After rotating till the approximate opening is obtained, place the clamp over the work with the middle spindle or screw as close to the work as possible. Adjust both screws so that the clamp grips the work lightly with the ends of the jaws not quite touching the surface. Apply final pressure by tightening the outside screws only, making certain to double check that pressure is being applied along the full length of the jaws-not just at the ends or along the edge.
Most hand screws are adjustable; that is, the jaws can be pivoted to clamp on surfaces that are not parallel. The work can also be gripped with one jaw advanced farther than the other where irregular projections are required. Nonadjustable types are also available, but these are more limited in use since the jaws remain always parallel. They cannot be used on irregularly shaped objects or where work surfaces are not parallel.
For clamping wide objects which are beyond the scope of ordinary C-clamps or hand screws, bar clamps are the tools to use. Steel units which have one fixed head and one movable head can be purchased in lengths from 18 to 72 inches. The fixed head at one end has an adjustable jaw mounted at the end of a steel screw with a crank handle for tightening. The nonadjustable jaw at the other end of the bar can be slid to the approximate opening desired. It is held in place by a friction clutch which grips the bar automatically wherever it is set.
Home handymen can easily fashion their own bar clamps in any length by buying special clamping fixtures which fit over 1/2 -inch or 3/4 -inch black iron pipe. These clamping fixtures are similar to the type used on regular bar clamps. The fixed jaw is threaded to screw onto one end of the pipe, while the movable jaw slides over the other end. One advantage of this type of bar clamp is that its length can be extended at any time simply by adding on another length of threaded pipe.
Short bar clamps (from 4 to 30 inches in length) with the sliding head containing the screw handle for tightening have become increasingly popular with home craftsmen in recent years. They can be used wherever C-clamps can be used, but they have greater capacity and are quicker to adjust.
Steel spring clamps which resemble oversized clothespins are also handy for many light-duty clamping jobs around the home or workshop. They are fast and easy to apply since the jaws are opened by simply squeezing the handles together. A built-in spring applies mild pressure when the handles are released. They are handy for those jobs where only one hand can be used while the other hand positions the parts or applies the glue. They are also handy where an extra pair of hands is needed to hold pieces temporarily in position while assembling.