This has worked for me several times but hasn't been needed at home so far. Not much room for swinging a mallet on mine so need to place stuff in the vice carefully. A mallet helps a lot and two stage vice tightening on a powerful machine with heavy cuts – the mallet for the 2nd one.
Clock the fixed jaw true. I'm assuming bright stock is being machined but it can work on black as well but may need repeating.
As nothing is dead accurate assume the fixed jaw is 1 degree out of square vertically sloping towards the back of the machine but the bed of the vice is parallel with the bed of the machine. There will be errors. Fit the metal, tighten fairly firmly and whack it down with the mallet to ensure at least one edge of the bottom is resting on the vice.
Fully tight and machine first face.
Rotate so that the machined face is against the fixed jaw and go through the same procedure again and so on.
What is happening is that the 1 degree error is being cut every time so the work finishes up square. As bright stock is pretty square and there usually is some slop in the moving vice all is ok. Otherwise this could be introduced with a number of things, piece of bar, a thinner style parallel which might pop out etc. What holding techniques need to do is ensure that a machined face is pressed firmly against the fixed jaw. Initially only an edge may be resting on the base / slides and may still be an edge due to vice errors and a mallet is the best way of achieving that.
Getting the ends right is trickier but how ever I did it I would check with a precision square against the light.That can be used to set the work in the first place which is the way I would do it.
John
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