Understanding helix angles.
But what factors should be applied when determining what is the best helix angle for a particular job?
If we think of gasoline as being sold by the gallon, bread by the loaf, and eggs by the dozen, then end mills have a 30-degree helix. If there was a recognized "industry standard" for end mill helixes, then 30 degrees was it, and generally it performs quite well in a wide variety of applications. But as spindle rigidities have increased, and toolholders have become more sophisticated, there has been a noticeable trend to faster helix angles that can improve results in any number of end milling operations. These advances have pushed the evolution of more modern end milling techniques such as taking two cuts at two or three times the feed in inches per minute when compared with the old "hog it off in one cut" approach.
An end mill's helix angle is defined as the angle of the flutes relative to the axial centerline of the tool (see Fig 1). Helix influences tool performance mainly by affecting chip flow and cutting forces. A 30-degree helix angle has always been the most general-purpose design that functions well in the broadest application area for the most common materials and cutting conditions. The 30-degree helix offers a "middle of the road" combination of strength, chip removal capacity, balanced cutting forces, and versatility. With increased rigidity, higher spindle speeds, and more and more end milling being done with solid carbide. We are probably in the process of a shift to where 45-degree helix will become the more widely accepted general purpose choice for typical edging cuts with end mills.
[FIGURE 1 OMITTED]
What are the advantages of a faster helix? If you've ever watched a rowing competition during the Olympics, you may have noticed that the rowers turn their oars so that when the oar enters the water it is almost parallel with the surface of the water. Then, once the oar is submerged, the rower rotates his wrist so that the oar is positioned perpendicular to the axis of the boat's direction and then the oar is pulled through the "cut" in the water. Faster helixes on end mills achieve much the same effect. The entrance to the cut is much smoother and more efficient. This lowers the radial forces that want to push the end mill and workpiece away from each other. The result is a smoother machining action with less deflection.
Many of the newer, faster helix designs for end mills are specifically optimized for particular applications, such as finishing of titanium. The material cuts best with very moderate chip loads, lowered cutting forces, and keen edge-line micro geometry. More and more manufacturers are also offering endmill programs with variable helix geometry. With these tools, an infinite variety of flute pitch and helical variability are deployed on the tool grind via sophisticated CAD and CNC grinding techniques.
In a broad sense, it can be said that variable helixes are almost always an advantageous feature, but it can be difficult to quantify the exact benefits all of the time. On some applications, the improvement in performance will be quite noticeable with variable helix end mills. On other jobs, it may be difficult to see the difference. Each shop probably has to decide how much time they want to invest in experimentation with the infinite number of possibilities for variable helix styles. Shops also have to accept that regrinding of these endmills presents an entirely new set of challenges.
In shops that utilize more modern end milling techniques, and where more difficult materials and sophisticated components are being machined, faster helix solid carbide end mills should be applied regularly. The advantages of these newer tool designs will significantly reduce deflection and forces-resulting in a more consistent cutting action and workpiece quality.
Mark Stover provides sales and marketing services to manufacturers and distributors in the metalworking industry. He's had chips in the soles of his shoes for the last 25 years. This column appears in T&P every other month. Mark can be reached at firstname.lastname@example.org
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|Title Annotation:||tooling around|
|Publication:||Tooling & Production|
|Date:||Nov 1, 2005|
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