Equations for enhancing cutting efficiency

Several situations in the last few months made me realize cutting speed calculations are not always well understood. However, knowing this information allows metalworking professionals build a base of knowledge that can enable shops to machine more efficiently. In last month’s column, I discussed the basics of understanding cutting equations. In this column, I’ll examine how understanding relationships among the variables can lead to significant efficiency gains.

Cutting tool manufacturers recommend ranges of cutting speeds and chip loads for their products based on testing and experience. More often than not, these values are accurate, but the ranges can vary a lot. It is common for manufacturing engineers and machinists to adjust cutting parameters while developing a machining process. Once they have determined the optimal cutting speed and feed rate for a workpiece material, this information is available for recall when similar applications arise. This eliminates the need for experimentation, resulting in faster development times.

Most if not all CAM packages offer some sort of tool library that allows the programmer to set up different cutting tools and cutting speeds for each type of material. Cutting speed calculations must be understood so the programmer, engineer or machinist can convert the rpm and feed rate values from the machine into information the software can use, which is usually in sfm and chip load per tooth.

With accurate information collected from the shop floor, the programmer can deliver programs that require minimal adjustment. These programs, when coupled with quick-change workholders and probing routines, enable shops to profitably run small jobs.

Images courtesy of G.W. Schultz Tool

Part manufacturers can increase feed rates when applying tools with a high number of flutes.

I have previously discussed the importance of managing tool wear and how proper management of cutting tools can significantly impact tooling costs. Cutting speed is the most significant factor in managing tool wear. As cutting speed increases, tool life decreases. In a controlled environment, it is possible to predict how an increased cutting speed will influence tool life. The ability to accurately predict tool life improves decision making when attempting to reduce cycle times. Nonetheless, increasing the cutting speed might reduce cycle time but increase overall costs by reducing the life of expensive high-performance tools.

When attempting to reduce cycle time, the first reaction is often to increase the cutting speed and feed rate. According to the feed rate formula (rpm × no. of cutting edges × chip load = ipm), an increase in rpm while maintaining the chip load yields a faster feed rate, which means the machine gets a part done faster.

Getting done faster is good, as long as the tool isn’t already being run at its limit and the increase would cause catastrophic tool failure. Understanding the relationship between cutting speed and chip load can allow shops to creatively approach cycle-time improvements and circumvent these failures.

For example, when applying a 4-flute, ½” endmill at 500 sfm and a chip load of 0.004 ipr to machine a circle with a circumference of 18.84″ (6″ in diameter), and using the equation of rpm = sfm ÷ diameter × 3.82 and the equation previously provided for feed rate, the following values are generated: 3,820 rpm and 61.1 ipm. Therefore, it takes 18.53 seconds to cut the circle.