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Milling – vices – testing
Testing a vice
For most vices the crucial surfaces that matter are the bottom, ie, the bit between the two jaws, and the fixed vertical surface. (Some vices are not even designed to have an accurate vertical face but only a flat bottom – see rack vice later.)
It is essential that both of these surfaces are flat.
These surfaces can be tested for flatness well enough by means of a flat ruler is good condition in conjunction with a DTI.
The jaw is usually hardened steel and is unlikely to have any dings in it. The bottom of the vice is usually cast iron. It might have holes drilled into it. These should be cleaned out but otherwise do not matter. However the bottom must be free of the raised part of any dings.
B It is essential that the bottom is horizontal, ie, parallel, to the surface of the milling table. This can be done by using a plunger type DTI
C It essential that the fixed face is vertical
The reason this surface needs to be tested is that the body of the vice is cast iron but the jaw is usually hardened steel. Since the jaws can be changed they can also be put back together again. The smallest piece of dirt will affect this. This test can only be done accurately enough using a DTI.
The height of the fixed jaw is relatively small so this limits the accuracy of any test. The solution to this is to extend this height by clamping something in the vice. The clamped surface must be absolutely flat.
In practice nothing can assumed to be flat. The most likely candidate is a parallel. The surface can be tested with a ruler that is in good condition. It will be found that most parallels are not flat enough on the wide side. The narrow side is most likely but still needs to be tested.
The parallel is clamped in the jaws of the vice as shown. The thin surface is then tested for being vertical by using a DTI fixed to the vertical head.
On vices that tilt in the forwards/backwards direction this test is an accurate way of setting the fixed jaw to 90°.
It seems to be traditional to assume that if the fixed face is vertical and the vice is properly made then when the vice is tightened up and the two surfaces of the workpiece are parallel then the workpiece will line up with the vertical face.
It can be very useful to know that the top surface of the moving jaw is horizontal. If it is horizontal, then this can be used to line a workpiece up by putting a parallel on this jaw and aligning the top of the workpiece, or a line scribed on the workpiece, with the top of the parallel.
But it cannot be relied on. This is especially true if the user changes the jaws from one type to another.
Testing the top of the jaw for being horizontal.
It is often useful to be able to set a workpiece to be horizontal by setting a line on it to be against the top of the moving jaw of the vice. But often the workpiece needs to be machined down to a line. In this case place a thin parallel on top of the jaw then line up the line on the workpiece with the top of the parallel then remove the parallel.
Very thin parallels, ie, less than 3mm, cannot be relied on to be flat along the wide surface.
The top of the jaw can easily be tested for being horizontal by using a plunger type DTI.