- April 23, 2014
Why you should ditch those old hobs for modern gear-making methods
Gears drive the world. Without them, our cars would do naught but idle in the driveway. Bicycles would be stationary, boats stuck in port, and trains forever at the station. Simply put, gears are used everywhere.
Suppose you want to break into the gear-making business. Be warned, this is complex stuff. Plan on some late nights boning up on gear technology. For starters, the sheer variety of gears manufactured today is mind numbing: spur, helical, worm, bevel and hypoid to name a few. Add to that a bewildering dictionary of gear nomenclature—chordal pitch and lines of contact, tooth profiles and root forms, obscure technical jargon describing how gears function. It's enough to give you a headache.
Once you've mastered all that, you'll need some equipment. Despite their complexity, most gears are machined using one of two basic methods: hobbing and milling. If you want to go the hobbing route, you can pick up a used manual machine off eBay for the price of a decent pickup truck.
The bad news is that you'll need a gear cutter, or hob, for most every gear shape and size combination ordered by your would-be customers. At a couple hundred bucks a pop times thousands of potential gear configurations, it's a big investment, one devoted to nothing but making gears.
Before breaking out your checkbook, be aware that the gear-making world is changing. Multi-function mill-turn machines and five axis machining centres are opening the door to low volume gear manufacturing without the need for dedicated cutters and special machines. And cutting tool manufacturers now offer indexable, multi-purpose gear cutters that blow away conventional high speed steel and powdered metal hobs, bringing flexibility to an industry that has historically been one of high volume production and dedicated equipment.
Aaron Habeck, marketing project manager for Sandvik Coromant, Fair Lawn, NJ, says that even small job shops are jumping on the gear-making bandwagon. "Where you see the biggest change these days is with those shops that have multi-task machines. By mounting a disc cutter in one of the live-tool stations, we can machine a range of gear profiles using the same cutter body, reducing se-tup time and investment cost. This makes it possible to economically cut a job for two parts today, and ten parts tomorrow."
Sandvik offers several flavours of these disc cutters. For example, the Coromill 172 cuts the full gear tooth form in a single pass. "The involute curve of the gear tooth, the root radius, etc., is generated by the shape of the insert. You cut one tooth, index the workpiece, and continue cutting each successive tooth until the gear is complete." An alternative, says Habeck, is Sandvik's InvoMill, which uses a straight-sided insert and the machine's C and Y axes to interpolate the gear form. "With a multi-axis lathe or machining centre, InvoMilling provides maximum flexibility, because you can use a very small number of tools to generate a wide range of gear profiles."
Another cutting tool manufacturer pushing hard on gear milling solutions is Seco Tools Inc., Troy, MI. Product marketing manager Todd Miller says that even high volume gear producers are transitioning to indexable cutters. "It's probably about fifty-fifty. A lot of the gear shops today are still using older equipment, with spindle speeds more appropriate to high speed steel cutters. You can't really take advantage of carbide unless you have have a machine with the rpm and rigidity necessary to drive it."
For those who can utilize indexable gear cutters, the results are impressive. Seco's gear machining technical guide boasts of 92 per cent cycle time reduction in 42CrMo4 ring gears used in wind power applications, and gearbox components machined 20 times faster than using HSS hobs. It's not all sunshine and roses, however. Miller says shops may need to adjust their processes. "You have to look at gear quality. As a rule, solid hobs generate a more accurate form than indexable cutters. So depending on the quality level required, you may end up roughing with an indexable cutter, then doing a secondary grinding operation to finish."
The indexable cutter bodies can be expensive as well. For very large gears, Miller says Seco has built tools with 400 inserts, and crazy long. "It can be a hefty investment, but all things considered, going the indexable route is still 30-40 per cent cheaper than conventional hobs." This is largely because multiple gear forms can be accommodated in a single body, as opposed to the need for a unique solid hob for every gear you're going to cut. Add to that the fact that, when an indexable gear cutter goes dull, you only need to swap out some inserts and you're up and running. When a solid hob hits the ropes, it's headed back to the tool room for a regrind. "Compared to high speed, indexable gear cutters provide dramatic improvements."
If you like the sound of that flexibility, you might reconsider that eBay hobbing machine purchase you were about to make. Nitin Chaphalkar, manager of Advanced Solution Development at DMG/Mori Seiki USA, says that multi-tasking machines allow machine shops to produce many gears in a single handling. "Take the NLX-type of machine, which is a three axis turn mill centre. It allows us to do spur and helical gears using disc cutters, hobs and InvoMilling, on a platform that, compared to CNC hobbing machines, is very low cost."
If you need even more capability, Chaphalkar suggests that a five axis multi-tasking lathe or machining centre might be in order. The advantages with any of these machines are obvious: when you're not cutting gears, the machine tool can be reprogrammed to make whatever else is on order that day. And compared to the craftsmanship needed to operate a gear hobbing machine, cutting gears on a CNC lathe or mill is a walk in the park. "Traditional gear making is an art," says Chaphalkar. "Ask someone in the gear industry how to adjust a gear profile or crowning pattern on a hobbing machine and you'll usually hear that the operator has 30 years of experience. He or she just knows what to do."
DMG/Moir Seiki attempts to replicate this skill set with gearMILL, a software package that allows the operator to plug in a number of known values—gear profile, root form, number of teeth, etc.—to generate a gear milling routine automatically. Aside from the cutting methods already mentioned, gearMILL can even program standard end mills and ball mills to cut certain gear geometries. "The software makes it visual and very easy to use, making it more of a science than art."
Mazak Corp. is another machine tool builder making a big push into the world of gear cutting. Its Integrex e-1550V/10 is a five axis multi-tasking vertical machining centre utilizing two 1,397 mm (55 in.) round pallets capable of loads to 4,989 kg (11,000 lb.) Process development engineer Mike Finn explains how one of Mazak's customers uses the Integrex to machine ring and pinion gear sets. "The pinion gears are machined from 8620 steel bar stock, while the rings start out as 4340 forgings. Both are rough machined to within 0.250 in. of their final dimensions, then heat-treated to 62 Rc, and 55 Rc respectively before going back to the 5-axis for final machining."
There are several benefits to this process when compared to conventional methods. One of these is time—this straightforward process delivers gear sets in days rather than months, using off the shelf tooling and machine that can perform "everyday" machining when not busy making gears. Best of all, the Integrex machines the gear sets so accurately that the need for "matching" of the ring to the pinion is eliminated.
"Of all the challenges involved with machining gear sets, imparting the correct involute form on the pinion gear is the most daunting," Finn points out. Yet Mazak is able to produce virtually interchangeable gear components, an important point when you consider that pinion gears wear more quickly. "When the pinion wears out, shops end up machining a whole new set, even though the ring gear may have additional working life left. This is because the odds of generating another pinion gear that meshes perfectly with the existing ring gear's pattern are extremely low."
If you're ready to try your hand at gear making, there are plenty of solutions available. Maybe you won't be making millions of gears using the methods described here, but low volume and prototype work is certainly within reach. All it takes is the right machine tool, the right cutters, and a lot of knowledge. You'll be geared up in no time. SMT