New to milling and have a question: Does anyone have any tips on how to rotate a cylinder for the bottom cut and ensure the rotation around the z axis does not change? I was thinking I should just scribe the top and bottom and indicate off that line after finish the top cut and rotate the cylinder for the bottom cut, but wasn’t sure that would be accurate enough. Thanks in advance for any tips!
If you hold the cylinder in a vertically mounted V block. Think <o| The center of rotation Shouldn’t change.
Indexing the part would be the hard part unless “close enough” on the degrees of rotation is ok.
If you need down to the second rotation control you’re going to need an indexing head or table.
Look up “rose index”
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you can mill a flat on it and trim it off later
Clamp or glue a bar on it and sweep an indicator to get it to zero before flipping, then re-indicated.
I had to google what a rose index was. That’s a formal version of what I was describing.
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Great ideas all! Thanks for the quick input. Clamping on a bar and indicating it should work great for now. I’ll put a rose index on “the list” 
Thanks again all!
You could easily machine one!
I 3d printed a few for non crucial stuff
Thanks again all. I ended up milling a quick 4-sided Rose Index and it worked great.
Used the following steps:
- Mill the top side.
- Slide on the Rose and indicate it parallel to the X-axis.
- Remove and flip the part.
- Indicate the Rose again to the X-axis.
- Fully tighten in the vise and re-indicate to confirm.
- Complete the bottom milling op.
Turned out great - thanks for all the help!
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Can save yourself alot of setup time here
- Slide on the Rose and clamp in squared/ indicated vice (doesnt need to be indicated just cant move during operations
- Mill the top side.
- Remove and flip the part.
- Clamp Rose again in vice that hasn’t moved.
- Fully tighten in the vise.
- Complete the bottom milling op.
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