Thanks guys for following along. I made some more progress today - and I'm happy to report it really is progress. Fair warning - far more pictures than necessary to describe a simple part, but I was enjoying myself.
Continuing with my step by step second go at the connecting rod, I got it set up on a 2-4-6 block held in the vise to get ready for slitting.
My reference surface was used to indicate the part and get it square for the cutting operation.
Side view shows a piece of the old broken connecting rod being used to pack the upper clamp to stop it twisting off the small clamping surface of the new connecting rod. The gauge pin was used to make sure the pilot hole was above the surface of the 2-4-6 block, in readiness for measurement later.
The gauge pin was also used to locate the slitting saw. I used a piece of paper to feel for a light drag between pin and saw to know I was very close to top of the hole. From that I could easily set the saw height to be right in the middle of the slot of stock material that needs to come out.
Part way through slitting. Remember that the hole is undersize significantly - it is only a reference at this point, which is why the slot looks way off. This is a 1/4" hole, and will be drilled and reamed 3/8" later. After slitting, the top of the connecting rod at the slit, once cleaned up, should pass right through the center of the hole. The slitting saw is 1/32" thick, and the total material to be removed is 1/8", which is why the slot appears above centerline in this picture.
Slit complete
Cleaning up. To understand end mill height, I used a feeler gauge between the end mill and a gauge pin in the partial hole to set the tool height (because it is too easy to chip a carbide tool setting it directly on the gauge pin).
Gauge pin in place just to show that the finished surface is right on the centerline of the hole.
Then added the oil hole while in this set up.
On to the end cap. Similar approach. The remaining amount of pilot hole is used to establish height. The reference surface I set up earlier on provides a very convenient locating surface to get the part square by sitting it on a parallel.
Again - setting tool height using a feeler gauge.
This is part way through cleaning up the mating surface. See how the hole just disappears on the final milling step - which is exactly correct. Means I nailed the height setting.
You can see only the chamfers remain - so I got this right on.
Now clamp it to the connecting rod:
Set it up to drill and tap the oil cup hole. Reference surface again comes in handy.
And again, getting set up for the final big end hole drill and ream op.
After locating the small end hole using a gauge pin and indicator, I lightly spot faced the big end to make sure that visually the spot face was centered.
After that I plunge milled out the half hole, and drilled and reamed the hole to 3/8". I guess I finally got bored of taking pictures because there aren't any more, until after test assembly of the connecting rod in the engine. I didn't take any pictures of slitting the little end either, which was a pretty straightforward job.
Happily, fits really well. The engine has the piston installed, and I can turn the engine over turning the crankshaft with my fingers, so this gives me hope - but at the same time it is not loose and sloppy.
So, finally getting the engine further along.
I enjoyed todays efforts. I take a lot of satisfaction in making parts by indicating features to locate them in space, as opposed to marking out. It's just an approach I like to take, I don't consider it superior to other methods or anything like that, it simply gives me satisfaction.
Anyway, it's time to get serious. I want the flywheel to be the last item made, so now it's time to make gaskets and oil cups, then it will be the flywheel, and then assembly and test.
But now it's Saturday night.
