Grinding Roughs Out a New Niche

In the past, grinding has been a process that was primarily relegated to machining for final dimension accuracy and surface finish. The grinding process did not enable large quantities of material to be removed quickly.

New grinding process developments by Tyrolit, and machine tool developments by Makino, are changing that thought process. Utilizing new grinding techniques, incredible material removal rates up to 300 mm³/smm respective, 28 cubic inches per minute, and one inch of wheel width are being achieved. And, it is being done in the aerospace marketplace where the grinding of tough, nickel-based alloy materials is common in aircraft engines and components.

Tyrolit History

Tyrolit is a global leader in grinding technology. The company is recognized as one of the top three manufacturers of bonded abrasives worldwide and is the market leader in Europe.

Tyrolit is a member of the Swarovski family of companies, which is a manufacturer of fine crystal. The company entered into the development of bonded abrasives over 100 years ago in its quest to make quality crystal. By the 1950s, Swarovski had developed such an expertise in bonded abrasives that it started Tyrolit, an independent business unit focused on the sale of bonded abrasives.

As is shown through this history, the Tyrolit approach to the market has never been to just sell grinding wheels. Instead, the focus is on understanding the customers’ specific needs, reviewing their processes and working jointly to create the best solution. This led to the development of upgraded solutions for coolant supply, component work holding/fixturing, as well as experimenting with the variables in bonded abrasive manufacturing and use.

Tyrolit’s primary goal is to achieve the optimal grinding results, and to be more than just a producer of grinding wheels and associated tooling. The company’s focus is on the whole package and solution: a much broader concept for conducting activities in the grinding research and development group.

Grinding Developments

During the early 1970s, creep feed grinding was first developed and utilized. In creep feed grinding, the machine table feeds the work piece in the direction of the grinding wheel rotation. While the table speed is slow—“creep feed”—the depth of cut is very high, which results in a high material removal rate.

The creep feed grinding process offered significant productivity enhancements over conventional grinding. This process soon was employed by a number of worldwide manufacturers of aerospace turbo machinery parts.

As more experience was gained concerning the variables surrounding the creep feed grinding process, there were two key attributes that emerged which would have a significant impact on the productivity of creep feed grinding: coolant supply and wheel dressing.

The Impact of Coolant on Grinding

The supply of coolant to the grinding wheel is vital to the grinding process. The coolant helps keep the work piece and grinding wheel cool so the material being machined is not burned. In addition, the coolant transfers away the swarf material, keeping it from fouling the wheel.

If the wheel becomes clogged with substrate material, the effective cutting edges of the abrasive are dramatically reduced. Once the wheel is clogged, the part manufacturer needs to redress the grinding wheel and expose newer, sharp abrasive crystals to reestablish the profile geometry on the grinding wheel.

A simple solution to surface cooling and grinding wheel flushing initially appeared to have been the use of huge quantities of coolant. But, this solution proved ineffective in that the grinding wheel would hydroplane over the substrate material surface with too much coolant applied.

To optimize the creep feed grinding process, testing proved it would require delivering just the right amount of coolant to the cutting zone so as not to permit the grinding wheel to hydroplane.

In order to achieve the proper balance in coolant delivery, extensive work has been done with Rolls Royce and Makino. The result of this work is the patented VIPER grinding process.

VIPER Grinding

VIPER (Very Impressive Performance Extreme Removal) grinding is a process that requires close coordination between the machine, the coolant supply and the composition of the grinding wheel. The principle behind VIPER grinding is that the coolant is injected into the grinding wheel under high pressure ahead of the grind, and centrifugal force then moves the coolant out of the wheel during the grind, cleansing the wheel and cooling the material.

The comparative results of VIPER are remarkable. The process saves time and money.

• A bearing housing using the VIPER process, as opposed to turning and milling, reduced the cycle time by 66 percent. The number of setups was reduced from 8 to 3, and the consumable cost was reduced by 50 percent.
• A casing that took 10 hours to mill was ground completely in only 4 hours.
• The cycle time of a compressor blade was cut by 50 percent from 8 minutes to 4 minutes using VIPER as opposed to conventional grinding, with a greater than 2 CPk quality measurement. Consumable costs were cut by 80 percent, and the overall cost was cut by 50 percent.
• A turbine blade manufactured with a twin-wheel creep feed grinding machine, which utilized very large and expensive grinding wheels, required 8 machining setups. With VIPER grinding, the blade was completed in 2 setups. The process used smaller grinding wheels that dramatically reduced costs and time.

Nozzle Control

The key to VIPER grinding coolant performance is the ability to deliver it at the proper pressure and orientation to the specially formulated grinding wheel. Makino developed an innovative, programmable coolant nozzle system that enables the jets to be positioned at any location around the circumference of the wheel. It further enables the machine to control the gap between the grinding wheel and the exit tip of the nozzle jet, as well as the impingement angle of the coolant hitting the grinding wheel.

The ability to control these factors and the coolant pressure and temperature enable the machine to precisely control the coolant delivery variables. In addition, the Makino system enables the coolant nozzle jet to be automatically changed during the grinding process.

When a grinding wheel with a new profile is used, the coolant nozzle jet can be changed concurrently to match the new profile. This optimizes the VIPER grinding results by ensuring the proper delivery of coolant to grind.

Utilizing the VIPER grinding process, manufacturers achieve metal removal rates up to 5-times greater than they achieved with traditional creep feed grinding. As it was initially developed, VIPER was intended as an intermittent dress grinding process. Recently, this process was enhanced further by utilizing a continuous dress of the grinding wheel, which is necessary for extreme rates of stock removal.

Continuous Dressing

Continuous dressing is a process enhancement that was developed for the creep feed grinding process and resulted in productivity enhancements of up to 90 percent over normal, intermittent dress creep feed grinding. The continuous dress process ensures there are always sharp, abrasive cutting edges exposed during the grind.

Continuous dressing maintains constant pressure between the dressing roll and the grinding wheel, which is adjustable to ensure the proper amount of wheel is exposed during each revolution. This is a process called micro-profiling, as only a micron or less of wheel is removed with each revolution. This ensures that fresh, sharp abrasive crystals are always properly supported in the bonding material of the wheel.

In an intermittent dress grinding process, substrate builds up on the cutting edges of the abrasive crystals on the wheel. As these edges build up, the cutting forces increase dramatically, and the force causes the grinding wheel to break down and large pieces are actually broken off the grinding wheel. Once this occurs, the grinding wheel must be redressed.

With continuous dress, the result is that there is no built up edge on the abrasive crystals, and the normal forces exerted on the wheel are minimal. In fact, when the dressing rate is properly set, continuous dress grinding actually results in reduced wheel consumption despite the continuous dressing with each revolution.

High-Speed CD Grinding

High-speed grinding occurs when the capabilities of VIPER grinding are combined with the continuous dress process. With high-speed grinding, manufacturers are achieving metal removal rates of 16 cubic inches per minute in grinding inconel. This type of metal removal changes processing decisions, such as when to machine the material with a cutting tool compared to a grinding wheel. Grinding is no longer just a finishing operation. Through extensive research and development, Tyrolit has developed a grinding wheel that performs optimally in a continuous dress VIPER grinding process. With high-speed grinding, it is possible to remove a greater quantity of material with reduced consumable expense compared to high-speed milling.

The following table provides a comparison of the productivity of the different grinding processes. These grinding processes are also separated between the conventional grinding processes, which currently have widespread use in industry, and the capabilities of the VIPER and high-speed continuous dress grinding processes.

The productivity factor Qw is the number of cubic millimeters of material removed per second, per millimeter of wheel width. This ratio accurately describes the efficiency of the grinding process. In addition, there are some key benefits and limitations to each grinding process.

This chart details a fairly large range in the productivity of each grinding process. The ability to fixture and hold the part being ground is the primary driver that determines if the actual application will be at the high end or low end of the range. The more rigidly the part can be held, the higher the potential Qw factor that can be achieved.

There are still limitations to grinding. Compared to milling, the limitation is not the material removal rate. With grinding, wheel access is always an issue. Traditionally, the part manufacturer has had to decide if a part would be ground or milled.

The optimal solution is to have the flexibility to choose either grinding or milling in the same machine. A machine with this capability changes the processing decision, allowing the manufacturer to select either or both processes as best for the geometry being machined.

Processing Flexibility

Rolls-Royce and Tyrolit jointly approached Makino about the viability of making such a flexible machine that would be optimal for an aerospace turbo part manufacturing. The machine had to be configured to perform VIPER grinding, but it also needed to have the ability to change all types of tools automatically. Makino reviewed the request and created a machine tool that enables VIPER grinding, intermittent dress grinding, continuous dress grinding and milling all on the same machine. The automatic changing of grinding wheels and dressing rolls is done in the same fashion as cutting tools are for milling.

Given the advent of the Makino machine, the aerospace part manufacturer can make some part features, like drilled holes, through a rotating cutting tool. Then, in the same operation, they can continuous dress and VIPER grind the profile of the shape.

This multiple process provides the manufacturer with the greatest degree of flexibility and productivity in the same operation. No longer does the manufacturer need to decide if grinding or milling will produce the shape; now they can have both in the same operation.

The ability to automatically change grinding wheels and dressing rolls rapidly also further changes the dynamics of grinding. It is now possible to select the right size of grinding wheel for the desired geometry. One composition wheel can be used for rough grinding while a different composition wheel can be used for finish grinding.

In the past, the manufacturer often had to make this compromise. If they chose a very large wheel, they could have multiple profiles on the same wheel. The wheel composition would also be compromised to best achieve the grind they were attempting.

The grinding machine tool simply did not have the flexibility to rapidly change wheels during the grinding operation. With the Makino machine tool, the wheel is sized properly, minimizing waste, and the wheel is optimized to the desired grind.

Tyrolit is very proud of its contribution to these advancements. These are the types of leaps forward a company can make when it is focused on the total needs of the customer, not just selling grinding wheels.

The collaboration of the Tyrolit, Rolls-Royce and Makino is dramatically advancing the field of grinding.

Grinding is not just a finishing operation anymore!