Part 006: The Case Front
This was probably one of the more labor-intensive parts of the Wren turbine. While straightforward, it did require some interesting chucking solutions.
The case front holds together most of the front end of the engine. The inlet cone of Delrin plastic is mounted in the middle of the piece, and the inner bolt-circle is drilled and tapped to accept this item. The larger bolt-hole circle contains 11 holes which attach the case front to the diffuser. Three of these are also service ports for fuel, gas, and lubrication. A fourth will probably become an air pressure tap. Additionally, the periphery is drilled and tapped to secure the outer case, along with an o-ring groove for a section of viton cord.
At the back of the case front, the profile is rounded to accept compressor output and direct it to the combustion chamber area.
|A short stub of 6061
aluminum was chucked and faced both sides. The outer
diameter was also turned to size, and the stock bored to
1/2" deep to accept the Delrin nozzle.
I elected to turn the rear portion first, which is nothing more than a 6mm radius fillet. The rear of the piece is first roughed out with a standard boring tool.
|Next, a form tool is
mounted, and the interior smoothed and a 6mm radius
fillet is cut into the periphery. Note the WIDE (but
thin) swarf generated by the form tool. The form tool
should be ground somewhat less than 6mm radius, as only a
portion of the cutting edge can be used at once, the
overall shape being generated by slight jogs of the
carriage and cross-slide levers.
The interior is deburred and polished. Not shown is the cutting of the o-ring groove on the exterior diameter using a fine parting tool.
|The entire stub is
mounted in the mill. Again, X,Y coordinates were
generated, and the 2 bolt-circles drilled, a total of 17
holes. I drilled through the case front and into
the stock enough so that after parting off, I could reverse
the case front and remount it on the same stub for
machining of the front of the part.
This need only be done for the inner circle which will hold the intake cone. The case front was parted off with enough stock for cleanup to final width, and the stub, with previously spotted holes, was drilled through for 3-48, the size I will use to secure the inlet cone.
|My camera failed me, so
I was unable to record a few of the steps. The inner
circle was tapped 3-48, and the case front was reversed
and mounted back onto the stub using 3 3-48 SHCS run in
from the back of the stub. This allowed me to
fully machine the front of the part after centering on
Machining required here was nothing more than a few lightening steps cut into the aluminum.
Pictured here is the rear of the case front which mates with the diffuser. The o-ring groove can be seen on the perimeter, and the 6mm fillet is apparent as well. The black delrin cone was inserted into its proper place in the bore.
|A frontal view. The case
front, with inlet cone attached, is resting on the
diffuser with the compressor wheel in place.
This part concludes the major work on the turbine. All that remains now is plumbing and a test stand!
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