A tip on lathe cutting direction by William Bassett

“We had a CNC shop for many years. One item required using 35° inserts for profiling, including machining up to a shoulder. We had a problem with breakage. Every machinist learns to feed a bit from the tailstock toward the headstock. We reasoned that as the insert curled a chip, that chip would get caught between the insert and the shoulder of the part, thereby breaking the tip of the delicate insert. The solution was obvious—reverse the feed direction. Machine the shoulder first and then feed toward the tailstock. The inserts lasted much longer. As a bonus, we discovered that we had much better surface finishes. The geometry of the cut was such that the lead angle of the insert in the direction of cut curled the chip away from the just-cut surface. We also obtained a better finish on the shoulder by programming a depth of cut equal to the tool tip radius. The tip radius gave us a pseudo lead angle.”

Some additional thoughts on tooling from an expert


End mills are not designed to work as drills. Because they cut sideways, they must be strong. This means that compared to drills, they are built short and stubby. This also means there is not as much space left for chips to clear and coolant to get to the cutting edges. The edges at the very center of an end mill do not form chips well. If you plunge an end mill directly into metal, a sort of work-hardened wad forms at the center that somehow has to mush through metal.


A better way to get down to depth is to slide down on a ramp if the pocket is rectangular. If the pocket is circular, or if you prefer, program a helix or spiral to get down to depth. This way, the end mill cuts on the outer edges, the way it was designed to cut. Some end mills are even made with a hole at the center where cutting edges would otherwise be. It is easier to sharpen these end mills if you do not have to worry about center cutting. Such end mills are often cheaper to buy.


As long as we are on the subject, use four-flute end mills if you cut tougher or harder materials. The end mills are stronger and stiffer and they have more cutting edges meaning they can be fed at twice the inches per minute for the same chip load per tooth. The down side is there is less room to clear chips and allow coolant in. Use two-fluted end mills for aluminum because there is more room for chip clearance and less need for strength. Studies have shown that there is no real limit to the speed at which you can cut aluminum. The only limitations are the rigidity and horsepower of the machine and the ability of the end mill material to absorb heat. Do not test this theory on Sherline machines because you can burn out the spindle and stall the stepper motors.


If chatter is a problem, it is good to reduce speed and increase feed as noted in the Sherline literature. For milling however, there is another choice that may help. End mills with a high helix angle are less prone to chatter. Here is an exercise that shows what is happening. Drag the edge of a card across a flat surface and it will chatter. However, if you drag a pencil or pen across the same surface, it will glide smoothly. A high helix angle is closer to a pencil point than the edge of a card.

–William Bassett, Clearwater, FL

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