The Engineer's Vice
I think I have puzzled you enough with the vice issue so let me clarify that I am not referring to the vice (opposite of virtue) but to a tool which is used to hold a workpiece firmly in position when some work is being performed on it such as filing and so forth. Just take a look at the picture below and you will get a fairly clear idea about the construction.
The vice consists of two jaws which press tightly on the workpiece so that it does not move. One of the jaws is stationery while the other is movable and is used to adjust the force on the workpiece depending on the type of work to be performed on it. The movement of this jaw is done with the help of a rotating handle which rotates a screwed spindle upon which the movable jaw slides.
Traditionally the leg type vice was mostly used which has the lower jaw pivoted at one end and this means that if the object to be held in between them is sufficiently large, the jaw would move back in a circular fashion and not parallel to the other jaw. Hence this would not give a proper grip as now the relative position of the jaws would give them a wedge shape as compared to the parallel sliding jaws which are mostly used in modern days.
Many modern vices also have a quick release mechanism which is used to instantly release the workpiece instead of having to unscrew the spindle using the handle. Also since the jaws of a vice are made out of steel it might damage some softer workpiece if held tightly within them. In order to avoid such a situation clamps normally made out of lead are used to soften the grip while retaining sufficient force to prevent the workpiece from moving when worked upon.
The body of the vice is strongly secured to the workbench and the bolts are checked frequently to check for any loose nuts. The vice is classified as per its size of the jaw and normally used vices could vary between say 3 to 9 inches of jaw width. There are pneumatic vices as well which saves the operator from applying force to tighten the jaws by using air pressure instead. As far as the maintenance of a vice is concerned simply keeping it well oiled should be sufficient unless some other damage occurs.