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You Must Play all the Angles! Part 1

Updated: Jan 2


Cutting Tool Angles, Functions and Importance! Part 1 of 3 - Lead or Approach Angle

All cutting tools have three angles; lead, rake and relief. When I say all cutting tools, I mean all of them; drills, turning tools, end mills indexable mills, cut-off tools, reamers, etc.… I really mean all of them. They may function a little differently based on the cutting application but all tools have them.


Let’s take Lead angle first. Lead angle can also be referred to as attack or approach angle. The definition is; “the inclination of the cutting tool as it enters the cut.” Drills are the easiest illustration to explain lead or approach angle. The point or chisel edge is the first part of a drill to touch or enter the cutting process and the drill point angle of has two cutting edges. A standard HSS high-speed-steel drill has a drill point of degrees 118°, but do not confuse the point angle with the lead angle. The lead angle of the cutting edge of any tool is measured from a datum line drawn perpendicular to the direction of the feed into the part. The illustration top the right illustrates the correct way to measure the lead angle for a drill. The drill is being feed down into the part. Draw a line perpendicular to the feed direction and measure the included angle between the cutting edge and the line drawn perpendicular to feed. The lead angle of a 118° drill point would therefore be 31°.


There are two main reasons why lead angle is important. The first is simple, cutting forces are always perpendicular to the cutting edge. Ok, so why is that important? The answer is simple, but first answer this question; in the movie STAR WARS, what did Obi-Wan tell Luke to do with the force? The answer is, “use the force.” The same is true in machining, you must “use the

force” generated during the cutting action to your advantage. It takes two things to make a chip, heat, and pressure. That pressure or force can work for you and/or against you. If you know that cutting forces are always perpendicular to the cutting edge, you also know that by changing the angle of the cutting edge you can change the direction of the forces. You always want the cutting forces directed to the most rigid

part of the set-up.

Have you ever had a situation where you could not feed the tool the as fast as you wanted to because the part would bow or cut a taper, or you would push it out of the chuck or away from the stop. By changing the lead or approach angle you can change the direction of the cutting forces thus making a more robust and ridged machining process. This is one of the most underutilized “truth about machining”. I can’t tell you how many times I have seen where a small change in turning tool holder style, a lead angle change of a milling cutter, a helix angle change on an end mill, or cut-off tool made all the difference between failure and success or losing or making money. But, managing the direction of the cutting forces is not all the lead or approach angle controls.


The lead or approach angle also controls the chip thickness. So, what is that important? What two things does it take to make a chip; heat and pressure. Where do you want the heat to go; into the chip. But for the heat to go into the chip, you must first have a chip thick enough to absorb the heat. As the lead angle changes so does the chip thickness. The closer the lead angle gets to 90 degrees “as measured perpendicular to the direction of the cut,” the thinner the chip gets. The thinner the chip, the less heat it will absorb. Therefore, as the lead angle Increases your feed rate must increase to ensure the proper chip thickness. At issue is you need a chip thickness thick enough to absorb the heat. The chart to above illustrates the impact of the lead angle and chip thickness. We will talk more about federate compensation and chip thickness when we begin talking about metal cutting application in turning and milling.

So, in summary, all tools have three angles; lead, rake, and relief. Today we discussed lead angle. Stay tuned for our next blog when we will discuss the tools rake angle and its function.


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