If you use a milling machine, you will need to be able use Edge Finders, Center Finders, and 3D Tasters to find edges, corners, and hole centers.  This is all part and parcel of setup work on these machines.  Often it’s done to locate Part Zero, so be sure to also check out our article on all the different ways you can locate Part Zero.

Edge Finders

The classic method of locating features on mills is to use an edge finder.  These fall into two categories–mechanical edge finders and electronic edge finders.

Here’s a typical mechanical edge finder:

mitutoyo mechanical edge finder

Mitutoyo Mechanical Edge Finder.  About $12.77 from Amazon...

Operating one of these isn’t obvious until you’ve learned how.  This video from Tormach will walk you through it:

You can also get conical tipped models.  The video also shows how to use an electronic edge finder.  There are a couple of styles:

Fowler Electronic Edge Finder.  About $33.59 on Amazon

They’re pretty accurate.  Claimed to be 0.0002″ (2 thousandths) repeatable.  Not sure I trust them to that degree.  Their worst issue is they’re very easy to break if you jog too far, at which point you just bought a new one.

They make one that has a little spring loaded ball so you don’t break the edge finder:

Fowler Electronic Edge Finder with moveable ball. About $150.46 on Amazon

I’ve never owned one because I was told they have lousy repeatability due to that moveable ball.  Plus, they’re not cheap either.  

These types of edge finder are fine and well, but once you’ve tried a Haimer 3D Taster (also called a 3D Sensor), you will never go back to this old style.

3D Sensors (3D Tasters)

These German-made measuring devices are my favorite aside from a 3D Touch Probe for edge finding and part zero setup.  They used to be called “3D Tasters”, which was such a charming name, though I am sure a mis-translation of the original German.  Now you look for them as “3D Sensors.”  Here’s the typical unit:

Haimer 3D Sensor.  About $408.46 on Amazon

They’re not inexpensive, but boy are they a time saver and a pleasure to use.  They do have a less expensive “Mini” 3D Sensor:

Haimer Mini 3D Sensor.  About $390.00 on Amazon

I haven’t tried a “Mini”, but I wouldn’t hesitate because the Haimers are very well made.  One thing you must remember to do is get at least 2 extra tips when you buy one.  It’s only a matter of time before you break a tip:

I’ve got a sign like this hung up in my shop…

You want to buy two tips so you can order 2 more as soon as you break one.  They cost $43 on Amazon, and they do add up.  But better to break a tip than the Haimer itself.

Something not to bother with–don’t try the cheap Chinese knock offs.  I used to start with Cheap Chinese on new tools and then buy something better if I liked the tool but wanted higher quality.  It worked out okay about half the time or more.  The simpler the item, the more likely it would be okay.  

But the Haimer knock-off I got didn’t work very well and it also broke in record time.  I think I only used it for less than a month.  Even though it looked like the real thing, it wasn’t.

Haimers are good for all sorts of things, so I will leave you with this video from Tormach on how to use one:

Be sure to adjust your Haimer for concentricity.  The video shows how.

DTI as Edge Finder with Accessories & DTI Center Finder

When accuracy is a must, you can use a DTI as an edge finder.  Particularly if you have some handy accessories like these chair edge finders:

Edge Finder “Chair” for DTI…

Corner Finder for DTI…

These gadgets should be high quality, ground tools.  They’re usually equipped with magnets.  These tools are from the days of jig boring machines and can be highly accurate.  This video from Oxtool gives a great explanation of their use:

Here’s a neat DTI holder that’s designed for finding circle centers:

The radius is adjustable with the knurled wheel on the left.

Blake Coaxial Indicator: Hole Center Finder

blake coaxial indicator

Coaxial Indicator.  About $59.99 on Amazon

Finding the center of bores is a common mill setup operation.  It also comes in handy with lathe tool setup in certain cases.  You can use a Haimer 3D Sensor for bore center, DTI rig similar to the above, or a Blake Coaxial Indicator.  

The Blake Coax dates from manual machining days, but it is super simple to use and handy.  Mine is a cheap Chinese knock-off, but it works great just the same.

Using a coaxial indicator is simplicity.  Position it in the spindle over the hole, put the probe edge on the hole edge, and start the spindle rotating SLOWLY.  Now adjust the X and Y handwheels until the indicator needle quits moving as much as possible.  

Done!  Wasn’t that quick and easy?

Here’s Tubal Cain’s video showing the Blake Coaxial indicator in use:

Coaxial Indicator Accuracy

Think of coaxial indicators as good to maybe a few thousandths.  If you need better, set up your DTI or Haimer 3D Sensor.  I’ve written an article about Blake accuracy that goes into the details of what’s going on.


This article is part of our Free Metrology Course.



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