Keys to Successful Milling Operations

It's not difficult to get agreement about what a successful milling application looks like. The application features a capable machine with cutting tools and fixturing matched to the workpiece material and part configuration to produce the desired and expected surface finish, tool wear, and tool life.

But for any shop, the challenge is to make the right choice from a host of competing technologies, identify the trade-offs, and, wherever possible, incorporate the latest advanced technology and programming strategies. Here's how some leading tech suppliers view the milling process and associated choices.

In a recent SME Advanced Manufacturing Now Podcast, Chuck Somerville, sales engineer-cutting tools, Ceratizit USA, Schaumburg, Ill., outlined what he calls the "Six Keys to a Successful Milling Application." They include:

Among the considerations are the complexity of the workpiece, skill level of the shop, and the need for communication among machine operators, programmers, and set-up personnel. "All of whom don't share a common knowledge," Somerville said.

"The programmer knows how to generate code. The machine operator knows how to load the machine and press the buttons," he explained. "But, in a lot of cases. the programmer doesn't understand what happens when the operator hits the button and tools start cutting."

That's why machine flexibility is so important. Ceratizit's new MaxiMill 273 tooling system, for example, features a high-precision prismatic insert with a positive clearance angle on both sides. The insert combines the advantage of negative inserts in which both sides of the cutting edges can be used with those of positive inserts with their low cutting forces. The new tooling system is targeted at the majority of milling workpieces that require less than 0.140" [3.6 mm] material removal.

The MaxiMill 273 version 2 features particularly close pitch with the maximum number of teeth on each diameter that make high feed rates possible. It is especially well-suited to high-volume production of cast-iron parts, mainly in the automotive outlined what he calls the "Six Keys to a Successful Milling Application." They include:

Among the considerations are the complexity of the workpiece, skill level of the shop, and the need for communication among machine operators, programmers, and set-up personnel. "All of whom don't share a common knowledge," Somerville said.

"The programmer knows how to generate code. The machine operator knows how to load the machine and press the buttons," he explained. "But, in a lot of cases. the programmer doesn't understand what happens when the operator hits the button and tools start cutting."

That's why machine flexibility is so important. Ceratizit's new MaxiMill 273 tooling system, for example, features a high-precision prismatic insert with a positive clearance angle on both sides. The insert combines the advantage of negative inserts in which both sides of the cutting edges can be used with those of positive inserts with their low cutting forces. The new tooling system is targeted at the majority of milling workpieces that require less than 0.140" [3.6 mm] material removal.

The MaxiMill 273 version 2 features particularly close pitch with the maximum number of teeth on each diameter that make high feed rates possible. It is especially well-suited to high-volume production of cast-iron parts, mainly in the automotive where short machining times are required at moderate cutting depths, according to Ceratizit.

Bryan Stusak, national product manager – milling, Iscar USA, Arlington, Texas, outlined the basics of tackling machinability, which is greatly affected by the workpiece material hardness condition (Rc). Choosing the appropriate cutting tool geometry and coating for a given workpiece material and application, he noted, is essential to success.

Cutting tools need to shear rather than "plow" the workpiece material to prevent work hardening and excess heat while forming the chip, Stusak explained. "The workholding must be solid for a stable and secure process. Any chatter and/or vibrations can cause poor part quality and reduce the performance of the cutting tool," he added.

"Incorporating basic milling application principles and strategies with CAD/CAM is essential to obtain a quality part with optimized tool life. For certain applications/materials, coolant is a must, acting as a lubricant, helping to evacuate chips, and cooling the cut zone. High pressure coolant (HPC) is always recommended if the machine tool offers this capability. HPC acts as a hydraulic wedge to the chip/rake face of the cutting tool interface, thus lifting the chip away with less effort to reduce BUE and increase the cooling action to the cutting tool and workpiece," Stusak said.

For applications as diverse as automotive, general engineering, and medical, Iscar's indexable milling cutter designs can be classified either as a versatile or application-specific solution. For job shops, Stusak noted, having a versatile milling cutter with a high, positive cutter-pocket inclination (axial/radial) allows operations such as facing, shouldering, closed and open slotting, helical interpolation, ramping, profiling, and copy milling.

Iscar's Heli3Mill design has advantages over the traditional APKT style insert that has been around for "eons," according to the company. First, the cutter has a true 90° cutting edge angle with three helical cutting edges to reduce the cost per edge compared with a two-corner insert. The Heli3Mill's V-shaped pocket captures the insert on two points of contact in the radial orientation for higher clamping rigidity.

"This cutter is ideal for light-duty machines due to the high axial rake angles incorporated into the cutter design," Stusak said. The Heli3Mill's triangular-shaped insert edge geometry offerings, along with numerous grade options, are available in five IC sizes: 5-, 7-, 10-, 15-, and 19-mm. These inserts are available on cutter body diameters ranging from 0.236"- 6.00" (6-152 mm).

For application-specific operations, Iscar offers the HELIDO H690 line of tools with either a double-sided triangular- or trigon-shaped insert. This reduces the cost-per-edge for production environments, Stusak pointed out. For heavy-duty operations, he said, the HELIDO H490 and HELITANG T490 exhibit a four-corner insert with a thicker cross section for increased feeds for heavy-metal removal rates (MRR).

Iscar recently introduced eight-corner inserts, such as the LOGIQ8TANG and NEODO S890, for "near-net-shaped expectations for any industry segment if a cost analysis is conducted up front to compare machining costs, cost/edge, and tool life for a given application," Stusak added.

He also noted the popularity of lighter-duty machines. "Less material removal in additive manufacturing and near net-shape castings/forgings are becoming commonplace. End mills designed for finishing applications, i.e., barrel mills can reduce cycle times from 50 to 90 percent versus traditional ball end mill designs."

Iscar's barrel mills come in solid carbide and Multi-Master. Milling inserts designed with shallow depth-of-cut offerings address this need too. Iscar's Mini Mills, (NANMILL, Heli3Mill, and Heli4Mill) offer a shallow depth-of-cut for 90° milling applications. For faster MRR, Iscar's indexable Mini Mill offering expands into fast feed as well, i.e., Micro3Feed and NANFEED.

Also, end mills with chip splitters have become commonplace in job shops and production environments. This technology reduces the radial cutting forces exhibited on the workpiece and manages chip control.

The most important factors in machining a part to print successfully start with "literally everything," asserted Brian Baker, product manager, Walter USA LLC, Waukesha, Wis. "You need to start with the correct machine tool, one equipped with a large enough work envelope, adequate hp, torque, and spindle speed for today's modern tooling. Then the programming needs to use the proper best-practice methods, like end mills or shoulder mills rolling smoothly into the material. And when applicable, yes, proper coolant volume, pressure, placement, and concentration are all important as well," Baker advised.

"But let's not forget the tooling. Having the correct tool for the application is most definitely a must," Baker continued.

"A failure in any one of these areas will make successful machining an uphill battle, if not impossible."For this purpose, Walter has an extensive selection of milling products for all applications, with micro-solid-carbide end mills starting at 0.0157" [0.4 mm] for medical; 7-flute long-reach end mills specifically designed for titanium alloys; to a large selection of insertable mills for almost every conceivable application.

Walter's current catalog of standard tools has more than 44,000 line items. As far as tooling drawings and models, Baker said, the majority of Walter's standard products already have drawings accessible on the company's website formatted in either PDF, DXF, STP, or all three. Walter's Xpress software allows sales engineers to design and quote some special tools at a customer site, delivering a quotation, print and model within minutes.

The supplier also is developing new and upgraded tools to handle higher machine speeds and horsepower to meet changing requirements, Baker said.

"An example is our M2131 mill for high-speed aluminum machining. We literally machined keys into each insert seat and molded keyways into each insert to secure them in the body from the centrifugal forces of today's high-speed spindles, allowing them to run at 40,000 rpm or even higher. But our specialty is our grades. WHH15X is our new cutting-edge grade for hardened materials targeting mold-and-die ,as well as aerospace and energy industries."

There also are significant market changes that tools must address. In the auto industry, for example, Baker cited the "massive shift" to electric vehicles, composite body panels, aluminum frames, and motor housings. "The 90 percent steel and iron composition of decades past is going away fast, and the tooling needs reflect that," he continued. "Walter is heavily focused on aluminum and titanium machining for auto, and aerospace, as well as hard material machining for mold and die and the energy sector which is also in its own similar transformation."

The trend in machine design is toward higher speed, higher productivity, higher accuracy, shorter lead times, reduced costs, and eco-friendly operation. All of which are considered in tooling design by insert stability at the pocket, pocket accuracy, cutter-body material and run out.

Efficiency is at a premium to enable lean manufacturing or job shops with small lot sizes produced on general purpose machines, according to Gil Getz, senior global product manager, Kennametal Inc., Latrobe, Pa.

The job shop is subject to the high cost of materials management, tracking and storing materials, increased production lead time, and distance in material flow resulting in heavy production cost, Getz explained. Kennametal's focus, he said, begins with an extensive selection of milling tools that provides "an efficient and high-performance solution" for any material or milling operation requirement.

Shops may require custom design tooling for several reasons. "The part may have a special need that cannot be performed by a standard tool, such as a long reach, an extremely tight tolerance, or even a particular size," Getz noted. "The most common reason for a customized solution is to combine several operations into one tool. This can often significantly reduce cycle time by eliminating multiple passes, tool change time, and having to perform several operations.

The main benefit of custom design tools is spindle time savings, according to Getz. As an additional benefit, he cited process simplification. "Implementing a custom solution tool shifts the complexity of manufacturing from the operator or programmer to the tool manufacturer," he continued. "The tool comes already equipped to account for the challenges of the operation."

Tool design largely depends on its function, Getz said, Tool design largely depends on its function, Getz said, noting that several factors need to be taken into account— some of which will have a large impact. These include:

Many shops also need versatility, Getz said. This could include machining different materials with one tool or performing multiple operations with one tool, as well as versatility for a single workpiece material.

"One indexable milling cutter can work in all materials for a wide variety of applications as shoulder, ramp, slot, plunge, and helical milling," Getz explained. "With the same cutter, customers can mount different insert geometries in pockets without modification and different insert grades for machining different materials, improving productivity and reducing cycle time and machining costs."

Kennametal's Mill 4-15 series is designed for multiple functions. For example, Getz pointed to "excellent surface quality and high metal removal rates in shoulder milling applications," and a "double-sided insert with four cutting edges ensures low cost per edge."

Describing the Mill 4-15 as a "definite game changer," Getz said the machines are capable of many operations and can accommodate most materials. The new SGE-R insert geometry also adds ramping and helical interpolation, making the tool even more versatile.

Another example is the KCFM45 face mill, which Getz said is designed for a single workpiece material (cast iron), a smart insert, and grade development that provides versatility in its space. It features fixed and adjustable pocket seats, providing the option for semi-finishing as well as fine-finishing applications. With carbide, ceramic and PcBN inserts, this tool is a flexible, cost-effective, and a user-friendly solution that is ideal for any type of CNC machining center, according to Kennametal. Carbide inserts are ideal for machining at lower rpm, or in thin-walled workpieces and less stable conditions, while ceramic inserts provide elevated surface speeds resulting in higher productivity.

There is a trend toward shops using high-feed milling applications, according to Jason Lutch, application engineer, Absolute Machine Tools, Lorain, Ohio. "As a result, we are called upon to set up a lot of our new machines with high feed-rate capability. That means taking lighter depths-of-cut (DOC) from 0.020" to .036" [0.5-.9 mm], radial engagement of 5 or 6" [127, 152 mm], and pushing them anywhere from 180 to 300 ipm 4.5-7.6 m/min. For the metal mold industry, especially for mold cavities with increasingly complex shapes, dynamic milling achieves high metal-removal rate (MRR) removing as much material as possible out of the cavity and then going in with finishing tools."

On something that is a straight wall application with simple pocketing, for example, dynamic milling usually produces the fastest removal rates, Lutch said. "We also run into other applications, which use the same principles. But rather than having DOC of .015" to .030" [0.38-.76 mm] or 040" [1.02 mm], they’ll have a side loader with axial DOC of .020" to .040 [0.5-1.02 mm] so they’ll bury a half-inch end mill an inch and a quarter deep and take .020" [0.5 mm] at a pass but at a very high rpm and very high feed rate. There is a disadvantage of going with a deep DOC when you have different slopes and contours in molds. A little bit more material is left behind for finishing passes because the DOC doesn't really contour to the wall as well as a high-feed cutter would," Lutch pointed out.

"A lot of dynamic milling and CAM software has been adapted to the lower-end spindles from 8,000 to 15,000 rpm range," he said. "For example, a lot of the Dynamic Milling or trochoidal milling that can be done with CAM software like Mastercam allow controlling feed and speed according to the horsepower constraints of the machine. Whether it's 10 hp or 50 hp, the maximum MRR can be dialed in so as not to overtax the machine."

With trochoidal or dynamic milling you are able to pick your pocket, the stepover, the DOC, and the rpm that will keep the cutter engaged to maximize metal removal, according to Lutch.

CAM software has come a long way with set-up sheets and data sheets. The programmer now is the point person when it comes to high MRR, Lutch said, noting that operators used to control a lot with the G-code and how they programmed at the machine. "What we’ve found in the milling world is that it has moved away to CAM software," he continued. "There are a lot of things you can do with software. You can set up tool tables and operations, allowing the front end of the CAM to be a lot more effortless.

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