Knowing the Problem is Only Half the Battle! Part 8
Determining Tool Wear Mechanisms and Correcting for Them!
Mechanical Failure, Cutting Edge Chipping! Part 8 of 10
Three weeks ago, we began discussing mechanical failure. We kicked it off with chipping of which there are three types; clearance or flank face chipping, rake face chipping and edge chipping. Today we will address the third and final of the three, edge chipping. But, before we do we must recap clearance and rake face chipping. When identifying both rake and clearance face chipping we must first look at the chip shape and determine which face; the rake or clearance the chip is larger on. This in turn determines the direction of the force that created the chip came from. Then we adjust our cutting geometry to reduce or redirect the cutting force to the strongest section of the cutting edge. Edge chipping is different. When we look at the cutting edge, the shape of the chip is more evenly dispersed on both sides of the cutting edge or the same size on both sides of the cutting edge. This tells us the force causing the chip is more of an oscillating force typically caused by vibration or movement, rather than a mono-directional force that causes rake and clearance face chipping.
This vibration and movement causes intermittent forces on both the rake and clearance face of the cutting edge thus causing similar shaped chips to form on both surfaces. This type of edge chipping is often seen when experiencing chatter. The vibration and oscillation of the either the part, tool or both causes micro chipping which can grow over time to eventually result in catastrophic failure. The first corrective action to take with edge chipping is to “tighten everything up.” You must first insure that everything; the fixture, tool, machine, etc. is tight and running true. Once we are sure everything is tight we must verify our rigidity.
Vibration causes edge chipping; therefore, every attempt should be made to ensure you have a rigid set-up. Remember the golden rule of tooling; “short and fat is where it’s at.” Always use the shortest tool overhang or gauge length as possible. Reducing your gauge length by 25% will increase the stiffness of the set-up by 2 ½ times. Reducing your gauge length by 50% will increase the stiffness of the set-up by 8 times. The equation used to calculate stiffness in not linear, it is logarithmic; therefore, every little bit counts. The same is true for the size or cross-sectional area of the tool. By increasing the diameter by 25%, stiffness will increase 2 ½ times. When you put these two factors together you can understand why “short and fat is where it’s at.”
When all attempts to stiffen, the set-up fail and you continue to get vibration and edge chipping you should strengthen the cutting edge. You can strengthen the cutting edge by increasing the edge preparation. By increasing the hone size or the angle and width of a T-land you can 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 move to a more negative rake angle.
Rake angle controls the strength of the cutting edge. A more negative the rake angle will also increase compressive strength. In many cases increasing the cutting edge strength will eliminate the edge chipping. The added forces the increased edge preparation and negative rake create can also have the positive impact of pushing the vibration out of the operation; however, they can have also the opposite effect.
In some cases, increasing the edge preparation and negative rake angle will create too much pressure increasing the vibration and chatter that caused the chipping to occur in the first place. When this happens; you need to move in the opposite direction. I realize this is counter intuitive, but sometimes given the machining dynamics of a particular situation it can happen. In these cases, you still want to keep the set-up as rigid as possible; however, instead of increasing the edge preparation and negativity of the rake angle to make the cutting edge stronger, you reduce the size of the hone or T-land and use a more positive rake face angle to reduce the cutting forces. In both cases, it may also be necessary to change the grade of the cutting tool material as well.
Typically, I will leave changing the grade composition
to the last variable changed in an operation unless it is believed that the existing material is a gross misapplication. Changing the grade of the material usually requires changing the surface speed of the operation thus impacting overall productivity. If changing the grade composition is required, you would want to use a tougher grade of carbide. Tougher grades of carbide have larger grains of tungsten carbide that are held in place by greater amounts of cobalt resulting in a more impact resistant cutting tool material. Keep in mind that the greater the amount of cobalt the less heat the cutting edge can absorb without plastically deforming. Thus, you will need to adjust the surface speed down to prevent more rapid flank wear.
In summary, edge chipping is when the size and shape of the chips on the cutting edge are similar on both the rake and clearance face of the tool. Edge chipping should not be confused with rake or clearance face chipping where the shape and size of the chip is greater on one of the two faces of the tool. Edge chipping is typically caused by mechanical oscillation and vibration of either the tool or part. First, all attempts should be taken to make the set-up as rigid as possible. The shortest gauge length projection and largest tool diameter or cross sectional area should be used to increase stiffness. Increasing the edge preparation and using a more negative rake angle will strengthen the cutting edge by redirecting the forces and putting the cutting edge into a more compressive state. Finally, use a tougher grade of carbide.
Stay tuned for next week’s blog when we discuss our next mechanical failure mode, depth of cut notching.