Part 025: The Turbine Wheel
The turbine wheel for the Wren 54 is one of those items best purchased. Early in the days of model gas turbines, the only way to get a functioning turbine wheel was to make one yourself, a very tedious, hit or miss process which starts with a sheet of stainless steel. The blades are bent and shaped by hand with a grinder. While I could theoretically do this on a larger engine, the Wren is small enough to require a turbine wheel matched to the compressor throughput and of the correct material, in this case Inconel 713. The turbine wheel can be had unmachined, or you can purchase it bored and balanced for an extra $75. A few hours work saved me that extra $$ when I bored this wheel myself.
|Like many machine tool
operations, the creation of a special jig and the setup
for this operation consumed 5X the time required to
actually execute it. The Wren operations sheet details a
clever and effective fixture to hold the wheel. I could
not think of a better way to do it with accuracy. I
considered a pot chuck (a combination of 5C collet and
expanding plate) but the simple act of closing the collet
lever is enough to throw the fixture out of truth. So in
the end, I followed the instructions.
First, a large aluminum round is drilled and tapped for securing cap screws. The Al round is chucked, bored, and a large ring is parted off. The remaining stock is left in the chuck and carefully faced with the carriage locked for best accuracy.
|A recess is bored to
accept the wheel for a sliding, accurate fit. This was
tedious, as it is difficult to measure the diameter of
the wheel with its odd blade number, so I had to sneak up
on it a few tenths at a time.
At the root of the blades is a ring which becomes the surface contacted at the bottom of the shallow boring. This ring can be seen here, with the wheel successfully placed inside the fixture.
|The previously parted
ring is reattached with 10-32 SHCS and snugged moderately
and evenly tight. I first gave the wheel a deep center
drilling. The Inconel is tough, even more than the SS
from the NGV, and one must be sure to keep cutting
pressure up and the chip-flow continuous.
I then chucked a bit especially purchased for this operation, a straight flute, carbide drill of 15/64" diameter. With trepidation I applied the drill, but it cut nicely in one continuous plunge.
From what I could measure, there was no run-out of the drill, and I could concievably have reamed it straight away to 1/4", bit I elected instead to bore it for 1/3 of its depth to .245"... this would start my cobalt, spiral-flute reamer properly, and any remote eccentricity would be corrected.
|The spiral reamer worked
properly. After facing the other side of the bore, I
slipped the wheel onto a 1/4" carbide end mill
shank, a perfectly ground diameter of .2498. When
rotated, I could feel a stop-and-go stuttering left over
from the reamer flutes. This is an inevitable result of
reaming rather than boring to size, and I have seen it in
many other pecision reamer operations. While the wheel
probably would have worked fine, I elected to lap the
bore to perfect concentricity. This was done by charging
a brass rod of scant .250" with 500-grit clover
compound, and using a pin vise or VERY slow speed drill,
to "pump" the lap into the bore. The resulting
bore was perfect, with a flawless feel when mounted on
the carbide shank.
I DON'T recommend lapping unless you have considerable experience with laps, as it is possible to bell-mouth the bore, or worse, freeze a too-aggressive lap in place.
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