Clever tools for finding an edge

Traditional edge finding on a milling machine is pretty straightforward. Edge finding establishes the milling machine’s measuring coordinate system relative to the newly inserted workpiece so the user can perform accurate work. “Picking up an edge” is the general shop description of this operation.

There is an array of mechanical and electronic tools for picking up orthogonal surfaces on a machine tool. Wiggler-type edge finders are common ones (Figure 1). They have a spring-loaded eccentric tip of known size that rotates closer and closer to concentric until it becomes perfectly tangent with the edge under interrogation.

Figure 1. Wiggler-type edge finders have a spring-loaded eccentric tip of known size that rotates closer and closer to concentric until it becomes perfectly tangent with the edge under interrogation. All images: T. Lipton.

Some shop workers debate the proper method to read a wiggler edge finder. The two competing methods are when the edge finder eccentric jumps sideways under the tiniest axis advancement and when, just prior to jumping, both the shank and eccentric are visually running perfectly concentric. The difference in accuracy for each method is very small, with several other factors contributing to the overall accuracy of the edge pickup.

When performing toolmaking or repair work on a milling machine, it is common to establish an accurate datum on nonorthogonal faces or edges of angled surfaces. The traditional wiggler edge finder does not work well for angular situations.

Kern Tool and Pyano, however, developed clever solutions (Figure 2). The round sections of these wire EDM-cut edge finders have a precisely known size. Both can be made from prehardened tool steel for long life and durability. The right-angle model (Kern Tool sine center) has two flat datum surfaces that intersect at the exact center of the round boss.

Figure 2. A Kern-type edge finder (left on surface in front of angled workpiece) and Pyano-type edge finder can be made for angular applications. Note the tiny corner relief at the center of each to clear any burrs on the workpiece.

With a standard wiggler edge finder and a Kern or Pyano-type finder, you can now pick up the precise corner on an angled setup using the standard edge-finding technique. The Kern edge finder also allows calibrating the vertical height of the corner under interrogation by touching off on the top of the known diameter, say, 0.500″ (12.7mm), and subtracting one half its diameter. The Kern style works best on acute angles or perpendicular corners.