I had a short session in the shop tonight after meetings and dinner with the team at work though reaching home a little late. The crank disc was done. This is a simple part measuring 1/2″ in diameter and 1/8″ thick. There were 2 holes to be drilled; a 1/8″ hole in the middle for the crank shaft and a 1/16″ for the little crank pin that will be driven by the piston. The supplied 12L14 steel measured exactly 1/2″ in diameter, which is what’s called for in the plan. I merely sand off the rust on its surface using sandpaper on the lathe running at slow speed. A #0 center drill was use before drilling the 1/8″ diameter hole.

The 1/8″ hole was then drilled.

I paused for quite a while thinking how the 1/16″ hole should be done. According to the plan, it should be 1/8″ away from the center of the disc. I thought of re-chucking the job in a 4 jaw and offset it like the way shown by some folks on YouTube. But I don’t have an idea how to offset it by the required 1/8″ from center. Since I’ve the mill squared up, I thought using one of the horizontal axes to do the offset. First, I’ve to find the center of the job.

This was what I did

The 1/8″ hole that was drilled was aligned with the MT1 center. Once the center was established, I moved the x axis by 3.175mm and drill the 1/16″ hole. This is the smallest hole I ever drilled since I started this hobby. I was afraid to break the tiny HSS drill but the operation went without a glitch.

Hole done

The stock in the chuck was transferred back to the lathe to be parted off.


But… the 1/16″ hole wasn’t deep enough to break through the bottom…

The challenging part was to mount the stock back to the 3 jaw to face the other side till the 1/16″ hole show up. This took me quite a while till I decided to hold a engineering square to the face with the motor turning at slow speed. After a few attempts, I managed to get the part turning without visible wobbling.

Facing was done carefully as I only have a little stock sticking out of the chuck.

I stopped with the hole showed up.

Test fit was done with the supplied pin & shaft.

The fit of both are rather tight. I’ll still use a drop of Loctite to avoid the parts falling apart.

I reported in my previous post on the flywheel that it’s 1/8″ hole was a sliding fit to the crank shaft. When I tried inserting the shaft to the flywheel, I couldn’t get the shaft into the hole. The part must be warm enough the last session to allow the shaft to just slide in. I think I’ll heat up the flywheel when inserting the shaft during assembly.

The completed disc with pin & shaft viewed from another angle.

That’s all for today. I’ll be making the piston in my next instalment before starting on the rest on the mill.

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