Yes the Alu plate is touching the table and the table top is wood and have on top a piece of MDF and the Al plate sits on top of the MDF

Jeff I have it like that but dont know if my spindle (Bosch Colt) is isolated or not

I attach pic of my Alu plate

Well some good news..........

I did switch the red wire to pin 13 which does not have capacitor connected, changed the pin number on the input pins screen  but again no change and so my theory about the capacitor been the culprit is wrong

Then decided something else; on the inputs screen I switch the probe to Active High (removed the green tick) and it works !!! but not properly.

Like when I click the button to auto zero, the Z starts to decent slowly and when it hits the plate I hear a grinding noise and I pressed the E-Stop because I thought she was going to damage the bit.

So at least now I know that the wiring works but don’t know what to do next

The wiring does NOT work.If you touch the clip to the plate, the LED needs to light up. If it doesn't, the tool will try to go through the plate. You need to get the digitize input to work correctly before you can run the auto zero code, or you could cause some damage.

Gerry what I meant by the wiring is working I meant that at least I have some voltage there because before I was getting nothing and I thought that perhaps my wiring is broken somewhere

So what I have to do to get the digitze input to work?

Kolias:
Take Ger21's advice and don't run the VB code any more until you get the input working. You're just asking for a broken tool or machine damage.

Simply put , the light should be normally off and if you touch the clip lead to the plate you should see the light go on. It should be off otherwise.

Did you install the pull-up resistor between pin10 (or whatever your using) and 5v?
Some BOB's need this resistor. The resistor should be something like 1k.

With the resistor in place you have a setup for active LOW to trigger the input. Make the settings in the software to match active LOW. If making and breaking the connection between the clip and the plate has no effect after that then something is not working with the input you are using. Please report your findings.


BTW you can't use the bare aluminum plate on the metal table or piece of work in the milling machine. Usually the plate is a piece of PC board (copper plate insulated from the table or work). The aluminum plate would be ok for a router I guess where you are always placing it on wood.


When you get that working you should get the code from CNC zone forum subject "aussie tool setter" where it was developed. Here is a link to some code. http://www.cnczone.com/forums/cnc_wood_router_project_log/36099-another_aussie_auto_tool_zero-35.html
You'll need to enter the thickness of your plate in inches. The code is metric. It's well described where to make the change.

Thank you Sage

No, I don’t intend to use the VB code until I find out what is wrong.

My limit switches are connected to the same points where I connect the zero plate and all my limits are working. Here is what the manual of my BOB says:

“The +5VDC logic supply rail connects to the limit switch and e-stop inputs through a jumper connected 1K pull-up resistor network. The pullup resistors are needed in most cases when switching the logic inputs through physical switches. Disconnect JP5 if you do not need the pullups”

So I think the resistor is there since my limits are working

Is it possible that my problem is the 1/8” thick aluminum plate I use?

Re-read my previouis post for more information. I was editing more into it when you posted.

What could possibly be wrong with an aluminum plate pray-tell??  

Are your limit switches N.O. or Normally closed? It could be they are normally closed and pulling the input low (shorted to ground). Have a look at the activation state of the limit switches in the software. If they are set to active HIGH then they are normally closed switches and are pulling the input to ground. If so you won't be able to use that input (unless you re-wire the limits).
 You should also be able to see the light go on and off when you activate the limits.

The VB code in your doc file looks too simple to do the job, but I'm no expert. Try the one Hodd posted and/or the one I linked to.

If the light works on the limit switches then try running the VB code and trigger one of your limits instead of using the plate and clip. This will tell you if the code is reading the input properly.

Sage

BTW.
I'm not sure using the same inputs as the limits is going to work in any case. I'm not sure if the limits have priority over the Zero setting code. If the tool zero and a limit are triggered at the same time (since they are the same in your wiring) which one wins?  Perhaps the limits are not payed any attention while the VB code is running I don't know.
 At best this would be a race condition. Sometihng best avoided. Is there another input you could use. Perhaps a home switch input.

Ps> It was Tweakie that posted some VB code for you Not Hood. Sorry.

Sage
 

Sage I think now we are getting somewhere

My limit switches are all NC and on the inputs screen I have ticked the the Active Low for all of them (The mark it is Green). The LED on the diagnostic screen is OFF for all of them and when I manually press the switch the light goes ON.

What I can do is to disconnect all the limits and connect only the plate to see if the light works as you say. Then if it works I can switch the wiring on limits to NO and change the input on the screen (remove the Green tick and it will be Red)

Does the above sound good? Would the limits work ok after I do that work?

I can aslo change the VB code to the one provided earlier by Tweakie

I dont think there will be a case where the limits and zero touch plate will work at the same time. I mean I only set the zero plate at the begining of a cut and thats it. There are no other inputs on my BOB

Can you trace and describe the wiring of your limit switches. I can't see how you can have NC switches AND have active LOW.

Usually you would have NO switches, the input would be pulled high by the resistor and the switch when activated would close and pull the input LOW (to ground).

Are you using an input for each limit? (because you can hook them all together on one input and free up at least 2 inputs).

You are not understanding what I mean about the limts and the zero setter being used at the same time.
The two are in conflict. The way you plan to use them they are ONE IN THE SAME. If you are using the zero setter and the clip touches the plate then you are triggering LIMIT and ZERO at the same time. Mach may pay attention to the LIMIT condition and shut down any further movement of the Z-axis and cancel or at best screw up your zero setting.
Get the idea??