New tool holders offer firm grip on progress – Sandvik Coromant - tool insert change cnc lathe

ATI Stellram  (615) 641-4200www.stellram.com Greenleaf Corp. (800) 458-1850www.greenleafcorporation.com Kennametal Inc. (800) 446-7738www.kennametal.com Sandvik Coromant Co. (800) 726-3845www.coromant.sandvik.com/us Seco Tools (800) 832-8326www.secotools.com

Luminoso has a bachelor of arts from Carleton University, a bachelor of education from Ottawa University, and a graduate certificate in book, magazine, and digital publishing from Centennial College.

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Also, a machine operator can vary a round insert’s approach angle to reduce chip thickness. “We’re always looking for thinning the chip,” Gardner said. “When we thin the chip with the approach angle, it gives us better productivity; we create less heat. The more heat we put into them, the more friction we create, the more [heat] these materials will throw back,” producing a burned-out insert. Protecting the tool from burn-out doesn’t mean applying it timidly, though.

Martin Gardner, global product manager—turning, threading and grooving for toolmaker ATI Stellram, La Vergne, Tenn., agreed. He suggested an RCMT insert, which has a 7° rake angle, for this application. “That’s probably the most popular insert we see in these types of applications,” he said. “They’re used for profiling and for narrow grooves.”

Nonetheless, Tisdall prefers a CVD coating when machining superalloys with a round insert. “The strength and chip thinning effect of a round geometry makes up for the inherent weakness of the edge line,” he said. “When machining superalloys with an insert with a point angle—CNMG, DNMG or VNMG in combination with a small lead angle tool—a PVD grade is preferable.”

1. Permanently damaging a metal by heating to cause either incipient melting or intergranular oxidation. 2. In grinding, getting the workpiece hot enough to cause discoloration or to change the microstructure by tempering or hardening.

About the Author: Joseph L. Hazelton is a freelance writer with 8 years of experience writing and editing articles for metalworking publications.

Round inserts can be worthwhile, though, because they’re strong and can be fed at higher rates than other types of inserts, more than making up for their disadvantages in certain applications.

"Sharp molded and ground cutting tool inserts provide less tool pressure and freer cutting," said Steve Easterday, Swiss applications engineer, Kyocera Precision Tools, Hendersonville, N.C. "Physical vapour deposition (PVD)-coated inserts provide excellent tool life and a sharper edge."

Workholders and machine tools must be rigid to apply round inserts because the tool’s radius, relatively large compared with a straight-edged insert, means more contact area and therefore more cutting pressure.

However, Graham said inserts should have CVD coatings when they’re machining softer superalloys, like Inconel 600, because the predominant failure mode is cratering and a thicker coating better protects against cratering.

For shops looking to take on small-part turning operations, there are a number of things that can be done to ensure success.

Conditioning of the cutting edge, such as a honing or chamfering, to make it stronger and less susceptible to chipping. A chamfer is a bevel on the tool’s cutting edge; the angle is measured from the cutting face downward and generally varies from 25° to 45°. Honing is the process of rounding or blunting the cutting edge with abrasives, either manually or mechanically.

Besides a round insert’s geometry, part makers for aerospace and oil and gas applications must consider whether they want a coated tool.

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Another concern with small parts is they get washed away with chips. For example, part ejectors that put small parts into the conveyor may have issues separating them from chips. A vacuum that removes the part when the chuck opens and puts it in a separator is a good option.

He also said round inserts need an edge hone that’s not too large because that would create too much cutting force.

Crystal manufactured from boron nitride under high pressure and temperature. Used to cut hard-to-machine ferrous and nickel-base materials up to 70 HRC. Second hardest material after diamond. See superabrasive tools.

On a conventional lathe you may be able to turn a length-to-diameter (L:D) ratio of about 10:1, and that is only if the machine has a steady rest or similar supporting device. On a Swiss-style machine, the L:D can be as much as 20:1, making it possible to produce very difficult parts.

Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.

Angle of inclination between the face of the cutting tool and the workpiece. If the face of the tool lies in a plane through the axis of the workpiece, the tool is said to have a neutral, or zero, rake. If the inclination of the tool face makes the cutting edge more acute than when the rake angle is zero, the rake is positive. If the inclination of the tool face makes the cutting edge less acute or more blunt than when the rake angle is zero, the rake is negative.

In a race, carbide—even a coated, submicrograin carbide—would be chasing ceramic at a long distance. “Say we take our best-case scenario of carbide—300 sfm on typical Inconel 718,” Greenleaf’s Hill said, adding that that speed involved light finishing, not roughing. “Ceramics are capable of running 800 to 900 sfm.”

Despite some major advantages, round inserts may be unattractive to manufacturers for several reasons. One is they can’t machine small-radius corners. For example, a 0.5 "-dia. round insert can’t turn a 90° corner with a radius of 0.004 ". Its smallest corner radius is 0.5".

Consequently, parts manufacturers have to maximize machine tool and workholding rigidity if they want to successfully apply round inserts.

Moreover, round inserts aren’t well suited to machining complicated profiles, like undercuts, and can’t create profiles not present in their geometry, according to Don Graham, manager of turning products for toolmaker Seco Tools Inc., Troy, Mich.

Tough, difficult-to-machine alloys; includes Hastelloy, Inconel and Monel. Many are nickel-base metals.

Engagement of a tool’s cutting edge with a workpiece generates a cutting force. Such a cutting force combines tangential, feed and radial forces, which can be measured by a dynamometer. Of the three cutting force components, tangential force is the greatest. Tangential force generates torque and accounts for more than 95 percent of the machining power. See dynamometer.

Looking for tooling that is purposely designed for small-part turning or microturning is a good place to start. These tools tend to have tighter tolerances and are more finely ground.

Included angle at the point of a twist drill or similar tool; for general-purpose tools, the point angle is typically 118°.

Turning small parts is nothing new for many machine shops. Industries like automotive, aerospace, electronics, firearms, and medical are pushing the envelope when it comes to small-part production. Typically, any application requiring a workpiece smaller than 1 in. dia. is considered small-part turning.

With small tools, a high-quality modular system, like Horn’s W&F Micro precision modular tooling, will allow operators to keep a close eye on the tool wear. Horn

"The screw could be an M2.5 or smaller, and in some cases, they can be easily lost to the bottom of the machine or conveyor system," said Tonne. "The other thing I would recommend is using torque-limiting drivers at all times. If you overtorque an M2.5 screw, you’re going to strip it, and that will add a lot of extra time getting that screw out and changing the insert."

Round inserts can operate at higher feed rates because they have the strongest geometry. “The larger the included angle of the cutting edge, the more inherent strength you have in that insert,” said Frank Battaglia, staff engineer–global machining technology with Kennametal Inc., Latrobe, Pa.

Any manufacturing process in which metal is processed or machined such that the workpiece is given a new shape. Broadly defined, the term includes processes such as design and layout, heat-treating, material handling and inspection.

Tisdall also described a “roll-in method” of plunging and turning. A programming technique, the method includes a radius as an insert comes out of a plunge. “It’s another way of keeping the insert engaged with the workpiece without doing a sudden movement in a different direction. Let’s say you’re plunging in Z, and then you go to the X-axis and make a move in that axis. There’s a lot of stress on the insert. Here, we’re making use of a radius to basically make the process gentler on the insert.”

"Common mistakes can include incorrect cutting conditions like incorrect surface footage, too high feed rate, and too heavy depth of cut, which can result in incorrect tolerances that include part bending and poor finish," said Easterday.

Some aerospace companies still prefer uncoated inserts, though, because they worry about the coating contaminating parts during manufacture. “Obviously, they don’t want anything that’s going to erode or deform that structure, however minute it is,” said ATI Stellram’s Gardner. “There have been many studies about the coating diffusion into the parent material. We’ve never found that diffusion into the material with a PVD coat. That said, some manufacturers of aero engine components still like to use uncoated inserts.”

With small components, tool pressure is a primary concern. Very small parts can easily deform during turning operations.

Workholding device that affixes to a mill, lathe or drill-press spindle. It holds a tool or workpiece by one end, allowing it to be rotated. May also be fitted to the machine table to hold a workpiece. Two or more adjustable jaws actually hold the tool or part. May be actuated manually, pneumatically, hydraulically or electrically. See collet.

Angle between the side-cutting edge and the projected side of the tool shank or holder, which leads the cutting tool into the workpiece.

Depending on the size of the parts and specifications needed, small-part turning can be done on a conventional CNC.

Battaglia cautioned, though, that a ceramic round insert can still wear out if run too fast. Also, he recommended ceramic inserts’ cutting edges receive a chamfer, also known as a T-land or radius hone. He said the typical T-land has a width of 0.002 " to 0.004 " on the rake surface and an angle of 20° to 25°. “When you have a sharp-edged insert, that edge is susceptible to crack propagation,” he said. “When you add an edge preparation, you tend to direct those cutting forces more into the bulk of the material, so it makes it more difficult for a crack to propagate and lead to chipping of the cutting edge.”

Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.

“Another key issue is a coating that will adhere to the edge line for a long period of time,” Tisdall said. “What you don’t want to do is finish a large aerospace component and find out that you’ve lost size because your coating has broken down halfway through the cut. You either are going to have to do a spring pass or you might scrap a part, and you are talking about a lot of money.”

Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.

Graham estimated that round inserts can be fed 20 percent faster than other types of inserts, depending on the round tool’s chipbreaker, which can be designed for different feed rates. Graham cited a chipbreaker with a neutral land and relatively wide groove as an example, saying that a round insert can take a heavier feed rate than one with a positive rake and narrow groove width. (See recommended feeds and speeds chart on page 39.)

Time saved via higher feed rates means money saved. “The amount of money you can save [by using round inserts] over the life of running an insert is phenomenal,” Tisdall said, adding that actual cost savings depend on a shop’s overhead, annual number of components and reduction of a part number’s process time.

A machine needs to have a rigid setup, regardless of whether it’s a conventional or Swiss machine, and it is vitally important that the correct tooling be used for small-part turning. EMEC

Tangential velocity on the surface of the tool or workpiece at the cutting interface. The formula for cutting speed (sfm) is tool diameter 5 0.26 5 spindle speed (rpm). The formula for feed per tooth (fpt) is table feed (ipm)/number of flutes/spindle speed (rpm). The formula for spindle speed (rpm) is cutting speed (sfm) 5 3.82/tool diameter. The formula for table feed (ipm) is feed per tooth (ftp) 5 number of tool flutes 5 spindle speed (rpm).

As good as they are, though, parts manufacturers may find coated, carbide round inserts insufficient for their use. “Their disadvantage is that they will be significantly limited in speed capability compared to ceramic cutting tools,” Kennametal’s Battaglia said.

Another thing to consider is keeping plenty of extra hardware for the holder around because typically the screws holding the insert tend to be very tiny.

Parts manufacturers can take advantage of a PCBN round tool’s ability to turn high-nickel alloys up to 1,000 sfm.

"With small tools, a high-quality modular system will allow you to keep a close eye on the tool wear," said Tonne. "If you have a feature that’s only 0.001 in. deep, and you lose the radius of your tool, you can have a problem on your dimensional tolerance, so keeping a close eye on the tool is very important."

Machining vertical edges of workpieces having irregular contours; normally performed with an endmill in a vertical spindle on a milling machine or with a profiler, following a pattern. See mill, milling machine.

Chip control and evacuation can be cumbersome in small-part turning. For the most part, the biggest tool possible is the better option, but it’s important to remember that there must be enough room, especially in boring operations, for chip evacuation. High-pressure coolant can be a huge help with this.

With a long part it’s always a good idea, especially with a small diameter, to try to grab it with the secondary spindle. While this may add a little bit of time, gripping and not letting the part hang out adds the stability needed to ensure tolerances are met.

A conventional CNC lathe is good for general-purpose turning but usually has spindle speeds of only about 3,000 to 5,000 RPM, whereas a Swiss-style lathe can be in the 10,000- to 15,000-RPM spindle range. That higher RPM enables the machine to generate the surface speed needed to cut small-diameter parts efficiently.

Sandvik Coromant’s Tisdall generally prefers that round inserts have a CVD coating in most applications. “A CVD grade will provide better tool life and higher-speed capability because the coating is typically much thicker.” He said, however, that a CVD coating is more prone to notch wear than a PVD coating because of the CVD process itself. According to Tisdall, the CVD process creates an eta phase in an insert’s carbide matrix, depleting the matrix of its cobalt binder, which acts to resist notch wear. Also, depleting the matrix of its binder weakens the carbide substrate.

The tool’s inability to create small-radius corners leads many parts manufacturers, and their programmers, to apply other insert shapes. “The CNMG is pretty much the first choice for any programmer because of the combination of flexibility and edge strength,” said Bill Tisdall, development specialist manager for toolmaker Sandvik Coromant Co., Fair Lawn, N.J. “You can OD turn, you can face, you can out-copy, you can turn to a square shoulder. With a round insert, you can’t turn to a shoulder. You can do all the other things.” (Out-copying is machine movement that combines a Z-axis movement toward the chuck with an X-axis movement away from the center line of the workpiece.)

A parts manufacturer has to consider whether to apply a round insert with a negative geometry, a neutral one or a positive one for machining a nickel-base alloy.

Machining grooves and shallow channels. Example: grooving ball-bearing raceways. Typically performed by tools that are capable of light cuts at high feed rates. Imparts high-quality finish.

He compared inserts’ different included angles; the 100°, 90°, 80° and 60° corners of various inserts, all the way down to a VNG-insert, which has a 35° angle. “The cross-sectional area, going from one side of that insert to the other, gets smaller and smaller as you go down in that angle,” Battaglia said. “With a round insert, it’s really just maximized to the point where you have the largest cross-sectional area going across from one side of the cutting edge to the other side.”

1. Spreading of a constituent in a gas, liquid or solid, tending to make the composition of all parts uniform. 2. Spontaneous movement of atoms or molecules to new sites within a material.

However, a negative rake angle tends to create greater cutting force, which may not be right for a particular application. Hill recommends the correct geometry for the job. “Use negative whenever possible for strength and economy—negative tools are generally double sided, allowing for more cutting edges. Use positive geometry if surface finish, tool force or built-up edge is a concern.”

According to Graham, producers of parts for oil and gas applications typically prefer negative or neutral rake angles on their round inserts because the angles provide extra strength. Those companies also like the inserts to have strong, heavy chip grooves.

A round insert can be fed up to 20 percent faster into a workpiece than other types of inserts because the tool has the strongest geometry of any insert shape.

Parts manufacturers should head straight to their high-strength round inserts to turn nickel-base alloys.

He added, though, that carbide inserts are sometimes needed when a machine tool’s limitations or a part’s fixtures require running at lower speeds or during final finishing of critical aircraft components. “Often carbide is explicitly specified to be run at a low speed so that any possible damage or ‘white layer’ formation on the part surface is avoided,” Battaglia explained.

Dimension that defines the exterior diameter of a cylindrical or round part. See ID, inner diameter.

Parts used in extreme conditions require extreme materials. The part may be a large ring or shroud in a jet engine, which operates at up to 1,200° F, or can be a down-hole component in an oil field, operating in a corrosive environment hundreds of feet underground.

High-temperature (1,000° C or higher), atmosphere-controlled process in which a chemical reaction is induced for the purpose of depositing a coating 2µm to 12µm thick on a tool’s surface. See coated tools; PVD, physical vapor deposition.

"Shops have been cutting small parts forever and have found innovative ways to do it effectively," said Mara. "It’s important to remember that a machine needs to have a rigid setup, regardless of whether it’s a conventional or Swiss machine, and it is vitally important that the correct tooling is used."

The enemy of precision machining is vibration and heat. Rather than heavy, deep cuts, small, higher-speed cuts can keep the workpiece as cool as possible, reducing heat deformation in the part. Any sort of deformation can make it difficult to maintain proper tolerances. According to Mara, Swiss machines usually use high pressure up to 2,000 PSI with water-based coolant to help keep the heat out of the part.

"For shops looking at turning long and thin parts, a conventional CNC lathe has its limitations and probably wouldn’t perform as needed," said Mara. "A Swiss-style machine offers the rigidity in the setup required to turn very small or long parts by doing the cutting as close as possible to the spindle nose and by feeding the material through a stationary tool as opposed to the tool moving across the part as on a conventional lathe. Choosing a Swiss-style machine is definitely the best option, if possible, for turning long, thin parts."

Swiss-style CNC machines even can perform other operations like this tapping of a small-diameter brass shaft. Phuchit/iStock/Getty Images Plus

Also, Graham recommended the tool have a PVD coating, not a CVD coating, when machining a high-temperature alloy with a hardness of greater than 32 HRC because PVD is usually thinner and therefore better maintains a tool’s edge definition. He added that PVD coatings are also preferred because of their resistance to wear, BUE and cratering—all common failure modes when machining nickel-base alloys.

"Because of this, the process needs to be planned to minimize tool pressure using sharp, positive tools," said Edwin Tonne, training and technical specialist, Horn USA Inc., Franklin, Tenn. "It’s important to look for well-defined cutting edges and a positive top rake. Fully sintered cutting tools can be problematic because they’re just not sharp enough to minimize the pressure. Look for finely ground cutting edges with plenty of clearance. A sharp wedge angle will help prevent deformation in the part. Form tools are a good choice because they can simplify the process."

According to Graham, a tool material exists for machining at even higher speeds than ceramics. “With CBN, you can run 1,000 sfm.”

Moreover, their strength and related long life mean round inserts are well suited to turning the large workpieces often manufactured by aerospace and oil and gas companies. “The round insert provides the best tool life and strength of any shape of an insert, so when you have a very large length of cut, you’re able to machine that full length with a round insert,” said Sandvik Coromant’s Tisdall. “With an angled insert, you’re typically going to get less tool life. You’d have to index the tool midcut.”

"When working with anything less than 0.250 in. dia., it becomes difficult to reach the correct speed for normal steel material," said Tonne. "And with RPM limitations on the machine, sharp cutting edges and very lubricious coatings are helpful. When it comes to depth to cut, typically the machine will perform a single finish pass. This needs to be balanced with tool pressure. First and foremost, keep your feeds very modest. Form tools and special tooling can help claw back some of the time."

Tonne agreed that when working with long parts, pinch turning is a great option. Pinch turning helps keep everything aligned. The external features are completed first to maintain that stability and then internal operations are performed.

Micromachining, on the other hand, is classified by part diameters that are too small to be fixtured using a standard collet. In some instances, this could mean parts that are 0.1 in. to 0.02 in. dia. all the way down to the size of a human hair, depending on the equipment.

Round inserts can’t cut small-radius corners and aren’t well suited to creating complicated profiles, but the tools have the strongest shape of any insert and can be applied at high feed rates. With appropriate rake angles and cutting strategies, the round insert is excellent at the extreme task of turning high-nickel alloys to create parts for the extreme environments inside jet engines and down-hole in oil fields. CTE

"The No. 1 thing to consider is chip control," said Easterday. "Having the correct chipbreaker is key. The chip needs to be directed away from the workpiece."

"If their machine is equipped with HP coolant pumps, internal coolant through tooling is a must," added Easterday. "Make sure that the workpiece can be done on the machine you have. In many cases when you are dealing with long workpieces, adjustments need to be made. Extended guide nose bushings or the capability to pinch-turn can be helpful."

Angle between the insert’s side-cutting edge and the line perpendicular to the milling cutter’s axis of rotation. Approach angle, which is also known as cutting edge angle, is used with metric units of measurement. See lead angle.

Round inserts can be made with different features, like a chamfer (upper left and lower right) and a positive rake angle (lower right), to reduce problems that can be encountered when turning high-nickel alloys.

Round inserts may be underutilized in some manufacturing sectors, but the aerospace and oil and gas industries shouldn’t be among them.

Chip control is essential for effective small-part turning where the chip needs to be directed away from the workpiece. Kyocera Precision Tools

"Everyone wants to automate their process," said Mara. "We can’t get skilled people, so the logical step is to automate. End-of-arm tooling is not typically as dextrous as it needs to be for small parts, and robot accuracy may not be sufficient. It’s difficult to hold very small parts with a robot, especially for loading and unloading."

"Many of today’s new machines come equipped to handle smaller parts," said Bill Mara, president, EMEC Machine Tools Inc., Mississauga, Ont. "One of the biggest challenges machine shops face when dealing with small-part turning is the machine’s capability to hold the parts effectively without damaging or distorting them, but that’s not really a problem anymore. For shops looking to get into parts that are smaller than 0.250 in. dia., a Swiss-style lathe is the way to go."

For example, a round insert’s maximum DOC shouldn’t be more than 25 percent of the tool’s diameter. At more than 25 percent, the tool will have too much contact area with the workpiece, resulting in too much pressure and heat, Gardner said. “We’re going to just blow the insert away,” he added, “or we’re going to damage the component.”

Operators need to be mindful of the cutting edge at all times. It needs to be sharp, and damage can occur very easily from general wear, handling, or even indexing.

1. Permanently damaging a metal by heating to cause either incipient melting or intergranular oxidation. 2. In grinding, getting the workpiece hot enough to cause discoloration or to change the microstructure by tempering or hardening.

Ceramics can run at such high speeds because their melting temperatures are much higher than the melting points of any metal they would cut. For example, a whisker-reinforced ceramic round insert—that is, a tool consisting of an aluminum-oxide matrix with silicon-carbide crystals—can have a melting temperature of slightly more than 2,000° C. Given such high melting points, ceramic inserts resist deformation and softening at very high temperatures.

Process of both external (e.g., thread milling) and internal (e.g., tapping, thread milling) cutting, turning and rolling of threads into particular material. Standardized specifications are available to determine the desired results of the threading process. Numerous thread-series designations are written for specific applications. Threading often is performed on a lathe. Specifications such as thread height are critical in determining the strength of the threads. The material used is taken into consideration in determining the expected results of any particular application for that threaded piece. In external threading, a calculated depth is required as well as a particular angle to the cut. To perform internal threading, the exact diameter to bore the hole is critical before threading. The threads are distinguished from one another by the amount of tolerance and/or allowance that is specified. See turning.

Given their strength, it shouldn’t be surprising that round inserts need to be applied carefully to workpieces, even to high-nickel alloys.

The carbide’s structure is the first consideration. Gardner recommended a submicrograin substrate for continuous cutting without vibration. Seco Tools’ Graham agreed. “The micrograin provides increased abrasion resistance as well as deformation resistance,” he said. “The latter is extremely important when you’re machining nickel-base materials. They generate a lot of heat and pressure [when cut].” A steel, for example, softens when it gets red hot, Graham noted, whereas a superalloy retains its strength and hardness when red hot. “So even when it’s red hot, it’s still putting a lot of pressure on the insert.”

With very tight tolerances, it can be challenging to get the tool back to size after indexing. Monitoring and controlling the size after indexing the cutting tool is a must for small-part turning.

Depressions formed on the face of a cutting tool caused by heat, pressure and the motion of chips moving across the tool’s surface.

Another area of importance is measurement and inspection. Small components require different inspection methods than their larger counterparts, especially because of the microtolerances and tiny features.

Round inserts for turning high-nickel alloys may be CVD or PVD coated depending on the tool’s performance characteristics relative to a particular alloy, such as the tool’s main failure mode.

Tool-coating process performed at low temperature (500° C), compared to chemical vapor deposition (1,000° C). Employs electric field to generate necessary heat for depositing coating on a tool’s surface. See CVD, chemical vapor deposition.

Automation is slowly making its way into all aspects of the shop floor. However, when working with small parts, automation may not be the obvious next step.

"Automation for small parts has historically been a challenge, but we are really starting to see some new and exciting advancements in automation," said Mara.

Cutting tool material consisting of polycrystalline cubic boron nitride with a metallic or ceramic binder. PCBN is available either as a tip brazed to a carbide insert carrier or as a solid insert. Primarily used for cutting hardened ferrous alloys.

Cutting tool materials based on aluminum oxide and silicon nitride. Ceramic tools can withstand higher cutting speeds than cemented carbide tools when machining hardened steels, cast irons and high-temperature alloys.

Their greater strength makes round inserts excellent at rough turning the scale present on forged and cast workpieces. Their strength also makes them less prone to chipping and breakage than other types of inserts.

"One big mistake is not checking the alignment of the machine," said Tonne. "Alignment is everything."

Joseph L. Hazelton is a freelance writer with multiple years of experience writing and editing articles for metalworking publications.

"You may want to think about using a vision inspection system, offline gauges, and those sorts of devices," said Tonne. "These take more time than using a caliper or bench-type measurement tool. Getting the part to the right size and back up and running after tool indexing is also critical."

Trochoidal turning, however, involves smaller DOCs. “It’s a light plunge into a turn,” Tisdall said. “With a lighter DOC, you don’t have as much engagement of the insert when you come up to that wall.”

Because of the low cutting speed required, built-up edge can be a problem. On larger parts burrs can be removed easily with the secondary processes, but it’s more difficult with small parts.

Groove or other tool geometry that breaks chips into small fragments as they come off the workpiece. Designed to prevent chips from becoming so long that they are difficult to control, catch in turning parts and cause safety problems.

Rate at which metal is removed from an unfinished part, measured in cubic inches or cubic centimeters per minute.

Measurement of the total angle within the interior of a workpiece or the angle between any two intersecting lines or surfaces.

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.

“There’s more material behind the force in a round insert, more material to absorb the force,” Graham added.

“Typically in aerospace, they will produce massive forgings,” Graham said. A 40 "-dia. ring that’s 0.5 " thick × 2 " wide may have started as a 48 "-dia. ring with a 6 " thickness and a 6 " width. “They remove massive amounts of material to produce that ring. That’s where a round insert is very useful because you can hog out a lot of material in an aggressive fashion,” Graham said.

Finally, a round insert can more easily damage a workpiece than a straight-edged insert. A round insert has a relatively large radius compared with a straight-edged insert, which has small radii at its corners. A large radius means more contact surface, which results in higher cutting forces. “This can be detrimental when applied in weak setups, extended tooling or on workpiece features that have a thin cross section,” said Dale Hill, applications engineer for toolmaker Greenleaf Corp., Saegertown, Pa. Detrimental effects include workpiece deflection and vibration.

To turn a pocket, Tisdall recommends trochoidal turning over plunging and turning to reduce potential damage to the round insert or part. In plunging and turning, an insert endures stress when plunged into a workpiece and moved across the material to turn it until it reaches where the pocket’s wall will be. The stress can lead to chipping and possibly catastrophic failure.

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Greenleaf’s Hill added that a negative rake angle is particularly suitable for interrupted turning. “When it engages the workpiece, the majority of that chip or swarf impacts that top surface of the insert, which is the strongest area of that cutting tool,” he said. “So, in interrupted turning you’re now presenting the strongest area of your cutting tool to that abusive situation.”

So aerospace and oil and gas companies have many of their parts made from nickel-base alloys (heat-resistant superalloys). To turn these materials, which include Inconel, Hastelloy, Waspaloy and Monel, manufacturers should have their eyes firmly fixed on that humble cutting tool, the round insert.