Milling: pushing carbide to the edge.
I. How to reduce carbide-insert failure
Inserts, to a great degree, determine the effectiveness of a milling cutter. To withstand high temperatures and cutting forces, cutting edges of carbide inserts must be tough to resist breakage, and hard to resist wear. Although both are required in a cutting edge, they are generally considered opposite characteristics. Increasing edge toughness will result in decreased hardness, and vice-versa.
In straight carbide grades, hardness and toughness are determined by two variables: grain size of the carbide particles, and percent of cobalt present. Although hardness typically decreases with larger grain size, toughness usually increases. Likewise, a higher cobalt percentage provides greater toughness, but lowers hardness.
In addition to straight carbide grades, there are also alloyed carbide grades, which may contain titanium carbide, tantalum carbide, or both, and coated-carbide grades. These various grades have been developed to resist specific types of cutting-edge failures. For example, straight grades of tungsten carbide and cobalt offer high resistance to abrasive wear. Alloyed grades help reduce cratering that is common when machining ferrous metals.
Coated carbide inserts can be either straight or alloyed grades that have a vapor-deposited surface layer that is typically 0.0002" thick.
Three metallic compounds commonly are used for these coatings. Titanium carbide (TiC) is silver-gray and offers high resistance to abrasive wear and cratering because of its increased hardness. The gold-colored titanium nitride (TiN) coating provides lubricity for the sliding chip and also helps reduce wear. It often is called a multi-coating because it's composed of up to four layers. Aluminum oxide (Al[sub.2 Osub.3]) is the third common coating. It appears black and withstands high temperatures.
Abrasive wear - Normal abrasive wear is caused by friction between the workpiece and the insert. Abrasive wear, or edge wear, appears as a uniform wear land on the flank side of the insert and is the only acceptable failure mode.
Lower speeds reduce friction heat at the cutting edge, and therefore reduce overall wear. On softer workpiece materials, a harder grade or a coated grade often will help reduce wear.
Crater wear - Cratering is caused by a thermochemical reaction as hot chips slide across insert rake surfaces. When this occurs, chips weld to particles of carbide in the insert, and, as chip flow continues, the carbide particles are actually pulled out of the tool. Cratering begins behind the cutting edge. Eventually, as the crater gets deeper and moves toward the cutting edge, the edge will break away.
Reducing speed will reduce heat levels. Feedrates can be increased to compensate because of their minimal effect on temperature. Also, an alloyed or coated cutting edge can reduce cratering because of greater chemical inertness. Grades alloyed with a high TiC content also reduce cratering.
Deformation - Deformation occurs when an insert softens because of heat and then distorts under the cutting forces. Deformation is usually seen under the cutting edge when tool material bulges out from the original surface.
As with cratering, this type of failure can be minimized by decreasing cutting speeds to reduce heat and decreasing the feed per insert to reduce tool pressure. These methods, however, are counter-productive and should be avoided, if possible. Deformation can also be reduced by changing to an insert with a high tantalum carbide content.
Built-up edge - This type of failure is identified by a ragged cutting edge produced by material buildup on the insert's rake face. During cutting of certain materials, particles separate from the chips and are forced, under pressure, to the rake surface of the insert.
As this buildup extends out to the cutting edge, it actually becomes the cutting edge. When the buildup breaks away from the insert, it pulls carbide particles away from the cutting edge, further reducing the effectiveness of the insert.
Built-up edges are most often caused by cutting at lower speeds on soft or gummy materials. Increasing cutting speeds and using TiN-coated inserts will help avoid this problem.
Notching - Notching is identified by a high degree of localized wear on the insert at the depth-of-cut line. Notching can be caused by machining materials that have hardened surfaces and softer material cores. This type of wear is produced by the hard surface accelerating the normal flank wear in the area of the depth of cut. It may also be caused by thermochemical reaction, which softens the insert material at the depth-of-cut line.
To minimize notching, harder grades and coated grades should be used. Higher lead angles help by distributing the cutting forces over a wider section of the cutting edge. Increasing the insert's hone and reducing the cutting speed can also help minimize notching.
Chipping - When the mechanical load of an operation exceeds the strength or toughness of the cutting edge, an insert may chip. Small fragments of carbide are broken off the cutting edge, not worn away.
Chipping can be minimized by reducing feed, increasing insert hone, or improving rigidity. Tougher straight or alloyed grades of carbide inserts will also minimize this type of failure.
Thermal cracking - Thermal cracking appears as fine lines that usually are perpendicular to the cutting edge. It occurs in situations where cutting temperatures cycle between high and low, often caused by setups where each insert cuts for only a portion of a cutter revolution. Each insert cools during the remainder of the cutter revolution until it enters the cut again. These temperature changes cause the carbide at the area of contact to expand and contract rapidly, causing mechanical stress.
Thermal cracking can be minimized by proper coolant application, but coolant may aggravate the rapid heating-and-cooling cycle if improperly applied. Tougher grades of inserts have a higher tolerance to rapid temperature change and also will reduce thermal cracking.
Fracture - Insert fracture failure occurs when a large portion of the insert breaks off, immediately rendering the cutting edge useless. It's caused when the mechanical load of the operation greatly exceeds the strength or toughness of the insert.
If fracture is a problem, first check the inserts for sign of other failure modes. If none are evident, try indexing the insert earlier in its life. Feedrates and cutting speeds can be decreased to reduce cutting forces. Tougher straight or alloyed grades of inserts also can be used to accommodate high mechanical loads.
The bottom line
By making use of information presented here, you should be able to decrease or even eliminate most types of premature insert failure. The most critical aspect of correcting any failure is the correct diagnosis of the failure mode. Choosing the correct insert will minimize the destructive effects of heat, cutting forces, and thermochemical reaction. You may even find that you have reduced the inevitable abrasive wear common in any machining application.
II. Face-mill basics aid tool selection
When milling with an indexable face mill, the workpiece, machine, and fixturing must all be examined for rigidity. This will help insure efficient use of this type of tool and produce the required results.
After this examination has been completed, you can address the subject of the cutter in terms of diameter, geometry, lead angle, and density and pitch. Only cutters using indexable carbide inserts will be discussed here.
Proper cutter diameter - For maximum efficiency, a face mill should be engaged in the workpiece approximately two thirds of the cutter diameter. Another way to figure this is that the cutter diameter should be about 1 1/2 times the width of cut desired.
Climb milling using this ratio of cutter diameter to width of cut will ensure a favorable entry angle into the workpiece. At the point of entry into the workpiece, the cutting edge will be taking an adequate initial bite.
When it's uncertain whether the machine has enough power to operate the cutter under this ratio, it may be best to divide the cut into two passes (or more) to maintain the ratio as closely as possible.
Applying cutter geometry - Cutter geometry is another important aspect of face-mill design. Insert cutting edges may be positioned relative to both radial and axial planes in either positive, neutral, or negative rakes. However, neutral rake is generally not used in either plane, because the entire cutting edge would impact the workpiece on initial contact.
The combination of radial and axial rakes determines the shear angle, and there are three basic combinations available: negative radial and axial; positive radial and axial; and negative radial, positive axial.
Double-negative geometry has been a traditional "starting point" in selecting a face mill for rough milling cast iron and steels when horsepower and rigidity are adequate. The double-negative insert allows the strongest possible cutting edges, able to withstand heavy chiploads and high cutting forces.
However, the increased cutting forces generated by this geometry will consume more horsepower. Double-negative cutters also require greater machine, workpiece, and fixture rigidity. Additionally, double-negative tools do not have the shearing angle needed to produce good finishes.
Double-positive geometry provides the most efficient cutting action because of its increased shearing angle. Although not as strong as double negative, entry impact and cutting forces are greatly reduced. This makes it a logical choice for older, less-rigid machines, or where horsepower is limited.
With double-positive geometry, the peripheral edge - both in the radial and axial planes - leads the insert through the workpiece, creating a true shearing action. Its shearing action also makes it the best choice for nonferrous materials and many soft, gummy stainless steels.
Negative-radial and positive-axial geometry combines some of the advantages of both double-negative and double-positive types. Negative radial rake provides strong cutting edges, while positive rake creates a shearing action.
Axial rake determines the direction of chip flow, and since this type of cutter is positive in the axial plane, the chips are directed up and away from the workpiece. This prevents recutting of chips and takes heat away from the milled surface and the cutting edge.
Effective lead angles - The lead angle is another criterion to consider when selecting a face mill. A 45-deg-lead tool, for example, reduces chip thickness about 30 percent. This allows maintenance of a given chipload at increased feedrates for higher metal-removal rates.
A proper lead angle also allows a cutter to enter and exit the cut more smoothly, thereby reducing shock on the cutting edges. Workpiece edge breakout, a common problem in machining cast iron, can be greatly reduced or even eliminated by use of increased lead angle. An optimum angle lets the cutting edge gradually exit the workpiece, often reducing radial pressure and minimizing the potential for breakout.
Remember, however, that increasing the lead angle to reduce radial pressure does increase axial pressure. This could cause deflection of the machined surface when the workpiece has a thin cross-sectional area.
Choosing cutter density - The density of the cutter must be such that it will allow the chip to form properly - and clear the cut. Inadequatee chip space can cause the chip to remain in the gullet and be carried around into the succeeding chip, welding, and then breaking the cutting edge and possibly damaging the workpiece.
However, it's necessary that the cutter have the density to keep at least one insert in the cut at all times. Failure to do this could cause severe pounding, which can lead to chipped cutting edges, a damaged cutter, and excess wear on the machine.
Coarse-pitch tools, with 1 to 1 1/2 inserts per inch of diameter, allow more space for chip gullets. These tools are recommended for soft materials that produce continuous chips, and for wide cuts with long insert engagement.
Fine-pitch tools, with four to five inserts per inch of diameter, should be used where lack of insert engagement is a problem. The fine pitch allows at least one insert to be in the cut at all times, even on extremely thin cross-sectional areas. Also, these tools are recommended for high-temperature alloys and hard steels, where light chiploads are taken. The chips are smaller, so less gullet space is required, allowing more inserts per inch.
Although selecting a face mill can be a complicated and somewhat subjective process, these guidelines can give you a good beginning. By using these principles, and expanding on them, you'll be able to select a face mill for any operation in your plant.
PHOTO : Abrasive wear
PHOTO : Crater wear
PHOTO : Deformation
PHOTO : Built-up edge
PHOTO : At Estee Mold and Die, an Ingersoll Max-I-Shear Plus face mill cuts ESCO 49C stainless steel, a material with low machinability rating and hardness of 29 Rc. At $10/lb, each 8" x 14" x 16" workpiece costs $6000. The face mill removes a total of 0.500" on the sides and bottom of the part during roughing. * The cutter runs at 0.250" depth of cut, 100 sfm, and 48 rpm. Feedrate is 4.6 ipm, and each insert takes a chipload of 0.007". Tool life is about 250", compared with 60" in a previous setup using a helical-flute indexable end mill operating at only 0.100" depth of cut. Tolerance is held to 0.002". * Each insert has four cutting edges and 0.100" wiping flats. Both coated and uncoated inserts work well in this setup, but engineers chose coated tools to complete the job. The workpiece material is mushy, tending to push the tool away from the cut; depth of cut must be limited to prevent work hardening. * Cutter geometry and lead angle combine to let the tool enter the cut smoothly and exit with minimum burr. Insert design provides high shearing action with positive axial rake, and special radial rake to increase edge strength.
PHOTO : Notching
PHOTO : Chipping
PHOTO : Thermal cracking
PHOTO : Fracture
PHOTO : Diagrams based on Ingersoll's on-edge insert design.
PHOTO : Climb milling requires adequate initial bite.
PHOTO : Double-negative geometry - radial and axial planes - allows strong cutting edges to withstand heavy chiploads and high cutting forces.
PHOTO : Positive-geometry tools are more efficient because they lead inserts through the cut, providing shearing action. They are best for nonferrous materials and gummy stainless steels.
PHOTO : Lead angles control chip thickness and determine how smoothly the inserts will enter and exit each cut.
PHOTO : Coarse pitch
PHOTO : Medium pitch
PHOTO : Fine pitch
PHOTO : Cutter density controls chip formation. There must be at least one insert in the cut at all times to prevent chipped cutting edges caused by pounding. However, coarse-pitch tools allow greater chip-gullet space.
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|Title Annotation:||insert failure prevention and face-milling tool selection|
|Publication:||Tooling & Production|
|Date:||May 1, 1990|
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