Tool and Cutter Grinding
Do you get the point?
By Robert B. Aronson
Keeping tools and cutters sharp is one of those inescapable overhead costs all manufacturers have to accept.
Like most other decisions in industry, deciding how and where to sharpen tools is a product-specific choice. It depends on overall cost, not just the rework fee.
Do-it-yourself sharpening is normally done by companies that can afford to have staff dedicated to this work or that need very specialized work.
But, overall, the trend seems to be greater use of outside sharpening specialists as manufacturers reduce labor costs. These services are frequently offered by tool manufacturers and by a large number of "regrind houses."
One company that has benefited from the trend to custom tooling is Acu-Grind Tool Works (Bradenton, FL). This operation specializes in both tool rework and the manufacture of specialized tools. Its president, Tony Antony, reports particular growth in the aerospace industry where tapered tools and tools for thin-wall cutting of aluminum are in demand.
In addition to reworking tools to custom specs the company also offers a design service to improve tool performance and life. "While this adds to up front cost, those who look at total cost per hole or cost per part will find we offer a cost advantage," according to Antony.
According to Ed Sinkora, Walter Grinders (Fredericksburg, VA), their machines are optimized for tool grinding with a design that combines complete geometric freedom with high rigidity. "For example, with our machines, you are always grinding near the center of the work envelope. Overhang is minimal so vibration is minimized. These are patented features.
Walter's latest machine is called the Helitronic Vision, a 3D gantry design that has linear motors driving the linear axes and frameless torque motors for the rotating axes.
"We also beefed up the base, which is a 14,000-lb [6350-kg] mineral casting, to accommodate the extreme acceleration of the linear motors," he notes. (The machine weighs 20,000 lb [9071 kg].)
"Measuring the finished tool is an important issue," Sinkora says. "We use two systems. For in-process compensation, we use a probe system built into the machine that measures and adjusts diameter, flute depth, rake angle, helix angle, and back taper.
"But you can't certify a tool on the same machine on which it was made or reworked. It has to be an independent, off-line unit. We offer a machine that can measure down to ±0.7µm, proven with a NIST-certified gage.
As to market trends, Sinkora says, "The hottest new tools are those with variable helixes within the flute or helixes that vary from flute-to-flute on the same tool. Handling this work requires special programming."
Walter offers Tool Studio software to meet this need and other development challenges. It allows the user to create a grinding program while modeling the tool in 3D--the two functions are completely integrated. When you change the model, you change the program. There is no need to rewrite codes.
"Our software also contains a number of proprietary programs," Sinkora says. "We have a number of partnerships with major tool developers.
"The overall goal is achieving good surface-finish and accuracy at high speed. For example, we are able to grind and measure a K land.
Anca (Farmington Hills, MI) is a grinding machine builder specializing in tool sharpening. According to VP, Russell Riddleford, "Our company serves three markets: tool manufacturers, resharpening houses, and individual companies that have their own sharpening operations.
"Both the resharpening and individual companies are expanding, particularly those operations that can't afford a long turnaround time. You can't wait two weeks for a tool with a million-dollar machine sitting idle."
Riddleford says much of Anca's development work is aimed at improving software. "We have the dual goals of making it simpler, and handling the more complex geometries that tool designers continue to generate.
"We provide the customer 'lights-out' capability. You load the tools, set the programs, and that's it. The user need only program the tool's major parameters such as tool type [mill, drill], and key dimensions. On the other hand, these machines can be programmed for one-off capability.
"A unique feature of our machine is automated dressing. The user simply sets the frequency [every five tools, for example] and the dresser does the rest.
Measuring the reworked tool is an important aspect of the process. One example of the equipment used for this task is the Genius 3 from Zoller Inc. (Ann Arbor, MI). An automatic system that measures and inspects tools using incident and transmitted light, it magnifies the tool up to 200 X . There is a measuring program for all critical parameters including radius contour, and tool contour and concentricity. It measures tools up to 600-mm long and 200 mm in diameter with repeatability of ±2µm and accuracy of 1µm.
"Removing the 'black art' aspect of tool sharpening is one of our goals," says Rolf Herrmann, Schneeberger-US Corp. (Elgin, IL). "We emphasize software improvement, particularly in the area of simulation." But, he cautions, "even with good software, you need someone running the machine who knows something about tools. It's not like running a lathe or a drill.
"I believe that linear motor technology is driving our market. It's a coming technology. They provide good acc and dec and give you a better tool in less time. Plus, maintenance issues for the machine are less. There is no wear and tear on ballscrews, belts, or bearings to worry about.
"Manufacturers are looking for ways to minimize or eliminate labor costs, and that includes resharpening," Herrmann says. "We often do this in ways not directly related to the resharpening process such as by adding pallet systems for loading and unloading, automatic chucks, and, in general, building more secondary operations into the machine.
MA Ford (Davenport, IA), a manufacturer of carbide and HSS tools, also offers a regrinding service, chiefly drills, end mills, countersinks, and burs. "We have multiple CNC grinders dedicated to that work," says Joe Kueter, products engineering manager.
"Our big advantage is being able to provide the point geometries exactly the same as a new factory-made tool. With point designs becoming more complex, it's tough for a company to do that on its own. Take edge protection, for example. You might be able to regrind the point, but the edge protection is a critical feature on the cutting lip.
"How often a tool can be resharpened is usually a matter of diameter change. The more you go back along the flute length, the smaller the diameter gets. You can lose one to two thousandths, and that can be an issue in high-accuracy operations. Reworking a tool 10 or more times is not uncommon if you have enough flute length. "Initially we concentrated on manufacturing new tools, but regrinding has become a larger part of our business."
"Business is growing, as manufacturers gravitate away from indirect labor costs," says Tim Knapke, general manger, Tru-Edge Co. (St. Henry, OH)."Our customers want 'one-stop shopping.' We do both solid tooling and inserts. Indexable tools are a totally different animal. They are a lot less forgiving than solid tooling.
"Our largest run is about 100 pieces. More often it's in the six to 20-piece range.
We have to do a lot of reverse engineering to turn these out, because tool suppliers don't give you total specs and some have totally proprietary data," Knapke says.
"When selecting a regrind shop, I suggest you first pay a visit. Look around. Look at the quality of the operation. How do they keep records? What is their print control like? Are they ISO structured? If not, your repeatability will not be there.
"We work on 5000 special tools from 180 companies, so we have to keep track of a lot of things including print changes. Everything has to be documented.
St. Gobain (Worcester, MA) focuses on grinding wheels used by the larger tool manufacturers. "We work chiefly with insert and round-tool manufacturers," says Tim Finn. "Most of our wheels use a polyamides resin bond. They offer rapid stock removal and provide a high-quality surface finish. We use a duPont resin that is very forgiving. It's able to handle high temperatures and feed rates while retaining a high form-holding capability. Most of the grit for this work is CBN or diamond when working with tungsten carbide or tools of other advanced materials.
"More recently we are using vitrified-bond technology, which has the ability to hold form almost as well as a polyamide. The advantage is that you don't have to take the wheel to an off-line dressing machine. You dress it on the machine. With other designs, you have to remove the wheel for dressing after about 30 tools.
"Although machines with vitrified wheels have higher up-front costs because of the dressing system, they offer much longer wheel life and productivity," Finn says.
"On the machine side, the newest thing is electrolytic dressing. It's an old concept, but has only recently been put into production. Basically it's a method for stripping the bond and leaving the grit behind, but more exposed. You don't touch the grit, but erode the bond. In this process, an electric current through the coolant strips away the metal bond. It allows us to use a finer grit that is much sharper so you get better removal rates."
"Generally, resharpening of high-end cutting tools has become more of an issue than sharpening standard cutting tools," explains Eric Schwarzenbach, president, Rollomatic (Mundelein, IL). In particular, we have observed the need for resharpening K-lands on drill points, and resharpening variable-lead, variable-helix end mills. Because of these special designs, a company will often resharpen new drills to their own specifications.
"For manufacturing applications, we offer the model CNC620XS," Schwarzenbach explains. "Most recently we have improved that machine's grinding and probing software. For the high-precision grinding of features such as K-land grinding, we have a gap control. It is an acoustic system that uses an ultrasonic signal to touch-off. The ability to do K-land grinding on CNC machines eliminates the need for hand work on these tools.
Our CNC6000XL version is intended for larger tools. This design features hydrostatic guide rails, rotary axes, and grinding spindle improve finish, reduce vibrations, and increase rigidity of the machine. "This type of bearing offers more precison than the ballscrew as well as less friction. Feed rates can be much higher because the higher the speed the less friction and less heat generation in the bearings.
"Wheels have been improving. For work on larger cutting tools there is greater use of metal bonds and also vitrified bonds with metal filler. When sharpening smaller tools the trend is toward polyamide bonds.
Lubrication is specialized. To ensure efficiency of these grinding operations, the fluids should be linked to specific requirements. Benz Oil (Milwaukee) is a major supplier to the tool grinding industry. Randy Bates, grinding fluid manager, notes that the fluids used for tool and cutter grinding applications are usually based on mineral oil or synthetic-based fluids. Water-dilutable fluids are seldom used in CNC tool grinding machines. The fluid requirements tend to differ from grinding carbide to grinding high-speed steels.
This article was first published in the July 2005 edition of Manufacturing Engineering magazine.