The correct central adjustment of the work may sometimes be attained, by a thin piece of wood fixed upon the surface chuck by screws or otherwise; this is turned away to the exact diameter to fit within the work, which is then fixed by the clamps. This method, which is employed only for accuracy upon the strong metal surface chuck, copies the constant practice in wood and ivoiy turning; the piece for the central adjustment is frequently fixed upon the wood surface chuck, while in the smaller examples the plain wood chucks, the chuck itself is turned away to leave the central projection to adjust, and in their case, to hold the work also. Temporary wooden beds of greater thickness, are used to chuck the irregularly formed pieces met with in machinery. Many of these can only be fixed by cutting out recesses in the bed to admit projecting parts, that some level or comparatively level plane in the work, may be placed in contact with the wooden bed and in that manner referred to the face of the chuck.
The holes and bearings in the sides of frames or parallel plates, to receive pieces for their connection, or for the support of axes and spindles, are conveniently bored upon the wood or metal surface chuck. The sides of the frames are sometimes marked and chucked separately, or they are pinned together, and drilled as one thickness, or else for pivots, the two sides are sometimes attached as in their permanent shape, and the whole frame chucked as if it were a solid block; the pairs of holes being then bored out with a long drill that will extend through the two. When the interval between the two plates is too wide so that the guidance of the drill becomes uncertain, the sides of the frame have to be pierced at two separate chuckings. A pin is fixed in the center of the chuck and turned exactly to fit the hole already made on the one side; the frame being reversed, the pin enters this hole which is thus exactly central, causing the hole then bored in the opposite side of the frame to be in the same axis. Should the pillars or connections terminate as screws or otherwise projecting beyond the sides of the frame, a parallel piece of wood or metal formed with corresponding recesses is interposed, that the side of the frame may lie solidly and exactly parallel with the face of the chuck.
A series of eccentric apertures at a common distance from a center, as in the collar plate fig. 128, is turned by means of a pin or stud fixed in the surface chuck, at the same distance from its center as the radius of the eccentricity required. The stud fits a central recess in the work, which is turned round and clamped to bring the position for every hole successively opposite the axis of the lathe. Eccentric pins are also temporarily fixed in the surface chuck to assist in turning studs, holes or recesses in exactly similar positions, in several duplicate pieces. Rectilinear adjustment of the work, may be greatly assisted by fixing it in contact with a slip of wood or metal, also fixed upon the face of the chuck, while sometimes a slip is used on either side of the work to form a groove; the slips are employed with or without the pins, for turning a series of apertures in a straight line, the rounded ends of slots, and many parts of models and machinery. In turning portions eccentric to the general axis of the work, the eccentricity of the latter places the chuck out of balance, and its equilibrium then frequently requires restoring, by means of a block of lead or some sufficiently heavy body fixed to its face, necessary to attain smooth and equal turning, and shown by the examples figs. 308 and 324; the driving band is generally removed when the counterpoise is fixed in position, in order that its exact effect may be ascertained.
Work which does not present a plain true, surface from projections, irregularity of shape, from the casting being in winding or from other cause; is propped or packed, by thin pieces of metal or hardwood, until it lies level on the surface chuck. So far as possible, the packing is placed beneath or close to the screws and clamps, and even then the process requires considerable care, that the fixing may not bend the work. Should that occur, the subsequent removal of the pressure allows the material to return to its natural tension, which deteriorates the truth of the surfaces that have been turned upon it. Should the surface chuck be weaker than the object to be turned, it may itself bend instead of the work, or the effect may partly result in each of them. The risk of error in fixing such work, is materially reduced when the casting is provided with projecting portions, also cast in the solid, in addition to those intended to be permanent; these are subsequently cut off from the finished work, but for the time serve as feet, that the work may stand upon the surface chuck without the necessity of packing. Previously to clamping, the extremities of all the projections or feet are corrected to truth, to bear equally upon a planometer, or in its absence upon the surface chuck itself.
A further and inherent source of error in large or thin castings, arises from the alteration in form these are found to pass through, while under the operation of the tool. Owing to its more rapid cooling, the exterior of the casting to a slight depth is both harder and in a condition of greater tension than the interior of the mass, and when this outer skin is removed by the tool, some change of form is almost certain. Thin works are especially liable to alter, and none can be said to be entirely exempt; but the more rigid the castings, the more uniformly they have been cooled, and the less the extent of surface to be removed, the smaller will be the risk of error from their elasticity. The difficulty can only be successfully overcome, by cutting away the outside from all the parts intended to be worked; after which but little further alteration in the material need be expected. This may be done with the chipping chisel and file or in the lathe; if in the latter, the work is afterwards rechucked with additional care in the packing and the surfaces turned a second time, when a much truer result will be attained.