Hard Milling: Not That Hard
- March 30, 2020
Still doing the “rough soft, finish hard” routine? There's a better way.
Just as it’s easier to push a stalled car downhill, it only makes sense to machine metal in its most agreeable state. For mouldmakers and tool and die shops, this has long meant removing as much material as possible from the workpiece while still soft, before sending it out for heat treatment, then EDMing or grinding the hardened die or mould cavity to size.
This paradigm changed over the years as cutting tools and CNC equipment grew more capable. Many shops have since discovered that they can skip the grinding and EDM departments entirely and finish the workpiece in the machining centre that roughed it. Others have taken the next logical step by simply clamping onto a hardened piece of tool steel and machining a die or mould in one fell swoop.
Both of these hard milling approaches offer significant benefits: shorter lead times, less work-in-process, lower costs, fewer fixtures and better machine utilization. For many in the industry, hard milling is now the only way to go, and the old days of multiple setups and lower productivity are just that: old-fashioned and less productive.
Hard milling by the numbers
But what exactly is hard milling, and when should it be applied? “The term ‘hard milling’ is relative and often misapplied,” says John Mitchell, general manager for Tungaloy Canada. “Some use it to describe metals that are difficult to machine—titanium or Inconel, for example—which can be challenging but are not, in fact, all that hard on the Rockwell (Rc) scale. Even for heat-treated materials, the term is still relative. Some consider 40 Rc to be fully hard, while others don’t consider a metal ‘hardened’ until it’s reached 60 Rc or higher.”
Of course, the softer the material, the easier it is to machine. This helps explain why some shops continue to embrace the “rough soft, finish hard” paradigm. When milling hardened material, on the other hand, the machine tool must be very rigid. The insert’s cutting edge must be strong and chip resistant. The tool must be running with minimum runout, and the workpiece held as securely as possible. On top of that, the cutting speeds and other application parameters adjusted in direct proportion to the hardness of the specific alloy.
“This is a key factor and will dictate the spindle speed at which the tool should run,” Mitchell adds. “When machining alloy steel in the annealed state, the typical starting parameters would be around 700 to 1,000 surface feet per minute (sfm). The same material in the 50 to 60 Rc range should be machined somewhere between 50 to 180 sfm, while 40 Rc material might go as high as 450 to 600 sfm.”
Cutting tool edge strength becomes increasingly crucial as workpiece hardness goes up. Always introduce the milling cutter to the material at the insert’s strongest location, he says, which means using positive lead angle tools such as the Tungaloy’s DoFeed or MillQuad-Feed indexable cutters. Positive lead angles not only make the chip thinner, but help drive cutting forces upwards rather than sideways, reducing deflection. The result is that a 45° face mill will generally do a better job than a 90° shell mill in hardened steel, while a 15° high feed milling (HFM) cutter will outperform both.
Thinning the chip raises another consideration—the need to increase feedrates, perhaps drastically. “With a 90° milling cutter, the chip thickness is equal to the programmed feed per tooth,” says Mitchell. “A 45° face mill, on the other hand, generates chip thicknesses 70 per cent that of the feed per tooth, while a 15° HFM cutter produces chips just 14 per cent as thick.” Because of this, Mitchell says feed rates should be increased in these situations, resulting in longer cutter life and shorter cycle times.
Andy Moon, milling and GM300 product manager at Guhring Inc., agrees, adding that geometries specifically designed for hard milling should be used with solid carbide end mills. “When you get into RC hardness levels from the low 50s and up, we typically suggest a negative rake tool,” he says. “Our GF300 T, for example, has a -7° rake angle, a special Torus grind, radiused corners, and nano-Si multi-phase coating, all of which improve tool life and productivity when milling hardened materials.”
Moon notes that trochoidal toolpaths are also effective when hard milling—a 5 per cent radial engagement would be a good starting point for steel in the mid-50s RC, with as much axial engagement as the tool or workpiece supports. And just as in Mitchell’s HFM example, increased feedrates must be applied to compensate for the chip-thinning effect.
Purchasing specialty cutters is costly, but generally a small price to pay in when the benefits are factored in. And hard milling offers significant benefits, beginning with simplified part processing.
“I would guess most of the die and mould industry is doing at least some hard milling, with much of it on five axis machining centres,” says Moon. “Between these two technologies, a shop can often complete the majority and sometimes all of the machining with a single part handling, avoiding the complexity that goes with multiple operations.”
On the ball
Cutting tool provider OSG also offers end mills designed to machine high hardness steels. Adam Dimitroff, application engineer for OSG Canada Ltd., says the company’s Exocarb WXS end mill series has long been a popular tool for hard milling due to its high edge strength, thicker core, and extreme accuracy.
That said, he also notes that OSG’s recently-introduced A Brand line of two and four-flute ball nose end mills are becoming popular with mould shops and others completing high precision finishing operations in 60 HRC and above materials.
“One nice feature of this new line is its variable flute spacing, which helps break up the harmonics that lead to chatter,” he says. “There’s also a steep spiral at the point to reduce cutting forces, and a multi-phase Durorey coating that helps stop cracks, something that’s common when milling hardened materials.”
If you look back five years, most cutting tool coatings had only a single layer. Because of this, any break in the coating would quickly travel to the carbide substrate below, leading to rapid wear. Dimitroff says the softer layers in today’s multi-layer coatings stop any cracks at the hard upper layer. “This layering also serves to increase wear resistance while making the tool tougher overall,” Dimitroff says. “And because we polish the tools after coating, they tend to hold lubricant well and generate less heat. All of these attributes are important when machining hardened steels.” SMT