Plans, projects and how-to's for home machinists

CNC: Automatically find edges, hole centers and more with Mach 3

This article describes how you can use Mach 3, the popular CNC controller software, to automatically

  • Find the edges of your work piece (and therefore the corners)
  • Find the centers of holes and their diameter
  • Set your cutting tool at a known height above your work piece or table

What’s involved

This feature is very easy to implement and use and it will cost you almost nothing.  You don’t even need a licensed copy of Mach 3.  Here’s what you’ll need to do:

  • Download and install a free screenset for Mach 3 that has the buttons you’ll need to operate these features.  There are a number you can choose from.  Keep reading to find some of them.
  • Connect a wire to an unused input on your CNC controller board.  This is the circuit board your steppers motors are hooked up to.  You probably have a spare input unless you’ve used them all up for home or limit switches.  If so there is probably an inexpensive way to add more.  You may also have to add an inexpensive and easy-to-find capacitor if have a problem with electrical “noise.”  I didn’t have to.
  • Go into Mach 3 configuration menu for ports and pins and enable the “probe” and assign the pin you hooked your wire up to.  You might have to also change the “active low” setting from its default but that’s easy to determine and do.
  • Connect the other end of your wire to a touch plate.  If you’re working with wood or other non-conductive materials you’ll just need a flat straight piece of metal for finding edges.  There’s at least a few different kinds of probes and touch plates you can use to find the center of holes and their diameters.  One is simply a short straight piece of metal rod mounted in your spindle.  If you’re working with metal then your touch plate or probe will need to be electrically insulated from your work piece.  So it will require a little more effort to make one.

It took me only about 40 minutes to get it working on my mill, although that doesn’t include all the research I did first.  I also made a quick-and-dirty touch plate and I still need to make a couple of better ones.  I’ll be writing more about that later.

More details

“Erniebro” on CNCZone didn’t come up with the idea for an auto edge finder, but he deserves an enormous amount of thanks for publicizing this method and making it easy for others to start using it.  He designed the first screenset for adding this feature to Mach 3, made the video above, posted excellent instructions on CNCZone describing how to install it and patiently provided help and support for those who needed it.  Others have built upon his work and made improvements, but if you want to try this I suggest you start with ErnieBro’s instructions.

  • One of the first things you’re going to want to do is download Erniebro’s Mach 3 screenset.  I just want to let you know that if you’re NOT a registered member of CNCZone and logged in, you’ll get an error message that might make you think the file is no longer available.
  • There’s a very good chance you won’t have any problems getting your machine to work properly.  If you do there’s also a very good chance that someone else has had the same problem and you’ll find the solution in Erniebro’s forum thread.  Unfortunately, the thread has grown to almost 400 posts and it takes a lot of time to read them all (trust me, I’ve done it).  So I suggest you take advantage of the “Search this thread” link you’ll find near the top of each page, just above the messages on it. You’ll need to be logged in to use it.

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