Knowing the Problem is only Half the Battle! Part 7
Determining Tool Wear Mechanisms and Correcting for Them!
Mechanical Failure, Rake Face Chipping! Part 7 of 10
Last week we began discussing mechanical failure mechanisms. We started off with edge chipping of which there are three types; clearance face chipping, rake face chipping and cutting edge chipping. For this session, we will address rake face chipping. Rake face chipping and clearance face chipping are often taught together as they have one thing in common. The chip on the cutting edge is larger on one of the cutting faces than it is on the other. This is a very important point; if you don’t understand this concept often times you will make the incorrect adjustment to the operation and make the problem much worse.
As you look at the graphic above please note that while both inserts have chipped cutting edges, their size and shape and placement on the cutting edge is not the same. The insert on the left has a chip that is larger on the rake face of the tool than it is on the clearance face. When the chip on the cutting edge is larger on the rake face than it is on the clearance face, it is rake face chipping. The opposite is true for clearance face chipping. The chip is larger on the clearance face than it is the rake face. You might ask yourself why this is important. To answer that question, I ask to look at the direction of forces illustrated by the red arrows. Chipping is caused by a force being exerted on the cutting edge that is greater than its compressive and transverse rupture strength. If we know where the force is coming from, we have half the battle won.
In rake face chipping the force is impacting the clearance face of the cutting edge and is being exerted from a direction perpendicular to the cutting edge. Once we understand where the force is coming from we can begin taking steps to reduce it and redirect it. One frequent cause that generates this force is not enough clearance angle on the cutting edge. This is often the case when machining work hardening materials. Work hardening materials such as nickel-based alloys, work hardening stainless and titanium have a tendency of spring back. This material spring back into the clearance face of the tool produces force, friction and heat. As the clearance face heats-up, thermal expansion kicks in to create more force which causes the rake face of the cutting edge to pop off or chip. Rake face chipping may also occur when machining soft gummy steels, aluminums, and other springy materials. Springy materials or materials with elastic memory have a high modulus of elasticity. This elastic memory causes the material to spring back onto the clearance face, creating force and can cause material to build up creating built up edge. As the material builds up the force on the clearance face becomes to great and the rake face chip will appear. With both cases, it is important to match the proper clearance angle geometry with the material being machined. In most cases increasing your clearance angle will reduce the heat generated at the shear zone thus reducing the likelihood of thermal expansion and prevent rake face chipping.
This table lists good starting points which should be adjusted as wear patterns indicate. Just remember, while increasing your clearance angle will reduce rubbing it will make the cutting edge weaker, so when making adjustments start small. Your clearance angle is not the only element that can cause thermal expansion and rake face chipping.
Your feed rate can also contribute to the heat generated thermal expansion and rake face chipping. Your feed rate must always be greater than the edge preparation, hone or T-land. When your feed rate is less than the edge preparation, the cutting edge is not engaged into the workpiece enough to create the required shearing action. Instead of cutting it creates a rubbing or burnishing affect. This in turn causes pressure, heat and thermal expansion of the insert resulting in rake face chipping. In addition to rubbing, if the feed rate is not greater than the edge preparation, hone or T-land the cutting force is redirected into the clearance face of the tool. The combined thermal expansion and higher clearance face forces will cause the rake face to pop off or chip. So, make sure your feed rate is always equal to or greater than the edge preparation, hone or T-land on the cutting edge. Thermal expansion is not the only cause of increased clearance face pressure.
We often see rake face chipping in boring applications as well. In boring applications, the cutting forces may cause boring bar deflection. When the boring bar deflects below the centerline, clearance under the cutting edge is reduced as the tool engages the arc of the bore ID. This reduction in clearance and increase in resulting force on the clearance face will cause the rake face to chip or pop-off. Again, the resulting chip on the cutting edge will be larger on the rake face of the tool and smaller on the clearance face. Remember “short and fat is where it’s at.” Use the shortest overhang and largest diameter bar possible. Positive rake cutting tools with a minimum edge preparation, hone or T-land will also reduce the tool pressure and reduce deflection. If deflection persists, you can set the boring bar above centerline. You can find the amount of offset above centerline using a standard deflection calculation. Using given parameters; DOC, feed rate, workpiece material, the length and diameter of the boring bar, you can calculate the amount the bar will deflect while in cut. When the bar enters the cut, it will deflect to the correct position on centerline.
In summary, you can identify rake face chipping when the size of the chip in the rake face of the cutting edge is larger than the size of the chip on the clearance face. The chip is created by a force being exerted on the clearance face of the tool in a direction perpendicular to the cutting edge. This force is created in four distinct ways. Two causes are thermal expansion of work hardening materials and springback of materials with high elastic memory. This thermal expansion interferes with the cutting edge clearance and increases clearance face pressure. Increasing the clearance angle will reduce the pressure and chipping. Feed rates less your edge preparation, hone, or T-land direct cutting forces into the clearance face of the tool. The force can be reduced and redirected by using feed rates greater than the edge preparation on the cutting edge. Finally, tool deflection often seen in boring applications decreases clearance and increases the clearance face forces. Use of short rigid overhangs and free cutting edge geometries set-up can reduce deflection and the resulting chipping.
Stay tuned for next week’s blog on mechanical failure when we discuss cutting edge chipping.