Monday, 15 June 2020

Tool length offsets and Zref - using TT and probe on the Centroid Acorn

Not again?
Yes, I've been here before and generally speaking when I've set up new tools and workpieces, I've avoided any major folks passes resulting from cocking the tool length offset process up. But often this has been at the cost of multiple paranoid checks and remeasurements (measure five times, cut once?). I've never 100% convinced myself I understand the process to the extent that I can just walk up to the machine and blast through the setup. With time, you'd hope I'd have it cracked but I never quite seem to get there....

I'm about to engage in some furious handle wanking on the knee crank as I work through a range of operations on the faces and edges of the various cover plates on the 4th axis build. Each time I change the table height, I will need to let CNC12 know where it is in the Z axis. This is something to do with the "Zref" value.

When you have both a TT-type tool setter AND a Renishaw-type probe:
  • How does it all work? 
  • How do you set up new tool lengths?
  • How (and when) do you acquire the work height (Zref)? 
  • If you change the table height, how do you tell CNC12 the new height?
  • If you need to measure a new tool length after raising or lowering the table, what do you need to do?
  • That's enough.
This is where I seem to get my wires crossed over....

The basics of Zref and tool length offsets:
Here, a young looking Keith (complete with era-defining pager) explains how to use a reference tool to set tool lengths - and introduces Centroid's horny new electronic tool touch sensor, the TT1. Phwoooaaar, look at that technology.


I get the basic principle fairly easily, as applied to your typical bed mill. Things get a bit more tricky when you move to the knee mill shown (about half way through the video). 

So my reference tool is actually my Renishaw probe, which means that when doing a Zref measurement, I'm actually using both touch tools. The Renishaw probe is in the spindle and is seeking the tool touch off TT thing. But who's the daddy?

Setting Zref:
It's fairly simple once you know - as you might expect. In Centroid's words, "the Z reference position...is the (MCS) value that appears on the DRO when the reference tool touches the top of the surface".

It's the touch probe that is your reference tool. In my case it's Tool #10 (the tool number is stored in parameter #12), which is the Renishaw probe that I also used for setting the workpiece coordinates. Quite simply, Zref is found by finding the height of the TT using the probe tool. The Renishaw probe triggers before the TT, so when these things come together, it's the Renishaw that triggers first, after negligible movement. 

If you change the table / knee height, CNC12 needs to know where the new TT height is located, so you run the Zref operation after each change. There's no need to overthink matters, as CNC12 requires the Renishaw probe for this to work. If you forget to plug in the probe, it tells you.

The "Zref" value is presumably stored in one of the system parameters and is displayed on the tool length screen. Lower the knee and the stored Zref value changes when you run the Zref operation.

Setting the tool length offsets:
So now that CNC12 knows where the top of the TT tool setter is, it can measure the relative lengths of any other tool compared to the length of the reference / Renishaw probe. I used to wonder if you could use any of the pre-measured tools as the reference tools but the answer to that one is no.

CNC12 knows the machine coordinate of the TT when the ref tool touches off on it, so it can now remeasure any subsequent tool without having to remeasure the entire library.

Setting workpiece coordinates:
Finally, you can acquire the workpiece height in the active WCS (G54 etc) in a couple of alternative ways:
  • Easiest - use the Renishaw probe to automatically touch off and zero the Z coordinate.
  • Alternatively (and for sanity / double checking), use MDI to activate the current tool (G43 TxHx, where x is the tool number), then jog the tool down to within 10mm of the surface and manually touch the tool off. Then save the Z coordinate as 10.000mm. This is a good way to check you have set your tools up correctly and I've caught a few cockups this way.

Any smart ass comments?
  • It doesn't matter what height you position the TT at. As long as you run the Zref operation in the TT's new position before automatically measuring tool length offsets, you could put them anywhere you want eg sitting on top of the vise or the workpiece, on the table or even on the knee. You can then move the TT to another place or move the height of the table without issue - as long as you rerun the Zref operation again before measuring the next tool.
  • Zref has no relevance to touching off on the workpiece - it's purely for tool length offset measurement.
  • The reference tool can be any length you fancy. Centroid recommend using a reference tool that is longer than any of the tools you plan to use. However, there's nothing to prevent using a short tool. It simply means the tool offsets will have a different sign in front of them.

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