Beam Engine Crankshaft

Having finished the crank, the obvious next step is the crankshaft. This will link the flywheel to one end of the beam and introduce the first interesting motion to the project. It is also the start of the wee small bits, at least according to my experience. Learning experiences were anticipated.

First I tackled the end that goes on a pin sticking out of the crank. I again used the method of milling the piece out of a larger bit of stock with the extra used to hold on to. In a flash of enlightenment, I realized I could up the game a bit by using my small toolmaker’s vise to hold the stock. I could then rotate the part 90 degrees without unclamping the stock.

The major work was done with the vise clamped to the rotary table. The fun bit was getting the appropriate spot centered on the table. I did this by center punching the spot, deploying my fancy new coaxial indicator which came with a tip for just such a purpose and gently tapping the vise around until it was centered. I then clamped it down and rechecked to make sure I didn’t shift it. Surprisingly, I also managed to get the thing square in the x-y plane to within 30 arc minutes. (I needed to know where to start and stop the rotation of the table to be planar with the straight sides of the piece.)

After I got the main body of the piece shaped and drilled, I needed to drill a hole in the flat end for the shaft. This was easily done by clamping the vise on its side in the mill vise. I found the edges by using a drill blank because there wasn’t room to get my center finder in there next to the vise jaws. I know that things are at right angles because the toolmakers vise has precision ground sides for just this sort of thing. I may have committed a sin by clamping on the jaws of the vise, though.

The last operation was just reclamping the piece wide side up and skimming off the back side. This was easy in this case because there are two parallel sides unlike the crank.

Next, I made up the crank shaft itself which was a simple matter of threading one end of a 3/32″ brass rod and cutting it to length. For the exercise, I used the lathe to square and shorted to final length the cut end. Here is the piece to date. I still need to do the other end which has a fork to go around the beam. I added my machinist scale to the photo for a size comparison.

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