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Author Topic: Spring loaded touch plates for Z auto-zero  (Read 7612 times)

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Spring loaded touch plates for Z auto-zero
« on: April 22, 2014, 05:38:45 PM »
There's lots of threads about auto-zero Z macros, but I'm wondering about touch plates used with them. I recently chipped a new V carving bit when the macro went a little wacky and brought the bit down too hard on the brass touch plate. I've tried several of the auto-zero macros and all at one time or another have gone  a little nuts...often the speed will be much slower than usual or the touch will occur w/o a retract. It doesn't happen often but it does happen.

I've seen a couple spring loaded touch plates and these seem like they could save a bit in such a circumstance. I can't figure how they don't introduce errors though. If there's a spring it must deflect at least a few thousandths before mach realizes the touch happened. I suppose this amounts to backlash within the touch plate.

Any way to compensate for this and get a true zero despite the spring?

Offline ger21

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Re: Spring loaded touch plates for Z auto-zero
« Reply #1 on: April 22, 2014, 06:59:24 PM »
First, you either have a problem with Mach3, or with the macros you're using. The speed should never change.
Often times, failure to retract and other similar issues can be caused by noise on the probe input pin. If you're macro has good messages spread throughout, you can often tell what went wrong by the messages displayed. Eben a simple "Finished" message at the end of the macro can be a big help. If you see the message, but the probing didn't work correctly, there's a very good chance that noised triggered the input.
Another source of trouble is if your macro doesn't account for being in incremental mode. A typical macro designed for use in Absolute mode can easily drive the tool e into the plate after probing.

There are hundreds of people using the macros in my 2010 screenset, and I've not heard a single report of an issue with the auto zero macros, in over 3 years.

While I don't believe that a spring loaded plate should be needed, the spring shouldn't affect the accuracy, unless you're probing too fast.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html

Offline stirling

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Re: Spring loaded touch plates for Z auto-zero
« Reply #2 on: April 23, 2014, 06:40:04 AM »
If a good macro is used correctly a spring loaded plate should not affect accuracy but will also be pointless. Perhaps you could post your macro(s) along with details of how fast you probed, your steps/per and accel settings of the Z axis.

Re: Spring loaded touch plates for Z auto-zero
« Reply #3 on: April 23, 2014, 07:34:30 AM »
This is the macro I'm using.

-------------
Message( "Auto Zeroing..." )
If IsSuchSignal (22) Then
code "G31 Z-3 F5"
While IsMoving()
Wend
Call SetDRO( 2, .75)
code "G1 Z1.5"
End If
-----------------
I had a soft brass plate (non-spring loaded). In use it would develop pits from the bit coming down onto it. I do V carving and the tips some of my bits are very fine and would sometimes chip from the pressure.

As mentioned there's a lot of macros out there. If there's a preferred one I'd appreciate someone pointing me to it.

Offline stirling

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Re: Spring loaded touch plates for Z auto-zero
« Reply #4 on: April 23, 2014, 07:43:14 AM »
OK - That is ONE of the MANY macros out there that is just plain wrong. However I need the other info I asked for also to explain why you're seeing the pitting.
Re: Spring loaded touch plates for Z auto-zero
« Reply #5 on: April 23, 2014, 07:51:12 AM »
Z axis  info:

Steps  - 1622.5 / inch
Velocity - 250 in/min
Acceleration - 8 in/sec

Speed in the macro was set as F5

Offline stirling

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Re: Spring loaded touch plates for Z auto-zero
« Reply #6 on: April 23, 2014, 10:03:29 AM »
OK first that particularly nasty macro!

1) Its accuracy is totally speed and accel dependant because it doesn't handle the actual trip point and instead ATTEMPTS to store the stop point.

2) Perhaps worse though is that because its attempt to store the stop point may well fail, the retraction will be pretty random/un-predictable and in worst case could seriously crunch your tool tip/plate.
Could be worse, you could have a Renishaw which could be destroyed by this little piece of macro genius. ;D

The probe speed and the accel you've posted will not show the errors behind 1) but faster probing or less accel certainly will. You will however more than likely see the effects of 2).

I'm left guessing then that the pitting you're seeing is probably down to not getting good electrical contact until you've pushed hard enough against the plate. If this IS the case then a sprung plate is not going to help for obvious reasons. Either way - I'd junk that macro if I were you.
Re: Spring loaded touch plates for Z auto-zero
« Reply #7 on: April 23, 2014, 10:11:26 AM »
Ok, understood, thanks.
Is there a macro that's preferred? I've tried a couple others with similar results so I obviously don't know a good  one from a poor one.

Offline stirling

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Re: Spring loaded touch plates for Z auto-zero
« Reply #9 on: April 23, 2014, 11:14:56 AM »
What -  all 44 pages of it Gerry?  ;D

Just a comment about the code presented on the first page.

The line:

Code "G4 P0.25" 'Pause for Dro to update.

would be better replaced by a while isMoving loop. The problem is I'm pretty sure that using a G4 for temporarily blocking the macro is version dependent and anyway, the wait is needed by the macro thread not by the Mach thread. A while isMoving() guarantees it's the CB that waits.

Ian