Gallery, Projects and General > Project Logs
The Return of No. 83, a Hot Air Engine
vtsteam:
I spent about an hour lapping and measuring. The oak lap did a really great job of straightening out the bore, and it also left a really fine finish, very unlike the aluminum lap. I used the same valve grinding compound with each as an abrasive. I can't account for the difference, but I'm definitely convinced oak makes the best lap on a cylinder of this size and type, and very easy to make. I haven't tried any other hardwoods. I don't think softwood would work as well, but haven't tried it.
I could see a reflection in the finish:
vtsteam:
With the cylinder lapped, it was on to the graphite piston. I had to cut the rod in half so it would fit in the spindle behind the chuck. I faced the end and started turning. I think I like the quality of this latest graphite even better than the original piston. It seems finer grained and more slippery. It cuts like butter, and it was only three cuts until I was at final dimension -- a hard slip fit for the cylinder. I'll lap the piston to a sliding fit later with a scrap of newspaper - that's all it takes.
vtsteam:
Then it was time to set up the vertical milling attachment, with a milling spindle to cut a cross slot in the base of the piston, where the connecting rod will go. It was getting late, and I hadn't set up a DTI for depth of cut yet, so I decided to wait until tomorrow to complete the operation.
vtsteam:
This morning I measured everything carefully, and decided to lengthen the piston by 80 thou and move the wrist pin further forward to compensate -- mainly to reduce the tendency slip stick in the cylinder. I slotted it .5" wide using the mill spindle in the milling attachment. That went really well.
vtsteam:
Time to drill the wrist pin journal. I didn't have a way (yet) to do that with the milling spindle. It's not yet possible to mount the milling attachment 90 degrees to the slots in the carriage. To do that I'll have to slot the present milling attachment's mounting holes to fit the table spacing.
But, no problem, we'll do it in the drill press.
I centered a chucked drill bit in a vee-block, then clamped a scrap of aluminum as a fence against the block. As long as the vee-block is tight to the fence, it will present its center to the drill bit. But it can slide along that fence. Then I clamped the graphite blank in the vee block, using a bit of cardboard under the clamp to protect the blank.
I checked the height of the piston slot to the drill table at both sides to ensure it was horizontal. Then I marked the top of the piston where I wanted to drill the journal hole the proper distance back from the end.
Then it was just a matter of sliding the Vee block along the fence until the mark was centered under the drill bit, and the hole was drilled.
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