- Bert Thorstenson and Bob Ferguson
No One Chuck Design is Ideal for Every Application
Many shop people visualize a magnetic chuck as a solid rectangular block with a uniform magnetic attraction over its entire surface, but this is erroneous. The top plate of a magnetic chuck consists of magnetic and non-magnetic materials arranged in some generally consistent and repetitive pattern. To obtain optimum holding of a ferrous workpiece it must be positioned so that it adequately completes the circuit between magnetic members of opposite polarities.
If a magnetic chuck was limited to holding only one type of workpiece, it would be possible for the magnetic chuck engineer to work up an ideal design providing firm holding with minimum electrical consumption. But in most instances the chuck is expected to hold almost an unlimited range of parts to be processed on the host machine. Parts can be thick or thin, large or small in area, long and narrow or fairly square in shape, and may be a variety of materials either with or without prior heat treatment. The net result is that no one chuck design is deal for every application. Selecting the proper chuck requires knowledge of the class of work to be processed and/or application of good judgement.
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.
Another common fallacy is to attempt to evaluate holding power of different electromagnetic chucks on the basis of their wattage ratings. There are many design details influencing the wattage ratings of a chuck, and unless absolutely identical designs are being compared, wattage ratings alone are meaningless in evaluating holding power.
If wattage alone was the secret to holding power, it would be a simple matter to design super-powerful chucks. However, what is really required is good engineering design in conduction with special care in manufacturing. Good engineering practice requires care to provide proper balance between cross-section areas of core irons and return paths, specifying satisfactory ampere-turn (NI) levels to accommodate the length and other characteristics of the magnetic path, and minimizing leakages (magnetic) and other problems resulting from accumulated tolerances. It is equally important to exercise careful control in manufacturing process. Because at first glance the internal construction of an electromagnetic chuck appears to be so utterly simple (almost crude), many metalworkers accustomed to working on more exotic machines and tools are likely to overlook the importance of producing the best possible flatness, surface finish, and dimensional accuracy. All are important if the end result is to be highly efficient, top quality magnetic chucks.
By altering details and proportions the designer can develop magnetic chucks resulting in deep magnetic penetration or a very shallow field pattern, or provide a chuck having just one or two major sets of poles or one with a multi-pole pattern. As in many instances, the final engineering solution is a compromise between several conflicting factors.
What is really required is good engineering design in conduction with special care in manufacturing
It might be appropriate at this point to touch on a sometimes-confusing aspect of the Magna-Lock USA family of electromagnetic chucks, their model designations. When the brand was founded, one basic type of rectangular chuck was made, the one we recognize as the Universal Laminated. It was given a model prefix “HR”, presumably to designate Hanchett Rectangular. Most HR chucks were eventually designated as “HU” models when the Hanchett Universal chucks were introduced. Later the Grid style chuck came on the scene, and this was dubbed “HG”. Again, we presume this stood for Hanchett Grid. You’ll find other old model identifications that remain to this day; such as “HB” for swiveling chucks, to designate Hanchett Bar (for knife bar), or “HC” for rotary chucks to designate Hanchett Circular. The model labeling for the Combination chucks saw the addition of a suffix letter “C” to the heretofore HR prefixed rectangular chuck designations. When rectifiers with variable holding control were introduced, the suffix letters “CV” were used to designate this feature. When the new line of Economag Chucks was introduced as a replacement for the old line of Combination chucks, the prefix “HL” was used. Its designated as Hitachi Low (for low profile).
The HU model chucks feature a closely spaced transverse magnetic pole pattern with holding power to extreme edges on transverse poles, and to the extreme ends on longitudinal poles. Available in both styles. These chucks are recommended for holding small or thin workpieces and ideally suited for use with parallels and V-Blocks. These chucks are Universal in that it can be made in any special pole pattern and size at near standard prices.
Our grid or HG model chucks are ideal for both large and small workpieces and are the Magna-Lock USA model that has the most uniform holding. These models have separators that are of special non-magnetic material which has been standard for years. The low magnetic flux pattern makes these chucks excellent for milling and other heavy machining applications. Magna-Lock USA grid chucks can be installed in tandem mounting or end-to-end to make up a larger machine table surface. The low wattage of these chucks means less heat which means closer grinding tolerances due to less expansion of metals in the chuck.
Our standard rotary HC model chucks are available with the popular concentric ring top plate pattern. The inserted brass ring design assures moisture proof construction and positive holding. However, Magna-Lock USA rotary electromagnetic chucks are by no means limited to concentric ring top plate patterns and have included top plates patterns of radial, heavy-duty crossover, clover leaf, tear drop, staggered pole, straight chordal laminated and grid.
HL model chucks are one of the most popular chucks in the industry and features a fine pole pattern that runs longitudinally resulting in a full magnetic surface to all edges of the chuck. These chucks have the lowest height in the industry allowing more head room on your grinder for jigs and fixtures. These chucks are ideally suited for small and thin workpieces and for parallels and V-Blocks.
Magna-Lock USA HR model features include the same top pattern as our HL Economag Electromagnetic Chucks in models less than 12” wide. These chucks offer excellent stability for heavy machining and grinding and are also ideally suited for use with parallels and V-Blocks and offer excellent holding for special fixtures and jigs.
Swiveling HB model chucks, known as knife bars, are available in many sizes and styles. The 5” x 10” and 6” x 18” models are provided with brackets only. Larger models are supplied with brackets, protractor dial, and worm gear assembly with handwheel. A ratchet handle is optional. Positive locking at desired angular setting is provided on all models. All swiveling chucks are balanced for easy rotation through 360°.
We repair all brands of electromagnetic chucks including Blanchard, Walker and Taft-Peirce.
Keep in mind that all Magna-Lock USA electromagnetic chucks have replaceable top plates, watertight construction under normal usage and waterproofing of interior of chuck. With our over 70 years of experience in electromagnetics, we offer repair services for all brands of electromagnetic chucks which includes our standard two-year warranty. We offer competitive pricing, quick turnaround times and discounting, when applicable, on all repairs.