MRO Today - Mulling over mills

Mulling over mills
When you consider all the traits and options,
end mill selection can be easy.

When choosing an end mill for a particular milling operation, there are many possibilities as to which tool is best for the job.

Tool material, characteristics, type and whether or not a coating is applied all play an integral role and can have a significant effect on whether tooling adds value or significant cost to your milling application.

Tool material
Perhaps the first thing to determine when selecting a cutting tool is the tool material. High-speed steel (HSS), cobalt-based HSS and carbide are among the choices.

HSS is a baseline tool steel. It is used for many basic machining applications and is useful for very short runs on older milling machines. However, it does not offer the speed and feed advantages of the cobalt HSS and carbide tools.

A much better alternative to HSS is the cobalt-based version. With a typical cobalt content of 8 percent, this tool is usually priced a bit higher than standard HSS but allows for an increase in speed and feed rates because of its increased hardness.

As computer numeric control (CNC) machines replace milling machines, some manufacturers now offer cobalt HSS tools at prices comparable to regular HSS.

Carbide tools, which can be run at speeds 2 to 2 1/2 times faster than HSS tools, provide an opportunity for substantially improved production output.

However, consider several things when using a carbide tool:

1) Ensure that your machine tool is rigid with a solid spindle and that holders have little or no runout. Due to the brittle nature of carbide steel and the speeds at which these tools run, rigidity is critical to prevent tool breakage.

2) The initial cost of carbide tooling is at least twice that of HSS. Savings in production costs due to increased output and fewer tool changes can recoup the initial costs and then some.

3) Carbide is more sensitive to chipping than HSS, so handle with extreme care. Also, be sure to evaluate the work environment.

Tool characteristics
The end mill’s basic design can be altered in many ways to tailor the tool for a specific application. Here are some basic characteristic considerations:

Length: Select an end mill as short as possible to minimize tool deflection during the milling operation. Select stub lengths, if possible, to save on tool cost.

Number of flutes: Generally, more flutes reduces chip load and improves surface finish if the feed rate remains the same. The most common flute numbers for general milling operations is two (maximum space for chip ejection) and four (better surface finish).

Rake angle: End mills typically have a 10 to 12 degree positive rake angle. This allows for efficient cutting performances of soft, gummy materials as well as high-tensile-strength materials. Depending on the material to be machined, it is possible to change the rake angle. Used in conjunction with variation of rake, clearance angles can be altered to provide added support and strength directly behind the cutting edge.

Helix angle: The standard helix angle of an end mill is 30 degrees. Higher helix angles provide better chip ejection and are useful in machining materials such as aluminum at a much faster rate.

Also consider the tool’s end style. Beyond the traditional square and ball end, there are end types that increase end mill life and perform special milling operations.

Since the sharp corner of the end mill is its weakest point, a ground radius on the cutting tips is useful to prevent chipping of the tool tips. These corner radius tools remove more material faster than traditional ball nose mills, but you must consider the shape of the cut.

Center-cutting square end mills are essential for plunge milling. Use non-center cutting mills only for side milling.

Special tool types
Choose the type of cutting tool based on the application for which it is needed. Many special styles exist in order to find the perfect tool for any application.

Roughers, or “hoggers,” are useful for rapidly removing large amounts of material. The chip groove design allows more cutting fluid to the edges and dissipates heat better. Manufacturers provide variations in rougher profile forms for different material groups. The coarser the pitch of the roughing edge, the more material removed and the less smooth the finish.

In addition, most roughers have eccentric relief so regrinding can be done by grinding only the cutting face. This saves you money if you have access to regrinding equipment.

Corner-rounding end mills provide a uniform finish on corners. They have a ground radius with relieved clearance.

Drill point end mills are multipurpose cutting tools. Use them for drilling, slotting, chamfering and profile milling.

Chamfer end mills eliminate hand deburring operations and provide uniform material finish.

Coatings
Coatings can enhance the performance and life of HSS or carbide end mills.

Base coating selection on the material you are machining. Keep in mind, however, that surface hardness increases with coatings. If the substrate material has poor surface finish, the coating follows the same contour.

Although they can take many brand names, tool coatings basically include three types of titanium.

Use titanium nitride (TiN) for more aggressive machining of steels. Speeds typically can increase 25 percent over uncoated tools.

For stainless steel and non-ferrous materials, consider titanium carbonitride (TiCN). This coating can increase speeds as much as 40 percent over uncoated tools.

Use titanium aluminum nitride (TiAlN) for stainless steels, high-alloy carbon steels, nickel-based high-temperature alloys and titanium alloys. Speeds can increase a whopping 80 percent over uncoated tools.

Aluminum titanium nitride (AlTiN) and diamond are newer coating types that could gain wide use in the future.

Article provided by Melin Tool Company. Writers include Michael Wochna, president; Bob Srail, manufacturing manager; and Rob Wise, operations manager.

This article appeared in the August/September 1999 issue of MRO Today magazine. Copyright, 1999.