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  #1  
Old Mon 04 July 2011, 08:40
giggler
Just call me: giggler
 
na
United Kingdom
Zero plate wiring and tool height?

Hi,

I'm considering a zero plate for tool height calibration during manual tool changes. I've seen a few design after a google search but they use sheet aluminium as one terminal and the router/tool bit as another. Touching the two seems to trip I/O that the connected PC can sense.

My table is grounded to earth along with the router body and to connect it up in this configuration, I'd have to link system ground (0v dc) to the mains earth. In all of the consumer electronics I've looked at, it's not done and considered a bad idea by the wiring regs since the earth is used during fault conditions as a return path this could potentially result in fried control electronics or worse. Not just by a fault on my machine but by other connected appliances, i.e. washing machine, iron, dust extractor etc.

So doesn't anyone have any options? I figured perhaps an opto-coupler with ground but even then it means putting current down the earth connection which may trip my RCD.

Has anyone got any alternatives for tool height adjustments during manual tool change?

Thanks

dc
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  #2  
Old Mon 04 July 2011, 09:50
JamesJ
Just call me: Jim #104 (retired)
 
Kansas
United States of America
Is this what you are looking for?
http://mechmate.com/forums/showthrea...t=2398&page=13
look at reply # 375
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  #3  
Old Mon 04 July 2011, 10:09
nisma
Just call me: chris
 
bz
Italy
Normally even the cnc electronic is grounded, mass and ground are connected on one point for safety reasons. Back to you´r question.
On a plasma system, where a 2-4KV during the high voltage ignition is present,
the so called ohmic height sensing system uses a 9v battery and a 10$ solid
state relay (SSR) in order to have a galvanic isolation. You could use a optocoupler instead of the SSR or if the board have already the optocoupler just a
battery. The other system
is to have a mecanical height sensor similar to this.
http://translate.google.com/translat...en_sensor.html
for downloading the pdf, you must use the german page
http://www.einfach-cnc.de/werkzeuglangen_sensor.html

Last edited by nisma; Mon 04 July 2011 at 10:11..
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  #4  
Old Mon 04 July 2011, 10:25
giggler
Just call me: giggler
 
na
United Kingdom
both replys - thanks, they are both what i'm interested in. I'm not going for a mechanical switch but will look at the external power source with an opto, seems a good idea

just hope i remember to replace the battries!

Thanks

dc
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  #5  
Old Mon 04 July 2011, 14:05
bradm
Just call me: Brad #10
 
Somerville(MA)
United States of America
I think you're going to find that the ground is common, even when you don't think it is. The PC itself has it's logic ground tied through to the mains power ground. Opto-isolators are still a good idea, but I wouldn't get overly obsessive about trying to float the grounds relative to each other. Be obsessive about being sure the grounds are solid enough that they never / can't spike instead.

I use a Z-Zero plate with my G-540, which has internal opto isolation. It's as simple as wiring an input pin to a plate, and making sure that the bit is grounded (usually comes for free)
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