Sunday, 13 January 2008

A new turbine wheel

I managed to get hold of some nice stainless steel round blanks 6mm thick and 66mm diameter - just perfect for the turbine wheel.

First step was to drill out the centre and remove some of the thickness from the blank around the middle of the wheel.

The hole in the middle is a potential weakness in a high-speed rotating component, thinning out the mid section of the wheel reduces the spinning mass without sacrificing too much strength.

After boring and thinning, I used a metal-cutting bandsaw to slot the blades.

Each blade was then twisted - I was aiming for an angle of 35degrees but the bandsawn slots weren't wide enough - I guess the final angle here is about 25 degrees.

I started out heating the blades to red-hot before twisting but ran out of gas after a couple. It didn't matter, as it proved perfectly possible to twist the blades without heat.

Note the tool I made for the job, it's mild steel, about a foot long.

I had to make a mandrel for the wheel in order to mount it in a vice - Just a couple of pieces of scrap aluminium and a bolt through the middle.

Now the frightening bit.

With the wheel clamped firmly in a vice, I used a hand-held grinder (125mm cutting disc, 4mm thick), to cut down between the blades, reducing each to a crude faceted-airfoil shape. (see diagram below - looking down on the edge of the wheel, I cut almost straight down but followed the blade edge to produce a thin tail to the airfoil)

The grinder is a heavy and powerful tool, designed for brute force rather than delicate work. I had to brace myself firmly and wear full polycarbonate face armour, dust mask, gauntlets and leather apron against the sparks.

I had to fight to hold the monster steady, It felt like cutting my toenails with an axe, but the end result was surprisingly neat

Final shaping was done by grinding with a 'Dremmel' tool.

Each blade is shaped to an airfoil section with the front of the airfoil blade making an angle of about 80 degrees to the disc. The blade is curved so that the 'tail' is nearer to an angle of 35 degrees and the underside of each is as evenly 'scooped' as I could get it (working between the slots there isn't much room for the cutting wheel).

Ain't it pretty?

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