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A Saw Blade That Gets Under The Skin

February 2012

Getting into production with his company’s new Amada bandsaw, Pat Schisler was happily cutting 316 stainless barstock with the recommended carbide-tipped blades when things started to go wrong.

As the blade wore, teeth chipped; teeth would give out on one side and start cutting on angles up to 45 degrees; and blades snapped. It didn’t all happen at once; when the blades worked, they worked great. But when they failed, they failed catastrophically. They were giving at least 25 percent more life than high-speed-steel (HSS) blades. But they cost twice as much.

Carbide-tipped blades often need fine-tuning, but the economics didn’t justify a lot of development time for Schisler. He was cutting through full eight-hour shifts but only one or two days per week. What he needed was some hassle-free, lower-cost blades.

Schisler is the CNC programmer, and half of the CNC machine-shop staff, for Eagle Stainless Container. Eagle makes a range of formed and welded containers for production pharmaceutical applications and a line of small bottles for pilot and testing work in the pharma industry. Beautifully finished, almost art-like objects, the smaller bottles are drilled and turned from solid 316 barstock, which begin as slugs sawn on the Amada hydraulic-feed cutoff bandsaw.

Manufacturing medical and pharmaceutical products often involves working with difficult or unusual materials, from aerospace superalloys to foamed titanium. There are few characteristics that these materials have in common but there is a dominant theme to them: Many of them are among the most corrosion-resistant, nonreactive structural materials.

And the most common among such materials is the familiar range of austenitic stainless steels – the 300 series – which present some challenges in machining.
Work-hardening is the culprit

The machinability of the higher 300-series grades averages around 45: far from free-machining, but not demoralizing, either. The machining challenges they present are based on their tendency to work harden, and the higher grades of austenitic stainless, with their high nickel content, are especially prone to develop a resistant, work-hardened skin while they’re being machined. If not approached properly, it can make machining especially difficult, stopping HSS drill and lathe bits, and saw blades, dead in their tracks.

Grade 316 is one of the tougher ones, and also a popular one for medical devices and pharmaceutical products. It passivates with a high degree of corrosion resistance and it’s more resistant to pitting and stress corrosion than lower grades, such as 304.

Schisler ran informal tests on a variety of saw blades before settling on a special-purpose Starrett VTH (for “variable tooth height”) bimetal blade, an electron-beam-welded combination of spring-steel backing and a band of M42 HSS from which the cutting teeth are milled and ground.

The M42 HSS used for the VTH is also known as “cobalt,” or “cobalt high-speed steel,” due to its high, 8 percent cobalt content. It’s longer-wearing than common M2 and Matrix II, and it can take more heat, although it still falls short of carbide on both counts. Because it’s tougher than carbide, it provides a good compromise in terms of cutting speed, tooth life and cost.

The story behind the VTH blade, however, is in the geometry and configuration of its teeth. It’s an all-out effort to deal with work hardening and frictional heat. Austenitic stainless grades from 304 on up can work harden even when cut cleanly with sharp teeth. When a cutting edge begins to dull, it compounds the work hardening. A normal sequence of even-height teeth in a conventional blade is always on the brink of cutting hardened material, and if the feedrate is not high enough, or if the teeth are not cutting cleanly enough, the blade can overheat and quickly fail.

The VTH is based on a strategy to get around it. It begins by minimizing the number of teeth in the cut, so each cutting tooth is taking a deeper chip, getting under any work-hardening on the surface. This it accomplishes by “ramping” the teeth. In any 4-in. or 6-in. section of blade, tooth height progresses from highest to lowest, at which point a new ramp of teeth starts with a high tooth. That high tooth is a raker that cuts on the end of the tooth, straight into the work in the direction of the cut.

If that’s all there was to it, the cutting would be exceedingly rough and the blade wouldn’t be stable in the cut. So the VTH has another tactic. While the first tooth is a raker that cuts straight down, the following teeth have increasing amounts of “set” and cut the sides of the kerf, using just the corner of each tooth to cut the kerf wider than the rakers. There actually are multiple teeth in the cut at any time, but they aren’t cutting in the same place. So the amount of stock left for the next tooth that does cut in the same place, is greater than it normally would be, giving that follow-up tooth a deeper bite into the work and helping to get under the work-hardening.

The series of teeth in one 4- to-6-in. progression actually includes two or three more rakers – they’re placed at every fifth, seventh or sometimes every third tooth. It doesn’t rely on just one raker in a progression. But the highest one does the most cutting. By alternately clearing the sides and the bottom of the cut, friction and heating are reduced.

“Eventually, as the blade wears, the top of the first set tooth will start cutting on top,” says Gene Ramsdell, Starrett’s production metallurgist and manager of saw R&D for North America. “But even if the teeth are worn, provided they’re not overcome by frictional heat, they’ll continue to cut. And as long as the teeth keep pulling a chip, the blade won’t rub and create a lot of heat. Blade rubbing is the kiss of death, especially in high-nickel alloys. You just have to increase the feed pressure as the teeth wear to avoid it, until the teeth wear excessively and you reach the point of diminishing returns.”

Friction and heat, as well as work-hardening, are the enemies in these alloys. Lubrication is especially important. Schisler’s machine is running a mist-coolant system with a vegetable-based cutting oil.

“It isn’t ideal,” says Schisler. “We had to compromise on the drilling of spray holes on the coolant manifold block, because the mist system we used, on our machine, wouldn’t allow us to mist all the way down to the teeth of the blade. So the blade runs a little hot, and it probably shortens our blade life. But it was necessary to fit everything into place.”

Starrett’s Ramsdell is not a fan of mist systems for these blades, although he prefers to leave it to the coolant experts. All else being equal, he prefers to see a flood of straight cutting oil or a rich mix of water-soluble oil.

The mist system does have its advantages. Watching the machine cut, we were struck by the lack of odor or fog, and the clean floor around the machine. In one eight-hour shift, the mist system uses just one bottle of oil, which appears to hold a liter or less. Sawn slugs were almost dry to the touch.

After cutting, slugs are transferred to a large vibratory deburring machine, and then into a pair of opposed-spindle CNC turning machines. Short slugs, 0.350 in. long, become the lids of the bottles. Longer ones, up to 8.25 in., become the actual bottles. They’re loaded into the tail-end chuck, machined on one end, and the chucks swap the part to turn the opposite end.

That’s it for machining. Only two operators, including Schisler, handle all manual parts-handling, and there are no robots except for a parts-handling robot on the parts washer. After turning, the bottles are laser-engraved, sometimes mechanically polished, and then electropolished. Finished parts are bright and mirror-like.

Starrett’s Ramsdell calls the VTH blade their “better,” mid-priced solution. They offer basic bimetal blades and carbide-tipped blades, as well. But the VTH, which has been in Starrett’s lineup for roughly 20 years, is an effective solution for many applications where the material tends to work-harden and to heat the blade. Besides stainless, it’s used on mold steels and other high-alloy steels, and particularly on high-nickel alloys that work-harden excessively if feedrates aren’t steady enough.

“It needs a robust machine, and it’s not a good choice on gravity-fed machines,” says Ramsdell. But that’s true for any effective cutting in work-hardening materials. A controlled feedrate that consistently gets the teeth below work-hardened surfaces is the first key to success in cutting high-end austenitic stainless, and it results in a cost-effective, medium-priced solution for Eagle’s work materials and work load.

Eagle Stainless Container
The L.S. Starrett Company