Creep-Feed Grinder

Overview

A creep-feed grinder removes large amounts of hard material in a single deep, slow pass rather than in many shallow strokes. A conventional surface grinder takes off a few microns per pass and traverses the table quickly. A creep-feed grinder does the opposite: it sets a depth of cut of one to six millimetres and feeds the work under the wheel at ten to a few hundred millimetres per minute. The wheel stays in the cut continuously, so the contact arc is long and the cutting forces are high. The machine is built heavy and stiff to absorb those forces without losing geometry.

The process suits parts that would be slow or impossible to rough by milling: turbine blade fir-tree roots, deep slots in hardened steel, forms in nickel superalloys, and profiles in ceramics. Because the wheel can grind a finished profile straight from solid or from a near-net forging, creep-feed grinding often replaces a milling and finish-grinding sequence with one operation. The trade is that the long contact arc generates a lot of heat in a small zone, so coolant delivery is as important to this machine as the spindle itself.

How it works

The operator clamps the work to a fixture on the CNC Work-Feed Table, a servo-driven slide that carries the part under the wheel on a Ball Screw and Linear Guide Rail. A glass scale feeds table position back to the CNC Control at 0.1 µm resolution, so the slow feed stays smooth and free of the cogging that would chatter the surface. The Grinding Spindle Head turns a large wheel at 30 to 60 metres per second, driven by a 45 kW motor in the Electrical Cabinet through an inverter.

The depth of cut is set on the Base, Column & Ways column slide, then the table feeds the part through in one stroke. Through the whole stroke the Continuous-Dress Unit dresses the wheel: a spinning diamond roll advances a few microns into the wheel face for every wheel revolution, peeling away loaded abrasive and exposing sharp grit continuously. This continuous dressing keeps the wheel cutting freely and holds the profile true even though the wheel wears down through the cut. The roll runs on its own Dress Roll Spindle and is fed by the Dress Feed Servo under control of the CNC.

Coolant delivery

The long contact arc puts almost all of the grinding energy into a narrow band of the part, and without enough cooling the surface would burn and crack. The High-Pressure Coolant System system answers this with high pressure and high flow. A multi-stage High-Pressure Pump delivers fluid at up to 70 bar through Coherent-Jet Nozzle units shaped to throw a coherent jet that matches the wheel surface speed, so the coolant rides through the grinding zone instead of bouncing off the wheel. A Pressure Sensor watches delivery pressure and the CNC will hold the cut if it drops. Spent coolant carries heavy swarf, which a Swarf Scraper Conveyor conveyor drags from the tank while a Magnetic Filter takes out the fine particles before the fluid returns to the pump.

Spindle and structure

The Grinding Spindle Head cartridge runs on a set of precision Ball Bearing pairs held in a heavy quill, sealed against coolant by Oil Seal elements. The large wheel stores a lot of energy at speed, so the head carries a Wheel Balancer that measures residual unbalance and shifts two internal masses to cancel it while the wheel runs. The whole head bolts to the Column Casting, which rides hardened box ways on the Cast Iron Base. The base is a box-section iron casting filled for damping, because any flex or vibration in the structure during a deep cut shows up as waviness or chatter on the ground form.

Guarding and control

A large wheel turning at full speed is a stored-energy hazard, so the Wheel Guard & Enclosure is a thick steel burst guard inside a sealed enclosure that also keeps the high-pressure coolant contained. The Interlocked Door is interlocked, so the spindle cannot run with it open. The CNC Control runs the part program, coordinating the feed servo, the continuous-dress feed, the spindle inverter and the coolant valves so the dress rate, feed rate and wheel speed stay matched through the pass. Each feed axis has its own Servo Axis Drive, and the Spindle Inverter Drive in the cabinet runs the main motor.

Variants and use

Machines range from small units grinding tooling and dies to large aerospace cells grinding turbine components in fixtures that index many blades per cycle. Wheels may be soft-grade vitrified aluminium oxide, which is cheap and dresses easily, or plated and vitrified CBN for superalloys, which holds form far longer but needs careful coolant and dressing. Continuous-dress creep feed gives the highest stock-removal rate but consumes the wheel quickly; some parts instead use intermittent dressing or no dressing with a long-life CBN wheel. The choice of wheel, dress rate and coolant pressure is set per part to keep the surface free of thermal damage while removing metal as fast as the structure and spindle allow.