Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

 I took a rusty old lump of some sort of steel - no idea what alloy - I liberated it when it was thrown out at work about 12 years ago. Anyway, this became the basis for a holding jig for the crankshaft. There now follows pictures, but not much explanation. (Further research leads me to believe it is 4140 HT. Partly because of the yellow identifier colour one end, and because all the materials at that little firm came from McMaster Carr - I believe that's their identifier - could be wrong).







I like my rolly-cart, which is handy for holding up drawings close at each machine, as well as essential tools. Like the engineers universal adjusting tool (coarse version).







To the mill - and centering with the very fun centering indicator.















I tried it in the 3 jaw first - the design of the jig (slightly different to mine) comes from PM Research, and the idea is to use it this way to get the crank throw. It works well, I only ended up 0.003" off.

However, I never intended to use the 3 jaw (although let's face it, that would work fine), but I got it to 0.0005" in the 4 jaw.










Crankshaft finished, and made a start on the connecting rod.























Holes in big end drilled.



Note that I left the casting gate in place, and also milled it flat as a reference surface.

Then re-positioned to get the little end hole drilled. I used that reference surface as an aid to squaring up the part. I used a pin gauge of the same diameter as the little end OD to do a visual centering alignment.



Drilled and reamed for a pin.



I then went and drilled a 1/4" hole at the finished distance (3.375") from the little end. This will give me a visual aid when I saw the big end in the next step.

Now - I was sorely tempted to make a 3/8" slot, to become a 3/8" hole after slitting the big end. However - I felt that was being too cocky. I've seen it done - but the guy who did it is a highly experienced production machinist. You can see the video on youtube.

For now I'm taking the simple approach.

Pleased with this though - the holes are nicely centered.



Next step is to slit the big end, bolt it back together and finish the big end bore.