The development of power-driven brushes specifically for use on conventional centerless grinders has increased significantly over the past two decades as new applications have developed. Six key areas have been identified as ideal uses for centerless brushes as opposed to abrasive wheels more commonly associated with centerless grinders.
- Edge blending
- Burr removal
- Removal of sharp edges or chamfering corners
- Blend tool and grinding marks
- Elimination of areas of stress concentration
- Increasing production finishing rates
Centerless brushes are large radial brushes filed with fine wire (0.005 through 0.010), medium wire (0.0118 through 0.016) or coarse wire (0.018 through 0.026) in carbon steel, stainless steel, brass or bronze. Centerless wheel brushes can also be filled with natural fibers including Tampico or abrasive nylon filaments, depending upon the job.
The two main categories of use involve improving surface finish or relieving stress concentration areas on parts. Typical dimensions for a Centerless Wheel Brush are 20’’ O.D. x 12’’ I.D. x 6’’ WIDE.
Surface Finish Improvement
Centerless brushes, of the fine wire type, will not remove metal from a cylindrical surface; therefore the part must be ground to size prior to brushing.
Wire filled centerless brushes are used for removing grinding burrs and for producing a minimum edge blend of 0.005 inch with fine wire. They can be used dry, but are preferably used with grinding coolant.
Fine-wire-filled brushes used with conventional grinding coolants are being used for removing feather grinding burrs and improving surface finish on such parts as control valves for automatic transmissions, rocker-arm shafts, and piston pins. In addition to removing feather grinding burrs, these brushes are capable of improving a finish originally measured at 24 to 35 micro-inches to 7 to 4 micro-inches without altering dimensions.
Stress Concentration Elimination
Manufacturers of jet engines have reported that certain failures experienced in well designed jet engine parts are the results of stress concentrations and progressive fractures that occur at scratches, sharp edges or burrs. With continued repetitions of stress imposed by engine operation, microscopic cracks are propagated, and the entire member may rupture if the cracks are not detected.
Data has been presented to show that a sharp corner or edge may reduce the endurance limit of a part by as much as 50%. A distinct “V” notch, such as might be made with a sharp cutting tool, can reduce the endurance limit of a part by as much as 60%. Centerless brushes are widely used to produce radii and improve the micro-finish.
Like all brushes, centerless brushes work best when the pressure between the brush tips and working surface is at minimum. The amount of pressure between the brush tips and working surfaces affects both the quality of the work and the brush life. The very ends of the tips of the wire or filament do the work. Brushes should be operated at the highest PRACTICAL speed with the lightest possible pressure. Excessive pressure can cause excess flexing of the filament which causes premature breakage and results in a wiping action rather than a cutting action.
These brushes are typically operated at 5500 to 6500 S.F.M.