The Combustion Chamber, part 1


Please be patient, lots of larger photos to load...

I may be wrong, but a lot of machinists are intimidated by any sheet metal work. I'd rather machine a tray from solid than try to bend one up from sheet metal. But there was no way around the fact that the gas turbine would take a bit of work with stainless steel sheet. Fortunately, with the purchase of the combustion chamber pack of parts, most of the hard work is already done. All that remains is rolling, extruding the air holes, and assembling the chamber.

There are three stainless cylinders which must be formed or rolled into tubes... the combustion chamber (cc) inner, cc outer, and the outer case. Each of these requires an overlap seam be formed in the end of the sheet. Note the green plan print which shows the overlap. This can be done with a lot of beating with a hammer, or it can be done neatly and elegantly with a die.

I constructed a die from the crs rectangles shown. This is a simple mill-drill job, with the step being milled on both sides of the die, so the steel is shaped into somewhat of a "z" shape when pressed. A scrap with a successful seam is show.

When you make your die, make it long enough to do all three parts. If you size it for just the combustion chamber, it will be too short for the case outer wrap!

With the seam in place, the trusty (but crusty) grizzly roll came into play. This was a lot easier than I thought it would be. Rather than roll it in one pass, I slowly crept up to the correct diameter through several passes. I was afraid that the roll would press out the seam, but it doesn't hurt the seam too badly.

Here, the cc inner is being rolled to the rather tight diameter required.

Ahh, thank goodness for cable ties! I was having a devil of a time wrapping up (and holding) the tube to the correct diameter for spot welding of the seam. The idea was to hold the tube stationary and at the correct diameter, and then deliver a couple of preliminary welds. A pair of cable ties did the job perfectly. I was even able to cinch the cable tie down rather tightly around the flange spun into the cc front, shown to the left as the disk at the bottom.

The first spot weld of the seam. Note that I am delivering a single spot as close to the cc front as I could get. Next, the cc inner was tested for size with the nozzle guide vanes, a portion of which grips this from the outer diameter of the tube.

After the second spot, the cc inner seam was welded along through its length.

The two larger sets of holes in the cc inner must be swaged out to a larger diameter. The main purpose of the swaging is to create a "crater" in the steel which will inject the air deep into the combustion chamber. Note that this is the cc inner, which surrounds the shaft tunnel, and is open through its middle to pressurized air from the compressor output; hence, the holes must be swaged as shown so as to inject the air into the combustion chamber.

I turned a 1/4" square of CRS into the required male 60 degree cone, and clamped it into a big boring bar, which I then held in a bench vise. The cc inner hole is positioned over the punch, and a female die is used to swage the hole. Note the shape of the resulting swage. Two rows of holes are completed in this fashion in the cc inner tube.

Now, we can proceed with welding the cc inner to the cc front. I had to modify my spot welder a bit to get into the tight confines of this joint. A couple of welds start the process, and then, when alignment is verified, the entire seam is welded tight. It is important to minimize air leaks in the chamber seams, especially at the front.

The cc outer is produced in much the same way as the cc inner, including a set of swaged holes. Once it was sized and seamed, it too is welded to the cc front. Still required on the cc outer are the swirl jets, and the two plug bosses. I plan on using a 1/4 x 32 spark plug for ignition rather than a glo-plug.

Here is a cool shot of the actual weld taking place. Each spot weld takes exactly one second.

Overall, the chamber is proceeding nicely, and is actually a lot easier than I thought it would be.

Gas Turbine | Home | Links | The Shop