by Kip Hanson
Making the most of cutting tools in hard milling applications
For decades, the process of mouldmaking has been one of rough machining the cavities, bases, and various inserts, pins, and cores, sending everything out for heat treatment, and then grinding and sinker EDMing the parts to size. This procedure has changed dramatically for some shops over recent years with the advent of technology that enables productive machining of tool steel in its hardened state.
As a result, hardened moulds can now bypass the EDM department in favour of finishing on much faster machining centres, which don’t require costly electrodes to operate. Simply drop in a carbide end mill, generate an appropriate toolpath, and mill the mould cavity or insert to size, a process that greatly increases flexibility and may save weeks of manufacturing time.
How much and how hard?
Most mouldmakers still pre-machine components prior to hardening, leaving just enough material for finishing and to compensate for any warpage during heat treating. But depending on the size, geometry, and expected production life, some opt to machine the entire tool in a hardened state, especially with lower volume mouldmaking steels such P20 pre-hard, available in off the shelf hardness levels of 30 Rc and up, a relative cakewalk compared to 60 Rc S7 or D2 tool steel, chromium-rich materials that test the mettle of mouldmakers and machine tools alike.
Of course, moulds aren’t the only hardened workpieces being machined these days.
A wide variety of aerospace parts are made of 17-4 PH stainless steel and CoCrMo (cobalt chrome molybdenum) alloys, both of which can be made very hard. So too can Inconel, a favourite in the oil and gas industry. None of these metals are for the faint of heart, least of all when machined hard. Being successful requires a number of supporting technologies, including specially designed and coated end mills, CAM software that generates ultra-precise and error-free toolpaths, and super fast, rigid, and accurate machine tools able to withstand the extremes that come with cutting tough, hardened materials.
Jay Ball, product manager for solid carbide end mills for NAFTA at Seco Tools Inc., Troy MI, says there’s a big push in the mouldmaking industry to eliminate soft machining. “During the heat treating process, materials like to expand and contract. Depending on how much material you’ve removed, parts may move substantially. Now when you come back to do your finished machining, you have to relocate part features that might not be where you left them. Another roughing pass might be required, and then semi-finishing passes to get the material back to near net shape for finishing. A lot of time and effort goes into that, so if you can just go directly to hard milling, there’s substantial benefit.”
This might not work for every application. Removing thousands of pounds of hardened steel from a mould for an automobile bumper or transfer case is cost prohibitive, no matter how robust the machining setup. Nor is the deep hole drilling needed for the cooling lines and complex water jackets surrounding the cavity of most injection moulds.
Ball says when and how much hardened material to remove comes down to a balance between how much setup time will be consumed by trying to dial each part in, based on heat treating induced movement, versus just how much material can be realistically removed in a hardened state.
“Each application requires some analysis to determine what makes the most sense.”
Once that decision has been made, mouldmakers and would-be hard milling aficionados should consider the following tips for optimal tool life:
• Hard milling raises the bar for acceptable cutter and insert quality. Avoid general purpose tools in favor of those designed specifically for hardened materials. Positive flute geometries reduce machining forces and improve cutting action. A PVD coating such as AlTiN, together with a cobalt rich micro grain carbide substrate is better able to handle the high heat generated when cutting hardened material, and helps prevent the abrasive wear, the usual cause of tool failure. Indexable insert cutters might do an acceptable job in roughing and semi-finishing, but solid carbide tools are the best bet, despite their greater initial cost. Budget accordingly.
• Modern CAM systems able to deliver trochoidal toolpaths, corner slicing, and similar programming techniques that produce thin chips and consistent cutter loads are the rule. Find a vendor with experience in hard milling; the toolpaths are similar to those used with heat resistant super alloys (HRSA), but mouldmaking has its own set of rules, particularly when machining the complex three dimensional surfaces common in this arena. Put them to the Pepsi challenge before signing on the dotted line.
• Whether you’re machining a hardened steel mould cavity or a forming die, your machining centre needs the right stuff. Contrary to what you might think, this isn’t some geared head muscleman but rather a ballerina, one with a motion control system able to maintain extreme geometric accuracy even at high feedrates, a spindle with enough rpm to drive small diameter ball nose cutters at the proper cutting speed but enough oomph to take a heavy cut, and a CNC control that can adjust trajectories and feedrates to prevent stalling and jerking when changing directions.
• The slightest amount of tool runout can spell disaster (or at least abbreviated tool life) during hard machining. Opinions vary widely on this subject–some say milling chucks and collets are the best choice, while others insist on shrinkfit or hydraulic holders. All agree, however, that accuracy is paramount. Runout measured at the tool tip should not exceed 0.005 mm (0.0002 in. ), and less is better. Roughing operations may require some sort of side lock or anti-pull mechanism to prevent tool creep, but this is less of a concern when using recommended trochoidal or high speed toolpaths, as tool pressures are lower. Check the manufacturers specs, test the finalists in your own shop environment, and document the results.
Tom Hagan, milling product manager at Iscar Tools Inc., Oakville, ON, agrees with most of these recommendations, but says there’s a better alternative to carbide: polycrystalline cubic boron nitride, or PCBN. “Carbide’s not really cut out for hard milling, certainly not in the upper ranges, say 60 to 65 Rockwell. Here, PCBN is the best choice. It’s very expensive, but provides tool life six to seven times that of carbide.”
However, warns Hagan, don’t attempt to hog out the bumper mould mentioned earlier. PCBN is strictly for finishing operations. For all else, he recommends one of Iscar’s newer PVD coated solid carbide grades, the blue-coloured IC702, which he says is designed for high speeds and light depths of cut in steels up to 65 Rc.
Because spindle speeds and feedrates are higher with these tools, he also recommends cutters with additional flutes–instead of the traditional two-flute design, end mills with six or even seven teeth are better able to maintain proper chip loads at elevated machining parameters, improving tool life and productivity. And variable pitch endmills, says Hagan, help reduce chatter and machine harmonics that sometimes occur when machining difficult materials. “We’ve also introduced a line of high feed mills, designed for roughing in die, mould and aerospace. These show great promise with hardened steel.”
Keep it cold
Emuge Corp., West Boylston, MA, is another tooling provider with its eye on hardened steel machining. Milling product manager Dan Doiron says the company’s line of hard milling tools have a high flute count and large core diameter, which lends rigidity when milling metals up to 66 Rc.
“We also break the cutter edges with a small radius, and add a slight hone to the flutes. This prevents the deterioration you get with sharp-edged endmills in hard materials.”
Because of the extreme heat generated during hard milling, Doiron says its critical to avoid thermal shock. For this he suggests turning off the coolant pump and cutting dry, using an air blast to remove chips. Better yet, use a cold air gun.
“If you apply cutting fluid in these situations, you’re just going to fracture the tool. A cold air nozzle system uses regular shop air and the Venturi effect to remove heat. We’ve seen tool life improvement of 50 per cent using these systems.”
Doiron agrees that trochoidal toolpaths are the right solution with hard milling and other difficult machining applications, but adds that it’s important to stay consistent with step-overs and cusp height. “Most CAM programs effectively calculate the toolpath to have a consistent value throughout the programmed part feature. The best starting point is to always reference the tool catalog for starting parameters and recommended step-over. Tool manufacturers have done the research on tool geometry and its proper applications on hard materials. In a lot of cases, if your setup is rigid and you have a good toolholding system, you can count on getting better than expected results.”
Finally, choose a tooling vendor with experience in this area. The days of “here, try this one, I’ll see you next week” sales tactics are over.
Don’t be afraid to ask for–and expect–good advice. SMT