Making a Crankshaft -2

by John

The first step was deciding which grade of steel to use, and the size and section of steel.

I have made several crankshafts, but only one was from solid steel. The others were all from pieces which were joined with pins and Loctite. And all of them were substantially smaller than the traction engine crankshaft. The solid steel crankshaft was for the model triple expansion steam engine. It had 3 cranks, so was a balanced design. I turned it from stainless steel.

The traction engine crankshaft has 2 cranks, at 90 degrees, so is not a balanced design. It turns relatively slowly, and the balance is provided somewhat by balance weights which are attached later.

I was advised to not use bright steel because of a tendency to change shape when machined, due to relieving internal stresses as material is removed. So I contacted several steel local suppliers about purchasing some black steel solid rod or square section.

The three small circles and black rounded rectangles are an end view of the crankshaft

The red circle with a diameter of 114.3mm is the minimum size of round bar stock if the crankshaft was turned with the mainshaft centralised.

The red circle with a diameter of 107.8mm is the minimum size of round bar stock if all 3 centres just fit within the bar.

If square section stock is used the minimum size is 76.2mm x 76.2mm.

In all cases the length of stock is 416mm.

Since steel bar is sold mainly by weight and grade, I looked for a supplier of square section black bar.

But square section black bar has radiused corners, as in the shape on the left. So, to end up with 76.2 x 76.2mm square section I needed to use 90x90mm with the radiused corners. Long story shortened (mercifully, if you are reading this), I found an engineering works locally who was prepared to cut off a 420mm length, shown in a photo in the previous post. It is heavy!

Next steps will be to mill the ends to 416mm, and 2 long faces to a sharp corner. Then to mark and centre drill the centres for the mainshaft and the big ends (the three small black circles in the diagram above). That will be today’s task.

Later that day…..

Squaring the ends of the 90x90mm lump of steel. Hmmm.. those radiused edges could be an issue…
Taking off 1mm per pass with a tungsten tipped milling cutter.
When the corners were squared, my 90x90mm bar just ended up 76.2 x 76.2mm. I always mark the waste with a cross. Don’t ask me why.
The end result of an afternoon’s machining. That bar is too hot to handle. I will measure it accurately tomorrow.
Oh. And a bucket of swarf after a quick clean up. A lot more to come.