Knowing the Problem is Only Half the Battle! Part 9
Determining Tool Wear Mechanisms and Correcting for Them!
Mechanical Failure, Depth of Cut Notching! Part 9 of 10
We are now getting close to the end of our series on cutting tool failure mechanisms with just two more failure modes to review. Today we will address “depth-of-cut-notching.” Depth-of-cut-notching is very easy to identify. Just as the name implies, a notch or gouge forms on the cutting edge at the depth-of-cut line location on the cutting edge. The notch forms on both the rake and clearance surfaces of the cutting edge and is typically larger on the clearance face than the rake. Picture taking the corner of a file to the corner of a piece of wood, the resulting notch would look very much like the type of notch that forms on the cutting edge of the tool. When you think about it, the notch is formed in pretty much the same way as the file and wood example.
What two things does it take to make a chip, heat, and pressure. Depth-of-cut-notching is caused by excessive heat and pressure applied to a localized area of the cutting at the depth-of-cut line. The elevated heat and pressure is created by a hard layer of material that exists on the surface of the part. This hard layer of material can be result of several different factors and in many cases, is workpiece material dependent. Let’s start off by looking at cast iron.
During the casting process the outside layer of the part; which is in contact with the mold, cools much faster than the core material. The rapid cooling of the outer layer of the casting creates a finer grain structure in the material that is much harder than the softer base material underneath which cools more slowly. This hard layer sometimes called scale, varies in thickness and can be anywhere from a few thousandths to 1/32 of an inch thick. Given the scale is harder than the base material it acts just like the file mentioned above and gouges or cuts a notch in the part due to the localized increase in heat and pressure.
I sometimes compare it to an “Indian Arm Burn" or a “Snake Bite.” Remember that childhood game where your friend grabs your arm with both hands and twists your skin in opposite directions? The same thing is happening the cutting edge of the tool at the depth-of-cut line. The soft-core material versus the hard outer layer create differing amounts of heat and pressure which in turn rips material off the cutting edge creating a notch. The good thing about cast iron is once you remove the outer layer all that is left is the base material which is easier to machine. Unfortunately, some materials are not that easy.
High carbon steels, stainless steels and superalloys or refractory materials tend to work harden when machined. This work hardened layer acts just like the hard scale on a casting. If proper precautions are not taken it never goes away. That’s right, work hardening materials are just that work hardening. Each successive cut, or pass made on the part can leave a work hardened layer on the surface. So, how do we reduce the impact of this hard outer layer of material on the cutting edge?
First you strengthen the cutting edge by increasing the edge preparation. By increasing the hone size or the angle and width of the T-land you strengthen the cutting edge by redirecting the cutting forces into the bulk strength of the tool. Remember, carbide likes to be compressed. Its’ compressive strength is greater than its’ transverse rupture strength. In addition, to increasing the edge preparation you may use a more negative rake angle. Rake angle controls the strength of the cutting edge. A more negative rake angle increases compressive strength. In many cases increasing the cutting-edge strength will not eliminate depth of cut notching.
The use of a larger lead or approach angle will reduce the impact of notching by spreading the increased heat and pressure across a larger section of the cutting edge. This example illustrates the impact increasing the lead angle has on the cutting edge. As the lead angle increases from 0° to 75°, the hard surface layer of the workpiece and associated heat and pressure engages a larger cross sectional area of the cutting edge. By dispersing the heat and pressure more evenly the localized gouging or notching effect is eliminated and a consistent cutting edge wear pattern can be achieved.
Round inserts are often recommended when
machining materials with scale or work hardening properties like high carbon steels, stainless, and super alloys. We know that cutting forces are always perpendicular to the cutting edge. This gives round inserts an infinite number of perpendiculars all directed to the core of the insert. While this added compressive strength can help fight the forces of notching, what we are really looking to achieve is a larger lead angle. As you can see in the illustration, when using round inserts, a large lead angle is only achieved when using very low depths-of-cut. Therefore, when utilizing round inserts to eliminate notching, depths-of-cut greater than 15% of the I.C. are not recommended and to achieve greater depth-of-cut capability larger I.C. inserts must be used.
When machining work hardening material, you can reduce the impact of notching by varying the depth-of-cut by using an alternating tapered/straight or flat/ramp tool path. When utilizing this type of tool path the work hardened layer of material moves across the cutting as the depth of cut changes during the cut. This action distributes the extreme heat and pressure generated by the work hardened layer over a larger section of the cutting edge preventing the notch from forming thus increasing overall tool life. While not quite as effective as ramping or tapered cuts, varying the depth of cut on each pass will also improve your tool life and is sometimes easier to program.
Finally using a harder grade of carbide or a grade containing TiC can also improve performance and tool life when machining workpiece materials with scale or that work harden. Notching is caused by localized extreme heat and pressure. A harder more wear resistant grade of carbide can manage the heat better than a tougher impact resistant grade. you should typically save changing the grade to last, unless it is just a gross misapplication. When depth-of-cut, notching is the primary failure mode, make every attempt to strength the cutting edge before moving to a harder grade, because harder grades are more susceptible to chipping.
In summary, depth-of-cut notching is identified by a v-shaped gouge or notch that appears on the cutting edge of the tool at the depth-of-cut line. The notch is typically larger on the clearance surface than the rake surface of the tool. Depth-of-cut notching is created by localized extreme heat and pressure being exerted on the cutting edge at the depth-of-cut line. This failure mode occurs when machining workpieces with a hard outer layer of scale or work hardened material that can be found in castings, forgings and heat treated materials. Work hardening materials like high carbon steel, stainless steels and superalloys or refractory metals can create the hard outer layer as well. Several actions can be taken to reduce and eliminate the depth-of-cut notching. First, strengthen the cutting edge by increasing the edge preparation, t-land, or hone and by using a negative rake face geometry. Using a large lead angle will engage a larger area of the cutting edge with the hard layer of the workpiece supporting more even wear. Utilizing an alternating straight vs. taper tool path or varying your depth of cut on each pass will spread the extreme heat and pressure along the cutting edge as well. Finally use a harder more wear resistant grade of carbide or one that contains TiC.
Stay tuned for next week’s blog when we discuss our final failure mode in the series, “Breakage.”