Creep-Feed Grinding Is Surprisingly Versatile

Manufacturing Engineering, Nov 2004 by Salmon, Stuart

It may be the only way to do the job

Creep-feed grinding is nothing like conventional grinding. It's best described as milling, but using a grinding wheel in place of the milling cutter. Originally, creep-feed grinding used a highly porous, vitrified aluminum-oxide wheel to take large depths of cut ranging from 1 mm to more than 25 mm. The process was targeted particularly towards the aerospace industry and other industries that used difficult-to-machine alloys.

A typical creep-feed grinding cycle is to take the majority of the stock in one or two roughing passes, then, after a dress cycle, one finish pass, leaving a surface that is precise to within 0.025 mm of size and 0.005 mm in form, with minimal burr. Moreover, the part would be cold at the end of the cycle. This is being done even in the most difficult-to-machine materials with high abrasion resistance, heat resistance, high-temperature creep-resistance, high potential for cracking and grain growth with localized high-temperature gradients, and a tendency to work-harden.

Invented in Europe, the technology spread slowly. It was found that fully hardened tool steels could be creep-feed ground, eliminating the soft milling stage. Just grind from solid, in one or two passes. It was recognized that the stock-removal rates were often equal to or better than those produced by milling with a lower piece-part cost. Plus there was the added bonus of a surface finish that was better than milling. Post-machining operations like deburring and polishing could be dramatically reduced or even eliminated. Creep-feed grinding was no longer exclusive to the aerospace industry, but also found applications in the more common steels, tool steels, and stainless steels. Applications spread into aluminum, titanium, and magnesium parts.

The switch from milling, broaching, and even turning, in some cases, to creep-feed grinding can be made with ease. A part designed to be milled will typically be toleranced and surfaces specified with the milling process in mind. Creep-feed grinding always produces a better finish and machines to tighter tolerances with no burrs. When calculating the overall piece-part cost, creep-feed grinding is a fierce competitor.

Creep-feed grinding relied on the porosity of a vitrified wheel to carry the swarf out of the arc of cut. As applications became more ambitious, and with the advent of continuous-dress creep-feed grinding, the stock removal rate increased by a factor of more than 10. The wheel manufacturers were under significant pressure to develop moreopen structures with higher bond strength. These are two quite juxtaposed wheel properties. Wheel manufacturers were reaching the physical limits of possibility with conventional vitrified bond systems. Today we see strong development in the area of new glass bonds, and bonds that key into the surface of the grains, not merely bind them together.

Continuous-dress creep-feed (CDCF), as the name implies, is a process whereby the grinding wheel is dressed, using a diamond roller, all the time the grinding wheel is grinding the workpiece. CDCF is ideally suited to large batch quantities of difficult-to-machine materials and expensive and metallurgically sensitive parts. It also works well on very long parts (press-brake tooling and wood-chipper blades, stacks of saw blades and scissor blades, corrugated paper-making rolls) as the grinding wheel stays in a constant state of sharpness throughout its life, and the process is never interrupted for dressing as the wheel is being dressed all the time. Stock removal rates can be increased in the order of 10 times those achieved by using intermittent dressing processes.

CDCF had the benefit of reducing the floor-to-floor time to grind a typical aircraft engine turbine blade root from 6 to 8 min to 20 to 30 sec. The downside was that part handling became a major issue. For a while, machine tool design in particular fell into neglect as the spotlight turned towards automation, robotics, and grinding "cells." We still suffer from that trend today. Many of the "modern" day machine designs have been compromised for economy, and now lack stiffness and vibrational stability. A recent push to create machines with a smaller footprint has prompted the redesign of machine-way configurations, not always for the better, though some such as Micron Machine Tools in Springfield, MA, have addressed the issue and have a machine that may appear to be small, but has a static stiffness more than three times that of a conventional creep-feed machine from the 1990s.

Many of today's milling processes stress high-speed machining. But grinders have been high-speed machining all along. It is not new to those involved with abrasive machining to deal with cutter mounting and balancing, proper fluid application, safe and proper guarding, and machine flexibility, while maintaining high stiffness and vibrational stability. Grinding is regularly carried out at 30 m/s, and with superabrasives there are production machines running 90 m/s and faster. That brings us to High Efficiency Deep Grinding (HEDG), which is a fairly new grinding technique. It is the same creep-feed grinding process, but uses an electroplated superabrasive grinding wheel, at a high peripheral speed, to minimize the length of the chip. For form work, the diamond roller, in fact the entire dressing system, can be eliminated when using HEDG. A plated wheel has the form accurately manufactured into the wheel periphery. It's a matter of properly mounting and trueing the wheel to the spindle and that is all that is required; no dressing at all, until the wheel wears out, and then it is changed.