Climb milling when you don't think you are

[digiex-chris]

Just a cautionary tale. Turned out well enough, but it was nearly a disaster.

I was milling the light end of a crankshaft off, to form the crankshaft weight. The crankshaft was setup in a fixture used to turn the crank pin, so the crank bearing journal was in a reamed hole in the face of a round chunk of bar.

Here's the fixture.



Here's the setup in the mill



With the endmill on the left of the line drawn between the center of the crankshaft and the crank pin, I quickly realized moving the endmill into the work using the Y axis would be climb milling, so I changed plan and came in from the left instead (moving the table to the left), with the X axis.

What didn't become obvious till later, is that if the endmill were to grab for any reason and anti-rotation pin were to shear, the crankshaft would rotate counter-clockwise, self-feeding into the endmill.

And that's exactly what happened!



It turned out ok, but I learned my lesson. The endmill is actively working to rotate something like that. I'll back up the crank pin with a clamp or a stop or something next time. I suppose roughing it out with a smaller endmill would have allowed a more even cut, slotting more of it. Thankfully, I was able to save the part. It knocked a corner off my endmill though.



Any other ideas on how to better secure that?

 

[velocette]

Hi Chris
You could approach this with the crank shaft upwards and a block with a hole bored a snug fit with the crankpin a button that is a light push fit into the recess in the bob weight long enough to fit a hole bored on the same centres as the crank throw in the block.
A hole drilled through the button for a clamping bolt to a hole drilled and tapped in the bottom of the recess centre.
A second clamp on the bob weight can be Used to secure the block and the crank to the table plus a clamp on the opposite end of the block.
Two clamps points are essential to secure ANY work to the mill table.
A problem may be with the length of the crankshaft so a long and large diameter cutter.
One worth considering is to use an adjustable boring bar set to the finished radius of the bob weight and light cuts and a light and steady feed moving the table until the correct shape.

Clear as mud I can see it in my head but putting into words on paper and the mud thickens.
Any questions and will attempt to make an equally unclear sketch.

Eric

View attachment crank-shape.pdf

 

[digiex-chris]

Eric, I like that solution a lot. infact I already had a button made to fit the crank center hole in order to dial it in parallel with the Y axis. I couldn't add more clamps the way I had it, because the opposite side of the crank (the "down" side, oriented like my photos) has a slight protrusion to stand off the crank web from the bearing surface a little bit, so putting clamps anywhere but the center would bend it. But your solution of flipping the crank over solves that, the problem of a too-small anti-rotation pin, and the ability to solidly clamp it nicely.

Thanks!