Grinding Does More Than Finishing
Grinders and grits get the job done
By Robert B. Aronson
Grinding is often considered a finishing operation, but it's gaining acceptance as a method of volume metal removal. A key reason is the improved versatility of grinding machines. Multiple wheels are available on one machine. It's possible to creep feed with one wheel then surface grind with another in a single setup. Grinders can mount several wheels on one spindle, or use multiple spindles. Grinding "centers" with milling, drilling, and turning capabilities that parallel the functions of machining centers are growing in popularity. Here's a look at some recent applications.
Most volume grinding is done utilizing creep-grinding technology, that is, relatively slow table movement with a deep wheel plunge. This process has its strongest advantages under these conditions:
High hardness: Materials above RC60 where conventional cutting tools are ineffective.
Heat sensitive materials: Grinding offers a viable alternative to milling, by generating less heat and stress to the part than conventional machining.
Setup advantages: Some parts require multimachining capability due to size, weight and accuracy. A machine which handles I/D, O/D, and surface grinding as well as boring and milling is required to reduce possible operator errors due to multiple setups on different machines.
Prototyping: Situations where prototype parts can be more effectively machined using a free-formed grinding wheel rather than conventional machining techniques.
These operations use square wheels that have been contour-dressed to any particular shape. This option is certainly more economical than utilizing a series of cutting tools. Customers who don't want to invest in a lot of costly tooling, instead, use universal dressing technology to dress any profile.
Difficult-to-machine materials: Some of those materials are considered "gummy," grinding is the only alternative on materials like nickel alloys, which take a heavy toll on conventional cutting tools. "We have been very successful grinding nickel-based alloys utilizing all ranges of available abrasives [aluminum oxide, vitrified. and plated CBN]," says Reinhard Koppen, United Grinding Technologies (Miamisburg, OH) application engineering manager for creep-feed grinding.
"Wheel selection is critical. We try to do all operations with a single wheel when possible.
"With the versatility of our machines, we have been able to replace broaching operations for a number of customers."
Mark Franckel, president, Edgetek/Jones & Shipman (Farmington, CT) notes a number of changes that have encouraged many companies to recognize grinding as a true metal-removal production process and not just a finishing operation. Jones & Shipman offers cylindrical grinders and creep-feed machines, while Edgetek machines are known for their capabilities in HEDG (high efficiency deep grinding) technology. Trends Franckel notes include:
Smaller batch sizes. This is sometimes true even with high-volume parts. A manufacturer who may need several thousand parts per year may make them in 100-unit batches to minimize storage requirements.
Single setup is paramount, particularly for complex parts, so machining functions have to be integrated into the grinding machines. The same machine can have plated wheels to rip off metals, and vitrified wheels for finishing and grinding radii. This includes greater use of toolchangers and quick-change capability for wheels and coolant nozzles. The customer wants to be able to choose from a wide variety of options.
More automation is demanded so robots are becoming more common.
Machine footprint is a big issue. No one wants to build or buy more factory space, so machines are getting smaller, and at the same time more rigid with flexible capabilities.
Customers are more aware of the need to know their total cost per part and compare that to other processes. When you know the cost of the wheels, labor, handling, and even maintenance, initial grinding machine cost becomes less of a factor.
K. O. Lee Co. (Aberdeen, SD) another company specializing in creep-feed grinding offers smaller machines, usually in the 5 to 15-hp range. According to K.O. Lee's Jeff Harris, "We gain customers when we can eliminate a step or a setup. For example, we can often eliminate costly broaching or milling operations. We can eliminate handling or multiple setups that can introduce inaccuracies."
"In our experience few creep-feed grind one-piece lots. It's effective with volume work using continuous dress. Where one-piece lots do occur, the utilization of CBN can justify the process. By using machines that allow multiple operations, setup can be reduced and very short production times can be realized. Typically we use multiple wheels on a spindle. We can creep-feed grind and surface machine on one machine, following the reduced setup and faster-throughput philosophy."
"We set up to creep-feed grind at one speed then reciprocate grind for finishing. For about 90% of our jobs we use CBN wheels. For the rest we have conventional wheels with a rotary or single-point dresser. With CBN we true and condition after several thousand parts; with conventional wheels we redress after 20 to 25 pieces. The compromise is wheel cost versus production time versus surface finish. The application and production volume will dictate the viability of one abrasive over the other."
Coated abrasive belts are becoming more popular as a means of volume metal removal. This development, according to Eric Askeland, senior technical service engineer, 3M Co. (Minneapolis, MN), is a matter of manufacturers becoming more aware of these products since ceramic grits were introduced.
"Earlier belts used the softer minerals such as garnet and flint. They were fine for polishing operations, but did not remove much material. This all changed when the ceramics, such as CBN, became available. These belts cut faster, can survive the higher pressures, and when they fracture, they produce new sharp cutting surfaces."
As with all grinding, much depends on the grinding machine and the setup. It's important to match the belt abrasive, belt speed, horsepower, and feed rate to the particular job and material being cut. For example, with most metals, the greater the speed, the more material is removed. But with titanium the opposite is true. Contact support and type of contact wheel are also important.
"A big advantage for this type of grinding is that contact time is less. You don't want heat to penetrate the workpiece," says Askeland. "And that is true with rapid belt grinding. The heat goes away with the spark."
Unlike wheels, belts need no truing or dressing. The use of coolant depends on the application, but generally the abrasive lasts longer when coolant is used.
"Precision of the operation depends more on the machine than the abrasive belt," Askeland believes. "With proper support and pressure control we can hold 0.001" [0.0254 mm]. The sophistication of the machine's control program is also a factor. It can reduce a lot of process steeps.
"As an example of how fast these systems are, in one of our demonstrations, we take a 12" long, 1" square [305 X 25 X 25 mm] cast iron bar and grind it down to 7" [178 mm] in just 10 sec. That's removing 0.5 in.3 of bar per sec," he concludes.
"We have replaced several grinding wheel applications with coated abrasive belts for typical centerless grinding applications such as hydraulic cylinder bar stock and various other tubing, pipe and rod material," says Joe Saad of Acme Mfg. Co. (Auburn Hills, MI). "Abrasive belts run cooler, are less expensive to operate than wheels and allows grinding at faster lines speeds of up to 60 fpm."
"The cost of belts averages $40 compared to a grinding-wheel setup that can cost up to $1000. Belts can be changed in 30 seconds while a wheel may take up to two hours. As an additional benefit, the belt doesn't require continual dressing. Cost savings can be appreciated when considering the total operating cost per ton of steel produced. This takes into consideration abrasive costs, changeover time, grinding speed capability and the flexibility of quickly changing abrasive grits to accommodate specifications of the application.
"In a typical operation we can remove 0.0010" [0.025 mm] of stock at 30 fpm [9 m/min]. With a bonded wheel grinding line speeds run at 5 fpm [1.5m/min] to remove approximately 0.0005" [0.0127 mm] depending upon the material hardness."
Abrasive belts run at constant surface speeds of 6000 - 7000 fpm (1829 - 2134 m/min) and at those speeds can remove 0.0010" (0.025 mm) from a bar moving at 30 fpm (9 m/min) in a safer process environment.
A possible down side is accuracy in diameter size tolerance, explains Saad. "Abrasive belt centerless grinding systems can generally maintain tolerances of plus or minus 0.0005" [0.013 mm]. If you require closer size tolerance, then a bonded grinding wheel must be utilized."
An additional benefit is the ability to configure multiple abrasive belt heads for a "One Pass" OD grinding. This allows the rough grind with high stock removal to be followed by final polishing in one continuous process at the speeds stated. Also these multiple head systems can be integrated with controls that allow accurate rapid setup and part changeover of 5 minutes or less.
Hardinge Inc. (Elmira, NY) offers the Kel-Curtis grinder known as a "universal centerless" machine. This is a new machine that, according to the company's Robert Winstanley, "Is not a typical centerless grinder, but for applications where you want a mixed bag of low-to-medium volume parts where frequent changeover is necessary. Typical applications are in the 100 to 500 part range.
"This unit extends the range of applications that can be performed by the centerless grinding products but is not a competitor for conventional centerless grinding machines," explains Winstanley. "It's not a through-feed, high-volume machine and can do angles and faces, double-angle approach grinding, and concentric grinding that a conventional centerless machine can't handle. It uses a wheel with four cavities that can make four parts in one shot. This machine's advantages are particularly strong when component length to diameter ration is high, making between-center grinding difficult."
Glockel America (Oxford, MA) offers a series of horizontal and vertical-axis volume grinding machines. To maintain rigidity, and therefore precision, the worktable is stationary and the grinding head is carried over the workpiece on a bridge unit. Large, long workpieces, such as industrial knives, are a specialty as well as large ceramic sections, plate steel units, ways, and large mold bases.
"Units up to 350 hp [263 kW] are available, and to meet a special need, a 500-hp [375-kW] unit was built," according to Glen Bergman. Maximum worktable size is 500 X 45" (13 X 1.1 m).
"For precision work the horizontal unit is recommended because both ends of the wheel spindle are supported. This unit can also do form grinding. The vertical configuration is best for volume material removal because a segmented wheel is used and greater pressure can be applied to the workpiece. These machines offer an accuracy of ±0.005 mm/m."
"After a relatively slow spell, there has been a lot of activity in abrasive development recently," states Mike Hitchiner, technology manager, Saint-Gobain Abrasives (Worcester, MA). One is a product called Altos. It's a needle-shaped ceramic grain with an aspect ratio of approximately 10 to 1 that is used in wheels with a porosity of about 60% or higher. It has, according to Hitchiner, "shown dramatic cost savings for difficult-to- grind materials such as Inconel. It has been used chiefly for rough grinding at standard speeds of 30 m/sec with a followup of CBN grinding for finishing where necessary. Our tests have shown these Altos wheels can remove material up to 10 times faster than CBN. In one comparative test, CBN removed material at a Q' of 5 while the grinding rate for the Altos wheel was in the 30 to 70 Q'. Studies indicate grinding at much higher speeds show even greater promise."
In another application, custom machines designed for high-speed direct-plated CBN has replaced turn broaching operations by removing material twice as fast at half the tooling cost. Wheel speeds were in the range of 150 to 160 m/sec with oil coolant.
To recapture some of the market lost to hard turning, Saint Gobain engineers are developing new families of vitrified CBN wheels and abrasives. In the US, because of EPA restrictions limiting coolant in many plants to those that were water-based, hard turning proved more cost effective. "The new wheels combine higher-strength bonds with a new generation of tougher CBN crystals that promise a three to four times improvement in wheel life using water as a coolant and makes grinding competitive to hard turning again," claims Hitchiner.
Mori Seiki USA (Irving, TX) is introducing the Taiyo Koki line of vertical grinders. These vertical-shaft machines have been popular in Japan for some time, but have only recently appeared in the US. According to company representative Fred Puzon, most of the advantages are in improved handling and space saving. They include:
- Smaller footprint than a comparable conventional grinder.
- In operation, gravity works to the user's advantage, so there is less part distortion, better concentricity, and less chucking force is needed.
- Setup as well as loading and unloading are faster than with a horizontal machine.
- Internal and external faces can be ground in one setup.
- Automatic loading/unloading can be easily adapted to the basic machine.
Several versions of the Taiyo Koki machines are available. One has a turret with two wheels so an ID and OD can be worked on in the same setup. The second has a toolchanger that can supply up to eight grinding wheels.
This article was first published in the February 2004 edition of Manufacturing Engineering magazine.