Millstar: Tech Articles

Die/Mold: The Leadtime Challenge

To deliver finished molds and dies in less time, mold and die shops are adopting high speed machining to eliminate steps and shorten the production cycle.

Few segments of the metalworking industry are under as much pressure as the die/mold industry. Foreign competition has been intense. Not only are some overseas mold and die manufacturers able to underbid their U.S. counterparts, but they are often able to deliver finished molds and dies in less time, too.

But many shops are fighting back–and winning. High speed machining is one of the most potent weapons these U.S. mold and die builders are using to retain and even to reclaim this business. HSM is allowing them to slash weeks from the process.

Convergence

HSM is an entirely new approach to making workpieces. It is a convergence of various technologies–certainly more than merely a convergence of high spindle speeds combined with faster and more accurate feed rates on a machine tool. To support such a machine, a shop has to create another convergence. A shop must bring together new and essential skills as well as new and essential capabilities. Both shop culture and shop infrastructure must undergo substantial change.

Yet HSM is only a means to an end. To the extent that it contributes to shortened leadtimes and to reduced costs, it is valuable. Any shop pursuing HSM is well advised to consider all other techniques that speed the production cycle, on and off the shop floor.

There is no cookbook recipe for successful HSM. HSM is still evolving. Almost every shop that is doing HSM will tell you that it is still changing, still experimenting, still learning, often from mistakes or failures. There is no one right way to do HSM. And there are no hard and fast rules about how to apply HSM in every case.

Finally, it is important to note that HSM is not a theory, a concept proven only in the laboratory. HSM is doable. Scores of mold and die shops are doing it now, and hundreds more are actively working toward its implementation. Profiles of a few shops deeply involved in HSM clarify this picture. This is HSM in the real world.

Molds In Five Weeks

High speed machining is important to Minnesota Mold & Engineering (or Minnmold, to use an abbreviation borrowed from the company's e-mail address), a 65-person mold shop in Vadnais Heights, Minnesota (near St. Paul). But more important to Minnmold than high speed machining is high speed mold making.

You can talk all you want about spindle speeds, feed rates, stepovers, and chip loads, but what counts for Minnmold is completing molds in five weeks–or less. These are complex, class A molds for high production applications in the plastic injection or die cast industries. Typically, these molds feature one to four cavities and weigh up to 10,000 pounds. The work ranges from cell phone bodies to wheel covers and larger parts.

At Minnmold, HSM must be appreciated in the context of high speed mold making. Bob Archambault, general manager, sums it up: "We streamline, streamline, streamline." Everything, not just machining, has to be streamlined. Unless upstream and downstream procedures or processes are streamlined in support of HSM, HSM can't happen effectively or have much effect, he says.

But when these processes are streamlined, HSM has a tremendous potential to lower costs and slash leadtimes. In fact, Mr. Archambault credits HSM as the major factor in the company's ability to attract work in this current period of soft demand for molds. The economies from HSM have allowed the shop to win jobs with lower bids while becoming more profitable, thanks to increased volume. The shop is producing 30 percent more molds per year than before HSM was introduced. The shop has been adding skilled staff and broke ground for a 18,000 square-foot expansion last spring.

Electrode Bottleneck

Like many mold shops, Minnmold first got involved in HSM when it upgraded its EDM capabilities. In early 1995, a newly installed ram unit proved 40 percent faster than the unit it replaced. To keep this machine busy, the shop needed to produce graphite electrodes faster.

Minnmold looked at machines for high speed milling of graphite, only to realize that generating the tool paths for one of these machines would create a bottleneck in its programming department. And the tool path files would be very long–too long to download effectively on the shop's DNC network. Before acquiring its first graphite mill, the shop had to update its programming software and its communication network.

Only then did they install this machine, a Makino SNC64. It featured top spindle speeds of 15,000 rpm and rapid traverse rates of 630 ipm. This machine was replaced by a brand new SNC64 this past April. An additional graphite mill from Roku-Roku rounds out the shop's graphite electrode milling capability.

Experience with high speed milling of graphite encouraged the shop to acquire a high speed machine for steel cutting. This machine, a VM5 from OKK, was installed in the autumn of 1998. Today, every hardened insert block the shop prepares is machined at some point on the OKK. This machine is used for some roughing operations in the high speed mode, but about 60 percent of its time is devoted to high speed finishing operations.

Off To Good Start

Where does successful HSM begin? At Minnmold, it starts with a 3D solid model of the customer's part that the mold or casting die must produce. Many customers can provide this model, but for those who cannot, Minnmold always constructs one from 2D data such as a print.

The next step, according to Senior Designer Brian Bussmann, is the creation of a completely toleranced, completely detailed mold design in a 3D, solid model CAD database. Every feature of the mold that is not purchased is represented in this database, with every dimension and tolerance defined. As Mr. Bussmann, points out, "Good design capabilities are critical to high speed machining and every other operation that follows. The [mold] design has to be good and it has to be fast."

The company has six full-time designers. Two of them work primarily with ProEngineer design software from Parametric Technologies Corp (Waltham, Massachusetts). The other designers work primarily with CADKEY 98 from Baystate Technologies (Marlborough, Massachusetts). Minnmold's designers complete the mold design around the 3D model of the customer's part.

A completely detailed and toleranced database in 3D is so necessary because the shop builds every component to meet the design within its tolerances–on critical parts, tolerances are held to 0.0001 inch. As Mr. Archambault explains: "We build to the database. There's no fitting of components in the shop because we want a customer to be able to come back to us for replacements or repairs of insert blocks and so on, and all we have to do is call up the database from an archive and reproduce a new component to the same exact dimensions and tolerances as the original. The customer can just drop in the new component to get the mold back into production."

Once completed, the mold design is archived on the shop's file server and is accessible from every PC on the shop's computer network (there are 30 PCs in this shop of 65 employees.) Paper prints rarely go out to the shop.

A finished mold design, however, settles what to build but not how to build. The next step is assembling the entire team that will be involved in machining and assembling the mold. At this "kick-off" meeting, designers, programmers, and the tool makers review the project and together decide on a build strategy. They determine, for example, what cutters to use, how many electrodes will be needed, about how long each step will take, and what scheduling contingencies to consider. They also settle where to use HSM and where to use EDM (see "To Burn Or Not to Burn").

Speedy Programming

What is said about mold design goes for programming as well. Fast turn around is essential, notes Mike Myers, Minnmold's programming supervisor. Programming assignments are scheduled for completion as precisely as each machining operation on the shop floor.

Programming is a round-the-clock affair at Minnmold. Five programmers are on duty during the day shift. Two are dedicated to 2D programming, and three are dedicated to 3D programming, with the department head filling in where needed and acting as a scheduler and liaison with the shop floor. Two more programmers, one for 2D and one for 3D, hold down an extended night shift five days a week.

Currently, programming is done on PCs. All programmers are experienced with the latest release of MasterCAM software from CNC Software, Inc. (Tolland, Connecticut). This software provides programming utilities that lend themselves to HSM, such as the HighFeed Machining feature. HighFeed Machining includes a "Multisurface Rough" and a "Multisurface Finish" option that Minnmold frequently applies. Two seats of WorkNC automatic 3D milling CAM software from Sescoi USA, Inc. (Southfield, Michigan) have also been installed recently.

Typically, roughing operations are set to leave 0.02 inch of stock for finishing. Roughing on the OKK with a 1-inch ball nose cutter feeding at 120 ipm and running at 3,500 to 4,000 rpm are typical parameters. Depth of cut, Mr. Myers reports, is generally 0.05 inch (10 percent of the cutter diameter is recommended, but the shop finds a cut this heavy to be less satisfactory than a lighter cut). Bull nose cutters are often used too, depending on the workpiece geometry to be machined, but these cutters require a lighter depth of cut due to the smaller radius at the cutter edges.

Programming finish cuts for HSM is trickier because of the number of variables involved. It's difficult to make generalizations. Depending on the surface finish desired, stepovers range from 0.010 to 0.015 inch for larger ball nose cutters (0.750 to 1.00 inch in diameter) and 0.002 to 0.004 inch for small cutters (down to 1/16 inch). Because the OKK is limited to 8,000 rpm spindle speed, feed rates of 120 to 180 ipm are typical, depending on chip load (0.004 or 0.005 inch per tooth is a common value). More aggressive cutting values are the rule for graphite milling, but the principles are the same.

In all cases, exact values are determined by formulas for various combinations of workpiece material, hardness, cutter size, surface finish required, and so on. The shop has developed these formulas based on recommendations from vendors but modified through experience. All of the programmers apply these formulas uniformly to maintain consistency in programming results. Mr. Myers believes that every shop has to be committed to developing its own set of standards for HSM from experience.

Finally, tool paths are verified with MasterCAM's simulation of the cutting tool in action in a solid model of the workpiece. Only simple 2D programs are not reviewed. Completed programs are posted on the shop's file server.

Shop Floor Network

HSM requires a suitable shop infrastructure. One of the most important elements is a computer network that supports high speed transfer of long tool path files and other communication needs.

Minnmold's network is based on a Dell PowerEdge 6300 file server. The server is linked to PCs throughout the company with fiber-optic cables, and to the Internet with an ISDN line for high speed access. FTP (file transfer protocol) connections to all of the PCs allow virtually uninterruptible flow of data across the company-wide network.

Downloading large tool path files, even when several stations are doing so at once, is fast and reliable. The machines for high speed operation have hard drives in the CNC unit, so entire programs can be stored and read in local memory.

Brian Bussmann, the senior designer at Minnmold who helped specify network needs, notes another value of this file server/network setup. All of the information relevant to a project is located in one central database in an organized and accessible fashion. "Everybody can find what they are looking for," he says. "Designers can access the part model. Detailers and programmers can access the mold design files. Tool makers on the shop floor can access tool path files."

Cutting Tools

As far as tool selection goes, Minnmold's practices are in line with the guidelines presented in the panel "Cutter Strategies." In the shop, tooling for HSM is maintained separately in its own cabinets. Only high-grade balanced toolholders are purchased for HSM in steel. Runout and concentricity of cutters are verified at the machine with every tool change. In general, following a disciplined approach to cutting tools is essential to successful HSM. However, Bill Carter, CNC process manager, encourages testing and experiments. "We'll try just about anything–new coatings, new inserts, new styles of holders," he says.

An interesting but effective strategy has been developed for graphite milling. The shop uses standard, uncoated fine-grain solid carbide bull nose end mills. These tools are cheaper and more readily available than tools with proprietary coatings. Cheaper tools can be changed more often, so worrying about tool wear is unnecessary. The shop finds that it's easier to use a new tool than to bother about resharpening.

New Generation Steel Cutter

The next move for Minnmold is to take delivery of a second OKK high speed mill, scheduled for later this year. This 50-taper machine, model KCV600/15L, will have a top spindle speed of 13,000 rpm and 30 by 60 inches of travel, large enough to allow the shop to apply HSM to the production of mold bases.

High Speed Machining For A Living

If HSM is only a means to an end (reduced leadtimes, for example) can you do HSM for a living? Mike Haverkamp and Brian TerBeek say you can. They're doing it. These partners are machinists turned programmers turned HSM entrepreneurs.Their company, Cad Cam Services, is located just outside Grand Rapids, Michigan. It is devoted (primarily) to contract out-sourced machining of large sheet metal forming dies, compression molds and other large workpieces. The company continues to offer CNC programming and 3D design services as well as rapid prototyping with fused deposition modeling, reverse engineering and complete manufacturing of die-cast dies and injection molds.

Haverkamp and TerBeek started out in business as contract NC programmers, forming their own company in 1991. Before that, both had been journeyman machinists. The operation flourished, but by the mid 1990s, it became clear that the emergence of high-powered, PC-based CAM systems would make their services less in demand. However, they saw that shops in the area would continue to have bottlenecks in the programming area during peak times, but likely have bottlenecks in the machining area at the same time as well.

Haverkamp and TerBeek saw the opportunity. A job shop that could provide roughing and finishing of large die and mold components on a fast turn-around basis would be sought after. Because die and mold work tends to be counter cyclical, demand for this service would stay fairly steady, they believed. They also believed that HSM made this machining service deliverable at an attractive rate.

They were ready for HSM, too. They had the essential programming skills. They had an established customer base which included many prospects for this service. And by 1995, they had an entirely new corporate strategy. Cad Cam Services was moving into the high speed machining business.

Literally, they built this new business from the ground up.

A Facility Designed For HSM

Originally, the plan was to have one large machine for roughing and semi-finishing and another for high speed finishing. Haverkamp and TerBeek worked with machine tool importer IMTA of Rockford, Illinois, to purchase a Rambaudi Ramspeed for finishing and a retrofit Droop & Rein vertical mill for roughing.

The partners invested in not only the appropriate machine tools, but also in a new plant–a building especially designed for efficient handling and machining of large components, with the structural features that would accommodate HSM. A site in an industrial park near the Grand Rapids airport was chosen because it was accessible to large trucks moving heavy loads by highway.

The foundations for the machines were designed to support their weight yet to isolate vibrations, which can degrade finishing operations. The original two machines were situated to face each other, to make it easier to move workpieces from one machine to the other and to allow one operator to tend both machines. The 10,000-square foot shop area has a ceiling high enough to accommodate a 50-ton overhead crane with 20 feet under the hook. Underground conduits were built in for the computer network cabling that reaches every machine or workstation in the shop. The front of the building has two levels of office space, each 5,000 square feet, but Cad Cam Services currently occupies only the lower level.

Several 15-hp vertical machining centers, a toolroom knee-type mill, a ram EDM and a pair of grinders are spotted along the wall of the shop area. These machines alone would make Cad Cam Services a modern, well-equipped mold and die shop.

However, the centerpiece of Cad Cam's machining capability is the Rambaudi Ramspeed B27L. This gantry-type vertical machining center has travels of 106 by 86 by 40 inches in X, Y, and Z. The 18-hp spindle motor has a top speed of 25,000 rpm. Rapid traverse is 600 ipm. This machine is designed solely for high speed finishing and that is strictly how it used. According to Mr. Haverkamp, this machine consistently and accurately performs finish machining at 250 to 275 ipm.

Across the aisle is the three-axis Droop & Rein vertical mill with a headstock that rides on a fixed column. This 50-hp machine has a table a little longer but not as wide as the Ramspeed. Top spindle speed is 2,000 rpm. Roughing operations on this machine are often performed with 4- or 5-inch inserted face mills, so the spindle speed is sufficient to generate adequate surface cutting speed for this kind of cutter in heavy roughing cuts.

A second large machine, a LEM 93-M5 from Italian builder FPT, was purchased new and installed in late 1997. This moving column machine has a bed 196 inches long. With a universal indexing head, it can cut vertically or horizontally, with 4,000 rpm at the spindle. Most of the time, this machine runs horizontally because it can reach farther with shorter, more rigid cutting tools.

The fixed table accommodates heavier workpieces than the other roughing machine and allows greater fixturing flexibility with angle plates attached to the table. Horizontal cutting also lets gravity act as a natural chip removal aid. The FPT has a lighter duty, 30-hp spindle but with higher rpm, and it often operates at twice the feed rate but half the cut depth compared to the Droop & Rein. "In a pinch, we can do finish machining on this unit, and that versatility is important to us," comments Mr. Haverkamp.

Hog Heaven

Although these machines are not in a class with the Ramspeed when it comes to spindle speed and feed rates, they are an integral part of this shop's HSM strategy. As Mr. Haverkamp explains, "One of the keys to successful high speed machining is how well you can do roughing and semi-finishing. You have to be able to remove a lot of metal efficiently, leaving the precise stock conditions for finishing at high speed."

Roughing and semi-finishing are big jobs as well as important ones. A compression mold, for example, may begin as a billet of 4140 steel the size of two refrigerators and weighing 75,000 lbs. By the time it gets to finishing, 20,000 pounds of material will have been removed. Only 0.020 inch of stock is left for finishing. No excess material can be left in corners or along radii, so geometric accuracy is critical.

A typical roughing operation on the FPT machine begins with a 5-inch face mill with TiAlN-coated inserts. Feed rates range from 150 to 175 ipm at 450 to 500 rpm, with a 0.050 inch depth of cut. Chips fly at an impressive rate. After roughing, most components are sent out for heat treating.

On return, semi-finishing takes place. Typically, a 2-inch ball nose cutter follows for semi-roughing, running at 2,000 rpm and 125 to 150 ipm. Stepovers are in the range of 0.125 inch for contour cutting conditions where lace or zigzag patterns are followed. A cutter 2.5 inches in diameter with a small corner radius would be used for Z-level machining at similar speeds and feeds and a 0.020 to 0.050 inch depth of cut. In addition, smaller cutters may be used to clear corners.

All three large machines have Fidia control units, which simplify operator training. Although Fidia is best known for its high speed machining applications, control features make a difference for roughing and finishing, too. In semi-finishing, for example, stock conditions may be inconsistent. Where excess material is encountered unexpectedly, the operator can enter axis travel limits to block out sections of geometry for reprogramming while completing the operation elsewhere.

For this reason, semi-finishing requires greater operator attention than roughing or finishing, says Mr. Haverkamp. "The better job you do at semi-finishing, the better your finishing operations will be," Mr. Haverkamp emphasizes. "When you know that the stock remaining after semi-finishing is right, you can do much of the finishing unattended. That makes finishing a very cost-effective operation."

Only The Speed You Need

Mr. Haverkamp has one guiding principle for high speed finishing, and it may be surprising. That principle is Faster isn't necessarily better.

"We rarely do finishing at spindle speeds above 12,000 rpm," says Mr. Haverkamp, even though the Rambaudi has a 25,000 rpm spindle. "A number of economic factors have to be considered, and they vary from job to job," he explains. "For example, we get the best insert life at these speeds and we can use readily available, very affordable standard cutter bodies and tool holders."

With an inserted 1-inch ball mill, typical high speed finishing involves 9,750 to 10,000 rpm. Feed rates at 275 ipm are common, with a depth of cut at 0.020 inch and 0.020-inch stepovers. The machine's top feed rate is 400 ipm, so a higher spindle speed could be used, but maintaining the desired chip load is critical.

However, at 10,000 rpm, balanced tooling is not critical. It is sufficient, the shop finds, to use a good grade of standard cutter bodies and to check concentricity of the cutter in the spindle (a runout of 0.001 inch TIR or less is acceptable.) At higher speeds, the balance of the cutter becomes a concern. Balanced tooling is costly, requiring expensive tool balancing equipment in house or a duplicate set of bodies to alternate with those out for rebalancing.

"Conservative" HSM is not just for tooling economy. Wear and tear on the spindle also is higher at the higher speeds. After more than three years of continuous use with normal maintenance, the machine is still using its original spindle with its original bearings.

"Unattended" machining is a better bet at the slower speeds as well. Although Cad Cam Services always has a machine operator on duty, the operator doesn't have to keep a close eye on every move the machine makes. Generally, two operators handle all three machines, working as team.

In addition, the machine has no trouble holding its targeted accuracies of ±0.001 inch on contours and ±0.0005 inch on details, at the lower speed range. Yet the acceleration and deceleration of the machine are very high. It moves smoothly and efficiently.

Programming For Shop Floor Flexibility

With roots as a contract CAD/CAM house, it's not surprising that Cad Cam Services is strong in its design and tool path capabilities. The company has five full time "CAD/CAM engineers" who can design and program molds and dies. Combining the design and programming functions results in highly manufacturable molds and dies, an advantage in this fast-paced setting.

Mr. TerBeek emphasizes the importance of a good programming staff to support HSM. "Programmers have to understand the shop floor and listen to the machine operators," he says.

Tool paths for roughing, semi-finishing and finishing are prepared with WorkNC from Sescoi Inc. Gary Thelen, head programmer, favors this system because it processes tool path commands quickly. Much of this programming is automatic, he says.

And here is where shop floor experience really matters. Mr. Thelen explains: experienced programmers can review cutter path simulation and quickly identify portions of the program that are not likely to work at the machine. For example, a cutter programmed to zigzag across a contour may encounter geometry for which climb milling is essential. The passes that have the cutter going in the "wrong" direction are unsuitable. The programmer can isolate the geometry in question and apply a different cutting strategy such as one that cuts in only one direction.

In many cases, the programmer may see several ways to machine a certain section of workpiece geometry. So the several ways will be applied to create separate cutter path files. In this way, the machine operator has a choice. If one approach appears to be better than the other based on how the cutter is performing, then the tool path is ready to go. There is no delay waiting for that section of the job to be reprogrammed.

Programmers also know when to apply tool path routines especially suited to HSM. For example, "radial lead-in" and "radial lead-out" create a path that begins and ends each pass with an arc. The tool steps over and changes direction in mid air, not on the surface of the workpiece. The result is a better blend between surface patches machined with different programming techniques.

Mr. TerBeek stresses that good communication with the shop floor is essential for HSM as well as for effective roughing and semi-finishing. He considers it so important that he and Mr. Thelen wrote a software program to create a documentation page that accompanies every tool path file. Written in the HTML format, each document is much like a "Web page" found on the Internet. This page identifies the program name, describes what kind of machining strategy it follows, gives details of cutting tool setup, specifies the cusp height to be obtained, and gives all other pertinent information. The page also includes a view of the pertinent geometry so the operator can visualize the entire operation. "Our operators have final say over the order in which cutter paths will be executed, so we want them to have complete information to make good judgments," Mr. TerBeek explains. "These decisions affect the entire process."

On the shop floor, Windows-NT PCs in industrial enclosures act as terminals for the company's computer network and as local file servers for the CNCs. The PCs communicate along high-speed data lines using TCP/IP, the standard protocol of the Internet. "Essentially, the Internet is our network," says Mr. TerBeek.

New Angle On Five-Axis Machining

The Rambaudi machine at Cad Cam Services can be used for full simultaneous five-axis machining. The spindle head can be programmed to tilt and swivel as it follows a path along a contour to maintain orientation of the cutter throughout. However, the shop rarely employs this kind of five-axis machining for contouring. It is most useful for operations impossible with only three axes such as machining undercuts that follow a contour along the side of a stamping die.

The shop finds it far more practical and expeditious to use the tilt and swivel of the indexing head in conjunction with a special software feature of the machine's Fidia control. This feature, called Rotational Tool Center Point (RTCP), allows the tool paths, calculated from the center of a ball nose tool, to be directly matched to the center of rotation of the corresponding axis. This feature eliminates the need to re-post the program for tool length changes or re-establish zero position of the workpiece for every axis rotation.

For example, the programmer selects a patch of workpiece geometry, and generates a three-axis tool path as usual, but indicates to the machine operator that the RTCP function should be turned on. To execute the program, the operator uses the handwheels at the control panel to position the spindle's A and C axes, angling the cutter for the most efficient machining. With RTCP turned on, new X, Y and Z values are automatically calculated so that the center of the tool tip will follow the original tool path centerline.

Another Rougher

Partners Haverkamp and TerBeek don't regret the decision to enter the high speed machining business. Keeping the high speed finisher busy has been the biggest challenge. Eventually, they see the company possibly investing in another roughing machine–the shop can be expanded another 15,000 square feet when needed.

"We manage to keep the spindles on all three machines in the cut 85 percent of the time or better," Mr. Haverkamp reports. "That's proven to be a realistic target for the shop."

Getting Ready For HSM

How does a shop get ready for HSM? Minco Tool & Mold, a large mold shop in Dayton, Ohio, plans to install its first machine designed especially for very high spindle speeds and high feed rates later this year. In the meantime, it has been taking all the steps necessary to make a smooth and effective transition to HSM. The shop has been systematically addressing both the "cultural" and the structural changes that set the stage for HSM.

Some of the things Minco is doing in anticipation of HSM are rather innovative. Assigning one person to focus on "R & D" of manufacturing tools and techniques, is one example. Going ahead and applying HSM techniques (light cuts with small cutters at numerous, closely spaced passes) on existing equipment is another.

Designers, programmers, and machine operators are all getting a taste of HSM, experiencing some of the challenges as well as some of the rewards. This experience will be invaluable when the high performance machine arrives. Some of the bottlenecks that HSM can create also have emerged. These have been resolved with new DNC software, an upgraded computer network, and a new shop scheduling system.

Minco has a good track record when it comes to staying up with the latest machining technology. Years ago, for example, the shop was one of the first to acquire high speed graphite milling machines to streamline electrode production. In fact, the shop's steady investment over the years in the latest machining centers and EDM equipment, along with skillful management of these production resources, has made it the preferred tooling provider for quite a number of prominent consumer and automotive product manufacturers. But, as Joe Kavalauskas, VP and general manager, points out, "You can't coast on yesterday's technology, even though it is serving you well today. You have to look for the next step."

He and John Levering, director of engineering at Minco, recognized that moving to HSM was the next step, but a big one because it affects so many fundamental aspects of shop operation. "We realized that we needed one individual to organize all the things we had to learn and have in place for the next level of high speed machining and other developments."

So, about two years ago, they created the director of R & D position. Jon Allen, who had been the head of Minco's programming department for over ten years, stepped in to do the job. Mr. Allen's duties include:

• Researching HSM and collecting information about best practices by attending seminars and training courses offered by machine tool builders, tooling suppliers and universities.
  
  
• Conducting test cuts and tryouts in the shop to help prove out programming techniques and experiment with different styles of cutting tools and tool coatings.
  
  
• Capturing HSM knowledge by compiling recommendations, formulas and tables of cutting data for programmers and other users.
  
  
• Conducting in-house training classes for programmers, machine operators, designers and tool crib managers.
  
  
• Reviewing and recommending new software to help improve the manufacturing process.

Programmer Readiness

The transition to HSM can be especially challenging to CNC programmers. From his years as a CNC programmer at Minco, Mr. Allen knew how this department traditionally approached its job. The emphasis was on selecting larger diameter tools and using multiple operations to work the stock down to the finished state. This approach was demanding on operators and required several tool changes.

Now they are learning to use smaller cutting tools so that stock can be machined to net shape in fewer operations. Careful control of the stock encountered by these smaller cutters permits the highest feed rates and relieves the need for constant monitoring by the operator.

Compiling a set of guidelines for this new style of programming has made it easier to follow this regimen. These guidelines include recommendations for cutting strategies (the best way to get the cutter on and off the workpiece, for example), formulas for calculating critical machining parameters, and recommended feeds and speeds for various sized cutters for different kinds of workpiece materials. These recommendations are based on those provided by tool manufacturers but also on experience with test cuts performed in the shop. Conducting test cuts in the shop has been particularly valuable, stresses Mr. Allen. "It's how we learn first hand what works for us and what doesn't."

Having these guidelines in a kind of "HSM Programming Manual" encourages programmers to learn from each other and to program consistently from workpiece to workpiece. Consistency helps machine operators know what to expect and what to watch for as they transition to HSM.

Software for creating tool paths has been upgraded. Minco uses Unigraphics (Unigraphics Solutions, Inc., Cypress, California) and WorkNC for tool paths. Unigraphics is favored for machining small mold details and other complex geometries where controlling and manipulating tool movements is required. This software has options for how to engage and retract the tool from the cut, for example. WorkNC is favored for highly detailed parts because of its ability to monitor stock conditions, use remaining stock models to generate tool paths, and create tool path data quickly.

New programming techniques made the need for tool path verification more apparent. The company uses Vericut from CGTech (Irvine, California). Although running verification programs adds to programming time, it ensures that machine operators can run a program with confidence while attending to other duties. Verifying tool paths is a positive trade off, given the productivity it gains on the shop floor.

Tool path optimization is also being considered, but it appears to have limited value for the one-of-a-kind workpieces that Minco produces. "It's a powerful tool, but the payoff is elusive in mold work," Mr. Allen admits.

Shop Floor Benefits

Using HSM techniques even on conventional CNC machines has been much more than a training exercise. It is providing Minco with some significant benefits, such as

• Reduced machining time. The appropriate tool paths allow the cutter to run at the fastest possible feed rates.
  
  
• Better surface finish. Time needed for hand polishing is reduced,
  
  
• Less EDM. Many small details can be milled instead of EDMed as in the past.
  
  
• Less workhardening of machined surfaces.
  
  
• Improved tool life. More consistent cutting conditions, consistent chip loads, and light cuts have extended the life of cutters, dramatically in some cases.
  
  
• More unattended or lightly attended machining. As a by-product of carefully verifying tool paths, operators can safely tend to other matters while a part is being machined at high speeds.

"Although these gains are modest, they're real," says Mr. Levering. "And our expectations for much bigger gains from a true high speed machine are more realistic because we know we'll be able to take advantage of this technology right away."

Looking Beyond HSM

With HSM just over the horizon, Minco is already looking ahead. According to Mr. Levering, the company's goal is to model the entire mold building process in software with 3D solids. This model would include not only how machining will take place, but also how all the components are to be assembled and how the finished mold will function for the customer. Simulation will allow this model to be tested and verified in virtual reality, yielding not only a better mold design but also a highly coordinated manufacturing strategy.

"With techniques like HSM, it will be harder and harder to find time savings on the shop floor so the search for efficiency will shift elsewhere," Mr. Levering says.

And it might be added that the search shifts but does not end.

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What Carbide Coatings Fit Best?

Coated cutting tools are composed of two basic elements: a substrate and coating. Selecting the right combination of substrate and coating for the material being cut is a critical early step in formulating a successful high speed machining process.

In high-speed cutting, reduction of vibration or chatter sources is a key consideration. This is why a more rigid substrate is generally recommended for high speed machining operations.

Rigidity is basically a measure of the density of carbide in the substrate material. For high-speed applications, relatively low, 6-8 percent cobalt content with micro-grain sized carbide, is a good starting point for selecting a substrate material.

A second consideration for cutters is what's the best coating for high speed machining? Fundamentally, coatings perform the double duty of insulating the substrate from heat and reducing friction between the cutter land area and the chip.

In high speed machining most of the heat is carried within the chip. Therefore the role of the coating in high speed machining is to aid in evacuation of the chip as quickly as possible so the heat can't transfer into the tool.

A low coefficient of friction between the workpiece material and the tool coating is the goal for rapid evacuation of the hot chip. Some coatings (TiAlN, for example) contain a lubricant as the top coating to help reduce this chip to tool friction.

"It's a misconception that the lubrication function of coatings such as TiAlN is critical at the tool edge," says Ron Field, manager product application and development at Millstar (Bloomfield, Connecticut). "Actually, the lubricant is gone from the cutter edge almost instantly upon contact with the workpiece. The advantage of this kind of coating is in its ability to lubricate the sliding motion of the chip over the insert so it can clear the cutter smoothly and quickly as possible with the least amount of heat transfer."

So what's a good starting point for high-speed application of coated tools?

Below is a chart that can guide you to match the more common tool coatings with general application parameters.–GCK

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