A CMM is a device used to take accurate measurements from physical objects–typically parts in your machine shop.  The letters “CMM” stand for “Coordinate Measuring Machine.”  They’re extremely useful to have around, so much so that many shops experience productivity boosts when they locate a CMM right on the shop floor where it can be easily accessed during the course of normal operations.  For CNC’ers, having an accurate probe and the software to make it sing also enables many CMM-like measurements to be taken right on the machine.  The operative buzzword is “in-process probing.”

A CMM is not unlike a CNC mill with a probe in many ways.  Here’s a typical example:


A typical Coordinate Measuring Machine.  Source: “9.12.17 Coordinate measuring machine” by Vulture19

As you can see, it has 3 axes in a gantry style arrangement, the table is a granite surface plate, and there’s an electronic probe instead of a spindle.

But none of this is cheap.  CMM’s are very expensive–used ones go for close to $20,000 on eBay and new ones are more.  Highly accurate probes are also not cheap.  But if you have a mill with an accurate DRO, you can do some surprisingly good CMM-type work with it.  This is probably not something a production shop wants to do much of–it is time consuming enough that it’ll be cheaper to buy a real CMM–but for amateurs, small shops, and in a pinch it can be a helpful tip.

Consider the task of accurately finding the location of a boss on a part.  You might set that up on the mill as follows:


The part with the boss is positioned in the vise and an indicator holder is in place to help find the exact center of the boss…

Use your handwheels to position the part until the indicator can rotate around the boss without a twitch–now you’ve located the center of the boss.  Zero the DRO and use an edgefinder to find the distance to the center of the boss from any edge of the part:


We’ve now marked up the workpiece with the dimensions measured on our makeshift “CMM”

Of course if you have a CNC, you can easily do this too, but you can also see how a probe would make it all very quick and easy to do.  In a pinch, you might find it is quick enough to use that old Bridgeport off in the corner to pick up a few measurements while all the machines and the shop’s CMM are in use.

BTW, this idea came to me from the Widgitmaster who was using it to adapt a turret to his manual lathe.


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Can’t Afford a CMM? Just Use a Mill With Accurate DRO
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