BY CHARLES BATES
Cutter grinding wheel performance depends on truing and dressing.
Off-line truing and dressing systems keep cutter-grinding wheel packs working.
A CUTTING TOOL is only as accurate as the grinding wheel that produces it. So, shops that manufacture cutting tools must precisely true and dress grindingwheel shapes to consistently produce high-accuracy tools to meet customer demands for evertightening part tolerances.
Shops true and dress wheels for cutter grinders with on-line or off-line systems, and each has advantages and disadvantages.
Off-line dressing, for instance, does not interrupt grinding operations because it is done on standalone, dedicated truing and dressing machines. These machines accommodate whole grinding-wheel packs, and they service multiple grinders within the same shop. They also keep debris from truing and dressing outside of precision grinding machines. "Most cutting tool shops service 10 or more grinders with our systems," says Bill Freese of Rush Machinery Inc.
(www.rushmachinery.com), a company that manufactures truing and dressing machines. Shops with these machines remove individual worn wheel packs from a grinder's automatic wheel-changing unit, true and dress the packs and reload them into the wheel changer — all while the grinder is working with another wheel pack.
Off-line dressing machines are especially beneficial for shops that regrind cutting tools. Those shops typically do short part runs and frequent grinding wheel changeovers, and off-line systems ensure that wheel packs are ready prior to these changeovers so machines are not kept waiting.
If a tooling shop is doing high-production runs, it typicallyuses two wheel packs, says Freese. While one is working, operators true and dress the other on an off-line machine. Wheels stay balanced and on their adaptors while the dressing machine restores the dimensions of each wheel relative to the other ones in the pack.
Rush's manual off-line systems are available in three varying levels: Level one is a basic model that shops use primarily for straight or angled wheels. The level two machine has a video-optic system with high-magnification optics and a video camera. Shops visually dress wheel radii according to various circle templates that are displayed on the machine's screen.
The level three machines are most often used by cutting tool shops and are equipped with vision systems that incorporate high-magnification video cameras and computers to run CAD programs for displaying on-screen overlays. The shops that use the level three machines can create any combination of radii and angles to produce the necessary wheel profiles. This level machine speeds the truing and dressing of difficult wheel profiles, such as those for grinding drills, says Freese.
ON-LINE DRESSING SYSTEMS are designed for long unattended grinding jobs because the systems are housed within grinding machines and operate automatically. The systems provide high-accuracy dressing that is programmed and controlled by CNC.
"I would always recommend on-line truing and dressing, mainly because the systems true and dress wheels to the arbors and spindles of actual grinding machines being used," says Tim Finn, senior corporate applications engineer at Saint-Gobain Abrasives Inc. (www.nortonabrasives.com). He says so because recent technological advances allow shops to dress diamond grinding wheels on-line using diamond dressing rolls, and such advances improve the on-line dressing process.
Diamond grinding wheels provide high metal-removal rates and dramatically reduce part cycle times. They last long, they require less reconditioning, and they need less machine power to run. But shops were not able to effectively dress these wheels online because grinding wheels were not free enough to cut and still maintain form. Diamond dressing-roll shapes deteriorated quickly and on-line dressing-unit spindles did not provide adequate speed.
However, three innovative products — the Univel G-Force diamond cutting wheels, the MaxTorq dressing unit and brazed profile rolls — from Saint-Gobain advanced the technology for on-line dressing of diamond grinding wheels.
Univel G-Force free-cutting diamond grinding wheels need no stick dressing after being profiled or dressed with a diamond roll. Stick dressing is an additional process that exposes a wheel's cutting abrasives and is necessary when running resinbond wheels that are rock hard. That type of wheel historically is used when shops grind HSS and carbide tools. The resin-bond wheels will not cut unless they are stick-dressed after being trued, Finn says.
His company's G-Force wheels are hybrid polyimide metal-bond types. What makes them free-cutting is that their bond is so strong the wheels do not need a lot of it. Dressing diamond grinding wheels with diamond dressing rolls requires speed, and until now, on-line dressing-roll spindles running at 3,600 rpm were not enough for the job. Saint-Gobain's MaxTorq dressing unit, however, delivers spindle speeds to 12,000 rpm through DC brushless motors that allow shops to dress the wheels at a rate of 18,000 surface feet per minute (sfm) with 6-in.-diameter dressing rolls.
Shops can true diamond wheels economically by running fast dressing spindle speeds and slowing grinding wheels down to 1,000 sfm, says Finn. But new abrasives have to be exposed on the grinding wheels, or they will not cut.
To ensure that new abrasives are exposed, Saint-Gobain developed the brazed profile rolls that feature one width of diamond grain that is 0.250-in. thick on the wheel circumference.
While most shops consider dressing traverse rates (the rate at which the dressing roll travels across the grinding wheel) the measure of productivity in the dressing process, the focus is shifting to overlap ratios. That is the measure of the traverse rates per grindingwheel rpm, a ratio of the number of times that the dressing roll "hits" each abrasive grit on the grinding wheel's face, Finn says. The ideal overlap ratio is one time, and anything more than that means that the dressing roll has hit the same spot on the grinding wheel more than it has to.
For instance, an overlap of 10 means the dressing roll hits a grinding wheel's diamond grain 10 times in the same spot before moving on. "The key to dressing diamond with diamond is using a thin roll that traverses across a grinding wheel as quickly as possible," says Finn.
Saint-Gobain's brazed profile rolls do not need re-lapping to restore their tip geometries. That is significant because machine downtime associated with re-lapping removal and installation is eliminated, Finn says. Using conventional rolls, shops must re-lap as many as four times during a roll's life at a cost of 30 percent to 50 percent the price of a new roll each time the process is done.
During field testing of these three products on Rollomatic (www.rollomaticusa.com), Anca Inc. (www.anca.com) and Walter Grinders (www.grinding.com) cutter grinders, Finn ground threads on carbide thread mills, knuckles on roughing cutters, and flutes and knuckles on HSS end mills using vitrified-diamond wheels dressed by diamond rolls running at high-speeds. In most instances, part cycle times dropped by 30 percent, and part output doubled per grinding-wheel dress. Prior to this, these test shops used resin-bond wheels that they had to remove from grinders and dress on off-line machines.