MRO Today - Grinding with CBN

Grinding with CBN

Maintenance profitability: Cutting tools, grinding and how to run a “sharp shop”

by Dr. Anil Srivastava

As with all work, there’s a right tool for every job. In manufacturing, successful machining depends on sharp, well maintained cutting tools that not only ensure precision, but also speed. Successful sharpening of cutting tools depends upon the use of the correct grinding wheel, tool set-up, and method of grinding for each type of tool. This article provides some basic but valuable information about cutting tool sharpening that can help you keep your manufacturing flow as efficient as possible, ensuring better competitiveness nationally and even globally.

Grinding with CBN
The most common method to sharpen a worn out tool is grinding. Both conventional (aluminum oxide and silicon carbide) and super abrasive (cubic boron nitride — CBN) wheels are used for this purpose. However, conventional wheels dull faster and become glazed and burnished, which reduces grinding productivity and can cause thermal/metallurgical damage to the tool being ground.

When choosing a grinding wheel and comparing the costs of improvement versus the effect to the bottom line, some major factors argue in favor of choosing CBN wheels over conventional wheels for sharpening cutting tools.

First, CBN grains have 55 times higher thermal conductivity, four times more abrasive resistance and twice the hardness of aluminum oxide abrasives. This combination makes CBN wheels especially well suited for grinding high-speed and super-alloy steel tools because they provide:

A) long wheel life at high material removal rates;

B) little or no thermal damage to the cutting edge because of the cool cutting action;

C) consistent sharp, burr-free cutting edges with no loss of hardness;

D) easy and more effective control over tool sizes, shapes, and finishes;

E) increased tool grinding productivity thanks to less downtime from wheel breakdown and conditioning, less time required for gauging, spark-out and wheel changes.

CBN grains do not chemically react with steels and retain their strength above 10,000 degrees C. Grinding with CBN wheels also improves the fatigue strength and extends the useful life of the cutting tool.

Factors to consider first
However, before replacing an aluminum oxide wheel with a CBN grinding wheel, it is important to make sure that the grinding system can take advantage of the productivity potential that a CBN wheel offers. The major factors to be considered are:

1) the grinding machine (which must have tight spindle bearings and close fitting slides to eliminate vibration and chatter, constant spindle speed to boost efficiency and reliable feed-rates to preserve wheel life);

2) horsepower requirements (material ground with CBN wheels are usually hard and require more horsepower to remove a given amount of material from the workpiece);

3) the proper use of coolant.

Wheel selection
When selecting a CBN wheel for tool and cutter grinding operations, the following points must be considered:

• Bond type: The bond is a primary consideration in selecting the proper CBN grinding wheel. Resin-bond wheels are used for most tool and cutter grinder operations. Electroplated wheels are used when form grinding.

• Grit size: When replacing an aluminum oxide wheel with a CBN wheel it is wise to follow the recommendations listed in the following table regarding grit sizes.

• Concentration: Wheels with 75 to 100 concentration are recommended because they provide good material-removal rates, long wheel life and are usually the most cost-effective.

Factors such as the grinding method, grinding mode, and the speed and feed rates used also affect the efficiency of a tool and cutter grinding operation. For example, the grinding method can be either wet or dry and both can be used with success.

Dry grinding with the resin-bond CBN wheel has proved to be effective. Low feed rates are recommended to prevent workpiece burning during dry grinding. Wet grinding with either straight oil or a heavy-duty, water-soluble oil is very effective. It is important that the cutting fluid be applied directly to the work/wheel interface for maximum cooling and lubrication.

As far as grinding mode is concerned, if the grinder has sufficient power and rigidity, creep-feed single pass grinding is recommended. Generally, this mode of grinding produces the highest productivity, longest wheel life, and best workpiece finish. Conventional multi-pass grinding is also very effective for most tool and cutter grinding operations.

The recommended wheel speed during dry grinding is in the range of 3,000 to 4,500 sfpm (15 to 23 m/s). Higher wheel speeds may cause burning of the tool edges. In wet grinding, wheel speeds in the range of 5,000 to 6,500 sfpm (25 to 33 m/s) provide excellent results.

In general, the higher speeds improve both wheel life and metal-removal rates. The traverse feed rate should be kept constant because of the cool, free cutting characteristics of CBN grinding wheels. Whenever possible, use the creep-feed single-pass grinding mode.

Use lower feed rates when grinding dry. Roughing cuts should be about 0.002 inches (0.05 mm) deep. Finish cuts are usually 0.005 to 0.001 inches (0.01 to 0.02 mm) deep.

Spark-out passes are not necessary when using CBN wheels because if the grinder is in good condition, the depth of cut setting determines what is removed from the tool being ground.

To make the grinding operation successful, CBN wheels should be mounted on a high-quality adapter and both should be kept together as a unit for the life of the wheel. A dial indicator should be used to true the wheel to within 0.001 inch (0.02 mm) or less run-out on the wheel face. Also, an appropriate truing device should be used for a given type of CBN wheel, and the wheel should be trued and properly dressed on the machine on which it will be used.

Recent developments
In recent years, new developments have been made in tool grinding machines that combine automation with flexibility, and promise increased precision and productivity. The use of CBN grinding wheels for the grinding of end mills, milling cutters, hobs, and a variety of hardened cutting tools can reduce the cost of grinding at the same time, producing better quality cutting edges.

CBN wheels last much longer than conventional grinding wheels and require little or no conditioning, so there is less downtime for wheel maintenance.

The savings in grinding costs with CBN wheels may range from 20 to 50 percent, or even higher depending on applications.

Dr. Anil Srivastava is manager, manufacturing technology at TechSolve, Cincinnati, Ohio. He has over 30 years of research, teaching and industrial/manufacturing experience, specifically in machining and grinding, and has published over 35 papers and articles in international journals, conference proceedings and technical magazines. Currently, he is managing a three-year, $6 million project awarded by NIST-ATP to develop intelligent optimization and control of grinding processes.

Dr. Srivastava is member of the American Society of Mechanical Engineers (ASME), the Society of Manufacturing Engineers (SME) and the American Society of Metals (ASM), among others. For more information, visit www.techsolve.org or call 513-948-2000.

Back to Uptime archives