Turbine Shaft Part I


Not counting the intermediate shaft, which requires only pressing of two gears together, there are two shafts to be turned, the power turbine shaft and the propellor shaft. Of the two, the turbine shaft is perhaps more critical, as it turns at a much higher RPM than the prop shaft, hence, excess runout will be evident in vibration and poor bearing life. Turn with care, and be patient. Shafts look easy, but in reality are challenging to make properly.

  A length of supplied EN24 steel is faced to a dead-true length, centered, and center-drilled both ends. It is important that the length is exact, as we will be swapping the shaft end-for-end, with the measurements for the shoulders taken from each end in turn.

After the earlier, disastrous efforts on my MW54 turbine shaft, I decided to approach this one differently, more in line with Mike's method. This entails roughing out the entire shaft, leaving perhaps .035 mm oversized in all locations; then, the shaft will be chucked between centers for the final work, to include turning and polishing the critical journals. Do not be satisfied with a poor shaft... if any of the surfaces go too small for the correct fit, scrap it and start over. Otherwise, you risk damage to some expensive components, namely the bearings, power turbine wheel, and possibly the gears.

Hereafter, for the roughing stage, when I refer to a diameter such as 12mm, I am implying 12.04mm or greater. If you doubt the accuracy of your setup, leave excess material in place for the finishing operation.

Chuck enough of the shaft blank so that you can turn the 12 mm portion (major OD) in its entirety. This will then be the portion gripped after we swap the shaft end-for-end. Use the tailstock for support; otherwise, the shaft will spring away from the tool resulting in odd diameters along its length. Then, the shoulders for the forward bearing and pinion are turned.

Since I am a primitive imperial toolist, my threads on this shaft will be 12-24 (~.220" / 5.6mm) for the pinion end, and 1/4 - 28 (6.3mm) for the turbine end. The blank thread portions of the shaft were turned appropriately.

The shaft is released from the chuck, reversed, and clocked true using, in this case, and adjustable 6-jaw chuck. Use a 4-jaw chuck if you cannot adjust, or ensure that the shaft is running within the correct tolerances given the excess material left. For example, if your diameter was turned to 12.1 mm. be sure runout is less than .05 mm, so that when you clean up in the finishing stage, all critical diameters are truly concentric.
Here, the power turbine end is attacked with a carbide roughing tool.
My roughing took 2 stages, the first stage to perhaps .010" / 0.25mm oversized, the second to the previously mentioned .035mm. For both the second stage roughing, and eventual finishing, I ground a HSS tool bit which worked very well. Quickly reviewing lathe tool geometry, we remember that top rake, as it increases, reduces the cutting pressure applied to the tool for a given cut. Most carbide tools are designed near 0 rake, and this is best for powerful, rigid, industrial machines, but for a lighter lathe, a bit of rake makes for a better cut, and an ability to shave off tiny amounts of material. The bit seen here has almost 40 degrees of top rake! Quite a bit for a lathe; care must be taken to ensure the tip is at or slightly below center height, and only light cuts can be made, or the tool may dig in. Hone the tip to razor sharpness; don't be satisifed with the tool as it comes off the grinder.

With this bit, no problems taking off 0.0002" / 0.005mm, continuously, in one pass. Dull tools or less rake would simply rub with this sized cut.

As an experiment, I made a crude toolpost grinder from a Foredom handpiece and a 4" wheel. You'll have to read Part II to see if it works!

Turbine Shaft Part II | TurboProp | Home