by Kip Hanson
Solid carbide roughing tools are productivity’s best friend
High speed steel (HSS) end mills are easily the lowest cost cutting tool solution on the market, according to suppliers. Unfortunately, they also offer the lowest productivity. This is especially true when removing large amounts of material, or with tough metals such as titanium, Inconel, and cobalt chrome alloys, known in the industry as HRSA, or heat resistant super alloys. For these, nothing but the best tooling available will do.
Don’t drop out
Still, HSS roughers, sometimes referred to as corn cob cutters, have their place. Brian MacNeil, milling products and application specialist for Sandvik Canada Inc., Mississauga, ON, says tool rooms and those making one-off components can save a few bucks by taking the less expensive route.
HSS roughers are very tough, and readily shrug off the abuse that comes with excessive toolholder runout and less than rigid setups. And they handle chip recutting–the kiss of death for most carbide tools–with the aplomb of a wood chipper facing a yard full of downed trees.
“If you have no need for productivity and very low part volumes, then HSS can be the answer,” says MacNeil. “However, carbide will always win the race against time and tool life, delivering the lowest cost per piece.”
Recognizing that high speed steel is anything but “high speed,” it begs the question: is it really more economical to spend five to six times more for a solid carbide roughing tool?
The answer, according to tooling suppliers, is a resounding yes. Compared to their pre-NC era counterparts, carbide roughers often deliver 10 to 12 times the productivity of HSS, with greater tool life to boot. But what if someone drops it, you say, or crashes it into the workpiece: there goes $500. And how can we be assured to get the longest tool life possible, so as to best amortize our tooling investment?
We can start by being careful. Because it’s relatively brittle, solid carbide is less forgiving than HSS, and unlikely to survive a fall to the shop floor or an ill-considered feedrate.
But once you’ve gotten past the nervous Nellie stage, there are some clear ways to maximize tool life and obtain the greatest productivity from solid carbide roughers.
Walking away from Weldon
You need the right toolholder. Regular users of HSS corncob cutters are also familiar with Weldon shanks, a toolholder technology invented in the early 1900s by Carl Bergstrom, owner of the Weldon Tool Company. In that ancient age, a Cincinnati milling machine capable of 1,200 rpm was considered high speed, and anything less than a couple thousandths tool runout (0.05 mm) was pretty darned accurate.
Granted, Weldon shank tooling today is far more precise than in Bergstrom’s time, but it can’t compare to the accuracy offered by shrinkfit, hydraulic holders, or mechanical milling chucks, many of which boast 0.005 mm (0.0002 in.) at a distance of four times the tool diameter. This is probably overkill for a HSS rougher, but equates to greatly improved tool life and part quality when used with carbide tools.
But what about tool pullout? End mills tend to creep during roughing operations, something that Weldon shanks are designed to prevent. Yet the side-lock screws used with Weldon holders have been known to back out during heavy cutting in titanium and other superalloys, potentially scrapping expensive workpieces. And imbalance of these holders causes problems at spindle speeds above 10,000 rpm.
A better solution is an anti-pullout toolholder such as the Haimer Safe-Lock system or equivalent, technology a number of toolmakers have now licensed due to its fail-safe spiral groove/locking pin mechanism.
Pitching a home run
Sumitomo Electric Carbide application engineer Shane Schultz says the irregular pitch and lead angle used in their tools reduces the vibration common with roughing operations. “By varying the end mill flute pitch a few degrees, and modifying the helix angle slightly from top to bottom, higher feedrates and cut quality are possible. Since the teeth aren’t perfectly symmetrical, it breaks up the machining harmonics somewhat, so you can take a deeper depth of cut in slotting, for example, and engage a greater length of the tool.”
A slightly rounded, double margin land at the cutting edge helps protect it from chipping, Schultz adds, and provides better surface finish to the workpiece. He also recommends a multi-layered coating such as TiAlN and AlCrN, or Si3N4, which, when applied in layers just a couple microns thick, retains the tool’s sharpness while improving heat and wear resistance, providing up to 80 per cent greater tool life. Because of these tools’ unique geometry and coating, Schultz suggests it would be best to return them to Sumitomo for sharpening. “Some local tool houses can do it, but the 3° variable helix is pretty tricky to grind. You need the right equipment.”
To those shop owners and procurement people who complain about the price of solid carbide tooling, Schultz offers the following advice. “One carbide rougher, properly maintained, will last as long as four or five high speed steel cutters. You don’t have to stop the machine as often, or risk a math error when touching it off. There are fewer mistakes, less interruption, and more consistency. And that’s just the setup side of the cost equation.”
“Workpiece clamping limitations should also be considered during the tool selection process,” says Sam Matsumoto, application engineer for OSG Canada Ltd, Burlington, ON. “For example, if the workpiece can only be clamped from the side, then you should consider selecting a fine pitch roughing end mill with a lower helix angle (17.5° to 22.5°) to suppress axial cutting forces and prevent the work piece from coming loose. Conversely, if bottom clamping the workpiece is the only option, select an end mill with a high helix geometry (40° to 42°) to reduce radial cutting forces and maintain stability during the milling process.”
Throw another log on the fire
Quite often, the benefits of carbide roughing tools aren’t immediately apparent. Even when not running production, 10 minutes are saved here, 30 minutes there, just because the tools run much faster. This adds up over time, but may require some organization to recognize, says Matsumoto. “Shops should keep track of how many minutes of cutting have been done, otherwise they might retire a tool before its time, something more difficult to do in a low volume than it is with production. Shop personnel need the right mentality to keep track of everything, and log what job, how long it ran, what material, that sort of thing. It’s tough, but far better than having a tool that explodes out of nowhere and then looking for the cause.”
One might argue this sort of discipline is a good idea with any cutting tool today. The expense and lost productivity due to less than optimal machining conditions can easily make the difference between business growth or stagnation, and usage data such as this gives management an opportunity to do root cause problem analysis and make continuous improvement on the shop floor.
Nor is a fancy software system necessary,” says Matsumoto. Much of the needed information is already logged into the machine, so it’s a simple matter of getting this into an Excel spreadsheet or even a notepad. Some shops even use a stopwatch to keep track of cut times. Aside from that, just engrave each tool with a serial number so you can trace it back to the job and cutting data. Simple is the best solution.
Keep cool, and carry on
Luke Pollock, product manager for Walter Tools, West Waukesha, WI, says the traditional deep gullet, serrated tooth geometry associated with roughing end mills is becoming less necessary for many metal cutting applications. That’s because modern programming systems use trochoidal toolpaths, corner slicing, and constant cutter engagement at lower radial depths of cut to reduce cutting pressures and increase feedrates. Because of this, some shops are seeing excellent roughing results with 5, 7, 9, and even 11-flute tools normally reserved for semi-finishing and finishing operations.
“Many CAM programs take advantage of light depths of cut and substantially higher feedrates. This means you don’t need as much flute space for chip evacuation, so you can engage more cutting edges. Even with long axial engagement, pressure remains low and the cutters are free cutting, an important factor with today’s lighter duty machine tools,” says Pollock.
Another factor is cutting fluid. Pollock says through-the-tool coolant is critical to tool life and predictable machining processes. If you’re wondering how toolmakers get coolant holes in a solid carbide tool, it’s not as big a chore as one might think. “Some manufacturers use EDM to create the holes, but we use extruded carbide stock, almost like a drill rod. This keeps costs down and avoids problems with micro cracking and HAZ [heat affected zone] like you have with EDM.”
Don’t have coolant-through capability? Get it.
Most newer machines come equipped with through-the-spindle coolant, and aftermarket inducers are readily available for older machines. And if your machine came with a 100-psi coolant pump, better supersize it to 1000 psi or more. This will improve performance in your rough machining operations, and in drilling, face milling, and profiling as well. SMT