3 bonehead moves in a row.

[Razzle]

I finished up putting my Seig x2d back together after installing glass scales for an upgrade to my touchdro. Before reinstalling my home brew x feed I wanted to remake the coupler used to engage the motor to the lead screw. Turning the blank on the lathe went without a hitch. But when I got to the milling it went pear shaped.
The coupler has a hex on one end and a rectangular paddle on the other. Blank went into the hex collet holder and into the vise. Started milling the hex and bonehead move one, didn't get the collet holder seated down properly in the vise on one op. Scrapped the part.
Remade the blank on the lathe and back to the mill. Making sure that I seated the collet holder firmly each op I began to cut the hex again. 3 go arounds and it was a hex but the size wasn't matching my math. After some head scratching I discovered bonehead move #2. I had the Dro in metric but my math and drawing were in Imperial. Didn't scrap the part this time but it was close.
Switched the part around in the collet holder to begin milling on the paddle part. First op went well. Then I rotated the collet holder 180 degrees and realized that a hex collet won't cut a rectangle. Bonehead move #3. Had to switch to the square collet holder and re zero the part.
I got the part completed but it was a close thing. I think I was just too excited about how accurate and easy the touchdro was after the glass scale upgrade. I just wasn't paying enough attention to what I was doing.

 

[Aukai]

Glad you were able to salvage it, good catches, and saves.

 

[jmkasunich]

Switched the part around in the collet holder to begin milling on the paddle part. First op went well. Then I rotated the collet holder 180 degrees and realized that a hex collet won't cut a rectangle.

Not sure if it would have helped in this specific case, but you can mill a rectangular feature in a hex collet block (or any other that can be flipped 180 degrees).

Mill one side using the END of the endmill. Flip 180, mill the opposite side again using the END of the cutter. Then machine one of the 90 degree faces using the SIDE of the cutter. Flip 180 again for the last face, again using the SIDE of the cutter.

YMMV, depends on the exact part and cutter...

 

[Razzle]

Not sure if it would have helped in this specific case, but you can mill a rectangular feature in a hex collet block (or any other that can be flipped 180 degrees).

Mill one side using the END of the endmill. Flip 180, mill the opposite side again using the END of the cutter. Then machine one of the 90 degree faces using the SIDE of the cutter. Flip 180 again for the last face, again using the SIDE of the cutter.

YMMV, depends on the exact part and cutter...

Hmmm. That sounds like something I should experiment with. Might have made hitting my dimensions more challenging in this case. But could be useful in the future.