Pocket NC Touch Probe – Motivation (Part 1/4)

When machining, you need to accurately position the cutting tool with respect to the workpiece. With the stock Pocket NC, there are two methods for doing so. The first is to rigidly locate the workpiece with respect to the B axis reference point using a fixture. The second, is to do manual touch offs. Nearly all of my work so far has relied on the former method, as using a manually touch off on a machine without manual controls isn’t all that precise or pleasant. And while possible, it is tedious to touch off against features more complicated than a single edge.

My Approach So Far

To make that first approach work, I’ve been making 3D printed fixtures (1, 2, 3, etc) to hold the work while simultaneously registering it to the mounting holes or alignment pins on the B axis. Simultaneously, I need to keep the machine well calibrated so that the X, Y, Z, A, and B offsets are all aligned as closely as possible to the center of rotation of the A and B axis.

About the best alignment I’ve achieved between calibration and my fixture is about 50-150 micrometers (2-5 thousandths of an inch). So for any parts or features which need relatively alignment better than that, I need to design it such that all the features can be completed in a single operation without moving the A or B axes.

To date, all the parts for the qdd100 servo were designed with that constraint in mind, so that I could prototype them locally. The rotor and stator need to be aligned with precision approximately 25-50 micrometers, and there are multiple pieces involved in maintaining that alignment, all of which need to be machined accordingly. However, designing the assembly to satisfy those constraints made actually assembling it a relatively time consuming operation, which I would like to improve.

Touch Probes

A third method of workpiece locating, not mentioned above because the Pocket NC doesn’t support it natively, is with a touch probe. A touch probe, is a stylus mounted in the spindle like a tool, which can detect when it is deflected in one or more axes. Then the machine can move the spindle around, and sense where the part is located with respect to the machine coordinates directly.

In this series, I’m going to explore adding a touch probe capability to my Pocket NC. We’ll see how it goes!

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