axis (X+). Furthermore, the offset error seem to vary with the radial position of the probe from the axis but even then, the resulting error has me beat.

As for touching off towards the tailstock (Z+), the axial error is at least predictable, being basically the width of the probe tip. The tip radius has to be accounted for, depending on the nose angle. I've figured out a way to make it work now.

So in essence, for touching off internal tools (ie away from the spindle axis in X+ direction) I will have to rely on good old fashioned manual methods until / unless the issues with the calculations are rectified. The Z+ moves will require manual correction.

BTW, positioning the TT on the centreline doesn't help. In that respect, the last 5 words in the thread title are perhaps misleading. The issue is the direction of the touch off ie in X+ and Z+

[slodat]

Any update on this? I’ve updated to v5 rc4 and I’d like to make use of the investment in the TT-4 on my lathe.

Thanks in advance!