Guides, slides, valves, and other objects that require accurate planing, are often bent during fixing, the bending resulting from an improper attachment of the plates and poppet-screws. Such distortion is the cause of a large amount of filing after the object is machined; because the plane surface is produced while the piece is in its bent condition under the pressure of the fixing apparata, and when released, the interior of the metal resumes the shape it possessed previous to being fixed, thus distorting the plane which the machine produced, and rendering a further planing or filing necessary. However thick the article may be, or of whatever metal it is made, it is bent in some way, to a small extent, by the holding plates; and because it is thought that thick heavy articles do not bend, the bad surfaces produced are sometimes thought to be the results of defects in the planing-machine. When a doubt exists in this particular, the smoothly planed surface is tried with a straight-edge, previous to unfastening or loosening the plates and poppets, and if the surface is then found to be right, any difference discovered afterwards will be through the interior particles of the metal springing back to, or resuming, their former relative positions. Such articles as axles, rods, thin surface-plates, and thin tubular objects, will bend with their own weight merely; and for a great quantity of work no attention is given to distortion, because it is not known to exist; but it is always indicated when a great amount of filing is necessary to finish a slide, guide, or plate, after it has been very smoothly planed while fixed on the machine.

Vices and vice-chucks are very effectual for preventing distortion. By gripping the chambered portion of a small slide-valve in a vice-chuck, with the valve-face situated upwards for planing, the risk of bending the face is avoided, because the direction of the grip which holds the valve is parallel with the face to be produced; but if the valve is held with plates, and its face adjusted to the planing-table by wedges beneath the valve, as shown in Fig. 749, the valve-face is liable to be bent while tightening the plates, because the wedges or other packing constitute fulcrums, on which the valve is bent. A similar result would follow if there were no packing beneath, in which case that portion of the table in contact would be the fulcrum; in either case, the portion of metal beneath the plate's paw is a portion of the lever by which the power is obtained, and bends the work. Large slide-valves cannot be gripped in vice-chucks to avoid distortion; and to prevent the ill effects of bending a valve, or other object, which must be held with plates, the fastenings are loosened, after the work is reduced to very near the finished size, and while gently held, a very thin cut is taken off, to complete the surface.

A vice-chuck of suitable length is also very efficient for holding a guide, or a slide, when it is especially desirable to avoid bending it, and if a slight distortion does result with the chuck's grip, the guide's face is not injured by the springing back of the metal, because the grip is parallel with the planed surface, as before mentioned.