CNC touch plate

The CNC router has achieved functional status. That is I can now with modest effort actually make useful things. These can be furniture pieces or parts for the router or other tools.

One of the hard parts has been zeroing out each of the  axis. X and Y i have been doing with a standard edge finder. I particularly like the 1/2 inch diameter ones like this one: Brown & Sharpe 599-792 Edge Finder Single. The smaller diameter (typically .2″ diameter touch) are bit small for woodworking due to the rougher surface of the wood and the large ones seem to be easier for me without false indications of the edge.

The way I am currently setting up my fixturing on the CNC table, I have a left size (x axis work zero)  fence and some movable stops from Rockler in the table slots. This works out quite well. I measure from the stops / fence to the home position and modify the zero references and I am off and running for several projects.

The hard part has been easily zeroing out for cutter height. This has to be done after each tool change and is just plain tedious. So I was determined to set up a touch plate and have that working with the Mach 4 controller software. This took a little bit of time. First was getting the Mach 4 “Probe” input  working.  It requires at least version 2803 of the Mach 4 software which I am using with the Ethernet Smooth Stepper (ESS) version 130 software.   The touch plate is simply a block of acrylic plastic about 3/8″ thick  with a sheet of brass mounted on top and connected to the “Probe” input.  The router spindle itself is grounded for safety and this conveniently completes the circuit for the touch plate.

The idea is to drive the Z axis down at modest speed until it touches the plate, and then it backs off slightly (~0.1″) and then moves downward very slowly to touch again for the final height.  Once complete, the height of the touch plate is subtracted off and you now have the height of your base (spoil) board on which the project will rest. This is all great in concept but the Mach4 software does not complete this for you. So I had to modify the UI / Screen set, add a new button and some scripting to do this.  This is one of the points where having an active user community helps tremendously. Searching for help on the Mach4 forums I came across a thread on this topic and some script code from another user “DazTheGas” see the message thread.  I had to convert from metric to inch measurements for the movements and enter the height of my touch plate (.487″). This works great. Much easier than manual zeroing and I don’t have to worry about breaking small diameter tools or chipping the carbide on bigger ones.  You can see the video of the process here