Mechanical Touch Off Tool Recommendation

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17 May 2012 19:20 - 17 May 2012 20:22 #20186 by crmorano
So, I am using a XYZ stage I retrofitted with EMC with a Z-axis that has no limit or home switch. For my application, I use it to move around various tools to precise locations, specifically heights (Z), rather than more standard milling, so I do not have a spindle or router. Now, the height between the bottom of the tool and the top of the stage is very important for what I'm using it for, and it changes on a semi-regular basis by a few mils, as does the stage itself. The tool I use is non-conductive and cannot have anything mounted to it, and moves up and down within its holder occasionally, depending on the current function, as does the distance to the stage itself.

Here is my question:
What is a good mechanical touch off/home switch tool that I can put between the stage and the tool to use as a home switch that will give me a fixed height within the system every time that does not require conducting through the tool? I need it at least precise to 0.001" or better. What I'm imagining is some small platform that rests on the stage between the tool and stage that has a fixed known height that it triggers/releases the mechanical switch, allowing for fairly rapid and repeatable homing.

Or is there a better way to do this?

I might have missed someone else's post on this, for which I apologize, I wasn't sure what to search for.

Edit: Besides the one I found for $1250.
Last edit: 17 May 2012 20:22 by crmorano.

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17 May 2012 20:05 #20187 by BigJohnT
Can you define "precise as possible"?

John

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17 May 2012 20:20 #20188 by crmorano
I think 0.001" or better would work.

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17 May 2012 22:28 #20189 by arch dude
How much downward mechanical force can the tool exert without causing a problem? it the tool is extremely sensitive and cannot exert any force at all, then you will need something sophisticated (perhaps the tool could interrupt an optical beam.) If you can exert a tiny force, perhaps you can build something using a piezo sensor. If more force is acceptable, jut use a high-quality mechanical switch.

For the piezo, Assuming this is a home-brew and not something off-the-shelf, you can remove the piezo element from a $2.00 buzzer and use it. For manual touch-off, you can just connect a voltmeter across the element and read the voltage as you jog the tool. For automatic, you will need to have a computer read the voltage. Piezos are extermely sensitive: a tiny deflection will produce a measurable voltage. I suspect you will get reproducability to within 100 nanometers or so. I have used a $2.00 buzzer piezo, a 9-volt-battery, and a junkbox potentiometer to move a morroe in an interferometer, getting about 100nm/volt. The effect is similar for converting displacement to voltage as for converting voltage to displacement.

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18 May 2012 09:00 - 18 May 2012 09:00 #20196 by ArcEye
Hi

I made a couple of Z touch off tools last year.

The first is mechanical, an adjustable arm actuating a very sensitive limit switch.
The arm is adjusted so that it is dead level at point of actuation, so that where precisely the tool touches is not important.

The second is electrical, two pieces of aluminium separated by a plastic and epoxy sandwich are connected by the tool touch.

I have found these repeatable to within a couple of hundredths of a millimetre (0.01mm) which is better or at least comparable to your requirements.

Cost me time and bits from scrap bin.
Could easily be changed to trigger signal to LinuxCNC rather than just optical LED





regards
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Last edit: 18 May 2012 09:00 by ArcEye.

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18 May 2012 09:01 - 18 May 2012 09:03 #20197 by ArcEye
Never did manage to get 2 pics in one post

This is the electrical one


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Last edit: 18 May 2012 09:03 by ArcEye.

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18 May 2012 11:16 #20202 by andypugh
crmorano wrote:

What is a good mechanical touch off/home switch tool that I can put between the stage and the tool to use as a home switch that will give me a fixed height within the system every time that does not require conducting through the tool?.


You could make something like the Renishaw patent devices, with three rods and 6 balls. That has the advantage that you can choose a spring to suit your tool force.
www.indoor.flyer.co.uk/probe.htm
Alternatively, you might be able to use one of the dial-indicator style devices:
www.ebay.com/itm/261024222481
Or possibly this
www.ebay.com/itm/150802223742 though that specifies "conductive" so an enquiry might be needed. I have bought a fair bit of stuff from CTC in HongKong.

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18 May 2012 11:30 #20203 by BigJohnT
I use and like the Edge Technologies touch off gauge on my mill but I think I purchased it directly from Edge

www.edgetechnologyproducts.com/

John

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18 May 2012 12:01 - 18 May 2012 12:11 #20204 by crmorano
I think I want to avoid having to actually make something, as I can think of exactly how I would make it, but don't have enough time.

While I don't know exact forces behind it, the tool itself is robust enough (Assuming you go slow, which is easy enough to do) that it can exert a decent amount of force as long as it isn't being pushed into something hard.

I'm hoping that someone has, for relatively cheap (The only one I found was $1250 from Centroid www.centroidcnc.com/cnc_tool_set.html), a spring loaded switch that always actuates at the same known place and can be connected to EMC to act as the home switch for Z. The company has tried both of the tools listed by andypugh. However, I'm trying to remove the human factor from this, except the need to place the switch and take it away once Z is zeroed, since it would save a bit of time when things get swapped around.

Edit: Another good example of what I am thinking of is for a manual/semi-automatic tool change in a CNC process that I've seen, where the system lowers the tool down to know the length of the tool once the tool is complete, then returns to milling. It just needs to be non-conductive though, which I don't think was the case here.
Last edit: 18 May 2012 12:11 by crmorano.

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19 May 2012 00:07 #20218 by andypugh
crmorano wrote:

I think I want to avoid having to actually make something, as I can think of exactly how I would make it, but don't have enough time..


Maybe something like this:
uk.rs-online.com/web/p/precision-position-switches/4460254/
That one appears to have a zirconium plunger and a repeatability of 0.0001mm

There appear to be ruby plunger options.

Not cheap, but all there basically.

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