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Holding round sections in vices
Though it is not obvious why, it does seem that a workpiece held along two lines of contact, for example a round bar is never as secure as one might expect.
Vices for use on drill presses often have v-shaped grooves in their jaws to help them to hold round sections. Milling vices never seem to have these grooves.
The solution might seem to be to use a V-block between the workpiece and the fixed face of the vice. Clearly the height of the bottom of the V must be well below the top of the faces of the vice. Most commercially made V-blocks are often too big to be useable.
One way of holding a round bar in a vice is to use a spacer between the round part and one face of the vice with a V-shape in it.
It is quite easy to make V-blocks whose width is less that the height of the face of the vice just for this purpose. Whereas on a V-block the angle of the V is always 90º this severely limits the smallest size of round shape that can be held
It is very probable that you will need a set of these because one that holds the biggest round shape will be too big for small round shapes. It is desirable that the intersection of the two sides of the V is parallel to the bottom of the vice and parallel to the fixed face of the vice.
Though it seems possible to hold longish round sections horizontally in a vice so they can be machined it is not possible to hold a shortish section without a risk of it slipping though this could be done with a square section of similar length.
Short round shapes can be held with their axis vertical by holding in the vice with a V-block on its side.
fig round shape being held in vice by using a v-block
Where the diameter of the workpiece is large compared to the length it is often possible to turn the ends flat (on a lathe) and then hold it on the milling machine by holding the flat faces in the jaws of the vice. This sort of shape is risky. If however a long V-shape is used to hold it then it is not a frictional force holding it and it is no longer a risky shape.