Internal Cylindrical Grinder

Overview

An internal cylindrical grinder finishes the inside diameter of a part to final size, roundness, and surface finish after that part has been hardened. It is the bore counterpart to outside-diameter grinding, and it does the harder job: the wheel has to work inside a hole, so it can only be as large as the bore will accept, and on small bores that means a wheel of a few millimetres. The typical work is rings and sleeves that cannot be turned to finish because they are too hard or too precise, bearing inner and outer rings, gear bores, hydraulic spool and valve bores, fuel injector bodies, bushings, and die bores. Bore roundness of 0.3–1 µm and size held to a few microns are normal production results on a machine like this. The bore is ground concentric to a chosen reference, usually the part's own outside diameter or a face, and that concentricity is often more important than the absolute diameter because it controls how the assembled part runs.

How it works

The part is mounted in the Workhead and rotated at roughly 100 to 600 rpm. The Wheelhead and Quill carries a small mounted wheel on the end of a slender Wheel Quill that reaches into the bore. Because the wheel is small, it must spin extremely fast to reach a usable cutting speed: a 10 mm wheel needs around 60,000 rpm just to reach 30 m/s at its rim, so spindle speeds of 30,000 to 150,000 rpm are standard, and the Spindle Cartridge is the defining component of the machine. Many machines float that spindle on an Air-Bearing Sleeve because an air film gives near-zero runout and almost no heat at those speeds. The Infeed Slide feeds the wheel radially into the bore wall to set diameter, while the Table and Oscillation Slide reciprocates along the bore axis so the narrow wheel sweeps the full length on each stroke. That traverse motion is what makes the bore a true cylinder rather than just a ring ground at one plane, and the Oscillation Drive adds a fast dither over the traverse to spread wheel grain and keep the surface even.

Holding the part

How the part is held decides the concentricity of the finished bore. A simple Work Chuck grips the outside diameter, and the bore comes out concentric to that grip to within the chuck's accuracy. For thin rings that a chuck would distort, an Magnetic Face Plate holds the part flat by one face. The most accurate method for bearing rings is the Shoe Support: the ring rides on two carbide shoes that locate it on its own outside diameter while a backing roller drives it, so the bore is ground true to the OD without any gripping force at all. The Workhead Spindle runs in preloaded bearings because its own runout adds directly to the concentricity error of the part.

Wheels, wear, and dressing

Small wheels wear fast. A bore wheel removes stock with a tiny contact patch and a short rim, so it loses radius quickly and must be dressed far more often than the large wheel on an outside-diameter machine. That is why the Diamond Dressing Unit sits permanently on the machine and trues the wheel between parts, sometimes between passes. The Mounted Wheels and Quills holds a range of mounted wheels sized close to each bore. Vitrified aluminium-oxide Mounted Grinding Wheel stock is cheap and easy to dress but wears quickly on hardened steel, so production work has largely moved to the CBN Bore Wheel. Cubic boron nitride holds its form for thousands of bores, cuts cooler, and shifts the dressing job to a powered Rotary Dresser instead of a single-point diamond. The CNC Control tracks every dress, since each one removes wheel radius, and re-feeds the infeed slide so the finished diameter does not drift as the wheel shrinks.

Sizing and gauging

Because the wheel is constantly changing diameter, the machine cannot grind to size by slide position alone. It measures the bore as it grinds. The In-Process Gauging head enters the bore and reads diameter live, and the control stops the infeed exactly when the In-Process Bore Gauge reports finished size. The Gauge Fingers ride the bore wall and read to better than a micron, and a Gauge Retract swings the head clear during load and unload. After grinding, a Post-Process Gauge re-checks the bore and feeds a slow correction back so the process holds size across a long run. A Face Grinding Attachment attachment can grind the part face square to the bore in the same setup, which keeps bore-to-face squareness tight and removes a second handling step.

Materials and structure

The work is almost always hardened, in the range of 58 to 64 HRC, which is the reason for grinding rather than boring. Bearing rings are through-hardened chrome steel, gears are case-hardened alloy steel, and valve and injector bores are nitrided or hardened tool steels. The machine itself is built for stillness: the Bed and Structure is granite or grey iron of 1.5 to 6 t for damping, the Quill Balancing Ring cancels quill unbalance that would otherwise print chatter at high speed, and the slides run on hydrostatic or precision rolling guideways. Heat is the main enemy of bore size, so the Coolant System system pushes filtered, chilled coolant through a Coolant Nozzle into the buried contact deep inside the bore where access is tight, and a chiller holds it within about a degree so the part does not grow during the cycle.

Variants and use

Machines range from small manual bore grinders for toolroom work up to fully automated CNC cells that load rings from a feeder through the Loader Interface, grind bore and face, gauge, and sort in a few seconds per part. Bearing plants run banks of these machines as the finishing step on inner and outer ring bores. Universal grinders combine internal and external grinding on one bed for low-volume precision work where both diameters must be concentric. Across all of them the constraints are the same: a small fast wheel, a slender quill that limits how deep and stiff the grind can be, frequent dressing, and live gauging to hold size as the wheel wears away.