Building a Disk Turbine
Cast disk blank. After the disk is initially surfaced on the lathe, the ports are being milled with the milling attachment. Two more lathe surfacing operations will create a raised hub section and final disk faces.
|April 21, 2003
Casting the Disks
I decided to give myself a little extra thickness to play with and settled on a disk gap of .017". Disk thickness would be .040, which seemed thick enough for adequate stiffness while machining the disk surfaces. This is probably a little heavier than needed, but if I wanted to cut things closer in future trials, it is would be easier to remove material than to cast and machine all new blades.
I made 4 disk patterns out of the scrap plywood from a small Clementine orange box left over from Christmas. Using a 3" hole saw, the pattern material was quickly cut into circles. Some auto-body putty to fill flaws, two coats of lacquer based sanding-sealer with talcum powder added, and two coats of spray lacquer finished the patterns. The patterns were sanded after each paint coat with 300 grit wet or dry paper.
Orange crate to turbine engine: cutout, finished pattern, rough casting, and machined turbine disk blank.
I bedded the patterns in a facing layer of greensand composed of #130 mesh sand and Bentonite clay. This is a little too fine for good permeability, so I backed the main with a coarser mix of fine masonry sand and fire clay. This was the same stuff I had cast the lathe and milling attachment with.
(c) Copyright 2003, Stephen Redmond, all rights reserved
The cope and drag sand was vented with wire right through the cavity faces. The sprue was 1" in diameter. I arranged all of the patterns around a single sprue. It is important with castings this thin that the runners be made wide. If not, the metal will chill before it fills the molds. Since the sections are so thin, you have to make the runners wider than normal in order to get adequate flow.
I got it right on the first pour of four disks, but failed on the second. The third and fourth pours had adequate runner width. Metal temperature was a little hotter than I normally pour with. I used fresh pistons rather than previously poured ingots or sprues, as I find that these tend to give the best castings with the fewest flaws.
April 28, 2003
Skipping ahead a little bit, I just cast and machined today a new stator for the turbine. I've got a lot more to say about the turbine disks, assembly and the the trials -- but I've done so much in the last week that I need to catch up with the current stuff.
The new part is a diffuser housing which will allow me to try the turbine as a compressor. It's basically a ring with set of twelve scrolls that mate with ports drilled through the back of the bearing plate.
Out of the Ashes
The casting method was the easiest and most basic method imaginable. My wife Cheryl was burning some downed limbs and brush in the yard for Spring cleanup and I decided to see if I could melt some aluminum in the fire rather than use the charcoal foundry. I thought I'd try to pour the stator if it worked out.