Compressor and Turbine Nuts


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Why am I showing such a simple turning operation? Like many items in a turbojet, these parts are deceptively simple. What would work fine in a static situation would be absolutely unsuitable for a dynamic operation, where concentricity and imbalances are critical.

The photos below show the compressor spinner nut. Similar care must be taken with the hex turbine nut of 316 SS. The main difference is the profiling of the nuts and the difficulty in centering the hex stock for the turbine nut.

A section of round aluminum, in this case 6061, is chucked and the outer surface cleaned up in the lathe. Turn enough aluminum so that the turned portion can be parted and rechucked with sufficient stock extending for the nut. Measure the diameter of the parted, turned stock to within .0005 if you can.
  The stock is mounted in a mill, and the exact center is positioned for cross drilling of the tommy-bar hole. The end of the stock must be also located for longitudinal position of the hole. I used a small end mill to spot the round shaft for subsequent drilling. This helps minimize drill drift.

I drilled mine .093 for the tommy-bar.

  Replace the stock in the lathe, and using a dial indicator, set it up for a dead-true turning operation. This will ensure the tommy-bar hole will remain centered.

My nut will be threaded 1/4 - 28 LH. The nut is drilled perhaps .010" shy of the correct tap drill size, and a small boring bar is run in to center the hole. A final drilling is made for the tapping using a #3 drill. Using a tailstock attachment, the tap is started for at least 4 or 5 turns to ensure concentricity.

  The nut is removed and the threading finished in a vise. A bottoming tap is required.
  Maybe this isn't necessary, but again for absolute minimal eccentricity, I reassembled the shaft with the bearing, spacer, and compressor in place. Before tightening the tapped nut, I checked the runout of the shaft and corrected for 0 TIR. Do not skip this step!
The profile isn't critical but should be smooth for the airflow. The outer diameter was turned down to match the compressor boss. Next, I used a homemade radius tool to generate a simple hemispherical shape at the nose. Form tools or freehand work will do fine, but be sure you don't use too much pressure and spring the shaft!

Follow the turning with files and paper to smooth and polish the aliminum.

The finished product, 0 TIR, shiny to boot!

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