Magnetic Chucks both electro and permanent types were originally developed for light machining and more specifically for grinding operations. This form of workholding is the best way of presenting the whole of the workpiece to the grinding wheel and at the same time clamping uniformly and very accurately.
Why, therefore, has the same philosophy not spread to other areas of machining – Particularly milling and turning disciplines? Actual clamp force is the primary concern.
As the magnetic chuck has evolved – particularly in the relatively new technology of Permanent-Electro Magnetism, clamp forces have become greater and predictable.
Specialist Magnet Engineers are now able to undertake feasibility studies with a high degree of success and then translate the information into easy-to-follow operator guidelines; in many cases the clamp force generated by today’s magnetic chuck will out-perform conventional workholding.
The illustration above shows a steel ring nominally 20” OD x 16.5” ID x 2” Thick. The material is pre-hardened alloy steel (4140, for example) and the surface condition although un-machined is relatively flat*.
Pole extensions have been utilized to present the workpiece for OD, ID and Face machining in a single operation. It follows that actual magnetic contact area is quite small.
Allowing for a suitable safety factor, this magnetic workholding set-up will comfortable withstand a 0.200” (~5mm) depth of cut at 0.020” (0.5mm) feed per revolution to all 3 areas of the ring.
It follows that as the workpiece gets bigger and the contact area increases, so the clamp forces increase.
When factoring the absence of distorting the part radially, and with both OD and ID exposed to the cutting tool in the same operation, greater machine accuracy and shorter downtime is the ultimate outcome.
‘* Un-flat materials can be magnetically clamped very well with the use of self-shimming pole extensions