Top of stock sensor/probe

CNC mill manufacturers such as Roland typically provide a fancy sensor for touching off the top of stock. These are pricey and integrate into the machine’s controller, making the process quite fluid, but hard to adapt to other machines. Fortunately, there is a simple but effective alternative:

I) Probe
Purchase or make a touch-off sensor (height probe) similar to this one or this one.

Two important points about these probes.

  1. The crucial part is knowing the height very precisely. A round number (e.g. 0.500" or even 0.495") makes life easier, but it really doesn’t matter as long as you know what it is.

  2. The top should be conductive (brass/bronze work well) and connect to a lead/wire, while the bottom should in insulated.

If you’re making one, a typical approach would be to aim to lather a metal cylinder of a certain diameter and height (e.g. 1.000" x 0.375"). Drill and tap a hole on the bottom for a #2 screw or similar and another hole in the side wall to connect the lead/wire. Then lathe a dimensionally stable plastic (Delrin/ABS) cylinder of the same diameter and 0.125" tall (ideally the height would be your target height for the overall sensor (say 0.500") minus the height of what the metal cylinder actually came out to be (e.g. 0.500" - 0.371" = 0.129"). Counter-bore/sink a hole to conceal the head of the screw, so that the head is well clear of the bottom surface. Assemble your sensor. The sensor could equally be made prismatic.

II) Continuity
The cutter is typically grounded to the CNC machine, so all that one needs to do is check continuity between the cutter and the probe as soon as one touches the other. A multimeter with audible continuity works great for this. If a more dedicated solution is desired, one may purchase something like this and mount it to/near your machine.

III) Top of stock location

  1. Turn on continuity meter.
  2. Attach one of the leads to the cutter, collet nut, or any grounded surface on your machine (such as a grounded screw).
  3. Attach the other lead from the continuity meter to the probe that you made/purchased.
  4. Making sure the top of the stock and bottom of the probe are both clean of any particles/dust, place the sensor on the top of the stock.
  5. Jog the X and Y until the cutter is over the probe.
  6. Slowly lower the cutter until it is just above the probe (be patient here if you want an accurate reading).
  7. Switch to the lowest resolution of travel available and jog step by step until continuity is made. Congratulations! You have found the top of your sensor! Write down this z-value.
  8. Jog the cutter up a few steps and carefully slide out the probe. Remember that cutters are razor sharp, so try not to make contact with the probe. Doing this will ruin the top surface of the probe leading to inaccuracies.
  9. Jog the cutter in the X or Y direction to a location where your stock is not present. You will soon bring the cutter down to the where the top of the stock should be and zero the z-axis.
  10. Once the cutter is in a location where it is clear to be lowered safely, jog the cutter back down to where you measured continuity (good thing we wrote it down, right?). Now, jog the cutter down the exact height of your probe. Do you now see why making it a nice even number like 0.5" makes life easier? Time to now zero the z-axis! Congrats again! You now know exactly where the top of your stock is!
  11. Raise the cutter to a safe distance and proceed as you normally would when machining (safely and carefully, that is).

Good luck and have fun!

There’s some information here: and I added a link back to your excellent post (so now we have a circular reference, my apologies).

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