Magnetic Finishing Technology for Finely Detailed Parts

20 Jan 11

Magnetic finishing is an important finishing technology. In this article we discuss the Magnetic Abrasive Finishing (MAF) process and the latest applications such as the polishing of cutting tool surfaces, deburring of micro-parts and polishing of finely detailed jewellery.

Harry P. Coats first patented MAF in 1938. Although US originate this idea, much of the later development of this process was done in the former USSR and Bulgaria. The Japanese then explored the technology for polishing purpose. The process was further developed in India, England, France, and Germany. Magnetic finishing machines for a variety of finishing processes and product applications are now readily available. Companies around the world are finding new applications for this important finishing technology.

How Magnetic Abrasive Finishing (MAF) Works

In a magnetic finishing machine, the work piece is kept between the two magnets & the air gap in-between the work piece & the magnet is filled with Magnetic Abrasive Particles (MAPs). The MAPs joined to each other, along the lines of magnetic force and form a Flexible Magnetic Abrasive Brush. The brush behaves like a multi-point cutting tool for the finishing process. The vibrational, rotary & axial motion is imparted to the work piece by the machine to enhance the performance of finishing operation. The flexibility of the 'magnetic brush' means it ability to modify itself as per workpiece contours. The magnetic flux density is stronger around the nonmagnetic workpiece (along the magnetic brushes) than through the workpiece. This keeps the magnetic particles of the brush in close contact with the workpiece and makes the 'brush' follow even the finest contours and detail of the workpiece.

Click here to find suppliers who have magnetic finishing machines or vibratory finishing machines.

The following are some applications for magnetic finishing:

Applying Magnetism To Smooth Cutting Tools On A Grinding Machine

An alternate surface finishing device available on a tool grinding machine uses magnetism to swirl abrasive powder across the surface of a cutting tool. This creates a smooth surface finish and precisely rounded cutting edges.

Cutting tool manufacturers often perform post-grinding operations such as honing and polishing to improve tool surface finish and generate precisely rounded cutting edges. These operations lead to better chip flow, longer tool life and improved adhesion prior to coating processes. They also reduce the coefficient of friction of coated tools by removing droplets and other imperfections left behind after CVD or PVD.

German company, Schütte TGM offers an alternate surface finishing method on its WU-305 tool grinding machines. The process uses magnetism to swirl abrasive powder across the surface of a cutting tool to smooth and improve its finish. The technology was originally designed for use on stand-alone equipment, but a magnetic finishing module has been engineered to be compatible with the wheel-changing mechanism used on the WU-305 machines. This enables the machines to both grind cutting tools and to treat them via magnetic finishing in one setup.

Click here to find toolmakers who do tool and cutter grinding.

The primary components of the finishing module are two revolving magnetic discs located on either side of an enclosure containing the abrasive powder. Each powder grain contains both abrasive and magnetic material. Once a tool is inserted into the enclosure, magnetism causes the powder to swirl around the tool and smooth its surface. This magnetic finishing technology is also being applied in aerospace and automotive applications to reduce friction between mating components such as gears and engine parts.

Schütte currently offers four powder grit sizes—400, 600, 1,000 and 1,500—which users choose based on their finish requirements. The company says the magnetic finishing process can deliver 0.02-µm Ra and 0.08-µm Rz. In addition, it is said to generate a reproducible radius of cutting tool outside edges and chipping edges between 3 µm and 50 µm.

The WU-305 machines can grind, mill, belt-sand and polish, so their coolant system has been designed to accommodate machining chips, grinding swarf and the powder used in the magnetic finishing operation.

Finishing Micro-Parts

Another application for magnetic finishing is for finishing small, swiss turned parts as described by US company GenSwiss: Magnetic Finishing Systems work great as a complimentary process in shops where small parts are being manufactured. The Magnetic Finisher works by spinning .5mm diameter proprietary grade 304 Stainless Stainless media in a centrifugical vortex producing a delicate rubbing action that can remove light burrs, increase surface finish and provide edge breaks of .002" - .003".

Typical applications would be parts manufactured on Swiss Type CNC Machines. The system works best on materials that are non-ferrous and do not posess any magnetic properties. Copper, Aluminum, Brass, Titanium, Non-Magnetic grades of stainless steel, and Plastics are just some of the possible materials the system works well on. Ideal applications would be RF Connectors, Stainless & Titanium Medical parts, Brass valve bodies etc.

Click here to find machine shops who do swiss turning of small parts.

Polishing Finely Detailed or Filigree Jewellery

Magnetic finishing also has applications for jewellery finishing. Digital Magnetic Finishing is very useful for polishing cast, hand made filigree jewellery without any metal loss.

Typically a stainless steel media is used and this highly efficient system burnishes and polishes quickly in working areas such as undercuts recesses and slots with fast cycles times. Depending upon the size and weight about 100/200 pieces of rings, earrings, and pendant like jewellery can typically be polished from on both interior and exterior in less than an 2 hours. The time can vary depending on the carat of the gold. Often finely detailed, filigree jewellery cannot be polished by other means.

Click here to find suppliers who do wax printing for production of jewellery prototypes.

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