And it plays a vital factor in successful turning of hard workpieces. When it comes to insert geometry, the experts agree that with respect to insert corner geometry, a wiper configuration allows for significant process improvements and productivity gains.

According to Howard, coatings also will help protect the insert edge from the heat generating during machining and will provide an excellent surface finish on the part. For example, a physical vapour deposition coating adds wear resistance, helps to improve cutting conditions, and extends the tool life.

“An important element of CBN is the wiper configuration, which helps greatly to improve surface finish and quality similar to grinding,” said Guruswamy. “Wiper inserts enable work with a higher feed and good surface finish on the workpiece.”

“This is because both CBN and ceramic inserts can tolerate high cutting temperatures. Typically, the heat generated when performing HPT operations is distributed into the chip (80 per cent) and insert (10 per cent), with only the remaining 10 per cent entering the workpiece.”

Beyond the cutting strategy, machining conditions also come into play. Having good machining stability and rigidity will help eliminate vibration, which is absolutely critical in HPT.

“Flank wear is the type of wear you would want to see on any insert,” Burton added. “It is the most predictable and progresses over time. Premature flank wear is not ideal, though. Large flank wear has a negative effect on surface integrity and dimensional accuracy. Even though wear is complex, there are ways to control it and maintain a consistent and reliable machining operation.”

Gang milling achieves multiple surface operations simultaneously by feeding the table against multiple cutters with opposing helical teeth, optimising machining time and efficiency, particularly in mass production.

Form milling produces irregular surfaces or contours using specialised form cutters, accommodating convex, concave, or custom shapes.

“Clamping and alignment of the workpiece are also crucial,” said Burton. “As a guide, a workpiece length-to-diameter ratio of up to 2-to-1 is normally acceptable for workpieces that are supported on only one end. If there is additional tailstock support, this can be extended.”

One of the major advantages of HPT as a process is the elimination of coolant. This is because both CBN and ceramic inserts can tolerate high cutting temperatures. Image courtesy of Sandvik Coromant Canada

Computer Numerical Control (CNC) milling machines represent the contemporary evolution of milling technology. They operate by interpreting user-input commands or G-codes to execute machining operations automatically, significantly enhancing productivity and operator safety. While CNC milling machines are primarily employed in industrial settings, scaled-down versions tailored for small shops and DIY enthusiasts are also accessible for smaller-scale applications.

For productivity, a one-cut pass would be the best option, especially because HPT should be more of a finishing process rather than a roughing process. After the correct cutting strategy is deter-mined, proper programming will help ensure long tool life. This will be enhanced if proper soft machining strategies have been put in place.

Sawmilling employs a generously sized cutter with a slender profile, designed for precision machining of narrow slots or the division of a workpiece into two segments. To prevent undesirable heat and stress buildup that can lead to tool and workpiece damage, the feed rate and spindle speeds are intentionally maintained at a lower pace compared to other milling processes. Typically, this operation is carried out on horizontal milling machines.

Careful preparation of the component in the soft (unhardened) state will significantly increase success in HPT. One way to achieve this is through machining as close as possible to the final dimension, leaving only a few 10ths of an inch for finishing. HPT requires a relatively small depth of cut, so tight dimensional tolerances in the soft machining stage will make all the difference.

“The toolpath is key to an efficient cut and achieving the best insert tool life,” said Howard. “It’s important to create a program that eliminates cutting conditions that will prematurely wear or reduce tool life. Key aspects to focus on are the entry and exit angles of the insert, sudden directional changes, and interrupted-type toolpaths.”

Guruswamy noted that machining on an older machine versus a new CNC machine really makes zero difference in the overall output. As long as there is a high degree of stability and no vibration, HPT will be effective.

Hard-part turning (HPT) has many advantages to help shops achieve better accuracy more efficiently in hardened steels from 45 to 70 HRC, superalloys, sintered materials, and grey cast iron. In recent years turning has eclipsed grinding as the preferred method for producing parts in their hardened state. As a process, it has been proven to reduce machining time and costs by 70 per cent or more over more traditional methods.

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Tool wear depends on a number of factors, including workpiece material, CBN grade, cutting conditions, edge geometry, and machine stability. In HPT, the most common forms of CBN tool wear are crater and flank wear.

“The insert geometry, as with any operation, should be chosen based on the strongest geometry allowable for the specific application while considering the cutting efficiency,” said Howard.

HPT demands CBN or ceramic grades to ensure successful processing. CBN inserts are designed to withstand the high cutting temperatures and forces associated with HPT while retaining their cutting edge. Image courtesy of Iscar

Angular or bevel milling machines have flat surfaces at angles different from the cutter's rotation axis, utilising single or double angle cutters as needed.

“HPT provides flexible machine utilization as the same machine can be used for external and internal machining,” said Kevin Burton, turning product and industry specialist, Sandvik Coromant Canada, Mississauga, Ont. “Complex component shapes can also be machined in a single setup.”

End milling concurrently generates peripheral and face milling operations, creating vertical, horizontal, or angled surfaces and is useful for slotting, grooving, and keyway milling.

“A one-cut strategy is feasible for both external and internal operations,” explained Burton. “However, a stable setup is important, and tool overhang should not exceed the bar diameter in internal turning (1xD). The big advantage of a one-cut strategy is minimal cycle times, although the trade-off is reduced tool life and potential difficulties in meeting stringent dimensional tolerances.”

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“CBN is, however, the ultimate cutting tool material for HPT,” said Burton. “Modern CBN inserts offer up to eight edges per insert and built-in security that further enhances the bondage of CBN cutting tips on negative inserts.”

T-slot milling occurs in stages, initially using an end milling cutter for plain slots, and subsequently, a T-slot cutter enlarges the slot and mills its bottom face.

When it comes to HPT, insert geometry should be chosen based on the strongest geometry allowable for the specific application while considering the cutting efficiency, said Howard. Image courtesy of NTK Cutting Tools

Manual milling entails the use of hand-operated milling tools to carry out the desired machining tasks, necessitating the manual adjustment of hand wheels and tool changes, which means they lack automation and advanced programming functionalities such as automated tool changes and feeds.

Gear milling involves shaping various gear types on a milling machine using form relieved cutters that match the gear tooth profiles, typically done with equally spaced gear teeth and a universal dividing head for indexing.

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An alternative is the two-cut strategy, which is ideal for unattended machining of high-quality surfaces. In this scenario, both inserts should have a wiper geometry. One of the main advantages of this strategy is it provides higher security, closer tolerances, and potentially longer runs between tool changes.

Applying regular cutting insert grades to hardened steel will simply fail. HPT demands CBN or ceramic grades to ensure successful processing. CBN inserts are designed to withstand the high cutting temperatures and forces associated with HPT while retaining their cutting edge. Ceramic grades can be used on materials between 50 and 60 HRC when surface finish demands are moderate. Ceramic grades offer good heat resistance and high wear resistance and can prove beneficial for light, continuous HPT.

“When using standard flat top CBN without chipformers, long, unbroken chips are often produced,” said Guruswamy. “It can harm the workpiece surface finish and even interrupt the machining process. CBN inserts with chipformers will provide improved chip control at various depths of cut.”

The experts agree that the use of features such as chamfers and radii on the part will help with entry and exit paths to maximize tool life, especially because sharp corners can damage or break the cutting edge. Also, operators should avoid burrs and use wiper inserts to achieve superior surface finish.

Milling operations are vital in manufacturing, shaping raw materials into precise components. Using rotary cutters, mills remove material from workpieces, achieving desired shapes and dimensions. These versatile processes find applications in industries ranging from aerospace to automotive, playing a fundamental role in modern production.

“Concentricity and perpendicularity of the part will be much better in parts that are finished using HPT with a single setup,” said Ashok Guruswamy, product manager, grip/turn/thread, Iscar Tools Inc. Canada, Oakville, Ont. “Machining a taper and radius profile surfaces and complex contours are possible in HPT. In grinding, this would require a custom dressed wheel.”

Beyond grades and geometry, edge preparation selection is important for protecting the cutting edge and achieving good tool life. However, Guruswamy added that sharp edges are generally not recommended on CBN since a sharp edge can chip or break. In situations where cutting forces need to be reduced because of an unstable setup like in internal boring applications, machine limitations, or workpiece clamping, a sharp edge can then be used.

CBN grades are available for continuous cuts, light interruptions, light and heavy interrupted cuts, and heavy interrupted cuts and unstable conditions, depending on what the project calls for. The binding force between particles and grain size determines whether the insert is suited for hard or tough applications.

Side milling is the preferred choice when you need to machine a flat vertical surface onto your workpiece. In this process, a side milling cutter is affixed to the arbour of a horizontal milling machine, and the workpiece-holding table is elevated to the desired depth of cut before being fed into the cutter. Additionally, the cutter is equipped with helical flutes to facilitate chip evacuation during machining, and its diameter is typically smaller than the width of the workpiece, similar to that of a drill bit.

Last but far from least among HPT considerations is a machining strategy, with the choice made very often a balance between accuracy and productivity.

Not only that, but there are significant environmental concerns when it comes to grinding, particularly in relation to the slurry left behind. The mix of coolant and grinding dust needs to be disposed of properly, and in many cases separating the coolant from the mixture can be time-consuming. Many shops will look toward HPT as an alternative to grinding because it can be dry-machined, eliminating this challenge. However, HPT has its own set of demands and challenges that need to be overcome before it can be effective. Choosing the right cutting tool for the job is necessary.

Conventional milling, or up milling, involves cutter rotation opposing the feed direction, yielding longer chips and increased material removal, but also higher vibration levels, shorter tool life, and greater workpiece clamping force. On the other hand, climb milling, or down milling, aligns cutter rotation with the feed, producing shorter chips, reduced vibrations, smoother surface finishes, longer tool life, and reduced clamping force requirements.

Milling machine operations are machining processes that involve the use of milling cutters to remove material from a workpiece's surface to achieve specific shapes and dimensions. These operations offer a diverse array of techniques for shaping, cutting, and machining materials with precision and versatility. These operations allow engineers to create intricate components, precision parts, and a wide range of products.

“Also, in many aerospace applications it is a requirement to use coolant to ensure surface integrity of the component,” Burton said.

Milling operations refer to the machining techniques wherein rotary cutting tools, typically equipped with multiple teeth or edges, are employed to precisely shape and remove material from the surface of a workpiece, resulting in the creation of diverse shapes, contours, and dimensions. These operations are integral to manufacturing and play a pivotal role in transforming raw materials into intricate components and finished products across various industries.

Primarily designed for pocketing applications with optimized geometries for ISO S, M and P. CoroMill® MH20 ensures secure and vibration-free machining with long overhangs thanks to its light-cutting action in combination with a robust shank design.

Thread milling is a machining process used to create threads within a pre-existing hole in a workpiece, employing specialised tools known as thread mills. This operation is compatible with both vertical and horizontal milling machines. Unlike tapping, thread milling is particularly well-suited for generating threads in holes larger than the diameter of the milling tool. Thread milling entails three distinct movements: one for rotating the thread mill, another for its axial movement into and out of the hole, and a third for advancing the workpiece to facilitate the cutting of the desired threads.

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Modern CBN inserts offer up to eight edges per insert and built-in security that enhances the bondage of CBN cutting tips on negative inserts, said Burton. Image courtesy of Sandvik Coromant Canada.

Guruswamy added that another benefit of dry machining is that during the turning process, the material becomes slightly softer as it heats, making it easier to machine. He noted that dry machining is not always advised, even though it offers economic and environmental benefits. For example, in cases of continuous turning, when high surface finish is required, coolant should be applied.

Lindsay Luminoso, sr. editor/digital editor, contributes to both Canadian Metalworking and Canadian Fabricating & Welding. She worked as an associate editor/web editor, at Canadian Metalworking from 2014-2016 and was most recently an associate editor at Design Engineering.

This article shall enlist and explain different types of milling operations. This topic in mechanical engineering is important for your upcoming examinations like SSC JE ME and RRB JE Mechanical Engineering.

Howard added that cubic boron nitride (CBN) inserts associated with HPT are much more cost-effective than purchasing a cylindrical and centreless CBN diamond grinding wheel. Although the grinding process can achieve higher tolerances, cylindricity, and roundness, there are a number of benefits with opting for HPT.

“With crater wear, you’ll want to try and get the crater about a third or two-thirds the way up of the negative land that’s put on the inside of the insert,” said Burton. “When the crater gets too big, the edge will basically chip away and break, because it weakens the edge.”

Flank wear can be limited by increasing both the cutting speed and feed.DRY MACHINING“Of course, one of the major advantages of HPT as a process is the elimination of cool-ant,” said Burton.

Straddle milling is the process of creating two vertical flat surfaces on both sides of the workpiece using two side milling cutters separated by collars, commonly applied for milling square and hexagonal shapes.

“Successful HPT demands not only suitable cutting tools, but also the right conditions, preparation, and environment,” said Guruswamy. “The soft state is the state in which the workpiece material is yet to be hardened to its final hardness (around 20 to 40 HRC).”

The high hardness of CBN enables it to machine hard materials at high speeds and feeds. However, successful HPT demands not only suitable cutting tools, but also the right conditions, preparation, and environment. Image courtesy of Iscar.

Various types of milling operations cater to diverse machining needs. Face milling creates flat surfaces, while end milling produces slots and pockets.

Plain milling entails the creation of a flat horizontal surface parallel to the cutter's axis using a plain milling cutter.

This discussion encapsulated different types of milling operations. We suggest that you appear in the SSC JE Mechanical mock tests and SSC JE ME Previous Years Papers. Also, get enrolled in the AE/JE Mechanical coaching to get a firm grip on the subject.

“A part design with specific features can be generated with the standard tooling and insert geometries,” said Steve Howard, engineering and marketing manager, NTK Cutting Tools, Wixom, Mich. “A grinding wheel applied to the same part would need to be specially made with features dressed into the face of the wheel, only adding to the already costly tool. In certain circumstances, where very fine finishes are required on the part surface, grinding will be necessary.”

He also noted that HPT shares a lot of basic principles with soft turning, so machinists can easily transfer their knowledge and skill from one to the other. Alternatively, grinding requires very specific skills and an experienced machinist to perform similar tasks.

Cam milling is a specialised machining technique primarily employed in the fabrication of cams. This method utilises a universal indexing head to both rotate and position the workpiece, allowing the cutting tool to remove material and achieve the desired cam shape. The cam axis can be adjusted within a range of 0 to 90 degrees to achieve the desired cam profile.

The two-cut strategy is ideal in situations where too much material was left on in the soft stage. If an operator is trying to remove too much material with one pass, the cutting forces and pressure involved could be detrimental. In this scenario, a two-cut strategy could be a good option.

Milling operations, based on milling mechanisms, can be categorized into two main types: conventional milling, where the cutter rotates against the workpiece's feed direction, and climb milling, where the cutter moves with the feed direction, impacting tool wear and surface finish.