The milling inserts are one of the significant components of a milling machine. Just like the name, the cutting tool is responsible for scraping material off the workpiece. It consists of every milling machine.
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It is required for a very high removal rate of material, in severe conditions and works on wet as well as dry machining.
In this topic, you will know what is the milling insert, what types of milling inserts are available, what material it is made up of, and what are some of the well-known milling insert models. So let's dive in.
WHAT IS A MILLING INSERT?
These are replaceable bits for machining the toughest materials like cast iron, stainless steel, titanium, plastic, etc. They are usually made out of carbide which is why they give maximum durability and also work under extreme temperature conditions. They make holes, are used for drilling and finishing, etc.
Previously they were available in limited shapes but now you can buy one of these in shapes like helical, frustum and elliptical, etc. While the milling process, they move at 90 degrees to its axis which allows them to remove material around the insert's perimeter.
TYPES OF MILLING INSERTS
Following are various types of milling inserts available that perform different tasks:
END MILLING inserts
The end milling insert has teeth on both sides and is beneficial for drilling purposes. The terminology 'end mil' is usually known for flat bottom cutters.
- ROUGHING END MILLING INSERT
Also known as 'Pippa' cutters, these are used to remove a huge amount of material from the workpiece. They perform under extreme operating conditions. These inserts have wavy teeth that give a rough finished surface.
The teeth present in this type of insert are at the circumference of the circular disc which is why they are known as periphery milling inserts. They only work in milling machines with a horizontal axis.
This type of insert has teeth on both face/end and periphery which is why it is used to make narrow slots or cut slots and is used for strand milling and face milling operations.
They have a cutter body with a large diameter where many insertion tools are fastened. Material is removed from them by axially narrow and radially deep cuts. The diameter of the face milling insert depends upon the body of the cutter and workpiece length. It is mostly used for down milling.
The gang milling insert is where periphery milling cutters with varying sizes are used to remove and cut material from the workpiece.
These milling inserts are staggered around the periphery having the option of left or right-hand helix angles.
It is a kind of formed insert and is designed with a specific shape for a particular workpiece. Its main use is to match a circular contour having a convex surface.
- CYLINDRICAL MILLING INSERT
It has a cylindrical shape and also consists of teeth on its perimeter.
It is similar to a pipe and consists of thick walls. It has bites inside the hollow surface and is used in screw machines.
MATERIAL OF MILLING INSERT
They are made of two types of materials; steel (FSS and HSS), and carbide. Following are the details:
STEEL (FSS, HSS)
The milling inserts made out of HSS perform better against wear response and heat as compared to ordinary carbon steel. It further breaks down into special and general purposes HSS and consists of characteristics like hardness HRC62-70, great cutting edge strength, great vibration resistance, etc.
With the use of this type of steel, it has comparably great forging, machining, and sharpness features. But in comparison carbide-made milling inserts, it has low hardness and wears resistance.
CARBIDE
These are tougher than HSS but do not have good strength. Their high stiffness properties make them good wear-resistant but their lower strength makes them prone to peeling and crack.
BEST MILLING INSERT MODELS
Following are the well-known milling inserts with basic specifications:
Carbide Inserts APKT TiAlN Coated Indexable Milling Inserts APKT Style
Used for end milling, indexable face milling, slotting etc.
Carbide made, TiAN finished, Square end cut type, 85 degrees parallelogram, relif angle 11 degrees, 4-7 times faster cutting speed than generic steel
Carbide Milling Insert RPMT Round
Feasible to mill stainless steel and mild steel.
Milling insert with groove, made with hard alloy casting, high toughness, 4.40 mm bore diameter, 4.80 mm thickness, weight 2.26 ounce
Carbide Inserts for Aluminum SEKTAFFN-LH2
Feasible for roughing and finishing process on copper, brass, wood, aluminum etc.
12.7mm size, 4.76mm thick, 5.5mm diameter, 16mm length, 0.8mm radius.
PDN-HQ-M IC28 Carbide Milling Inserts CNC
Mainly used for aluminum milling on medium cutting speed and large chip section. Very sharp cutting edge
10.35mm cutting edge length, 4.48mm insert thickness, 0.4mm corner radius, 10.35mm inscribed circle diameter
Carbide Insert milling Inserts APMTPDER
Used for stainless steel and iron etc. milling. Made with 12.9 grade alloy steel, good cushioning and 90 degree cutting angle
11.18mm length, 3.5mm thickness, 0.8mm edge radius
SEHT 43 AFSN Carbide Insert Face Mill
Can mill at different angles or rough turning. Grooved design removes chips faster, and gives smooth finish. Best for milling aluminum alloy
5.50 mm bore dia, X83 chip breaker type, 0.50 mm thick, 0.50mm wide, 0.19 mm long, 0.13 pound weight
CONCLUSION
The milling inserts are used for milling extreme tough materials like stainless steel, cast iron, etc. and used to drill and finish these materials. They can mill and horizontal, vertical and inclined angles and can remove the chips extremely fast from the workpiece.
The milling inserts are rotary tools having one or multiple teeth. During the milling process, each cutter tooth cuts the workpiece one by one. They are used mainly for making grooves, steps, milling planes, forming surfaces, etc.
Selecting the right milling requires considering various factors. In this article you will know what factors to look for:
WHICH TOOL IS SUITABLE FOR AN ORDINARY MILLING MACHINE OR CNC MACHINING CENTER?
If you are using solid carbide inserts, then you need to use them on CNC machining centers. This is because carbide inserts have the great abrasion resistance and thermal rigidity but they have low impact resistance because they are made from alloys like powder metallurgy. They have a hardness of about 90 HRA and thermal rigidity of about 900- degrees.
For using inserts with ordinary milling, go for white steel milling inserts. These inserts are softer in comparison, have good toughness, and are economic. But the strength is not good enough which is why they have low heat hardness and wear resistance. Their thermal rigidity is approximately 600 degrees and 65HRC of hardness.
DIAMETER OF MILLING TOOL
The diameter of inserts varies depending upon the product batch. The milling insert's diameter depends upon the equipment's specification as well as the workpiece processing size.
Following are examples of milling inserts with standard diameter specifications:
FACE MILLING INSERT
The consideration of diameter depends mainly on the size of the processing workpiece and the power required to work on that workpiece. The diameter of the insert can also be selected based on the insert's spindle. The typical diameter range is 40 mm ' 250 mm.
The typical formula for calculating diameter is D = 1.5d where d is the diameter of the spindle.
SLOTTING MILLING INSERT
The standard diameter for slotting milling inserts starts from 1.5 inches, 2 inches, 3 inches, etc.
END MILLING INSERT
For slot milling inserts with a small diameter, the maximum number of revolutions is considered if it can reach up to 60m/min cutting speed. The standard diameter ranges from 5 mm ' 10 mm.
MILLING TOOL BLADE
There are various types of blades for milling inserts. Choose a grinding blade for fine milling. It has good accuracy in dimension therefore the cutting edge is higher in milling and delivers good surface roughness.
To attain roughing, you should use a pressed blade because it can reduce the cost of processing. Its dimensional accuracy and sharpness are not good compared to grind blades but give great edge strength and also resist impact during roughing during the machining process. It also can bear high feeds and large cutting depth.
For viscous materials like stainless steel, you can select inserts with sharpened large rake angles. Because during the cutting action of the sharp blade, there is reduced friction between the workpiece and blade, and chips are easily escaped from the front of the blade.
If you want to achieve a better-finished surface, use a scraping blade to remove rough machining marks.
MILLING TOOL BODY
The higher the diameter of the milling tool, the costlier. For instance, a face milling inserts with a 100 mm diameter costs above $600. Therefore, careful selection is required:
Consider the number of teeth
Coarse-tooth milling insert with a 100 mm diameter has 6 teeth. But the dense teeth insert of the same 100 mm diameter has 8 teeth. The size of the pitch tool is determined by cutter teeth which affect the smoothing and cutting rate of the insert.
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A coarse milling insert is usually for rough machining because it consists of a large chip flute. After all, a small chip flute, will create difficulty for chip curling and removing.
The load of the cutting tool of coarse-toothed milling is larger compared to the dense-tool milling tool.
NOSE RADIUS OF INSERTS
RE (nose radius of an insert is another crucial factor in the selection of inserts. It is available in different nose radii. Its selection depends on surface finish, depth of cut and feed, insert length, etc.
ENTERING ANGLE
The lead angle or entering angle is between the feed direction and the cutting edge. It is necessary to select the entering angle for a successful turning operation. It affects:
- Cutting force direction
- Cutting edge length in the cut
- Formation of chips
USES OF MILLING INSERTS
It is used since the late s and is used in the following sectors:
Surgical tools ' Doctors and surgeons rely heavily on accurate tools, therefore, inserts with the base of stainless steel or titanium are selected and the tip of the tool is manufactured with tungsten carbide.
Jewelry ' The inserts are used here for shaping the jewelry. Since the tungsten is second hardest material than diamond, it is perfect and economic for the shaping of jewelry rings, etc.
Nuclear industry ' The inserts made with tungsten carbide are the best neutron reflectors and are heavily used for investigation and research on nuclear chain reactions for weapons etc.
CONCLUSION
Choosing the right milling inserts requires careful selection from various factors like entering angle, machining requirements, tool diameter and blade, nose radius, etc. Selecting the wrong milling insert will not only increase your cost of production but also may damage your workpiece.
CBN inserts incorporating reinforced, chamfered edges eliminate the edge breakout common when cutting materials harder than 50 RC.
Materials hardened to 60 RC can have carbide particles hardened to 90 RC. When milling such materials, common coated carbide inserts suffer rapid flank wear.
Round carbide inserts afford advantages when machining hard steels. The profile provides a stronger tool without vulnerable sharp corners.
Fully hardened steels, hard powder-metals, heat resistant superalloys, and bimetals are all gaining broader acceptance in industry. While such materials deliver practically indestructible parts, they come with this difficulty: how to machine them to final shape at a reasonable cost per part. Fortunately, cutting tool suppliers have made dramatic advances in inserts for milling and turning the difficult materials. Today's coated carbide, cermet, cubic boron nitride (CBN), and polycrystalline diamond (PCD) inserts all play a role. Advanced material inserts with special geometries and coatings withstand mechanical shock and heat while resisting abrasive wear. However, using these inserts productively can require various external factors'one of which may be a partnership with a knowledgeable tool supplier.
Because the cost of cutting inserts is relatively low'typically just 3 percent of total machining costs with carbide inserts and 5 to 6 percent with CBN'using cheaper inserts may be a false economy. Advanced material inserts can pay for themselves in shortened cycles times or more good parts per shift.
On the other hand, filling a large milling cutter with exotic inserts unnecessarily is a costly mistake. CBN inserts can cost eight to ten times as much as carbide. And running these advanced material inserts at the wrong speed and feed rate compromises part quality and tool life. With difficult stock, picking the right inserts requires an appreciation for both machining economics and the overall process.
Payoffs And Tradeoffs
Consider the entire application. Less expensive carbide inserts that can do the job in terms of tolerance and surface finish may be costly when the time spent indexing and replacing inserts is considered. Real productivity results from an understanding of the tradeoffs in throughput, cycle time and insert performance.
In one specialized, low-volume example, a sintered titanium carbide gas turbine blade was milled successfully with coated carbide cutting inserts. At 120 sfm, the carbide cutting edge cut well for just 5 to 10 minutes. Acceptable insert life is typically placed at 15 to 30 minutes in high volume production with difficult materials, but with a low-rate part, the short insert life and frequent tool changes are not major drawbacks. Longer insert life does become important in full production, however, to decrease tool-changing downtime and labor and to increase machine utilization and throughput. Carbide works well for the turbine blade for now, but should the part go to higher volume production, the application may justify harder, more costly inserts made of CBN.
Productivity with advanced material inserts requires adopting the right feeds and speeds. Sandvik Coromant's CBN inserts incorporate reinforced, chamfered edges to eliminate the edge breakout common when cutting materials harder than 50 RC. Yet even despite this toughness, CBN inserts demand cutting machine parameters held to tight tolerances. Cutting speeds 10 percent too low or 10 percent too high can dramatically hamper performance.
If faced with the need to machine a difficult material, consider contacting your cutting tool supplier. Suppliers can offer solutions based on how others have approached the same problem. When experimentation is required, careful trial-and-error generally starts with carbide inserts and moves on to harder and more costly cutters. Modern insert geometries, rigid toolholders and refined machining routines often make less costly carbide inserts suitable for tough jobs. When to move beyond carbide will vary from application to application, but broad classes of materials do pose common machining challenges.
Hardened Steels
Steel alloys for many applications are becoming harder. While tool steels were once considered hard at 45 RC, steels hardened to 63 RC are now common in the die and mold industry. Mold makers who once cut parts only before heat treating are now precision machining tool steels in the fully hardened condition to avoid heat treating distortion. The heat and pressure encountered when milling the fully hardened alloys can cause plastic deformation in cutting inserts and rapid insert failure.
Even so, fully hardened steels can be machined economically with carbide. One example involves aerospace machining. A major aerospace manufacturer switched to Sandvik GC carbide inserts to re-bore a massive forging of hardened Type 300M steel, modified. Most of the metal is removed before heat treating when the steel has a hardness of 30 to 32 RC. However, to correct for distortion, precision holes in the big workpiece must be re-bored once the stock is fully hardened to 54 or 55 RC.
One particularly challenging feature deep within the part requires three re-boring passes to achieve the required tolerance and finish. The hard material combined with interrupted cutting wore out cermet cutting edges after less than one pass. This was particularly alarming given that a broken edge could ruin a part. In contrast, advanced fine-grain carbide inserts with their tough physical vapor deposition (PVD) coating and sharp cutting action lasted from six to nine cuts. To exploit the carbide inserts, the tool supplier recommended reducing the cutting speed from 300 sfm to 175 sfm but retaining the same depth of cut. Three passes through the bore at this lower speed took about 20 minutes with carbide inserts versus more than an hour with cermet cutters. More important, the added edge security of the carbide inserts minimized the risk of a broken edge scrapping an expensive workpiece.
To establish machining parameters to mill hardened steels with carbide inserts, generally start at 100 sfm. Test cuts can build up to speeds from 150 to 180 sfm. Usual feed rate is 0.003 to 0.004 inch per tooth. Insert geometries with a neutral or slight negative rake typically provide stronger edges than positive-rake inserts. Round carbide inserts also afford advantages when machining hard steels. The profile provides a stronger tool without vulnerable sharp corners.
When choosing among carbide grades, consider toughened grades. They provide edge security against the high radial cutting forces and severe entry and exit shock encountered in hardened steels. Alternatively, specially formulated high-temperature grades can withstand the heat generated by steels hardened to 60 RC. Shock resistant carbide inserts with an aluminum oxide coating can also counter the high temperatures generated by milling hard steels.
Sintered Metals
Advances in powder metallurgy are producing extra-hard sintered metals for a range of applications. One manufacturer developed a powdered nickel composite alloy containing tungsten or titanium carbide to achieve hardnesses from 53 to 60 RC. The carbide particles within the nickel-alloy matrix can reach 90 RC. When milling such materials, coated carbide inserts suffer rapid flank wear, and their primary cutting edges wear flat. Extra-hard particles within the microstructure create 'microchatter' that accelerates insert wear. Carbide inserts can also fracture under the shear pressure of machining the hard stock.
CBN inserts provide a productive means to cut hard powder metals containing tungsten and titanium carbides. Advanced geometries can overcome microchatter. One user milling the powdered composite alloy found that an advanced CBN insert lasted better than 2,000 times longer than the best carbide inserts. A five-insert face mill running at 200 sfm and 0.007 inch feed per edge completed test cuts in the hard stock 75 percent faster than electrical discharge machining.
To make best use of CBN, cutting parameters must be maintained within a tight band. Speeds around 160 sfm and feeds of just 0.004 to 0.006 inch per tooth appear slow, but they are highly productive when machining sintered materials. Exact machining parameters are best determined by 30- to 60-second test cuts. Start at low speeds and build up until cutting edges show excessive wear.
Difficult materials should generally be machined dry to maintain consistent temperature on cutting edges. In most cases, a rounded cutter with double-negative geometry is most effective, and depth of cut is typically limited to 0.04 to 0.08 inch.
Milling cuts are by definition interrupted cuts. The constant hammering in materials hardened to Rockwell 60 or higher causes unique machining stresses. Machines and tooling must therefore provide maximum rigidity, minimum overhang and maximum strength to accommodate the high shock loads during machining.
Superalloys
Heat resistant superalloys (HRSAs) developed for the aerospace industry are gaining broader acceptance in automotive, medical, semiconductor and power-generation applications. Familiar HRSAs such as Inconel 718 and 625, Waspalloy, and titanium 6Al4V are now joined by newer titanium matrix and aluminum-magnesium matrix materials. All pose machining challenges.
Superalloys are hard; some grades of titanium are machined at 330 Brinell hardness. With conventional alloys, cutting zone temperatures greater than 2,000°F soften molecular bonds and create a flow zone for chips. In contrast, the heat resistance that makes HRSAs so desirable keeps them hard throughout the machining cycle.
HRSAs also tend to work-harden as they are cut, notching cutting inserts to premature failure. The difficulty cutting HRSAs is compounded where unpeeled stock is covered with abrasive, knife-edged scale that wears cutting edges down even more quickly.
Given their machining difficulty, superalloys are cut slowly. For example, Inconel 718 is milled for brake keys with Sandvik GC grade carbide inserts at 200 sfm. Turning speed for the same alloy with Sandvik CBN inserts in an outside turning/facing application is 260 sfm. By comparison, uncoated carbide inserts typically cut tool steels at 400 to 800 sfm. Feeds for HRSAs are generally comparable to those used when machining tool steels.
The choice of cutting inserts to machine HRSAs depends on the material and the workpiece. Carbide inserts with positive rake geometries will cut thin-walled HRSA stock effectively. However, thick-walled parts may require ceramic inserts with negative cutting edge geometry to create a more productive plowing action. While dry machining is preferred in most difficult materials to maintain uniform edge temperatures, titanium requires coolant even at very low speeds.
The sustained hardness of HRSAs accelerates wear on the nose radii of cutting inserts. A round insert with no sharp corner provides the strongest cutting edge, but the work-hardening common to HRSAs leads to progressive insert notching. Varying the depth of cut for consecutive machining passes avoids work-hardened zones, eliminates notch buildup, and prolongs the life of cutting edges. The depth of cut could vary from 0.300 inch on one pass to perhaps 0.125 inch and 0.100 inch on subsequent cuts.
Bimetals
Bimetal components put hard materials in select wear areas surrounded by or mixed with softer alloys. They are gaining popularity in the automotive industry and elsewhere, and they pose special machining challenges. The CBN inserts that are so productive cutting alloys with greater than 50 Rc hardness can fracture if they hit softer materials. PCD inserts able to machine abrasive aluminum suffer excessive wear cutting ferrous metals.
Machining bimetals productively calls for refined machining routines developed by the user, tool supplier and machine vendor. In one application, the hard powder metal composite alloy described earlier was hot isostatically pressed onto a less costly 316 stainless steel substrate. A helically interpolated tool path programmed into the machine control applied optimum feeds and speeds to machine the powder metal zone first, then the backing.
To machine bimetal cylinder blocks productively, automakers must contend with both abrasive aluminum alloys and cast iron cylinder liners. The design of the part means hard iron wear zones cannot be isolated from the soft aluminum. However, machine programs providing very low speeds and very light depths of cut enable abrasion-resistant PCD inserts to machine both aluminum and iron without frequent tool changes.
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