Crankshaft Grinder

Overview

A crankshaft grinder finishes the bearing surfaces of an engine crankshaft to final size, roundness, and surface finish after the forging or casting has been turned, heat-treated, and roughed. A crankshaft has two families of journals: the main journals, which sit on the engine block centerline, and the rod or pin journals, which carry the connecting rods and are offset from that centerline by the crank throw. Both must be ground concentric, round, and parallel within a few microns, because clearance in a plain bearing is set by the journal diameter. This machine grinds both families on one setup using a Wheelhead & Spindle carrying an electroplated CBN Grinding Wheel, a work drive in the Work-Drive Headstock, and an in-process In-Process Diameter Gauge that ends each cut at the target diameter.

Everything mounts on the Machine Bed & Ways, a mineral-cast bed chosen for its damping and thermal stability. The crank is held between the headstock and the Footstock, supported along its length by Steady Rest units, and the wheel is fed in by the Wheelhead & Spindle under CNC Control.

How it works

For a main journal the crank turns on its own main-bearing axis. The flange end is gripped by the Work Faceplate, a Driving Dog transmits rotation, and the free end runs on the Live Center in the footstock. The Work-Drive Headstock rotates the work slowly while the wheel, spinning at 60 to 120 metres per second, is fed in by the X Infeed Slide. The slide uses a Ball Screw on Linear Guide Rail rails driven by a Servo Motor, with a X-Axis Glass Scale reporting position to 0.1 micron. As material comes off, the In-Process Diameter Gauge rides the shrinking journal and tells the control the instant size is reached, so the cut stops on diameter rather than on a programmed depth.

Pin journals are harder, because each one orbits the main axis at the crank throw radius. Two methods exist. On the older eccentric method the Crank-Pin Chuck uses an Eccentric Setting Plate to shift the work center sideways by the throw, so the chosen pin journal turns on the spindle axis and is ground exactly like a main; a Balance Counterweight cancels the resulting imbalance. On a modern CNC machine the crank stays on its main axis and the wheelhead chases the orbiting pin: the C-Axis Encoder reports crank angle and the CNC Control interpolates X infeed against the C axis so the wheel oscillates in and out in step with the pin orbit. This follow or orbital grinding removes the need to re-clamp for each pin and keeps indexing between pins accurate.

Wheelhead and spindle

The cutting edge of the machine is the Wheel Spindle Cartridge, a precision cartridge holding the wheel arbor on angular-contact Ball Bearing sets sealed by Oil Seal elements. The CBN Grinding Wheel runs cooler and far longer than a vitrified wheel on hardened journal steel, which is why it is standard on this class of machine. Wheel unbalance at these speeds prints straight onto the roundness of the journal, so a Auto Wheel Balancer with in-spindle balance heads trims residual unbalance while the wheel turns. The Wheel Spindle Motor drives the spindle through a Drive Belt.

Wheel dressing

A CBN wheel needs truing to keep its face round and its profile correct. The CBN Wheel Dresser feeds a powered Rotary Diamond Disc across the wheel on its own slide. Because CBN is expensive and slow-wearing, dressing removes only a few microns at a time and is scheduled by part count rather than by every cycle. The dresser slide shares the same Ball Screw and Linear Guide Rail hardware family as the infeed slide, which keeps spare parts common across the machine.

Workholding and support

A crankshaft is long, slender, and out of balance, so support matters as much as drive. The Footstock advances its Footstock Quill hydraulically to clamp the crank between centers at a controlled force. Along the span, Steady Rest rests close their Support Shoe elements onto the journal under spring and hydraulic load, holding it against the wheel so it cannot deflect or chatter. The shoes are bronze or carbide and ride directly on the finished surface, so coolant must be clean where they contact.

Coolant and thermal control

Grinding puts most of its energy into heat, and heat distorts the very geometry the machine exists to hold. The High-Pressure Coolant System system floods the grind zone through Coherent-Jet Nozzle jets aimed tangent to the wheel, then filters and recirculates the oil. A Coolant Chiller holds the coolant at a fixed temperature so the bed and work do not grow during a shift. Clean coolant also protects the journal surface from scratches and keeps the steady shoes from picking up swarf.

Control and variants

The CNC Control coordinates the X infeed servo, the C-axis work drive, the gauge, the dresser, and the coolant. Its core job on pin journals is the tight X-C interpolation that traces the orbit. Machines of this kind range from single-wheel general-purpose grinders found in engine rebuild shops to multi-wheel production grinders in engine plants that grind several journals at once. The bed, headstock, wheelhead, and gauge described here are common to both; production machines add wheel slides and automation, while a rebuild-shop machine keeps one wheel and relies more on the operator and the In-Process Diameter Gauge to hold size across a wide mix of cranks.