# Tool touch off - the homemade version



## koenbro (Jul 14, 2022)

I am interested in making a tool touch-off part for my mill. Acorn has an easy interface which basically is a one-wire input; you use a standard height (I use the KP-3 probe) and then reference everything off it as a relative height. Although you can buy a factory made tool touch off system, they are pretty expensive and that's why I want to explore the home-made option. So I made a crude prototype, basically a piece of wood from an old bed and some 1/8" alu plate:




And it works!

After touching off all my cutters, the surface got marred and the height might vary a little between tools. Of course soft wood is also a  poor base as it yields a little when the tool is pressed into it. I was wondering if a harder metal (SS or should I just use plain carbon steel?) might be better. Has  anyone made a tool touch off system like this? Any advice is greatly appreciated.


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## graham-xrf (Jul 14, 2022)

I take it your one-wire continuity detector completes it's circuit via the machine frame.
You only need something insulating that has a stable dimension better than wood. It's all a near zero force thing, so use a piece of poly-carbonate (plexiglass?), acrylic (perspex). If you don't mind something softer, then nylon ( Delrin). You might raid a chunk off the kitchen cutting board, though that might lead to some domestic disapproval.

Of course, you can hide the insulators in bolt holes, using insulating washers sunk in counter-bores, and a plastic sleeve, in much the same way power semiconductors ate mounted on heatsinks. There is also the option of setting the probe into a small epoxy potting box (which can be made of metal), ending up with a construction like the standard solid state relay (SSR). Come to that, you could hijack a standard SSR (less than $6 worth), bolt it down, and just (mis)use it's terminals.

You could mount your metal onto just about any insulating terminals you fancy. Have a look on eBay.



	

		
			
		

		
	
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## Boswell (Jul 14, 2022)

you might want to consider adding some sort of spring to the touch plate so that if you accidently overtravel you don't break things.


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## koenbro (Jul 14, 2022)

Boswell said:


> you might want to consider adding some sort of spring to the touch plate so that if you accidently overtravel you don't break things.



Yeah that sounds like a great idea if there is some dirt preventing electric contact. My Avid CNC router has that. Wonder how I would wire it in? Pressing the spring triggers an e-stop perhaps. Will strongly consider it in a future iteration. 


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## koenbro (Jul 14, 2022)

Boswell said:


> you might want to consider adding some sort of spring to the touch plate so that if you accidently overtravel you don't break things.




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## koenbro (Jul 14, 2022)

graham-xrf said:


> I take it your one-wire continuity detector completes it's circuit via the machine frame. […]
> Come to that, you could hijack a standard SSR (less than $6 worth), bolt it down, and just (mis)use it's terminals.
> 
> You could mount your metal onto just about any insulating terminals you fancy. Have a look on eBay.
> ...



Wow I like the idea of the relay. If I choose to make my own, does the type of metal matter? On one hand Aluminum will prevent chipping tools OTOH a will be imprecise with time. 


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## RJSakowski (Jul 14, 2022)

Some years ago, I was working with a project that required using tools like .009" drills, etc.  and I needed a touch off point that wouldn't break the tools.  I made my touch off using a silver contact from a heavy duty relay.  The comparatively light contact pressure prevented any significant gouging of the surface.   I set the x, y coordinates of the contact point at 0,0 ABS in my DRO so I always contacted the same point.  My work reference was set at 000,000,000.  When I changed tools, I reset the ABS z axis of the touch off at 000 which automatically entered the correct offset for that particular tool.

That was back before I started the Tormach TTS tool holders.  Each tool couldn't be loaded to a repeatable offset so they had to be referenced before use.  With the TTS system, the tools are preloaded into ER25 chucks and the chucks are precisely referenced to the spindle face so I can load all my tools for a project and set offsets for them prior to beginning work.

Here is the setup.  The relay contact was mounted to a pece of 1/4" aluminum plate via an insulating block from the relay and the plate mounted on the mill table at a convenient location.  The contact was connected to the positive terminal of a 3 volt battery and the negative terminal was connected to athe negative lead of an LED.  the positive lead was connected to a 470 ohm resistor whuich as connected to a plate with two magnets.  The plate was placed on the mill table, completing the wiring.  When the tool touched the contact, it completed the circuit and the LED lit.  This setup was repeatable to +/- 2 tenths which was the resloution of my DRO.


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## RJSakowski (Jul 14, 2022)

While the TTS system eliminates the need for touching off each tool prior to use, were I to go back to using the touch off, I would use a transistor amplifier or an op amp to increase its sensitivity.  I could then add an audiuo signal as well so I wouldn't have to be watching the LED when touching off.


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## koenbro (Jul 17, 2022)

RJSakowski said:


> While the TTS system eliminates the need for touching off each tool prior to use, were I to go back to using the touch off, I would use a transistor amplifier or an op amp to increase its sensitivity.  I could then add an audiuo signal as well so I wouldn't have to be watching the LED when touching off.


I do not need to pay attention to the light and/or sound because the Acorn has a routine that touches off (senses continuity), retracts and brings the tool down slowly a second time, then records the touch off distance, and substracts from the reference tool height, and records that as offset, while doing a rapid retract to Z0.


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