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Creep-Feed Grinding is Surprisingly Versatile

 

It may be the only way to do the job

 

By Dr. Stuart Salmon
President
Advanced Manufacturing Science & Technology
Rossford, OH

            
            

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 more-open 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.

Plated wheels are becoming more popular for the reasons cited. As spindle speeds and power increase, the emphasis on the cutting fluid, both in terms of its chemistry as well as the application methods, becomes more critical. HEDG grinders have been built for plated superabrasives to run as high as 250 m/s and initially were run using straight oils. That is beginning to change as water-based fluids are performing as well as, if not better than, some oils without the fire and health hazards.

The proliferation of electroplated superabrasive wheels, particularly CBN, has the industry in a dilemma due to the perceived short life of CBN in traditional water-based grinding fluids. Most companies will run plated CBN in straight oil and deal with the fire and health hazards stemming from the oil mist based on the historical perception. Electroplated applications, in the aerospace and medical industries, use a jet cleaner, a high-pressure (70 - 100 lb or 1000 - 1500 psi) blast of fluid at the wheel periphery to keep the wheel from loading with the gummy material, and so extend the wheel life based on the wheel loading being the predominant wheel-wear factor. Recent research by Master Chemical Corp. in Perrysburg, OH, has shown that there is no need for the jet cleaner if a hard lubricant coating is applied to the wheel. When used in combination with advanced water-based fluids, coated wheels have shown equal and even longer life than uncoated wheels, while using water-based fluids instead of the traditional straight oil.

CNC controls opened up a whole new world of shape generation. Creep-feed grinding was no longer just a flat/form/slot machining process. It can contour the most accurate concave as well as convex radii and shapes in three dimensions. A popular application is the machining of glass molds for CRT, television, and computer screens as well as the intricate shapes related to the molds for automotive head, side and rear lamps, and reflectors.

Electroplated superabrasive wheel technology has opened a new area of creep-feed grinding on a machining center, whereby premounted wheels are stored in the toolchanger and then brought out to contour complex shapes into punches and dies, for example, all in one clamping, bringing JIT technology to the fore.

Machining centers even support continuous-dressing for special applications using small wheels. This has been championed by Makino (Mason, OH), along with UGT (Miamisburg OH), who both showed machines at IMTS having grinding-wheel and toolchanging capabilities.

If multiprocess machines are to be used, then some compromises must be made in the areas of fluids and swarf. A particularly good grinding fluid may not be a good milling or drilling fluid. The chips that are generated by milling are far different from those generated by grinding, and need special filtration to keep the fluid as clean as possible. Many milling operations are going dry, or at least MQL (Micro-Quantity Lube), the flood fluid actually being a disadvantage to surface finish and tool life when milling. Grinding seems to be staying very wet, however. The mix of machining and grinding on one machining center is still in its infancy, but it is finding its place and growing in the ever increasing market for flexibility and small-batch quantities.

One of the newest creep-feed-style grinding processes is VIPER (Very Impressive Performance Extreme Removal).This is a process born out of creep-feed grinding and developed for and by the aerospace industry in the UK by Rolls-Royce and in cooperation with Tyrolit in Schwaz, Austria. The VIPER process uses grinding wheels with a porous structure but in combination with high-pressure grinding fluid. The combination provides the stock removal capability of continuous-dress creep-feed grinding without the heavy wheel loss, and in many cases exceeds the productivity expected from CDCF. Its niche is in the aerospace industry but the process could show significant benefit in the medical and electronic industries also. The technology has been licensed to a number of companies worldwide.

Grinding is very much a materials-driven process. As materials technology moves from metals to cermets, to ceramics and whisker-reinforced metals and plastics, grinding, or abrasive machining, is the only process for such a task. Although creep-feed grinding is a process that competes with milling and broaching, when it comes to ceramics and whisker-reinforced materials, it is the only process that works. It is critical, therefore, that the industry understand that abrasive machining may be the only way they will be able to machine the next generation of materials. It will be the principles of creep-feed grinding that will machine the next generation of "super" materials. The fast feeds and small depths of cut used in conventional grinding tend to chip and crack these materials, whereas creep-feed grinding can operate in the ductile region of even the most hard and brittle materials. Conventional practices will not do the job. Creep-feed grinding offers a new world of performance opportunities that may surprise even those in the grinding industry.

 

This article was first published in the November 2004 edition of Manufacturing Engineering magazine. 


Published Date : 11/1/2004

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