Milling of Hard Materials
Mr. Corey Greenwald ran into this problem. He understands that milling hard materials has become a major issue when producing injection molds in the factory. He knows how to do it in a shop he created himself?Hard Milling Solutions, Inc.? with its unique? Hard Milling Solutions, Inc. s unique method of milling hard materials, effectively, consistently and with confidence. As a tool and die shop, he understands the problems of milling hard materials that must be encountered, and it would seem that they would need such a specialized machining shop in order to solve the problems of milling hard materials that they neither have enough time to machine on their own machines, nor are they prepared to mill hard materials on their own equipment.
Unfortunately, when it comes to milling hard materials, many mold shops are not prepared. They continue to use traditional methods of mold machining. They spend a lot of manual time polishing the molds after they are finished machining them, and then they spend a lot of time trimming, testing, and blending the molds point by point.
Mr. Greenwald said: ? These extra added hours make a big difference between competitive and non-competitive mold industries in the global marketplace," said Mr. Greenwald. He concluded: ? If these mold shops don't change their machining methods, it will be very difficult for them to continue to survive.?
Mr. Greenwald is convinced that the most important issue now is to address the milling of hard materials, especially for small molds and core inserts. He said that the use of hard material milling technology can help shops to start machining hardened steel molds, and the surface of the mold cavity no or little need to carry out polishing and other operations. This allows them to eliminate several time-consuming and labor-intensive machining processes in mold machining production. He added that after the adoption of hardened material milling technology, the level of machining of the other side of the mold's closed face can be improved, so that it can achieve ? Negative stock? Conditions. This means that a very small gap is left between the mating surfaces of the mold. This results in significant savings in final commissioning time and reduced labor intensity of the die when it is first placed in the press. Also, the life of the die is increased and the speed of the run is increased.
Mr. Greewald said: ? Properly applying hard material milling technology, the mold shop is able to machine the mold parts directly from the machine tool, then assemble them and finally install them on the injection molding machine, so that the first injection can produce a very good part. After the use of hard material milling technology, mold shop from the injection mold processing and commissioning process to save a lot of time and cost.
So what does it take to adopt hard material milling? According to Mr. Greenwald, the first step in adopting hard material milling technology is to have a new way of thinking about how to machine metal. Milling in hard materials requires a machine tool, a tool and toolholder, and a program. All of these technologies must be in place in a timely manner and should ideally work together in a comprehensive manner.
Mr. Greenwald said: ? As long as everything is executed smoothly and you know how to work with each other, hard material milling can be a reliable and productive process," said Mr. Greenwald. As it turns out, the two vertical machining centers used in his workshop, which are dedicated to the milling of hard materials, actually run 24 hours a day with very little human involvement.
The Birth of the Die Shop
How did Mr. Greenwald become a respected practitioner and a strong advocate of milling in hard materials? Here's an unusual story. Initially a production engineer, he later dabbled in mold shop management at a mold factory outside Detroit. One of the projects he went through was the installation of a machining center for milling hard materials for hot forging and forming stamping dies for metal connecting rods.
The results were impressive. Production of the stamping die was reduced from 30 days to 6 days, which was far more than the shop owners had hoped for, and it was exactly what they had asked for. Moreover, Mr. Greenwald even found a more promising and cost-saving milling technique for hard materials.
A friend of Mr. Greenwald's in the Detroit mold industry, Mr. Graig Sizemore, was also very interested in this hard material milling technology. His plant, Cut-Rite EDM, specializes in advanced wire EDM machines, and he was also looking for a hard material milling technology to complement or replace EDM machining technology. With Mr. Sizemore's support, Mr. Greenwald opened Hard Milling Solutions, a hard material milling company in Shelby, Michigan, about 30 miles north of Detroit. It was adjacent to Mr. Sizemore's shop. Although Mr. Greenwald did not have the training as a machinist or toolmaker that Mr. Sizemore had. His only experience is that he has worked closely with his former employer in hard material milling technology.
It was this that encouraged Mr. Greenwald, who as a production engineer knew enough about milling in hard materials to realize that he had to work hard. The task before him was that he had to work hard to learn the practical knowledge of how to mill hard materials and how to equip a shop so that it could work efficiently. To that end, Mr. Greenwald contacted Single Source Technologies in the Auburn Hills, Michigan, area, which is a distributor of machining centers and EDM machining equipment supplied by Makino (in the Mason, Ohio, area) and other fabricators.Single Source Technologies has been in the Detroit Metro area to aggressively market high-speed machining and hard material milling machine tools, and along with its previous owner, helped Mr. Greenwald establish a hard material milling facility.
He attended a shop floor training program in high performance machining technology at Single Source Technologies, which set him up for success in hard material milling technology. When Hard Milling Solutions opened for business, one employee was responsible for operating a Makino V56 machining center.
Although Mr. Greenwald started out with a little help, he soon realized that hard material milling still required him to break new ground on his own. Since the shop was set up specifically for hard material milling, he began to mill hard steel up to 60 HRc. Some of the machining data sheets supplied by Single Source Technologies provided the basis for his work, but most of the work required hands-on experimentation and trial cuts.
Today, most of the work in the shop is machining plastic injection molds, most of which are made from H13 and S7 tool steel. The shop also specializes in the machining of medical device parts, and also includes forging dies as well as mold parts, casting mold cavity inserts, powder metallurgy parts and any other parts up to a hardness of 65 HRc. A V56 vertical machining center has been added to the shop's machining equipment, and Kevin Hunter, a tool and die maker with 30 years of experience, has been hired to operate the machine.
Because his shop specializes in milling everything in hard materials, Mr. Greenwald says he's become almost a high-level expert in this area. Selfishly and altruistically, he'd like to see hard-material milling technology become widely used by U.S. moldmakers. As more mills invest in this technology, he's certain that their company's workload will grow, especially on difficult jobs. More importantly, he sees hard material milling technology as the future of U.S. mold manufacturing. He said: ? Faced with more and more molds imported from low-wage countries, there is really no better way to make mold manufacturing profitable.?
A mold block made from H13 hardened tool steel, produced by a Makino V56 vertical machining center. For this machining project, a 1.0mm diameter spherical end mill was used with a length to diameter ratio of 12:1, which is the smallest machining tool available, and the use of this mill resulted in a significant reduction in the amount of machining done on the EDM machine, and eliminated the need for the customer to polish the machining process.
Suitable for processing in the walnut shell hard material milling technology
Hard material milling technology is a branch of high-speed machining technology. The core of high-speed machining technology is to carry out a number of light cutting in close steps, so that the machining surface leaves only a very small trace of the tool machining. The goal is to make the machined surface significantly reduce the subsequent machining requirements. For the tool, in order to achieve effective chip loading, the tool feed rate and spindle speed must be greatly increased in order to exceed the normal machining speeds applied in conventional machining processes, hence the name of the process as high-speed machining. The high speed feed also results in a much higher tool travel, so the workpiece is machined faster than it would be using conventional methods.
Milling in hard materials is another step forward than the concept of high-speed machining. The combination of high-speed light cutting and high-speed spindle can effectively cut and process hardened steel, which can maintain the original characteristics of the material. At the same time, the use of small-diameter, round-ended face milling cutters for closely spaced high-speed machining produces a surface finish that approaches the level of hand grinding and polishing. (According to Mr. Greenwald's report, surface finishes of 10 to 12 rms can often be achieved using hardened material milling techniques when necessary.) Because the steel is machined hardened, there is no need for later processes such as heat treating, stress relieving or grinding. What's more, this process can also replace many EDM machining processes that are extremely expensive to process.
Another advantage of using hard material milling technology is that it can guarantee very precise tolerance dimensions (?0.0004in), which is invaluable for mold machining. The machined molds do not need to be trimmed by hand point by point. Moreover, because the mold is machined to a point where no manual trimming is required, the geometry of the mold exactly matches the CAD design model. Moreover, the mold mating surfaces are machined to a level that does not require any trimming. The concept is to machine a precise and slight point along the parting line, generally at one end of the mold cavity, so that it is slightly below the nominal size. This allows for a small gap between the mold contact surfaces when the mold halves are closed. However, this gap is very small (typically only 0.008in) so that, when injected, some of the plastic can be allowed to flow out, although the mold remains effectively closed. Also, this gap allows air to be removed from the gap when molten plastic is injected into the mold, without the need for prior ventilation. Small contact pads on the corners continue to retain this gap, which would otherwise prevent the mating surfaces from contacting each other. Normally, the problem of interference between these mating surfaces would have to be solved during the point-by-point trimming and matching process, which is not a problem now. When the core and cavity are closed, this gap eliminates the interaction between the surfaces, so the parting surfaces are completely protected and the parts produced do not have flying edges.
Efficiency from the tool tip
Mr. Greenwald said that, like any CNC machining process, hard material milling depends on a high-performance machine tool, the right tool and shank system, and an effective tool-running program. However, unlike other machining processes, the interaction of these elements has a complex dynamic relationship, but it is certainly not insurmountable, and new users can change the relationship and solve the problem if they are determined and not afraid of the difficulties.
He noted: ? Mastering milling in hard materials can be challenging. But there is a way to center your thinking around that goal.? The way to understand hard material milling technology, he says, is to know how to protect the tip of your tool.? Concentrate on trying to make the tip of your tool safe and effective for cutting. If one understands this, then one is able to work against the grain, and one understands the requirements of the system and how they interact with each other. If all of this is managed in an organized manner, then the tip of the end mill can be fused to the material being machined and cut freely. Everything else can be executed smoothly and in an orderly fashion.? If any part of the system is neglected or interrupted, he adds, then the tool's tip suffers and the entire process fails.
The cavity surfaces of this mold are milled entirely in the hardened state, resulting in a surface finish of 12 rms, which eliminates the need for further machining by a clampman, thus saving point-by-point trimming to match machining time. The matched core also adopts hardened material milling technology. After machining, the closed face on one side of the cavity was machined to an accuracy of 0.0008 in. Using this tooling, the part was produced well the first time
Mr. Greenwald lists a few key factors to keep in mind when starting with end mills.
Tooling: Hard Milling Solutions generally uses round-ended end mills for roughing, semi-finishing and finishing. Round end mills with two chipformers are used exclusively for finishing, and these finishing cutters are a critical factor in milling hard materials. Round end mills for finishing must fulfill two key requirements: the tool must have a near-perfect radius and a virtually flawless cutting edge. The radius accuracy of the cutting edge must be extremely high so that neither a high nor a low chipformer will cause an uneven state of metal removal that can affect geometry, reduce surface quality and tool life, and end mills used in the shop for finishing should have a radius accuracy of at least ?5mm (?0.0004in). Occasionally, 0.012in-diameter round-ended end mills with a radius accuracy of ?0.0002in have been used in the shop.
The cutting edges of chipformers are bound to show some trace amount of chips, fractures or other irregularities. The presence of these defects indicates that accelerated wear occurs when they come into contact with the workpiece. This situation can lead to the appearance of a rougher finish and shorten the life of the tool. Tool life is a very critical factor because when the machine is in unmanned operation, the shop is completely dependent on the end mill being able to work for the desired duration.
Suppliers such as OSG Corporation (in Glendale Heights, Illinois) and NS Tool (distributed in North America by Single Source Technologies, Inc.) can provide cutters that meet the above specifications, but at a much higher price than standard cutters. The use of this quality level of tooling is absolutely essential and cost should not be an issue, said Mr. Greenwald. These tools are the foundation of mold machining, using them eliminates the need for mold polishing and point-by-point dressing of the mold, and are a very important investment in the hard material milling process.
Shanks: Shanks protect the radius and edge quality of end mills, and Mr. Greenwald is confident that hot-sleeve mounted shanks with HSK interfaces provide the best protection. The use of hot-sleeve clamping has an extremely low offset, superior to any clamping method available today, minimizing the eccentric rotation of the tool. This is because offset errors can cause excessive cutting in one chipformer of the tool, increasing the cutting load on that chipformer, which can shorten tool life.
Hard Milling Solutions utilizes the Haimer Hot Sleeve Mounting System. According to Mr. Greenwald, it takes less than a minute to change and reclamp the tool and then return to the automatic tool changer. In his experience, the use of the hot-sleeve clamping method results in a clamping offset accuracy of less than 0.0001in.
The company only purchases HSK shanks that have been balanced to prevent further unnecessary handling, and the HSK interface is a mandatory use because it is more robust, more accurate, and allows for safer clamping to the spindle than other taper sleeve clamping methods.
Spindle: Just as the shank is used to protect the cutting edge and precision radius of the end mill, the spindle is used to protect the tool and shank assembly as a whole. Of course, the spindle should also be designed to provide the high speed rotational performance required for milling of hard materials. It is important to control the heat and vibration generated by the spindle. Direct-drive spindles (without gear or belt drives) and their internal cooling are specifically suited for milling of hard materials.
These two V56 vertical machining centers, installed at Hard Milling Solusions, have spindles with these characteristics. Both machines have spindle speeds of up to 20,000 rpm.
Machine construction: The question of spindles on this type of machine can be discussed separately from the overall construction of the machine, even though they are an integral part of the machine, says Mr. Greenwald. There is no doubt that a machine used for milling hard materials must be very rigid. And, of course, the overall accuracy is very important.
The thermal stability and rigidity requirements of the company's machines were designed and built specifically for their application. Certain structural features of these machines differ from general-purpose machines, including:
● Equipped with a heavy-duty base and column (these machines weigh more than 9072kg).
● Equipped with a center-cooled spindle.
● Linear roller bearings are fitted in the spindle housing.
● Double supported screws.
Mr. Greenwald said: ? In my opinion, the most important thing is to minimize the vibration and cumulative error of the machine, which may affect the cutting accuracy of the tool. Whether we are using a 40 ipm 0.5mm end mill or a 380 ipm 6mm end mill, the accuracy is always 1/10th, but the variable effects of the system are magnified, so each of these factors has to be strictly controlled," Mr. Greenwald said.
The CNC processor and attendant system on these machines is also specifically designed for milling of hard materials. The features are as follows:
● Two RISC processors, one of which is dedicated to the necessary conversion of data to turn the programmed tool machining routes into service instructions.
● 120 program block look-ahead function to avoid upstrokes or downstrokes of the tool stroke.
● Efficient interpolation of high-resolution encoder feedback for precise positioning control.
● Encoder capable of feedback in 50nm increments.
Programming software
However, the performance of a rigid, responsive machine depends on the programming inputs to the NC. In hard material milling, Mr. Greenwald is relatively uneducated in this area, but has studied the toolpaths that drive the CNC.
He explains: ? The question goes back to the tool tip issue that we explored at the beginning.
When machining hardened materials, the ideal fillet radius and cutting edge of the tool is important for safe, accurately positioned cutting, but it also depends on the machine tool? smooth operation? It is for this reason that most CAM software is not suitable for milling of hard materials. The algorithms that generate toolpaths are not designed specifically for this type of hard material milling, which requires smooth, precise movement to tolerance, allowing the mold shop to bypass the full machining steps of the mold making process.
As Mr. Greenwald sees it, typical CAM software is designed to generate toolpaths very quickly, so most system applications can effectively generate coded processing shortcuts. Such software is very useful for normal milling operations, as these shortcuts do not generate great efficiencies. But the problem is that, according to him, this advantage turns out to be a disadvantage in hard material milling.
They apply CAM-TOOL as their programming software in their workshop. This software was developed in Japan and is distributed in the United States by Graphic Products of North America, Inc. The company is based in Windsor, Ontario, and has a new president named Randy Nash, a sales and applications specialist, Mr. Chris Renaud, who has worked with Hard Milling Solutions and is therefore very knowledgeable about the use of this software. Mr. Corey's shop represents a need in hard material milling, especially in the die and mold industry," he said.
According to Mr. Renaud, the software cannot create a triangular approximation of a surface mesh with contoured geometry because these approximations affect the millionths-of-an-inch accuracy required for hard material milling. Instead, the software calculates toolpaths based on individual measurement points taken directly from the geometry. In fact, he says, these points can be connected by mathematically defined curves that best fit the points, and then by straight line segments that connect the center points of each triangle distributed across the contour map's mesh, which are then plotted against each other. Since the resulting path is a series of curves, the motion defined in the tool machining route lacks steep variation in the direction created by the short line segments. Attempting to have a machine tool with micron-level feedback resolution move unambiguously along these line segments is bound to produce undesirable results and put the machining tool at risk.
Following Mr. Renaud's thinking, there are other requirements for effective tool machining lines for milling in hard materials. They must:
● Control how the tool moves in and out during the cutting process.
● Keep the chip load constant by controlling the degree of contact between the cutting edge and the workpiece material.
● Provides constant stock conditions for each subsequent roughing or finishing process.
Algorithms for accurately analyzing geometry in a machinable manner are key to achieving these goals, Mr. Renaud said. Tool routes suitable for milling hard materials cannot create conditions that are the opposite of what the tool is capable of doing in order to make the mold's machining accuracy and surface finish fully compatible with the press.
A complete and accurate model of the cutting tool helps the programming software to check for possible collisions or planing grooves in the workpiece with up to 100% accuracy, enabling unmanned cutting.
Focusing on all the advantages
Milling technology for hard materials achieves the desired reliability when there are no weak links in the chain connecting the tool tip to the programming software. It is for this reason that unmanned operation becomes practical. This is what Hard Milling Solutions does on a daily basis," says Mr. Greenwald. Each of our machines, on average, works more than 100 h per week, mostly unmanned. About 90% of the machining work is completely unmanned. He says: "If you have to stand in front of the machine, you can't do it. If you have to stand in front of the machine, then there is something wrong with the programming of the machine. It's only during test cuts with smaller tools, or when machining hard materials he's never encountered before, that he, or Mr. Hunter, will stand in front of the machine and watch the material being machined, so that he can learn and master the system's performance.
He emphasizes, however, that unmanned operation is not an option or a bonus for milling hard materials on a daily basis. The purpose of running the machine almost 24/7, he says, is to pay for the machine itself, so that there is a healthy payback on the investment. Unmanned operation further reflects the machine's goal of low labor input and high output. And the process also replaces hundreds of hours of mold grinding and point-by-point trimming by a clampsman, which requires expensive labor.
The benefits are obvious to customers who purchase molds, which is why they are so eager to make hard material milling an essential process for mold makers. Milling with hard materials can replace some of the most expensive and time-consuming machining steps in the mold manufacturing process, eliminating or greatly reducing the need for electrode milling, EDM machining, grinding, polishing, and point-by-point dressing on presses. For this reason, Mr. Greenwald considers hard material milling to be a critical technology that will make or break a U.S. mold shop.
Their existence ensures his survival, knowing that there are more milling tasks in hard materials waiting to be accomplished, and more unusual difficulties awaiting him along the way.
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