An important rule governing magnetic behavior is that tractive force is proportional to the number of magnetic lines of force passing through the part. Holding force is, therefore, dependent on the mass of the part, not contact area alone. Obviously, a thin part cannot accommodate as many lines of force as a thicker part, though it may present an identical area in contact with the chuck. Fortunately, cuts taken on thin parts are seldom as heavy as those taken on thick parts, so there is usually adequate holding for the reduced forces involved.
But this explains why it is not practical to express a chuck’s holding force simply as “pound per square inch” alone. There are many factors to be considered, such as: the type of material for the workpiece, the prior heat treatment, size and configuration of the part-especially the thickness of the part, flatness and surface finish of the areas coming into contact with the chuck, etc. All of these factors would have to be qualified in order to rely on a PSI rating alone.
It can be logically concluded, then, that it is more difficult to obtain effective magnetic holding of thin parts than of similar thick parts. But what do we mean by “thick” or “thin”? What limits apply? A generally accepted rule of thumb is that the thinnest part that can be effectively held on a magnetic chuck is one whose thickness is equal to the width of the non-magnetic separators in the chuck’s top plate.