How to Choose a Magnetic Chuck
A magnetic chuck holds a steel workpiece on a machine table. The right chuck holds the part firmly, gives the wheel clear access, and lasts for years. The wrong chuck lets thin parts lift or leaves the work covered in swarf.
Answer these five questions before you order.
1. What shape is the machine table?
Rectangular chucks fit surface grinders and milling machines. The workpiece sits flat and the wheel passes back and forth across it.
Circular chucks fit rotary grinders and lathes. The poles run in rings or in radial lines, so a round part is held evenly as it turns. SMAG makes circular chucks up to 600 mm in diameter.
Match the chuck to the table. A chuck that overhangs the table cannot be clamped safely.
2. How large is the workpiece?
The chuck should be a little larger than the largest part you grind. A part that overhangs the chuck is held only on the section that touches it, and it can tip under the wheel.
Small parts have the opposite problem. A small part must sit across at least two poles, one north and one south, or the magnetic circuit is not complete and the part is held weakly.
3. Which pole pitch?
The pole pitch is the distance between the poles on the top plate.
Standard pitch suits medium and large parts. The field reaches deep into the workpiece and gives a strong hold.
Fine or close pitch suits small and thin parts. The poles are close together, so a small part always sits across several of them. The field is shallow, which also means less magnetism is left in a thin part after grinding.
If you grind both, choose the fine pitch. It holds small parts well and holds large parts adequately.
4. Permanent or electromagnetic?
Permanent magnetic chucks use neodymium magnets inside the body. They need no power and no wiring. The operator turns a lever to switch the hold on and off. They cannot fail during a power cut, and they generate no heat. SMAG chucks use N35 grade neodymium.
Electromagnetic chucks use coils. They can be very large, and the holding force can be adjusted. They need a power supply and a controller, and they warm up during a long run. Multicoil versions are built to customer sizes.
For most grinding shops, a permanent chuck is the simpler choice. Choose electromagnetic for very large plates or where variable holding force is needed.
5. How will you demagnetise?
A steel part keeps some magnetism after it leaves the chuck. It then attracts swarf, sticks to gauges and can upset later machining. A table top demagnetiser removes the residual magnetism in a few seconds. The SMAG Maxx-Demag bench unit has a 150 x 100 mm plate and runs from a 230 V single phase supply. Larger surface demagnetisers are built to order.
Looking after a chuck
- Keep the top plate clean and free of swarf before every job.
- Wipe the plate with a light oil at the end of a shift to prevent rust.
- Regrind the top plate when it wears. A thick all-metal top plate can be reground many times.
- Do not strike the chuck. A crack in the top plate lets coolant into the magnets.
The SMAG range
Santosh Magnetic Works makes rectangular and circular permanent magnetic chucks in premier and universal ranges, electromagnetic chucks to order, and the Maxx-Demag table top demagnetiser. See the workholding range or contact us with your table size and part sizes.