High precision grinding produces medical implants and devices such as shoulder and hip prostheses.
PHOTO ©i3d_vr | ADOBE STOCK

Shops that swap out ceramic, vitrified, or resin-bond grinding wheels for metal-bond wheels increase productivity up to 30% and reduce grinding-wheel wear by as much as 70%. However, the difficulty of dressing small wheel radii and special profiles, while maintaining high grain profusion, has traditionally made them more expensive and difficult to use than they are worth.

Fortunately, a recently developed dressing system based on wire electrical discharge machining (EDM) technology provides a simple, machine-integrated, efficient, and cost-effective solution that will allow more shops grinding surgical implants, tools, and other precision components to reap the benefits of metal-bond grinding wheels.

Metal-bond wheels

Grains of diamond or cubic crystalline boron nitride (CBN), firmly embedded in a sintered metal matrix, produce very high material removal rates on the workpiece. Metal-bond wheels easily handle tough materials such as high-strength alloys, tungsten carbides, ceramics, and hardened steels. Better heat dissipation leads to cleaner, faster cuts than alternatives.

However, high material removal rates cut both ways – the extreme abrasion provided by metal-bonded diamond or CBN grinding wheels wear out dressing wheels quickly, reducing geometric precision. The silicon carbide wheels used in conventional dressing systems simply cannot achieve suitable results or create the intricate profiles that make metal-bond grinding wheels so attractive. The complex mechanical process of extracting grains from their metal matrix sometimes damages them, preventing selective grit exposure from its metal bond.

Studer WireDress technology dresses metal-bonded grinding wheels with high precision.
PHOTO COURTESY OF UNITED GRINDING

Dressing challenges

Conventionally dressing metal-bonded grinding wheels requires taking them off a grinding machine, moving them to a secondary piece of equipment for processing, and remounting them on the grinding machine. Repeated handling increases the likelihood of dressing and remounting error and imprecision. For shops with dressing machines, preparing grinding wheels still requires time and handling and rarely achieves precise results. Many shops send their grinding wheels out to subcontractors for dressing, magnifying complications

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The primary traditional option for generating intricate metal-bond wheel profiles uses plating technology which cannot be dressed and must be shipped back to the manufacturer for re-plating. Another option is contact-based dressing methods with sintered metal-bond diamond wheels. However, geometry limits intricate profiles, preventing wheel shapes that hold complex contours and very fine, precise threads necessary to grind carbide parts, for example.

EDM dressing

A newly developed EDM-based dressing system addresses those problems, simplifying dressing and enabling greater use of metal-bonded wheels. The EDM integrates directly onto the grinding machine, eliminating handling errors. The non-contact system creates high-protrusion grains unachievable with other dressing methods for metal-bond wheels, generating the cutting performance of electroplated wheels.

EDM dressing uses quick sequences of extremely short, direct-current pulses to generate a discharge in dielectric oil (grinding oil) within the gap between electrode and workpiece. Tiny areas of the metal bond are melted and flushed away as small particles.

Studer’s WireDress, incorporated into some of the company’s grinding machines, allows shops to dress their own metal-bond wheels directly on the machine at full operating speed. Integrated into the grinder’s CNC control, it eliminates wear on dressing tools and labor-intensive reinstalling and resetting processes that follow external dressing.

A Studer S41 cylindrical grinding machine.
PHOTO COURTESY OF UNITED GRINDING

Improved operation

The integrated EDM wheel dresses at 15mm/sec to 25mm/sec (0.59ips to 0.98ips) axial feeds, creating free geometries and intricate contours with 0.2mm (0.008") internal radii and 0.05mm (0.002") external radii while retaining the original shape of the grain. For example, dressing a 10µm radius on a 400mm (diameter) x 10mm (width) wheel takes approximately three minutes.

Integrated EDM wheel dressing saves energy as well as time, using 700W while it dresses and 60W in standby mode. Rotary dressing with a diamond wheel requires up to 1.5kW with an additional 1kW for sealing air.

United Grinding