By HaJo Franken

Machining the Trestle Steam Engine with straight lemniscate guide was challenging. I had to go beyond my usual machining limits in the area of the shut-off valve and throttle; the normal range of my turning and milling with a flywheel with a flywheel of 330mm dia.

The first thought was to get a steel ring 330mm on the outside, 306mm on the inside and 8 mm thick and then to use form-milled spokes and hub. After about a year of not intensive and unsuccessful search, I decided to try to mill it myself.

Since my RC10BF milling machine and a rotary table of 100mm diameter would probably be overtaxed with a heavy flywheel, aluminum was considered using the lowest cutting depth and feed which  resulted in a machining time of approx. 35 hours.

I compensated for the loss of centrifugal mass due to the light weight with a groove of 9x6 mm in the outer ring, which I filled with lead. Roughly calculated, the aluminum wheel would have been approx. 350g, and the lead-filling 950g where 2/3 of the mass is in the outer ring and thus forms a good centrifugal mass in relation to the total weight.

So two aluminum plates 340x340 mm, one as a reserve in case of milling errors, were bought and get started (only one was needed in the end).

First outlined a circle of 335 and milled off the waste corners.


Then the plate was clamped tightly on the turntable with four screws always remembering not to touch the screws until all radii have been completed. All milling work was carried out with 8 mm roughing, finishing and radius milling cutters.

First the outer contour with a roughing cutter and then with a finishing cutter to size.


Then the groove cut with a roughing cutter so that the cast lead gets a better hold with that finish.


Next, I marked the spoke contours; it is easier to mark on the work table.

Finally the inner radius of the wheel was milled out and the fit for the hub is spindled out in the centre before unclamping. To do  this I had to grind away 1 mm on the mill column. If I had known that I could have made the wheel 328 mm!


Through the hub and on the outer edge, I now had two points for secure clamping. The milling cutter was aligned with the marking lines and the segments were milled out. Since the spokes are conical, all eight edges had to be aligned separately.


Then did the same again to work out the spoke contour with the radius cutter.

Finally, just poured lead, glued in the hub, followed by a deburr and light grind. I can only say the self-made flywheel is more fun than a finished bought one.

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Editor: David Carpenter