Edge Finder (Milling machine)

A wiggler, edge-finder, or center-finder is a tool used in the spindle of a machine such as a mill. The device is used to accurately determine edges or markings and therefore the center of a workpiece or a previously machined feature during the set-up phase of a machining operation.

Edge finder

A rotating tool, meaning the machine spindle must be turning for the tool to work. On one edge a cylinder shape is spring-loaded, when the edge finder touches an edge a kickoff of this cylinder is noticed. On the other end a cone shape is also spring-loaded and is used to locate the center of a previously drilled hole. Most accurate of all edge finders. Improved accuracy is obtained by use of collet. Repeatable to 0.0002 in (0.0051 mm) or better.

Electronic edge finder

Electronic edge finder

Sometimes also (erroneously) referred as a digital edge finder, this instrument can locate edges of work pieces and also height offsets. It works in a non-rotating spindle, which is a great advantage over its mechanical counterparts. It is battery operated and works by lighting up its internal LED (usually red) when the electrical circuit formed by the instrument, the workpiece and the machine is closed. The light is thus illuminated when the edge finder is touching the workpiece and is visible through openings in the case. A repeatability of 0.001 in (0.025 mm) is obtained.

Center finder

Center finder

A pencil like tool used to locate markings on stock. In contrast to the edge finder, the tip is not spring-loaded and it works with the spindle stopped. Sometimes a magnifying glass is used to assist in marking location. Only as accurate as the lines that are drawn on the part.

Why it’s so important!

A student was working and was quite upset. Apparently he'd drilled some of the holes in the brackets wrong; not once but three times. So he basically totally messed up the one bracket. He just couldn't see how his measurements were causing him to drill his holes so far off. Basically, he was measuring and laying out his measurements with only a ruler and caliper. Not very accurate!

So today we took some scrap and proceeded to practice finding an edge to set the mill to a known baseline using the mill's "X" and "Y" axis dials for accurately drilling holes in flat stock. I took a few pics and though it might be of interest.

An edge finder is used to accurately position the centerline of the mill spindle over a known reference point of the workpiece. In most cases that is a fixed corner from which all subsequent measurements can be accurately made.

This is a standard double-sided edge finder. It has a 3/8" shank and two swivel ends for finding edges and centers. For accurately finding an "X" "Y" axis intersection on a corner the side that is outlined in red is used. This bottom piece is exactly 0.2" in diameter and swivels on a spring (more on the importance of that later).

It is mounted in a milling collet and looks like this when properly setup in the mill

The metal stock is then mounted securely in a milling vise. In order to use the edge finder, it is necessary to leave a top edge showing above the vise jaws; hence the stock is mounted on a set of "machinist's parallels" to keep it square (a good riser block will do as well)

At this point, it doesn't matter which axis is indicated first, but we chose the left side of the workpiece (i.e. the "X" axis) to start. The bottom of the edge finder is brought slightly below the horizontal plane of the workpiece, and the mill is turned on. Since the bottom of the edge finder is spring loaded, it will wobble (that's good). Now, using the mill table's "X" axis handwheel, the workpiece is moved slowly to the left and brought in contact with the "wobbling" edge finder (VERY SLOWLY). At this point, the "wobble" in the edge finder will become less pronounced the more the workpiece is brought in contact with the tip of the edge finder.

At some point the wobble will completely disappear, and the edge finder will run "true". By carefully tapping the handwheel a few more degrees, the edge finder tip will begin to "kick off-center" -- STOP any further handwheel movement!!! The mill spindle is now exactly 1/2 the diameter of the edge finder tip away from center. Since the tip of the edge finder has a diameter of 0.2", the spindle centerline is half that amount from the edge of the workpiece.

Now lift the spindle up to get edge finder up and out of the way, and move the mill table another 0.1" (half of 0.2") to the left -- Now set the mill's "X" axis handwheel indicator dial to "0".

The "X" axis has now been properly referenced to the centerline of the spindle.

NOTE: We can now move the "X" axis, but we need to accurately keep track the number of revolutions that the handwheel has been moved from "0" in order to reverse the procedure and get it back in alignment with the quill.

Here is a bad movie of the procedure. You can see the edge finder run very smooth and then suddenly move slightly off center when the center of the edge finder passed by the centerline of the spindle.

Now it's time to set the "Y" axis. Move the mill table so the edge finder is close to the "top edge" of the workpiece (NOTE: now we MUST keep track of how far the "X" axis is moved with the handwheel) and then lower the edge finder just below the horizontal plane of the workpiece.

Same as in the previous section, start the mill and bring the top edge of the workpiece in contact with the "wobbling" edge finder. Once it "kicks off-center" raise the edge finder and move the "Y" axis handwheel forward another 0.1" to set the centerline of the spindle with the edge of the workpeice. Now as before, set the "Y" axis handwheel dial to "0".

Here is a bad movie of the "Y" axis being aligned

If you properly counted the number of handwheel turns it took to get the "X" axis back to "0", then you are now perfectly aligned with the upper left hand corner of the workpiece

The next pic shows a center drill perfectly set on the corner when the "X" and "Y" dials on both axis are set to "0".

From this reference point, any hole or slot can be accurately machined by simply using the mill dials as a guide -- (no more haphazard measurements)

NOTE: It takes longer to explain this procedure than to execute it. Once mastered, this can be done in minutes. Those fortunate enough to have a Digital Readout on their mills can do this in seconds.