Part 035: Cast Nozzle Guide Vanes

The NGV came as a nice lost-wax stainless steel casting. This particular alloy machined reasonably well, but did call for a rigid setup and very sharp new tooling. Certain portions of the casting were turned or bored with ground HSS bits, while others were turned with indexable carbide tooling. In any case, be sure the bits are either new inserts or carefully honed HSS. Like most stainless steels, the casting will work harden, which means if your tool dulls, or the feed isn't reasonably aggressive, the steel will toughen and harden, causing your bit to rub and make the situation worse!

Concentricity is vital, and as the casting must be reversed to machine each side, some method of centering in the lathe is crucial. I used a Buck Adjust-Tru chuck... a 4-jaw chuck will do the job as well. A normal 3-jaw chuck simply will not have the accuracy for this procedure. You cannot unchuck a part, reverse it in the lathe, and expect anything better than .003" or worse out of concentric truth. I selected the shaft tunnel boring, which was complete during the first chucking, as my "datum" to center the part for subsequent turning operations.

The plans called for some lightening cuts/undercuts in certain areas which I chose to forego. This was a personal and not an engineering choice, as I don't mind an engine which weighs a few ounces more than spec. Additionally, as I had the combustion chamber rear of spun SS on hand (purchased), I turned that portion of the NGV casting to fit this part, which turned out to be ever so slightly (.010") undersized.

The casting is first chucked with the aft side out. Centering was done with an indicator using the turbine shroud portion of the casting. The plans called for a skim cut only to clean up the casting. After a most careful centering, the skim cut, just enough to get to bright metal and form the turbine shroud area, ended up .010" oversize. WOW! As soon as I measured that ill-fated cut, I grabbed my cast turbine wheel, and with the casting still chucked, checked to see if the tip clearance would be excessive. The turbine JUST slipped in, so I think I may be OK. Excessive clearance between the turbine blades and the wall would be fatal unless I could add a sheathe of SS to close the gap. Anyway, I think Wren needs to add a bit more meat to the casting in this critical area.

With the turbine shroud bored, I faced off and cleaned up the rest of the rear of the NGV to print.

Next, the center hole which mates with the shaft tunnel is bored. Again, concentricity is critical. Take great care with your boring tools to assure a fine, accurate finish. With as much as possible to be machined off of the rear accomplished, the plans called for the NGV to be reversed and lightly gripped on the thin turbine shroud or rim, neither of which is by this time very stout. As the forces involved in machining this SS are high, I elected instead to drill and tap the periphery for 8 x 4-40 holes, which allow the NGV to mate with the case rear. My plan was to bolt the casting for subsequent machining to a strong, beefy ring to be held in the lathe chuck rather than the casting itself.

On the mill, the NGV is centered and the 8 4-40 holes are drilled and tapped, and the burrs cleaned. Tapping is scary in this material. Use a new tap, preferably a set of 2 new taps, one taper and one plug. The taps can be swapped out as necessary to finish the job, alternating the taper and the plug tap. Don't break a tap! Go easy.

Back in the lathe, you can see the NGV casting is bolted to a ring of mild steel. I could then aggressively tighten and center the casting for the coarse operations on the front without fear of slippage or damage to the thin steel walls on the rear.

On the front, 2 critical diameters are turned, the larger for the combustion chamber outer, and the other which will grip the inner. The paths for the exhaust gasses are turned to a 30 degree angle, and in general the NGV is machined to print.

Four separate pictures of the same casting are seen here. On the left is the rear of the NGV. Next is the front, and thirdly, the front half with the spun combustion chamber rear slipped into place. You can see the 12 holes for the sticks pre-drilled. Lastly is the rear of the NGV, this time with the unbored turbine wheel slipped into the turbine shroud. A close check shows the current tip clearance to be perhaps .002", which is not optimum but which should allow the engine to run fine.

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