A Precision arbor for turning compressor nuts
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One of the real headaches with either hall IC tach sensing or the use of a magnetoresistive sensor is the requirement for a magnet to be installed somewhere on the rotor shaft. The easiest method is to create a custom compressor nut. My previous nut works fine but is very labor intensive and not suitable for any quantity of production. To correct that, I spent literally days looking for a source of rare earth (Neodymium or Samarium Cobalt) ring magnets. The ring must fit over the shaft, but also have an OD under 0.500" so that it will not extend beyond the compressor nut. The real problem was to find said ring with the poles through the diameter rather than on the faces of the ring. In the end, I found the ideal magnet. This is a class 40 Neodymium rare-earth magnet with an ID of 0.250"/6.35 mm, and an OD of 0.437"/11.1 mm. To set up my shop for repetitive machining of compressor nuts to accept these rings, I needed a rigid, dead-accurate arbor with the correct 6mm LH thread. This page details this bit of work.

A piece of 5/8" dia 12L14 steel was chucked. The shank of the arbor was then turned to exactly 0.500" for subsequent chucking via a precision collet or adjustable 3-jaw for dead-true turning. I made the shank 1.5" / 38.1mm long. The end of the shank was then turned in preparation for the threading.

Proper relief was turned with a parting tool at the base of the threaded portion. Next, the ends of the threaded shaft (both ends) were relieved at a 60 degree angle with a normal threading tool. I am using a mini-thin shank and tool tip to do this work, which is done freehand.

Thread cutting is done using normal lathe practice. Since this is a left-hand thread, the travel of the tool is from left to right. The tool tip is positioned in the relief cut next to the base of the threads, and with the lathe turned on, the leadscrew is engaged so as to force a left to right travel. Left hand threads are easier than right hand threads because when the cut is complete, the tool tip doesn't bang into any shoulders! It simply coasts to the right into the clear area of the lathe.

Getting close! As you approach the correct depth, it is important to check the fit of the nut. To do this, I was using a stock Wren compressor nut. I wanted a fairly tight fit of the nut, but more importantly, I wanted the nut to easily be able to bottom out on the precisely faced shoulder at the base of the threads. This is important... if you simply screw the nut onto a threaded shaft, but not allow it to bottom, it will be badly canted out of truth and will not be in balance when installed on a turbine.

Success! A tight, but not overly so, fit of the Wren nut. The plan for this arbor is to create nuts from round stock, embed the magnet in the base, then tap. The raw nut will then be parted off, and installed on this arbor for final turning and contouring.
After the successful threading, the arbor is parted off of the raw shaft, and the rear of the arbor faced and deburred.
The final arbor, ready to go to work for me