On the machine, you don’t use brute force, you manage your energy. You create heat through friction that needs to be released, you don’t just add more power to push against that resistance. Getting feed rates and speeds right is what makes all the difference between running a tool for hours or having it fail within seconds. Taking too aggressive of a cut at speed with a carbide end mill in stainless steel can cause it to shatter, then you’ve got a rough surface on the part, and scrap.
To start with RPM, the chart shows a series of listed values of tool diameter versus surface feet per minute. Take the suggested speed for the material, multiply it by a constant, and divide by cutter diameter. It’s just basic physics: A bigger cutter mean longer circumference, so it needs to be spun more slowly to keep up the same rate of cut on the edge of the cutter compared to a smaller one. Starving or overloading the cut is simply failing to respect this relationship.
Tips for Machining
Pre-calculated tables for common cutter diameters are included in the infographic. It save some time on the shop floor. It takes the guesswork out of set-up.
The material makes all the difference. The reference guide differentiates hard metals, such as Inconel and titanium, which is more difficult to machine. It contrasts them with softer metals, such as aluminum, which you can cut deeper and faster while keeping the chips flowing.
Titanium holds heat like a sponge and conducts it poorly, so when you start turning a piece of titanium, the cutting edge gets very hot very fast. If you’re not careful and try to turn titanium too fast, you will soften the tool and cause it to fail. Harder alloys needs to be turned much slower (check out the chart) to avoid breaking tools.
The material of both the tool and the work piece are equally important. For instance, HSS (High Speed Steel) is great in applications like interrupted cuts where the tool could collide with a clamp or on a manually operated machine because it’s not only flexible but also tough. This means it will flex a bit first and then break. Carbide is harder, which means it will run faster and stay sharper longer. However, it is brittle, so if you shock it, it will shatter.
So choose the right tool based off the geometry of the part and the rigidity of your machine. Using HSS on a high speed CNC mill is wasting money on tool changes. High speed steel on an old wobbly lathe won’t do you any good either.
A big part of that equation has to do with coolant strategy. For stainless and steel, flood coolant moves heat out of the cut area. Aluminum likes mist coolant which does not wash away lubrication but keeps edge clean. Cast iron usually goes dry, the chips are like a lubricant, putting fluid on it can mess things up or damage the surface.
The clue comes from seeing the smoke color. No smoke is good, blue means you got a problem.
The last variable is chip load, which is how much chip each tooth grabs with every revolution. Too little, and the tool will just rub along, creating heat from friction instead of actualy cutting. Too high a load, and you run the risk of chatter and breaking off the edge. The chart has ranges for both heavy roughing cuts and finishing cuts, but most people start out by cutting too lightly, believing that’s the safe way to go. Often a bit heavier cut will clear away the chips better and last longer.
It’s a dance between the material, the tool, and the machine. Start where the numbers take you, but listen to what the machine sounds like. Is it a smooth, high-pitch whine? That’s good. Or does it have a rhythmic thump-thump or grind-grind? That’s bad. Your ears say whether you’re heading down the right path or not. Set the dial, check the chart, and listen for the clean cut: success.
