Grindability, a test worth taking

Oct. 1, 1999
Passing this test reduces costs and increases productivity.

Passing this test reduces costs and increases productivity.

Timken's personnel use 1/2-in.-diameter drill blanks to test, compare, and measure the grindability of high speed steels.

Timken Latrobe works with cutting tool manufacturers such as Greenfield Industries to improve grindability of material grades used in cutting tools.

To improve its customers' flute grinding operations, materials manufacturer Timken Latrobe, Latrobe, Pa., is using a robust experimental technique to test, compare, and accurately measure the grindability of various high speed steels. It involves a flute grinder, 1 /2-in. diameter drill blanks, and G-ratios.

Guided by test results, Timken Latrobe Research in Canton, Ohio, then works its metallurgical magic to produce material grades more consistently and with improved grindability. "For some grades," says Jim Maloney, manager of steel product development at Timken, "we have upped grindability by as much as 10%."

Little thought is usually given to a material's grindability, an important property that affects the production of drills and taps. Grindability is generally gaged by ease of grinding. This means the material rapidly grinds to the desired accuracy of form and dimension with low wheel wear and grinding injury to the material.

Over the years, several methods of defining and measuring grindability have evolved. Two of the more common methods of measurement are specific grinding energy and G-ratio.

Specific grinding energy or Kz is a measure of the energy required to remove a unit volume of material under a given set of grinding conditions. A high grinding energy roughly correlates with high heat generation during grinding, and ultimately with an increased probability of grinding injury of the workpiece, such as distortion, burn, surface roughness, and cracking.

G-ratio is the volume of material removed divided by the volume of wheel lost during the grinding process. G-ratio directly relates to form loss and grinding wheel life, two things that are important to the toolmaker. As the G-ratio value increases, so does the longevity of useful wheel life.

According to Peter Glaws, senior research specialist at Timken, the test lets them quantify a material's grindability, then tweak the material composition or the process to achieve incremental steps of improved grindability. "Individually, these slight modifications," he says, "can go unnoticed on the shop floor, but made in groups, can bring on quantifiable results." The test tells Timken researchers that they have found the best way to process a certain material for a customer's particular use along with the highest level of grindability.

All for the toolmaker
Greenfield Industries, a cutting tool company in Augusta, Ga., has a close working relationship with Timken Latrobe. The steelmaker relies on such toolmaking companies for input as it develops steels and for feedback on its final products. As a tool company, says Ray Moring, the principle engineer in the firm's research, development, and engineering department, Greenfield must consider the entire life cycle of a tool, not just grindability in the plant. While improving production is important, the performance of a drill or tap must not suffer because of it. According to Glaws, material grindability can improve without having a detrimental effect on tool life.

For example, a high vanadium content in tool steels, he says, provides some of the properties needed for good tool life and performance, but lowering the amount improves grindability. Timken, using its grinding test, can then quantify and fine-tune these changes in vanadium content.

Besides being chemistry-related, changes can also be process-related, adds Maloney. The way steel is made, the ingot size, rolling temperatures, pressing and forging sequences, and annealing all impact the final material. Timken studies each segment of material processing to manipulate them for the best wear-resistance along with improved grindability, says Maloney.

Flute grinding is the most aggressive grinding in drill and tap making, comments Moring, and if a material is hard to grind, it places more stress on the grinding machine. When there is less stress, less horsepower is required, and operators have better

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control over total geometries. Harder materials also cause grinding wheels to break down faster, which translates to more dressings and extra efforts to keep geometries constant.

As for grinding wheels, says Moring, Greenfield can spend less time grinding and get better surface finishes on drills and taps by switching to finer wheel grit sizes. Often, the company uses a heavy grit to remove flute material fast and efficiently, but this can take away from the finish. A fine-grit wheel grinds just as efficiently and produces a better finish.

Currently, Greenfield relies on the expertise of its operators to monitor variations in material. While running a new batch, operators may begin dressing wheels more often, seeing burns or grinding damage where there wasn't before, or experiencing a deterioration in surface finish. All of which indicates a change in material properties. Timken hopes to eliminate this guesswork.

Through internal testing and technology, Timken is already changing some of its material processes and learning exactly what to control for a more consistent product. "This means the toolmaker can set up machines and wheels to grind as aggressively as possible without damaging the tool. Once these parameters are established, there will be no surprises as far as material quality is concerned, and Timken's material will be consistent enough to support the setup," says Maloney.

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