Tech Talk
Why do endmills have different numbers of teeth?
Standard end mills come with 2,3,4,6 and 8 cutting edges. More often than not, getting the desired results depends on selecting a tool with the right number for the job.
The 2-flute end mill, is one of the hardest-working members of the end mill family. Since it has only two cutting edges, its gullets are deeper and longer than end mills with more cutting edges, providing more chip-carrying capacity. This feature makes the 2-flute end mill an ideal tool for milling operations that begin with a plunge cut and, in general, cuts where the tool is "buried" in the work and chip removal is a problem.
The 2-flute end mill is a center-cutting tool. It can be used for multiple operations - plunge cutting and slotting, spotting, drilling, profiling etc. - which can minimize the number of tools required to complete a given job as well as the equipment downtime required for tool changes.
As discussed in an earlier chapter, when the 2-flute end mill is used as a slotting tool, results are characteristically superior to anything obtainable using a cutter with a greater number of teeth. Proper slot size and location can be more easily held.
The 3-flute end mill is a compromise between the chip-carrying capacity of the 2-flute end mill and the better finish capability of the 4-flute end mill. At any given instant, at least one and frequently two of the teeth are solidly in the cut; the cutting action is smoother than that of the 2-flute end mill, with less chatter and flutter. The 3-flute end mill has ample chip-carrying capacity, and at the same time the finish is usually fine enough that a second finishing cut is unnecessary.
the 3-flute end mill is probably the most under-used standard end mill. End mill users who are currently using a 2-flute end mill for a cut and having to make a second pass to achieve an acceptable finish may be able to get acceptable results in a single pass using the 3-flute end mill.
For a given feed rate, a 4-flute end mill, will produce finer finishes and will last longer than a 2-flute end mill. This is because the amount of metal which must be removed per revolution of the tool is distributed over a greater number of teeth and the cutting load per tooth is reduced. The same is true of multiple-flute end mills generally.
For the same reason, a 4-flute end mill is capable of higher metal removal rates than a 2-flute end mill. For a given feed rate per tooth the feed rate per revolution of the cutter is directly proportional to the number of teeth on the cutter. This means that a 4-flute end mill can be fed at double the rate of the 2-flute end mill, and a 6-flute end mill can be fed at triple the rate of a 2-flute end mill.
Of course, there's a natural limit to this effect: the more teeth there are on the periphery of the cutter, the smaller and shallower the gullets become. Theoretically, an end mill could have an infinite number of teeth - but there would be no place for chips and the cutter would strangle itself.
The higher production rates possible with the 4-flute end mill make it the most widely-used end mill in high volume milling operations. In fact, the 4-flute end mill is as commonly used in milling operations on the more difficult metals as the 2-flute end mill is used for aluminum and other relatively soft materials.
The 4-flute cup-end mill is used for operations that do not involve plunge cutting. It is most often used after a roughing cut to finish off slots, recesses and cavities. With the smaller chip load per tooth, this tool is particularly effective for making interrupted cuts and machining of thin-walled parts.
One would use the 4-flute center-cutting end mill for plunge cutting applications, particularly where fine finishes are required and moderate to difficult to machine metals are involved. Typical applications are plunge cutting and slotting, cam milling, tracer milling, die sinking and just about any other operation where axial cutting is involved. The ball-nose version of this tool is widely used for die and mold work where machinging of contours and compound curves are involved as well as in production machining where part design dictates a rounded surface.
The 4-flute end mill is perhaps the most versatile member of the end mill family. Because it can be fed at higher rates than the 2-flute end mill, it can reduce the time required for a given operation - an important feature in production milling operations. The same tool can be used at reduced fee but higher speed to deliver exceptional machined finishes. The 2-flute end mill is most frequently associated with aluminum and softer stock, while the 4-flute end mill is used for both the easy and difficult-to-cut metals.
End mills with 6-flutes and 8-flutes, offer the high production rates and superior finishing capabilities that generaly characterize multiple-flute cutters. These cutters are available only in larger sizes (3/4 inch diameter and up). One reason for this is the fact that the 6-flute and 8-flute end mills are more difficult to manufacture in small sizes. Another reason is that the limited chip carrying capacity of such tools would greatly reduce their effectiveness.
Most tool manufacturers offer multiple-flute end mills with four flutes in sizes up to a given diameter. For larger sizes, the tool is offered with six or eight flutes. At end mill diameters where the number of flutes increases, tool manufacturers will frequently offer the tool both ways. Thus, in the larger end mill sizes, the user is frequently able to choose between a 4-flute, a 6-flute tool and an 8-flute tool. When making the selection, bear in mind that the tool with the higher number of teeth can be fed at a higher rate with better wear distribution; this makes for higher production rates an/or improved tool life which can make the tool more cost-effective than the cutter with fewer teeth.
