This will be a quick update on the status of the crankshaft — The only operations I have left are threading each end. I have not cut threads on my new lathe and don’t want to learn on my crank with its many hours invested. I will practice on scrap and then finish off the crank later.
We left off on the design and placement of the counterweights
Since the pistons do move in unison the engine can be modeled as a single cylinder engine with twice the mass of one of the con rods, pistons, rings and wrist pins. This the two yellow rings in the assembly above. The formula I found is as follows: Weigh the top half of the connecting rod and add it to the weight of the piston, wrist pin and rings. Then take a percentage, say 55%, and add it to the weight of the bottom half of the rod. Place this weight in the CAD model at the center of both connecting rod journals on the crankshaft. Then adjust the weights of the counter weights to balance the entire rotating assembly.
The counterweights needed to be slightly wider than the crank webs, they could not extend toward the ends of the crank because they would interfere with the sides of the crankcase, but there is room inside the crank webs toward the conrod. I use two 4-40 cap head socket screws to secure each counter weight.
Before I set to work on the counterweights I did some cleanup and material removal on the crankshaft. I fabricated two plates that I clamped to the sides of the crankshaft to insure I did not knock the crank journals off center with further machining. I reduced the radius of the crank webs and trimmed the sides of the crank webs, all to reduce the mass at the conrod end of the crank. In hindsight I should have done this machining before I finished the main and center journals, but I was impatient and wanted to see how well the crank ran installed in the crankcase.
Then on to the counterweights.
First I rough cut some 1/4″ mild steel plate.
I used the mill to drill three holes to align with three threaded holes in an arbor mounted in the lathe. I then turned the outside radius of the counter weights.
I used a cutoff wheel to slice the round disks in half to give me two counter weight blanks. I then mounted them in the mill, zeroed the Z-axis against the parallel under the blank and machined the top side as shown below.
Below is the first counterweight test fit on the crank.
I then used the following process to accurately locate the mounting screws for the counter weight where I wanted.
- I marked each counter weight and its associated crank web so they would not get mixed up later
- All of the following work was performed on the mill, I drilled two holes in each counterweight so they would be centered on the crank web, so the holes ended up slightly off center on the counter weight. The holes I drilled were the diameter of the drill bit used for the 4-40 tap.
- Then I mounted the crank in the mill vise and match drilled one of the holes. through the counterweigh, into the crank web.
- I then tapped this hole in the crank web.
- I drilled out the one matching hole in the counterweight to a tight clearance fit and mounted the counter weight to the crankshaft with a 4-40 screw.
- I then mounted the assembly in the mill vise again and match drilled the second hole in the crank web.
- I tapped the second hole in the crank web
- I mounted the counterweight in the mill vise and drilled out the second hole to 4-40 clearance size.
- I used a 3/16″ end mill to create my counter sinks for the heads of the socket head cap screws.
- I then repeated this process for the remaining three counter weights.
Below is a close up of the mounted counterweights
The final operation I performed was drilling the two oil holes that will deliver lubrication to the large conrod end bearings from the crank center bearing.
This is the current state of the crankshaft. So far I have not screwed it up and it still turns true in the crankcase.
