Regular fly cutters (one tool bit, swept diameter usually less than 100 mm) are widely sold in machinists' tooling catalogs. Fly bars are rarely sold commercially; they are usually made by the user. Fly bars are perhaps a bit more dangerous to use than endmills and regular fly cutters because of their larger swing. As one machinist put it, running a fly bar is like "running a lawn mower without the deck",[2] that is, the exposed swinging cutter is a rather large opportunity to take in nearby hand tools, rags, fingers, and so on. However, given that a machinist can never be careless with impunity around rotating cutters or workpieces, this just means using the same care as always except with slightly higher stakes. Well-made fly bars in conscientious hands give years of trouble-free, cost-effective service for the facing off of large polygonal workpieces such as die/mold blocks.

By eliminating the need to change out one mill for another and employing more cutting-edge techniques, today’s machinists can go faster, which leads to increased productivity.

Tonne agreed with Strauchen that modern CAM software is what helps make possible processes like trochoidal milling and high dynamic milling. “CAM software has gotten really good at high dynamic milling, where it’s managing the chip thickness,” he said. “So, you can use that in a roughing operation.”

There’s even more encouraging news for smaller shops when it comes to multi-flute end mills and CAM software. “They’re great tools for low power and smaller taper machines like 40-taper because with multi-flutes we’re taking deep axial cuts and balancing the radial forces by using the length of the tool for stability,” said Matt Clynch, national product specialist-milling, Iscar USA, Arlington, Texas. “With those smaller taper machines, if we take deep radial cuts with the large widths of cut it would start to bend and the assembly topples.”

“There are many modern machining techniques and strategies that really turn [that wisdom] on its head,” said Strauchen. Now, faster machines with more horsepower and faster, more precise spindles have made possible aggressive machining strategies like high-efficiency milling (HEM), also known as dynamic milling, and trochoidal milling.

“Once these conditions are met, it is possible that not only high efficiency is achieved, but also that tool life and the life of the machine spindle are greatly extended,” said Hashizume. “In such an environment, it is less important to consider chip evacuation by enlarging the chip pocket of the end mill, but rather how to increase the number of flutes to increase the tool rigidity and feed rate to achieve high efficiency.”

By adding flutes, the machinist can decrease the feed per flute and still maintain the same feed compared to an end mill with a lower flute count. For example, for a four-flute end mill running 0.002" (0.005-cm) per flute, substitute a five-flute end mill and maintain the same feed with decreased pressure per flute. “So, you get a little more flexibility in your tool wear without decreasing your productivity,” Tonne said. “The linear feed rate can stay the same and the cycle time will stay the same but you’re decreasing the section each flute has to take.”

Edwin Tonne, training and technical specialist, Horn USA Inc., Franklin, Tenn., said the perceived application for multi-flute end mills is for semi-finishing and finishing workpieces. “But, actually, if the shop is willing to re-program the job multi-flutes can be used to rough and to pocket as well,” he said.

A face mill is a cutter designed for facing as opposed to e.g., creating a pocket (end mills). The cutting edges of face mills are always located along its sides. As such it must always cut in a horizontal direction at a given depth coming from outside the stock. Multiple teeth distribute the chip load, and since the teeth are normally disposable carbide inserts, this combination allows for very large and efficient face milling.

Tonne also agreed the software enables processes that lead to higher productivity. With trochoidal milling “you have some unproductive time in the cut but [the process] more than makes up for it because you can take a really big axial DOC, even with a small end mill,” he said.

The higher the number of flutes per given diameter, the smaller the flute space on the end mill, Fiedler said. Depending on the material and its specific chip formation behavior, sufficient chip evacuation is critical and needs to be closely observed. In general, cut-off material moves down to the core and then breaks or rolls into chips, but it’s helpful to know how to read different materials and their tendencies. Steels up to 45 HRC, depending on the type of alloy, tend to roll and then break. Hardened steels are brittle and create thin chips. In general, stainless steels have less tendency to roll, but this is also heavily dependent on the alloy. Cast iron breaks into dust particles. Titanium tends to curl and fills up the available flute space quickly.

Cutter location is the topic of where to locate the cutter in order to achieve the desired contour (geometry) of the workpiece, given that the cutter's size is non-zero. The most common example is cutter radius compensation (CRC) for endmills, where the centerline of the tool will be offset from the target position by a vector whose distance is equal to the cutter's radius and whose direction is governed by the left/right, climb/conventional, up/down distinction. In most implementations of G-code, it is G40 through G42 that control CRC (G40 cancel, G41 left/climb, G42 right/conventional). The radius values for each tool are entered into the offset register(s) by the CNC operator or machinist, who then tweaks them during production in order to keep the finished sizes within tolerance. Cutter location for 3D contouring in 3-, 4-, or 5-axis milling with a ball-endmill is handled readily by CAM software rather than manual programming. Typically the CAM vector output is postprocessed into G-code by a postprocessor program that is tailored to the particular CNC control model. Some late-model CNC controls accept the vector output directly, and do the translation to servo inputs themselves, internally.

These cutters are a type of form tool and are used in hobbing machines to generate gears. A cross-section of the cutter's tooth will generate the required shape on the workpiece, once set to the appropriate conditions (blank size). A hobbing machine is a specialised milling machine.

Hollow milling has an advantage over other ways of cutting because it can perform multiple operations. A hollow mill can reduce the diameter of a part and also perform facing, centering, and chamfering in a single pass.

End mills, traditionally made with two to four flutes, are used in one of the oldest mechanized machining processes—milling. Cutting-edge software, machine tools, novel strategies, ever-improving techniques and design updates in the tools themselves keeps milling useful in the 21st century. The machinist who masters the art of these metal eaters can save their shop time and money while producing superior parts.

A fly cutter is composed of a body into which one or two tool bits are inserted. As the entire unit rotates, the tool bits take broad, shallow facing cuts. Fly cutters are analogous to face mills in that their purpose is face milling and their individual cutters are replaceable. Face mills are more ideal in various respects (e.g., rigidity, indexability of inserts without disturbing effective cutter diameter or tool length offset, depth-of-cut capability), but tend to be expensive, whereas fly cutters are very inexpensive.

In contrast to the linear radial toolpath in conventional machining, trochoidal milling uses a spiral (or D-shaped) toolpath with a low radial DOC to reduce load and wear on the tool. Since trochoidal milling uses a tool to machine a slot wider than its cutting diameter, the same tool can be used to create slots of varying sizes. This can free up space in the tool carousel and save time on tool changeouts, depending on the requirements of the part.

There are several common standardized methods of mounting shell mills to their arbors. They overlap somewhat (not entirely) with the analogous joining of lathe chucks to the spindle nose.

Selecting a milling cutter is not a simple task. There are many variables, opinions and lore to consider, but essentially the machinist is trying to choose a tool that will cut the material to the required specification for the least cost. The cost of the job is a combination of the price of the tool, the time taken by the milling machine, and the time taken by the machinist. Often, for jobs of a large number of parts, and days of machining time, the cost of the tool is lowest of the three costs.

Newer software and machining techniques can even help make an older machine perform like a shiny new model. “If they have machines even with moderate speed, a lot of times an older machine that has moderate capabilities—if it’s partnered with a new machining strategy—can still take advantage of modern high-efficiency machining and toolpath strategies,” said Strauchen. “The best way to figure it out is to bring specialists in that are savvy with modern programming techniques … and help customers maximize what they have.”

Although there are many different types of milling cutter, understanding chip formation is fundamental to the use of any of them. As the milling cutter rotates, the material to be cut is fed into it, and each tooth of the cutter cuts away a small chip of material. Achieving the correct size of chip is of critical importance. The size of this chip depends on several variables.

Dynamic milling relies on the ability of CAD/CAM software to create a trochoidal milling program; a milling machine to read complicated trochoidal programs at high speed; and a machine that can rapidly move the spindle and table.

Is there a limit on the number of flutes for one end mill? The primary method of manufacturing end mills is grinding using automatic NC grinding machines, said OSG’s Hashizume. As long as they are manufactured using such machines, the capabilities of CAD/CAM applications and the grinding machines themselves (especially the size of the grinding wheel) impose limitations on the number of flutes it is possible to create. “The larger the OD of the end mill manufactured, the bigger the space that can be used for one cutting edge, and the more cutting edges that can be manufactured,” said Hashizume. The maximum number of flutes depends on the diameter of the tool, Tonne agreed.

“Dynamic milling is defined as a method that is done with a large axial depth of cut (DOC) and small radial DOC to reduce the engagement time of the cutting edge of an end mill, which reduces the force load on the tool and spindle and the generation of cutting heat, while increasing the amount of material removed,” said Tyler Hashizume, product engineer II, OSG USA Inc., St. Charles, Illinois.

A bull nose cutter mills a slot with a corner radius, intermediate between an end mill and ball cutter; for example, it may be a 20 mm diameter cutter with a 2 mm radius corner. The silhouette is essentially a rectangle with its corners truncated (by either a chamfer or radius).

The side-and-face cutter is designed with cutting teeth on its side as well as its circumference. They are made in varying diameters and widths depending on the application. The teeth on the side allow the cutter to make unbalanced cuts (cutting on one side only) without deflecting the cutter as would happen with a slitting saw or slot cutter (no side teeth).

Trepanning is also possible with a hollow mill. Special form blades can be used on a hollow mill for trepanning diameters, forms, and ring grooves.

Another important quality of the milling cutter to consider is its ability to deal with the swarf generated by the cutting process. If the swarf is not removed as fast as it is produced, the flutes will clog and prevent the tool cutting efficiently, causing vibration, tool wear and overheating. Several factors affect swarf removal, including the depth and angle of the flutes, the size and shape of the chips, the flow of coolant, and the surrounding material. It may be difficult to predict, but a good machinist will watch out for swarf build up, and adjust the milling conditions if it is observed.

Milling cutters are cutting tools typically used in milling machines or machining centres to perform milling operations (and occasionally in other machine tools). They remove material by their movement within the machine (e.g., a ball nose mill) or directly from the cutter's shape (e.g., a form tool such as a hobbing cutter).

A shell mill is any of various milling cutters (typically a face mill or endmill) whose construction takes a modular form, with the shank (arbor) made separately from the body of the cutter, which is called a "shell" and attaches to the shank/arbor via any of several standardized joining methods.

“All the CAM systems have different names for this type of programming,” he said. “HEM, VoluMill—there’s all sorts of them. If you really want to give it a blanket name it’d be ‘optimized roughing.’ These CAM systems have made it so easy, you just say this is my percentage of diameter width of cut and it does a lot of the back figuring for elevated surface footages and correcting of feed rates. It figures the tool path for you. It’s just made it so much easier for the small guy to be competitive.”

Using these tools in an HEM/VoluMill environment (toolpath software from Celeritive Technologies, Moorpark, Calif.) will mitigate assembly disasters, he said. This approach can make the smaller tapered machines competitive with larger taper machines. “The metal removal rates we can achieve are very close if they aren’t in fact beating the standard way to rough material out on bigger taper machines like 50-taper and HSK A100,” Clynch said. As a result, a wider segment of the industry can be competitive because smaller taper machines are less expensive and easier to learn.

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Cutters of this form factor were the earliest milling cutters developed. From the 1810s to at least the 1880s they were the most common form of milling cutter, whereas today that distinction probably goes to end mills. Traditionally, HSS side and face cutters are used to mill slots and grooves.

There are 8 cutters (excluding the rare half sizes) that will cut gears from 12 teeth through to a rack (infinite diameter).

What to do if chip formation doesn’t look right? “Unfortunately, there is not a simple answer to this. It depends on the application and material,” said Fiedler. In the case of chips changing their color, the coolant supply into the work zone needs to be improved. Vibration might be the cause of all this, so the tool and workpiece clamping need to be checked. Modifying the feed rates and axial DOC can also help. Curling and ruffling of chips often indicates feed rates are too high, so adjusting the feed rates can help, but it is important to maintain sufficient average chip thickness

Most fly cutters simply have a cylindrical center body that holds one tool bit. It is usually a standard left-hand turning tool that is held at an angle of 30 to 60 degrees. Fly cutters with two tool bits have no "official" name but are often called double fly cutters, double-end fly cutters, or fly bars. The latter name reflects that they often take the form of a bar of steel with a tool bit fastened on each end. Often these bits will be mounted at right angles to the bar's main axis, and the cutting geometry is supplied by using a standard right-hand turning tool.

A common use of a hollow mill is preparing for threading.  The hollow mill can create a consistent pre-thread diameter quickly, improving productivity.

From what he’s seen in the industry, 20 flutes on a 1.25" (3.18-cm) tool is the maximum. “With that many flutes, the radial engagement due to the limited usable flute volume is much less than 10 percent,” he said. “So, you start to diminish the practicality in most applications or limit your work to pure finishing and not much else.”

An adjustable hollow mill is a valuable tool for even a small machine shop to have because the blades can be changed out for an almost infinite number of possible geometries.

The machinist needs three values: S, F and Depth when deciding how to cut a new material with a new tool. However, he will probably be given values of Vc and Fz from the tool manufacturer. S and F can be calculated from them:

However, Clynch offers several cautions. When using this strategy, a machine tool’s acceleration/deceleration rates have to be higher because with the smaller moves the tool makes, the machine has to ramp up and down more to adjust the speed. The machine tool needs more memory for longer programs and it also needs enough “look-ahead,” or buffer space, to run smoothly. If the machine can’t read the code fast enough, it jerks, stalls or dwells trying to keep up, he said.

Scientific study by Holz and De Leeuw of the Cincinnati Milling Machine Company[11] made the teeth even coarser and did for milling cutters what F.W. Taylor had done for single-point cutters with his famous scientific cutting studies.

How can the use of multi-flute end mills lead to higher productivity if they take such small bites of metal? “Because the normal operation of the multi-flute is decreased radial engagement, let’s say less than 25 percent of the diameter, the arc of contact is smaller,” said Horn’s Tonne. This allows the use of two to three times the normal cutting speed range.

These strategies change the way a machinist tackles a job, and it’s becoming popular for machinists to use multi-flute end mills—those with five or more flutes—to do both roughing and finishing, eliminating the need to fill up the tool carousel with an array of different end mills. These modern strategies mitigate the need to bury the tool into a part and any worries about getting chips clogged up in the flute gullets, which can lead to a broken end mill and the failure of the part in progress.

Ball nose cutters or ball end mills (lower row in image) are similar to slot drills, but the end of the cutters are hemispherical. They are ideal for machining 3-dimensional contoured shapes in machining centres, for example in moulds and dies. They are sometimes called ball mills in shop-floor slang, despite the fact that that term also has another meaning. They are also used to add a radius between perpendicular faces to reduce stress concentrations.

Whereas a hob engages the work much as a mating gear would (and cuts the blank progressively until it reaches final shape), a thread milling cutter operates much like an endmill, traveling around the work in a helical interpolation.

Both convex and concave spherical radii are possible with a hollow mill. The multiple blades of a hollow mill allow this radius to be produced while holding a tight tolerance.

The DOC can also be increased. For example, the machinist could run a process using a 5/8" (1.6-cm) diameter, two-flute end mill on titanium 6AL-4V at 130 sfm using full slotting and 1× the diameter DOC for productivity of 1.49 in³/min. (24.4 cm3/min.). Doing the same process with an eight-flute end mill with 230 sfm and 0.019" (0.048-cm) radial engagement or width of cut and 2× the diameter depth of cut increases productivity to 1.57 in³/min. (25.7 cm3/min.). “So, the net gain on productivity is significant,” said Tonne.

“In a typical, traditional approach, when machinists use end mills for more material [removal], more roughing, and if the tool is buried more, the fewer flutes they would use,” said Drew Strauchen, executive vice president at GWS Tool Group, Tavares, Fla. “With conventional wisdom, roughing uses two- or three-flute end mills and semi-finishing and finishing operations uses more flutes—four, five and beyond.”

Hollow milling cutters, more often called simply hollow mills, are essentially "inside-out endmills". They are shaped like a piece of pipe (but with thicker walls), with their cutting edges on the inside surface. They were originally used on turret lathes and screw machines as an alternative to turning with a box tool, or on milling machines or drill presses to finish a cylindrical boss (such as a trunnion). Hollow mills can be used on modern CNC lathes and Swiss style machines. An advantage to using an indexable adjustable hollow mill on a Swiss-style machine is replacing multiple tools.  By performing multiple operations in a single pass, the machine does not need as can accommodate other tools in the tool zone and improves productivity.

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This modular style of construction is appropriate for large milling cutters for about the same reason that large diesel engines use separate pieces for each cylinder and head whereas a smaller engine would use one integrated casting. Two reasons are that (1) for the maker it is more practical (and thus less expensive) to make the individual pieces as separate endeavors than to machine all their features in relation to each other while the whole unit is integrated (which would require a larger machine tool work envelope); and (2) the user can change some pieces while keeping other pieces the same (rather than changing the whole unit). One arbor (at a hypothetical price of USD100) can serve for various shells at different times. Thus 5 different milling cutters may require only USD100 worth of arbor cost, rather than USD500, as long as the workflow of the shop does not require them all to be set up simultaneously. It is also possible that a crashed tool scraps only the shell rather than both the shell and arbor. To also avoid damage to the shell, many cutters, especially in larger diameters, also have another replaceable part called the shim, which is mounted to the shell and the inserts are mounted on the shim. That way, in case of light damage, only the insert and maximum the shim needs replacement. The shell is safe. This would be like crashing a "regular" endmill and being able to reuse the shank rather than losing it along with the flutes.

More advanced hollow mills use indexable carbide inserts for cutting, although traditional high speed steel and carbide-tipped blades are still used.

Roughing end mills quickly remove large amounts of material. This kind of end mill utilizes a wavy tooth form cut on the periphery. These wavy teeth act as many successive cutting edges producing many small chips. This results in a relatively rough surface finish, but the swarf takes the form of short thin sections and is more manageable than a thicker more ribbon-like section, resulting in smaller chips that are easier to clear. During cutting, multiple teeth are in simultaneous contact with the workpiece, reducing chatter and vibration. Rapid stock removal with heavy milling cuts is sometimes called hogging. Roughing end mills are also sometimes known as "rippa" or "ripper" cutters.

With a higher number of flutes, though, chip formation and evacuation become concerns. Mitigate these concerns by adjusting radial engagement and table feeds to the application and target material; choosing the correct tool for a specific application; and selecting tools tailored for a high number of flutes—for example, those with a specific core design enabling bigger flute space toward the front end, or a design that optimizes chip formation.

Don’t equate cycle time with tool life, though, said Clynch. Just because a machine ran six hours, that doesn’t mean it used six hours of tool life. The end mill may have only been engaged with the workpiece a fraction of that time due to the short arc of contact. “Pay close attention to this to make sure you are getting the maximum [life] out of your tools,” he said. “If not, you may be leaving money on the table!”

Interpolation is also not necessary when using a hollow mill; this can result in a significant reduction of production time.

The most common type of joint between shell and arbor involves a fairly large cylindrical feature at center (to locate the shell concentric to the arbor) and two driving lugs or tangs that drive the shell with a positive engagement (like a dog clutch). Within the central cylindrical area, one or several socket head cap screws fasten the shell to the arbor.

Hollow mills offer an advantage over single point tooling. Multiple blades allow the feed rate to double and can hold a closer concentricity. The number of blades can be as many as 8 or as few as 3.  For significant diameter removal (roughing), more blades are necessary.

Slab mills are used either by themselves or in gang milling operations on manual horizontal or universal milling machines to machine large broad surfaces quickly. They have been superseded by the use of cemented carbide-tipped face mills which are then used in vertical mills or machining centres.

Don’t forget about the material you’re removing, said Clynch. “In theory there is not a limit, but you’ve got to have some place for the chip to form correctly,” he said. For normal, everyday materials like ISO P, ISO M, and high-temperature alloys there has to be a limit on the number of flutes. The rule of thumb is for every millimeter in diameter of a tool you get one flute, he said. For example, for tools with a ½" (12.7-mm) diameter, the maximum flute number to be effective is 12 and for tools with a 1" (25.4-mm) diameter the maximum flute number to be effective is 25. “For practicality that’s a good way to do it,” Clynch said.

Woodbury provides citations[9] of patents for various advances in milling cutter design, including irregular spacing of teeth (1867), forms of inserted teeth (1872), spiral groove for breaking up the cut (1881), and others. He also provides a citation on how the introduction of vertical mills brought about wider use of the endmill and fly cutter types.[10]

“Very important is the amount of coolant that can be provided and ensuring that the coolant stream direction maximizes the chip evacuation out of the cutting zone,” said Bernd Fiedler, senior product manager-solid end milling, at Kennametal, Fuerth, Germany. “Sometimes high-pressured air can be a good option to remove chips out of the working area and prevent chip clogging, especially in pockets. ”

The newer processes enabled by modern CAM software are a boon for machining hard materials that need to be machined with low radial engagement, which otherwise would risk breaking the end mill. For example, when machining steel greater than 50-60 HRC, a two-flute end mill probably will snap.

Milling cutters come in several shapes and many sizes. There is also a choice of coatings, as well as rake angle and number of cutting surfaces.

Another type of shell fastening is simply a large-diameter fine thread. The shell then screws onto the arbor just as old-style lathe chuck backplates screw onto the lathe's spindle nose. This method is commonly used on the 2" or 3" boring heads used on knee mills. As with the threaded-spindle-nose lathe chucks, this style of mounting requires that the cutter only make cuts in one rotary direction. Usually (i.e., with right-hand helix orientation) this means only M03, never M04, or in pre-CNC terminology, "only forward, never reverse". One could use a left-hand thread if one needed a mode of use involving the opposite directions (i.e., only M04, never M03).

The history of milling cutters is intimately bound up with that of milling machines. Milling evolved from rotary filing, so there is a continuum of development between the earliest milling cutters known, such as that of Jacques de Vaucanson from about the 1760s or 1770s,[3][4] through the cutters of the milling pioneers of the 1810s through 1850s (Whitney, North, Johnson, Nasmyth, and others),[5] to the cutters developed by Joseph R. Brown of Brown & Sharpe in the 1860s, which were regarded as a break from the past[6][7] for their large step forward in tooth coarseness and for the geometry that could take successive sharpenings without losing the clearance (rake, side rake, and so on). De Vries (1910)[7] reported, "This revolution in the science of milling cutters took place in the States about the year 1870, and became generally known in Europe during the Exhibition in Vienna in 1873. However strange it may seem now that this type of cutter has been universally adopted and its undeniable superiority to the old European type is no longer doubted, it was regarded very distrustfully and European experts were very reserved in expressing their judgment. Even we ourselves can remember that after the coarse pitched cutter had been introduced, certain very clever and otherwise shrewd experts and engineers regarded the new cutting tool with many a shake of the head. When[,] however, the Worlds Exhibition at Philadelphia in 1876, exhibited to European experts a universal and many-sided application of the coarse pitched milling cutter which exceeded even the most sanguine expectations, the most far-seeing engineers were then convinced of the immense advantages which the application of the new type opened up for the metalworking industry, and from that time onwards the American type advanced, slowly at first, but later on with rapid strides".[8]

The question is how can a machinist seemingly defy the laws of physics and use the speed of a higher-flute tool without clogging it with chips and causing it to break? The answer is in new programming strategies. Today, CAD/CAM software, with sophisticated toolpath generation built in, allows programmers to generate more efficient toolpaths that are speedy but prevent the tool from getting into danger zones. The software’s approach is very specific, so the mill is never over-engaged with the part. Users can tell CAM software, “I don’t want to exceed this amount of tool engagement,” and the application will create the toolpath necessary to ensure the tool never gets engaged beyond the point he defined.

End mills (middle row in image) are those tools that have cutting teeth at one end, as well as on the sides. The words end mill are generally used to refer to flat bottomed cutters, but also include rounded cutters (referred to as ball nosed) and radiused cutters (referred to as bull nose, or torus). They are usually made from high speed steel or cemented carbide, and have one or more flutes. They are the most common tool used in a vertical mill.

“But using a multi-flute end mill with high-speed techniques and low radial engagement you can mill a slot or any kind of feature in it,” Tonne said.

Most shell mills made today use indexable inserts for the cutting edges—thus shank, body, and cutting edges are all modular components.

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“There are several general indicators that chip formation is insufficient,” Fiedler said. “Chips are very curled or rippled, have no uniform edge, or they are deeply colored. For instance, when the side that rolls over the cutting edge is no longer shiny but shows color changes.”