# Electrical Probe for CNC



## Don B (Jul 11, 2014)

I'm looking for some input please on the pros and cons of using a Electrical Probe for for setting the Z axis on my CNC, is anyone using one of these devices on their CNC?

I've come across this.



The setup instructions are here.
http://www.autoleveller.co.uk/cnc-probe-guide/

It seems a little to good and simple to be true, I have the connections open on my CNC (NM 145)
Setting aside the potential for physical damage to the machine can I hurt anything electronically by trying this?
On the drawing there is a 10K pullup resistor, is this a special resistor? Or is this terminology for the resistors function?:thinking:

I would appreciate any advice or guidance with this, my apologies in advance but I'm electronically challenged so any answers will really need to be dumbed down.

Don B


----------



## John Hasler (Jul 11, 2014)

It will work if your touch plate is insulated.  It might or might not work if it isn't, depending on details of the construction of your machine.  In any case it can do no harm.  "Pullup" describes the function of the resistor.

Rick Sparber has a pretty good paper on one of these:

http://rick.sparber.org/rctf.pdf


----------



## Don B (Jul 11, 2014)

John Hasler said:


> It will work if your touch plate is insulated.  It might or might not work if it isn't, depending on details of the construction of your machine.  In any case it can do no harm.  "Pullup" describes the function of the resistor.
> 
> Rick Sparber has a pretty good paper on one of these:
> 
> http://rick.sparber.org/rctf.pdf



Thank you very much John, I'm planing to make a touch plate with a spring under the plate, I'm a little leery of a tool coming down on a solid plate, so an isolated design shouldn't be a problem, I've been drawing up a low profile Z tool (1 inch high) that utilizes a test indictor  but this looks a much better way to go if it's reliable.


----------



## Analias (Jul 11, 2014)

I have three probes on my mill and on my router.  They are all wired to the same input pin on their respective controllers. One probe is my fancy 3D probe, which I swap between the two machines as needed. Each machine has a fixed plate for measuring tool lengths during tool changes. Finally, each machine has a "mobile" plate for probing the Z height of the work piece. On the mill, this is a piece of single sided 3mm PCB board. Single sided to insulate it from metal of the table or work piece. On the router I use a piece of aluminum bar stock since the table is wood. I find using the non-3D probes very reliable and simple. The 3D probe has been a bit troublesome since it was abused by the prior owner and had a tendency to not trigger. 

I have found that if your axis motors can handle the weight of your spindle and head, and your probes reliably trigger, these kinds of probes are worth having. I would recommend wiring your probes to use active low signal since most spindles are grounded. If you use conductive tooling and/or your spindle is not insulated from ground, you don't need to clip it to ground. 

Think about how you are using the probe. Do you want to do just Z, or also X and Y? If the "mobile" plate is small enough you can place it against the side of your work piece and probe X or Y. 

My setup allows for plugging in arbitrary mobile probes. On the mill I have a probe made of delrin round turned to fit my TTS holder. I bored the center to hold a 3/4in copper pipe cap pressed inside. I use it to probe and center round tools mounted on the table. 

3D probes can be problematic. You get what you pay for. Unfortunately, if you have trouble with reliable triggering your machine can turn an expensive probe into trash, or destroy probe tips - which are not cheap. I have gone through many tips myself before I was able to make my 3D probe serviceable. Again, make sure you have the electrical side sorted to provide reliable triggering. I try really hard to make it a habit to test the probe signal before I initiate any probe operation. Testing before probing is better than being fast on the E-stop - which I am not. 



Sent from my Xoom using Tapatalk


----------



## Don B (Jul 11, 2014)

Analias said:


> I have three probes on my mill and on my router.  They are all wired to the same input pin on their respective controllers. One probe is my fancy 3D probe, which I swap between the two machines as needed. Each machine has a fixed plate for measuring tool lengths during tool changes. Finally, each machine has a "mobile" plate for probing the Z height of the work piece. On the mill, this is a piece of single sided 3mm PCB board. Single sided to insulate it from metal of the table or work piece. On the router I use a piece of aluminum bar stock since the table is wood. I find using the non-3D probes very reliable and simple. The 3D probe has been a bit troublesome since it was abused by the prior owner and had a tendency to not trigger.
> 
> I have found that if your axis motors can handle the weight of your spindle and head, and your probes reliably trigger, these kinds of probes are worth having. I would recommend wiring your probes to use active low signal since most spindles are grounded. If you use conductive tooling and/or your spindle is not insulated from ground, you don't need to clip it to ground.
> 
> ...



Thanks for the info, I think I mostly understand what you mean, for now I'm just trying to do the Z axis, I usually just use paper which is OK when every thin is dry, but after some coolant is splashed round........          soggy paper doesn't work so well for tool touch offs..!
Is your probes wired like the diagram I posted, using the resistor? 
Usually by the time my brain tells my thumb to push the E-stop bad things have already happened and it's finished anyway.


----------



## acourtjester (Jul 16, 2014)

HI
If your running mach 3 you can use this very simple and works great.


----------



## Don B (Jul 16, 2014)

acourtjester said:


> HI
> If your running mach 3 you can use this very simple and works great.



Thank you very much, yes I'm using Mach3, that "VB Code for Auto Tool Zero Button" is something I've been trying to pin down, seems to be several examples around and there all just different enough to confuse me, do you use this yourself? The reason I ask is that I've ben contemplating making a spring loaded touch off plate in case the tool should run past, and I'm wondering if I'm wasting my time?
If you do use this method have you ever broken or damaged a tool?
I started to work on this Monday, then aggravated a ruptured disk in my back and things have been at a stand still, hopefully I can complete this by Christmas, LOL, but at the moment it's not looking good.

Thank you very much for the post.

Don


----------



## acourtjester (Jul 16, 2014)

Hi Don
Yes I have used it but now a lot since I just found it too.
If you look at the routine the number after the f is for speed so you can really slow it down.  Also to est it put  some foam under the touch plate when you try it.  remember to check the thickness of your plate to put the into the routine so it makes the correction to be at 0" after.
I too had make a spring loaded touch point but discarded it when I found this routine.
Sorry to hear about your back that is some bad stuff to deal with

Tom:jester:


----------



## Don B (Jul 16, 2014)

acourtjester said:


> Hi Don
> Yes I have used it but now a lot since I just found it too.
> If you look at the routine the number after the f is for speed so you can really slow it down.  Also to est it put  some foam under the touch plate when you try it.  remember to check the thickness of your plate to put the into the routine so it makes the correction to be at 0" after.
> I too had make a spring loaded touch point but discarded it when I found this routine.
> ...



Thank you very much Tom)

Don


----------



## acourtjester (Jul 16, 2014)

Happy to help
have fun
Tom:jester:


----------



## Don B (Aug 6, 2014)

This is just an update on my progress with using a touch plate to set the Z height on my CNC using Mach3.

I wired this using the wiring diagram from post #1 and the info from post #6 "How to setup touch plate.doc" posted by acourtjester.
It's working great through repeated testing and I'm very happy with the setup, I did overlook one thing even though I read the document repeatedly I overlooked the fact that it states to use a piece of "PCB material" and I soon found out why, the plate can not ground out or the touch off plate will not work properly, I used a piece of brass plate which worked great when used off pieces of foam for testing but as soon as I tried it off of a piece of aluminum I soon realized my error, a small piece of screen protector on the bottom of my brass plate fixed this issue.

There is one thing though that anyone considering a setup such as this, is the fact that to high of a feed rate will cause the tool to travel on even after the signal is detected.
The VB Code for Auto Tool Zero has a value of 20 for a feed rate which was a little fast for my comfort, so I slowed it down to 15 but I was still concerned that the tool may travel after a signal was detected.
My testing setup is admittedly crude but the results where very consistent and repeatable throughout a couple of hours of testing.




Feed rate 15 --> Over run .0019
Feed rate 10 --> Over run .0010
Feed rate 05 --> Over run .0003
Feed rate 02 --> Over run .0001

Regardless of the tool over run results using the 4 different feed rates the tool height constantly finished at the exact same height as near as I could detect with my .0001 indicator.

Just something to think about if your considering a setup like this, I'll be making a new touch off plate that will allow some over run of the tool.


----------



## Boswell (Aug 6, 2014)

I will not have a chance to test this anytime soon so use with caution but here is a slightly modified version of the code in the doc that was attached above. this will use a high feedrate initially. Once it touches it will back off .05 and then go down again for a touch at a very slow feed rate. This should eliminate the overrun in the final z setting. It will NOT prevent an actual overrun in the first touch. It might be wise to lower the 20ipm to something lower so that there is no damage to the tool when it overshoots slightly. The good thing about a copper clad PCB is that is that it will probably absorb a small overshoot. Eventually it would need to be replaced.
Anyway, remember that I have not tested the modifications below. It seems pretty simple but you never know until you test it.

Message( "Auto Zeroing..." )
If IsSuchSignal (22) Then
code "G31 Z-3 F20"             ' first touch with feedrate of 20  
While IsMoving()
Wend

Call SetDRO(2,0)    ' this temporarily sets the z axis to zero where it first touched
code "G0 Z 0.05"     ' Back off .05

if IsSuchSignal (22) Then
Code "G31 Z-.1 F1"              ' do it again at feedrate of 1
While IsMoving()
Wend


Call SetDRO( 2, .060 )     ' now reset the Z-Zero with the offset of touch plate.
code "G1 Z1"
End If


----------



## Don B (Aug 23, 2014)

Thank's for posting this info, I'm not getting time in the shop right now but I'll have a closer look when I have time )



Boswell said:


> I will not have a chance to test this anytime soon so use with caution but here is a slightly modified version of the code in the doc that was attached above. this will use a high feedrate initially. Once it touches it will back off .05 and then go down again for a touch at a very slow feed rate. This should eliminate the overrun in the final z setting. It will NOT prevent an actual overrun in the first touch. It might be wise to lower the 20ipm to something lower so that there is no damage to the tool when it overshoots slightly. The good thing about a copper clad PCB is that is that it will probably absorb a small overshoot. Eventually it would need to be replaced.
> Anyway, remember that I have not tested the modifications below. It seems pretty simple but you never know until you test it.
> 
> Message( "Auto Zeroing..." )
> ...


----------

