Common Processing Methods of NdFeB

The main shapes of sintered NdFeB magnet products include square, cylinders, rings, tiles, sectors and various special-shaped products. In actual production, large blanks are usually produced first, and then the final products are obtained through post-processing according to the user’s size requirements.

Sintered NdFeB is made by powder metallurgy. The material has high hardness, high brittleness and is easy to break. In addition, heat release, corrosion and defects during processing will damage the magnetic properties. Therefore, it is necessary to select a suitable processing method based on these characteristics. At present, sintered NdFeB machining is mainly based on traditional cutting, grinding, chamfering, drilling, etc., and there are also other methods such as electric spark cutting, laser processing, and ultrasonic processing.

1. Slicing (Cutting) Process

The cutting process is usually completed using a slicer, wire EDM machine, wire saw or laser cutting machine.

(1)Slicer: Use high-speed rotating thin-sheet inner circular diamond drill tool to automatically cut NdFeB magnet steel, and use cutting oil as cutting coolant during the slicing process. The advantage is that it does not require customized special tools, has strong flexibility, and is suitable for sample processing and truncation processing. However, due to low processing efficiency and yield rate, and weak verticality guarantee ability, batch slicing production has gradually been replaced by multi-wire cutting machines (wire saws).

(2)Multi-wire saw cutting: Use a fixture to fix the workpiece on the workbench, and use the roller cloth diamond wire. The high-speed diamond wire (wire diameter 0.15~0.2mm) rubs against the magnetic steel to achieve material cutting. The cutting process is cooled by cutting fluid. The main feature is that it can cut multiple workpieces at the same time, with high production efficiency, good product rate and material yield rate, strong verticality guarantee ability, and is suitable for batch continuous processing. However, special rollers need to be customized for products of different specifications.

(3)Wire EDM: Use molybdenum wire electrodes to generate high-frequency electric sparks on NdFeB magnets, causing them to melt locally. Computer control allows the electrode wire to cut along a predetermined trajectory. The advantage of wire EDM is high processing accuracy. It can be used for slicing tile-shaped and special-shaped products and cutting large magnets. The disadvantage is that the cutting speed is slow, and the molten area on the cut surface has a greater impact on the magnetic properties.

(4)Laser cutting: The laser beam is focused on the magnetic material, the magnetic material melts and vaporizes, and the disappearing area forms a slit. Laser cutting is a non-contact processing, with the characteristics of small impact on the environment, high processing accuracy, and the ability to process inclined surfaces. It has broad application prospects. However, the changes in temperature and stress during the processing have a certain impact on the performance of the magnet, and when cutting thicker products, the divergence of the laser beam causes a slope in the cutting section.

2. Grinding Process

Mainly refers to the processing method of grinding the workpiece surface with a grinding disc or a grinding wheel. Common grinding methods for square NdFeB magnets include: vertical grinding, flat grinding, double-end grinding, etc. Cylindrical and annular NdFeB blanks are often processed by centerless grinding, square grinding, internal and external grinding, etc. Tile-shaped, fan-shaped, and special-shaped magnets can be processed using multi-station forming grinders.

（1）Surface grinding machine: It is used for surface grinding of magnetic materials and can also process multiple surfaces. Generally, a horizontal axis rectangular table surface grinder (flat grinding) or a vertical axis circular table surface grinder (vertical grinding) is used. The flat surface of the magnetic steel is neatly stacked as the reference surface and fixed on the magnetic disk workbench with a baffle clamp, and a grinding wheel is used for reciprocating surface grinding.

（2）Double-end surface grinder: The product is continuously passed through by a conveyor belt, and two grinding wheels are located on both sides of the product. The horizontal axis double grinding head is used to rotate and drive the grinding wheels (the two grinding wheels produce an inclination angle), and the two planes of the product are ground under the rotation of the grinding wheels. The double-end surface grinder has high processing accuracy and small surface roughness. It is the most widely used symmetrical plane processing equipment in NdFeB machining.

• Centerless grinder (or square grinder): Centerless grinder is used for external grinding of cylindrical blanks, and square grinder is used for grinding of square magnetic steel. The magnetic steel passes through the guide wheel and grinding wheel in sequence through the feeder and guide rail. The guide wheel drives the magnetic steel workpiece to rotate on the pad iron, and the grinding wheel grinds the outer circle of the magnetic steel to the required diameter.

• Internal and external grinding machines: Fix the magnetic steel workpiece with a fixture, and then move the grinding head along the inner or outer circumference of the workpiece to grind the magnetic steel workpiece to the set inner and outer dimensions, and grind the surface flat and remove burrs. It is mostly used for internal and external surface processing of ring products.

• Forming grinder: It can grind various flat surfaces, curved surfaces, or complex forming surfaces through specially shaped grinding wheels (wheel dressing). It is suitable for grinding processes that do not require automatic feed, meeting the shape requirements of different types of products. It is commonly used for mechanical chamfering of products or processing of irregularly shaped products.

3.Drilling (Punching) Processing

The sintered NdFeB is very prone to breakage or fragmentation during the drilling process, so specific equipment and processes are required for drilling operations. Commonly used equipment for NdFeB internal hole processing includes boring machines, instrument lathes, and benchtop drilling machines.

• Hole-boring machine (bushing machine): This equipment uses diamond ring tools to process inner holes. The product is fixed by a chuck and rotated by the spindle, while the tool feeds into the product to machine the inner hole. Hole-boring lathes are typically used for machining neodymium iron boron products with inner holes larger than 8mm. By using specially designed boring and reaming tools, it can complete both boring and reaming operations.

• Instrument lathe: The instrument lathe clamps the magnetic steel product through the fixture, drives the product to rotate continuously through the spindle motor, and the fixed alloy tool drills the rotating workpiece. It is mainly used for drilling and piercing of cylindrical, circular and small square products, and the processing hole diameter is less than 5mm.

• Benchtop drilling machine: a kind of equipment that uses self-made tooling to position the product, and carbide tools to rotate and feed, to achieve drilling processing on the product; the main difference from the instrument lathe is that the instrument lathe workpiece rotates and the tool is fixed; while the benchtop drilling machine workpiece is fixed and the tool rotates. Therefore, the benchtop drilling machine can be used for the processing of through holes, blind holes, and stepped holes of special-shaped products.

• Ultrasonic drilling machine: The transducer concentrates the ultrasonic energy to the drill bit, and the high-frequency mechanical vibration of the drill bit drives the abrasive suspension to achieve impact perforation through high-speed impact, friction and cavitation. Ultrasonic drilling has high accuracy, efficiency and qualified rate, and can be used for small hole processing of magnetic steel.

4、Chamfer

During the processing of grinding, slicing, punching, etc., the sharp corners of NdFeB magnetic products are very likely to cause edge knocking and corner chipping, and the tip effect during electroplating will deteriorate the uniformity of the coating. Therefore, after machining, the magnetic steel is usually chamfered, including mechanical chamfering and vibration chamfering. Common chamfering equipment includes vibration grinding chamfering machine and drum chamfering machine.

• Vibration grinding and chamfering machine: Through the vibration deviation generated by the vibration motor, the magnetic steel and abrasive in the working tank are driven to move up and down, left and right, or rotate and rub against each other, so as to make the surface of the product smooth and round the edges. Commonly used abrasive media include silicon carbide, brown corundum, etc.

• Drum chamfering machine: NdFeB magnetic steel products, abrasives and grinding fluid are placed in a sealed horizontal drum. The rotation of the drum causes centrifugal rotation and friction between the product and the abrasive to achieve chamfering.

We will select the most economical and efficient processing route according to the product size specifications and geometric tolerance requirements. For the quality of processed products, we should focus on dimensional tolerance, geometric tolerance and appearance. Common defects and defects in processing include: dimensional tolerance, poor verticality and contour, missing corners, knife wires, scratches, grinding marks, corrosion, hidden cracks, etc.