Accurate PTL with Locate TCP Special Function?

[skymoco]

Hi Dave.
I recently re-surfaced my table and remounted my TCP TP. This got me thinking and I wanted to make sure I have the concept correct. If I am using Master Tool Mode and non-repeatable holders, do I really need to accurately measure the PTL before running the “Locate TCP Special Function” . My thinking is that for this configuration, it’s not that critical as the PTL gets re-measured with each tool change and it is relative to the Master Tool.

My logic is along the following lines. When I first start a new job, I mount the master tool with the MIDI. This re-measures the PTL relative to the TCP TP position. I then use a mobile touch plate with the Master Tool mounted to measure the Z height of the part using the probe screen. I then mount the appropriate non-repeatable tool and start machining.
This has proved to be very accurate, so I'm thinking my logic is correct. If I wasn't using master tool mode or using repeatable holders, I would need to more accurately set the PTL before using the "Locate Special Function".

[DaveCVI]

Hi,
Well, sort of and sort of not....

What you are describing is working for you, but not quite for the reason that you have stated... I'll try to explain.

May 13, 2012 16:16:12 GMT -8 skymoco said:

Hi Dave.
I recently re-surfaced my table and remounted my TCP TP. This got me thinking and I wanted to make sure I have the concept correct. If I am using Master Tool Mode and non-repeatable holders, do I really need to accurately measure the PTL before running the “Locate TCP Special Function” .

It IS critical that the system know the correct, physical location of the top of the TCP TP in MCz. If that location is wrong, then every measurement of PTL that is made with the TCP TP will also be wrong.
So, please accurately measure the PTL of the tool that is used to calibrate the the location of the TCP TP via the “Locate TCP Special Function” .

But it sounds as if you did not do this and things are working for you - so you have to be wondering why.... IF (and only if) you are using Master tool mode, and ONLY master tool mode, you can get away with the TCP TP calibration being incorrect (I'd rather not tell you or anyone else this, but since you asked.... )

How can that be? Because when you measure a tool with the (uncalibrated) TCP TP, you are getting incorrect PTL values for every tool that you measure - the PTL values will be off by the amount that the TCP TP location is off - and that error is a constant amount.

But in MTM, each tool's TLO value (which is NOT THE PTL value) is defined as the difference between the MT PTL and the tool PTL.
Equation wise this would be:

Tool TLO = MT PTL - Tool's PTL
But since BOTH the MT PTL and the Tools' PTL as wrong BY THE SAME AMOUNT, the tool's TLO will end up being correct anyhow.
IF we do the algebra we see that:
Tool TLO = MT PTL - Tool's PTL
Tool TLO = (MT PTL+PTL error) - (Tool's PTL+PTL error)
Tool TLO = (MT PTL) - (Tool's PTL) +(PTL error -PTL error)
Tool TLO = (MT PTL) - (Tool's PTL) +(0)
and we are back to
Tool TLO = (MT PTL) - (Tool's PTL)

Thus, you will be OK, ONLY AS LONG AS YOU ONLY USE MTM!

The key is to remember that while the tool change operation measures PTL, it is not PTL that mach uses to position a tool. It is the TLO value that mach uses to position a tool. as long as the TLO value is correct, you are ok.

In MTM, the process you described yields correct TLO values, but incorrect PTL values (so the PTL values in the tool table will be physically wrong).

In TLO=PTL mode, you'll be in trouble as the TLO value will = the PTL value and then the PTL error will show up as the tool tip being at an incorrect height.

It is much to easy to forget later on that the TCP TP is uncalibrated and then go to use a non-MTM mode..... if you switch MSM to using PTL as the TLO value, you will be in trouble.

Dave

[skymoco]

That's what I had kinda/sorta figured out, only you explained it so much better. I really appreciate you taking the time to answer. It's one of the major reasons I bought MSM in the first place and will continue with my support agreement.