Once, polycrystalline diamond (PCD) cutting tools were so dear that it was almost as though there needed to be a Brink’s truck running between the tool crib and the machining line. And once the tool was received at the line, the person handling it had to have the dexterity of a safecracker lest the exceedingly expensive tool found itself chipped or shattered on the factory floor.
Nowadays, it seems, in machining operations PCD tools are the order of the day. Every day. Day in. Day out. And the reason is really quite simple: Aluminum.
“Aluminum is more prevalent everywhere,” says Robert Trent, central zone manager, WALTER USA, Inc. (walter-tools.com/us). “Prevalent in all parts, not just engine blocks and heads. It is being used for chassis components and steering components, as well.”
And while the cost of PCD tools has become somewhat less expensive, Tom Chevalier, Sandvik Coromant Co. (sandvik.com) business development specialist for Automotive, acknowledges, “It still commands a premium price—but it provides high metal removal rates, so there are not a lot of other options. You can run polished carbides, but at lower speeds.”
Not only are there higher metal removal rates, but Trent points out that there is an overall economic advantage: “You can achieve a cost savings with diamond compared with carbide. For some round tools”—by which he means rotating tools—“that are monoblock, brazed tools, we’ve seen them run months and months without being changed.”
What’s more, Chevalier points out that when milling aluminum with PCD, “the cutters run with a lot fewer teeth.” For example, there might be four inserts in a 160-mm cutter, a considerable decrease of what would be the case with other types of material.
And speaking of other types of material, Trent points out that there is an increasing trend away from cast-in cast-iron cylinder liners for aluminum blocks and to a spray-in material. The sprayed-in liner is much thinner than a sleeve, but, Trent points out, whereas cast iron is easy to machine, this high-velocity, sprayed-on material is thin, hard and more difficult to machine.
What’s more, Trent notes that the increase in the number of turbocharged engines means that there is a need for machining high-temperature superalloy materials like Inconel, which, while familiar to those in aerospace, are less so in the automotive arena.
By and large, Walter is producing more monoblock tools, tools that are all one piece, not, say a toolholder with an adjustable insert in a pocket. This is facilitated by the long machining time that PCD provides.
However, because there can be chip evacuation problems (generally, in aluminum machining, coolant is used more for washing away chips than for cooling the tool), Walter has developed chipbreakers for PCD tools that are put into the face of the PCD material with a laser.
At Sandvik, Chevalier says, they have developed some tools specifically for finishing aluminum engine components, especially those with several cavities (e.g., heads, valve blocks), where burr formation can be a problem. One example is the company’s CoroMill 5B90 milling cutter, which is specifically engineered for applications. The inserts in this face mill are specifically oriented both axially and radially for the given part. One of the tools is always a wiper. Because the tool doesn’t require inserts adjustments, setup time can be reduced by 66%. In machining a cylinder head made of AlSi9-Cu-1 with a 6-in. diameter 5B90 cutter with nine cutting edges, they were able to increase the speeds and feed rates of the operation compared with the existing cutter, while maintaining the same depth of cut*, and ending up achieving a cost savings on the order of $26,000.
Cast iron isn’t completely out of the picture by any means, particularly for manufacturers of powertrains. Trent notes that they’re seeing more compacted graphite iron (CGI) being used for blocks. This material, he explains, is more abrasive than conventional cast iron, which leads to a need to machine it at a rate that is “much slower” compared to cast iron.
Of course, it is not all milling that’s going in part production, by any means. Chad Miller, Turning Products Manager at Seco Tools Inc. (secotools.com), says “We’re still machining steels.” He says that given the experience that has been built up over the years, people in industry have gotten really good at it.
But—and there is always a But—Miller goes on to point out that there is beginning to be a transition to other materials in some instances: “The questions now come up regarding turning Inconel and stainless steel components. We’re seeing materials coming over from aerospace.”
An issue with machining the super-alloys, Miller says, is chipbreaking. The materials are gummy, so not only have they found it necessary to have specific chipbreakers designed into the inserts, but they have developed what they call “Jetstream Tooling,” which directs high-pressure coolant directly at the cutting edge so that it helps break the gummy chips and washes them away from the cut surface.
However, Miller notes that there is an increase in the amount of aluminum being turned: “Components that were once made with iron are now aluminum alloys.”
*For those interested in the numbers:
Other tool 5B90
Cutting speed: 10,300 fpm 12,500 fpm
Spindle speed: 5,000 rpm 6,000 rpm
Feed rate: 325 ipm 355 ipm
Depth of cut: 0.02 in. 0.02 in.
Tool life (hours): 30,000 45,000
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