Application Development in High-Speed Internal Grinding

An internal grinding wheel has to fit inside the bore it is cutting. A Ø20 mm bore leaves little room, so the wheel often ends up at Ø10 mm or smaller. Small wheels lose surface speed fast. Spin a Ø10 mm wheel at 10,000 rpm and the rim runs at roughly 5 m/s, well below the 30–45 m/s that vitrified aluminum oxide wants. The result is a wheel that rubs instead of cuts.

Low surface speed shows up in three places: heat goes into the workpiece, the wheel glazes, and cycle time stretches. Bore size drifts as the part grows from thermal expansion. That is the gap high-speed internal grinding closes.

A high-speed spindle is not simply a faster motor. It needs a rigid quill, oil-air or oil-mist lubrication, and a frequency converter that holds rpm under load. Typical production spindles run 60,000 to 120,000 rpm for small bores, and the surface speed stays in the useful range even with a Ø6 mm wheel.

Higher speed also changes the chip. Each grit takes a thinner cut, so the specific grinding energy drops. Less energy in the contact zone means less burn risk and longer wheel life, provided the feed and coolant keep up. Speed alone is not enough.

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  Speed buys grit penetrationThin chips, lower forces, less spring-back in a slender quill.
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  Speed does not fix setupWheel balance, truing and coolant aim still decide the finish.

The workhead carries the part, usually in a chuck, collet or shoe arrangement. For through-bores and bearing rings, shoe-centerless workholding gives the best roundness because the part floats on its own diameter. For a housing with an off-axis bore, a chuck or fixture plate holds the part and the wheel head indexes to the bore.

The wheel head does two things at once: it spins the wheel and oscillates it along the bore axis. Oscillation stroke is set slightly longer than the bore so the wheel exits both ends and the edges do not wear a step. On a machine with a 4,000 mm maximum processing size class of travel, the same control loop still governs a 20 mm stroke, just with finer resolution.

Feed axes move the wheel head in and out. Coarse infeed brings the wheel to the bore wall, then a controlled infeed rate takes over. Most modern machines split the cycle into rough, semi-finish and spark-out. Spark-out, running with no further infeed for a few revolutions, is what removes the last of the elastic deflection and brings the bore to size.

Coolant delivery is a design decision, not an accessory. Through-spindle coolant aimed at the contact arc reaches the zone where the heat is generated. Flood coolant from outside rarely gets past the wheel and the bore wall. On small bores, oil-based coolant carries the heat better than water-based fluid but needs a fire-safety plan.

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  WorkheadChuck, collet or shoe-centerless, depending on bore and roundness needs.
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  Wheel headHigh-speed spindle plus oscillation stroke and programmable infeed.
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  CoolantThrough-spindle or high-pressure jet aimed at the contact arc.
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  GaugingIn-process or post-process, feeding size back to the control.

Aluminum oxide and silicon carbide wheels still do the job on soft steels and cast iron. They are cheap, easy to true with a diamond dresser, and forgiving if the coolant flow is imperfect. Their weakness is wear. On a long run, the wheel diameter shrinks, the surface speed drops, and the operator compensates by raising rpm.

CBN (cubic boron nitride) changes that trade-off. A CBN wheel holds form far longer, cuts cooler on hardened steel, and tolerates higher surface speed. The catch is cost and truing. CBN is trued with a diamond roll or a brake dresser, and the dress must be done before the wheel loads, not after.

Diamond is the choice for carbide, ceramic and some hardened tool steels. It cuts those materials cleanly but reacts with iron at high temperature, so it should not be used on plain carbon steel at high speed.

Bond type matters as much as abrasive. Vitrified bond is porous and self-sharpening, good for soft ductile steel. Resin bond is tougher and often used with CBN for high-speed work. Metal bond holds form best and is common on small superabrasive pins for carbide dies.

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  Aluminum oxideSoft steel, cast iron, low volume, easy truing.
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  CBNHardened steel, long runs, high surface speed.
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  DiamondCarbide, ceramic, hardened tool steel; not plain carbon steel.

A grinder that stops on a timer will drift. Wheel wear, thermal growth and coolant temperature all push size around. In-process gauging closes that loop. A pair of contact heads sits in the bore and measures diameter as the wheel cuts; when the reading hits the target, the control retracts the wheel.

Not every part allows in-process gauging. A small blind bore may have no room for the heads, and a very short bore gives the gauge nothing to sit on. In those cases, post-process gauging on a separate station measures the part and sends a correction back to the grinder offset.

Feed control is the other half. A fixed infeed rate works until the wheel contacts the wall, then forces spike. Modern controls detect the touch point, switch to a programmed rate, and finish with spark-out. The rate itself is set from the target material removal and the wheel's safe specific removal rate.

Truing frequency is a schedule, not a guess. On a CBN wheel running hardened steel, truing every 200–500 parts is a common starting point. Check bore size and roundness across the run; when roundness drifts before size does, the wheel is loading and needs a fresh dress.

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  In-processHeads in the bore, real-time size, best for high volume.
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  Post-processSeparate station, offset correction, fits small or short bores.
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  Spark-outNo infeed, a few revolutions, releases elastic deflection.

High-speed internal grinding pays off when the bore has to be round, straight and fine-finished at the same time. Hydraulic valve bodies and spool bores are the classic case. A spool that fits at 2–4 μm clearance will leak if the bore is out of round, and grinding is one of the few processes that holds that band in hardened steel.

Bearing rings, fuel injector bodies, and diesel injector nozzles also land here. So do medical cannulae and surgical instrument bores, where the finish drives both function and cleanability. On the automotive side, EV motor housings and transmission bores with tight roundness call for the same capability.

The process is not for every bore. A soft aluminum bracket with a ±0.05 mm bore belongs on a boring head or a reamer. A large, shallow bore with a loose finish requirement is faster on a CNC mill. Grinding wins when the tolerance is under about ±0.01 mm, the material is hard, or the finish has to reach Ra 0.8 μm or better.

GreatLight runs 127 high-precision CNC machines across three wholly-owned plants, with capability to ±0.005 mm and finishes to Ra 0.2–0.8 μm on grinding work. When a bore is better served by 5-axis milling or turning, we say so before quoting. That is part of the application work, not a sales step.

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  Good fitHardened bores, tight roundness, fine finish, high volume.
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  Poor fitSoft material, loose tolerance, short run, shallow bore.