Time to start working on the crankshaft. I cut a length of 1 1/4″ by 5/8″ bar to length and laid out the crankshaft from a 1:1 printout.
Then using a combination of drilling, a compressed air driven cut off wheel and the mill, removed most of the material for turning the first crank pin.
I marked off and drilled the holes on the ends of the work piece with a center drill for turning on centers. Then turned the first crank pin.
I only remove material with the drill and mill as I machine each feature to leave as much strength in the work piece as I can for each operation.
I had a bit of a brain fart and accidently used a dead center in the tail stock when I started turning the first crank pin. I caught my mistake before too long, but may have ruined the dead center. I installed a live center and continued machining.
Then I turned down the second crank pin as I did the first. This was all pretty standard lathe work turning between centers. Since the force of clamping the work piece between centers was straight through the crank pins being turned there was not distortion introduced by this clamping action.
I turned the crankpins to .500″ finishing the last thou with emery paper. I am not completely happy with the finish, but they got better as I progressed with the fabrication of the crank. I ground a cutter from a piece of HSS as shown below. The intent, though not necessity executed well, was a broad faced tool at exactly 90 degrees to its length, with a radius on each side and a notch in the middle to reduce the amount of the tool cutting the work piece. I have generous reliefs on the face and sides.
When the crank pins were finished to proper dimension measured with a micrometer, I made some precision spacers to fit between the crank webs. They may not look precision, but I was very careful to make these a good fit, not too tight and not too loose, milling first then running them on emery paper on my surface plate (my band saw table). I made a crank once where these were an interference fit and they actually flexed the crank so that when they were removed the crank sprang back. I bonded them in place with JB weld. Not sure if this was the best choice, the JB weld got soft when the work piece got hot from machining, but it seemed to work out OK.
Here is the setup in the lathe preparing to turn the center main bearing journal.
Here is the center journal for now. I left it .010″ over sized because I will finish it to final dimension in a setup where I turn all main journals at the same time to insure concentricity.
To determine the actual dimension between the two main crank bearing inner races, I fully assembled the crankcase, sump and main bearing holders with a dummy shaft with two bearings able to float as shown below.
Then reaching through the cylinder holes in the crankcase I spread the bearing and hot glued them in place. This gave me the actual dimension I needed to turn on the crankshaft shoulders that will rest against each of the main bearings. The design dimension was 2.3125″ and the actual measurement was 2.331″. I split the difference to maintain a centered crankshaft.
This worked exceedingly well as I ended up with a smooth crank with no end play.
The final operation on the lathe brought the three main journals into concentricity. They were all initially turned to between .005″ and .010″ over final dimension. I painted them with Dykem and used a thin strip of aluminum in the shape of a tool to rub on the journals to give me an indication of how far out I was. I was surprised that the journals were not perfectly concentric as I had been really careful. So I took very light .001″ cuts off each journal walking the carriage back and forth in multiple spring passes. Fortunately I had enough material on them to allow me to produce nice concentric journals. I brought the two outside main journals down to about .0005″ over, then removing and replacing the part in the lathe on centers test fitting them to the main bearings. I only used emery paper after I removed the part from the lathe to maintain my concentricity.
I am very pleased as to how smooth the crank turns in the engine now. There is no play in any axis and the crank turns under its own weight.
I have a few operations to finish the crankshaft, I need to drill oil holes that will deliver oil from the center main crank bearing to the big end rod bearings, I need to drill holes through the center of the crank pins (the ends will be sealed with plugs as there will be oil in there) to reduce rotating mass, and I need to add the counter balances.
Crank model with counter balances
I have started researching how to calculate the size and mass of the crankshaft counter weights. This is not a performance engine, won’t rev very high, and it is impossible to balance an inline twin with both pistons moving in unison anyway. But I want to get as close as I reasonable can. I am not going to wait to weigh the actual pistons and con rods, but can estimate their weight and center of gravity with the CAD program.
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. 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.
I will let you know how it works out.