Precise Touch-off Method for CNC Machine PCB Routing

by Michael Shaw, October 2011

This article explains a method I use for determining the exact location of the top of an electronic Printed Circuit Board (PCB) when machining traces using a CNC machine. I have been using this method to perform the Z-axis “touch-off” for a year now with great success and thought it would be great to share it with you. 

Determining the top of a PCB is important for controlling the depth of the bit when machining a PCB. The router bit will wear less and you can achieve better detail and cleaner traces by limiting the depth of the router bit.

Methods I used in the past were hit or miss, at best. You could eyeball the height of the bit over the PCB, but you risked breaking a bit or being way off on the depth. You could also use a method I once read in an electronics magazine. This method used a piece of paper (paper is about .005” thick) placed between the PCB and the bit. After each incremental jog of the Z-axis, moving the paper around would determine if it was captured between the bit and the top of the PCB. If it was, then you were at the top of the PCB, minus .005”, give or take.

The method I developed is more precise. It requires a multimeter, ohmmeter or other device for checking continuity. The meter will be used to detect a complete circuit when the router bit touches the top of the PCB.

To perform this technique, clip one lead of the meter to the bit and the other lead to the top of the PCB. In the figure 1 illustration, you can see in the upper left-hand corner the black lead of the Fluke 27 multimeter clipped onto the bit and in the lower right-hand corner you can see the red lead of the Fluke meter secured against the top of the PCB under the clamp.        

Figure 1, meter connection

Switch the meter to the “diode” mode so the meter will produce a tone when the two leads make continuity (touch).

Note of caution: you might want to verify the correct operation of the meter to make sure it beeps when the leads are touched together before performing the next step.

Slowly jog the Z-axis down until you hear a beep from the meter (I use a jog increment of .001”). When you hear the meter beep you know the bit is touching the top of the PCB. Now you can define your touch-off point from this location.

One last note about using this technique, although this method can precisely locate the top of the PCB in a given location, if the CNC table or the PCB is not level, then the top of the PCB is going to vary. So this method could also be used as a tool to help with leveling the CNC table or PCB.