Machinists like to live on the edge, but they always want to know precisely where it is. If you’ve watched any machining videos (*cough*) then you’ve seen heavy use of digital readouts on machines. A “DRO” (as the cool kids call them) is a little computer that knows where the slides are, and thus where your cutter is on the piece. However, there’s a catch. DROs don’t know the absolute position of the spindle, they know the relative position of it. The bottom line is that a DRO is just a fancier version of the graduated scales on the hand wheels. The key difference is that the DRO doesn’t suffer from backlash, because it is measuring the slides directly (via glass scales similar to your digital caliper) rather than inferring position from rotations of the leadscrews. With traditional hand wheels, you have to compensate for backlash every time you change direction, and a DRO saves you from that (among other convenience features).
The point is that, whether old school or new, you still only get a relative coordinate system on your part. You need to establish an origin somehow. A useful way to do this is to set an origin at one corner of the part, based on its physical edges. How do you tell the DRO (or hand wheels) where the edges are? Enter the edge finder.
Precision Beyond Measure
The humble edge finder is one of those tools that are so commonplace in manual machining that everyone has forgotten how clever they are. It’s just a shaft with a precision ground stub on the bottom of it that is free to float around laterally (with some spring tension on it). This shaft is of a precise (and known) diameter — typically something conveniently halved like 200 thou or 6mm. You’ll see why this matters in a moment.
When chucked in the mill, the edge finder’s stub will wobble around like crazy, because it’s free to do so. To use it, you run the spindle at a moderate-to-high RPM (say 1000) and bring the wobbling end up to the workpiece. As the edge finder contacts the part, the wobbling end will become more and more concentric with the arbor of the tool.
So here’s the trick, and what makes the device so clever. In principle, when the wobbling end is precisely touching the surface, the edge finder tool will be running perfectly concentric. However that’s pretty much impossible to see by eye. If it’s close-but-not-quite concentric, you won’t be able to tell. To get around that, the spring tension holding the wobbling end is calibrated such that the spinning surface will “grab” the part and gain traction right as it becomes concentric, causing it to kick off to the side as it tries to roll along the surface. This is visible as the wobbling end “kicking-over” to one side. There is some technique here to get a perfect reading, because if you continue cranking the table past that kick-over moment, your reading won’t be precise. However, with proper technique, this very simple device is extremely precise and repeatable.
Going Halfsies From Tip to Spindle
Once you’ve found that edge, you know that the edge of the tool and the edge of the material are in the same place. However, what you want is for the center of the tool (and thus your spindle) to be on that edge. This step is easy, because as we said earlier, the diameter of the edge finder’s tip is known. Simply raise the tool clear of the work, and continue moving in that direction by the radius of the tool (as shown on your hand wheel or DRO). You can now set your DRO (or hand wheel) to 0 in that dimension and repeat in the other dimension. You’ve now got an origin in a known location, and by extension a coordinate system across the whole part. There are many types of edge finders (including electronic ones commonly used in CNC), but the basic cylindrical wobbling type is common, inexpensive, and precise. Three things that machinists love!
Avoid Tolerance Stacking
One final note — when making your own mechanical drawings, it’s always helpful to pick one corner of each surface from which to mark all your dimensions and locations. Edge finding is a main reason why. If all your numbers are relative to the same corner, the machinist only has to edge-find once, and there will be less accumulated error from resetting the origin. Either that, or the machinist has to do math to recalibrate all the numbers you gave to be relative to the same place. Both scenarios will make said machinist angry. Help heal the generations-old rift between engineers and machinists by minding your origin. If you don’t, expect to someday find a chuck key on the wrong side of your broken window.