How to use the surface finish Ra calculator
The fine helical marks left on a turned surface come from the tool nose radius sweeping along at the programmed feed. Guessing the feed when a print calls for 63 microinch usually means recutting the part, so enter the feed and nose radius here to get the theoretical finish and the feed required to hit a target.
The relations used are Ra = f squared / (32 R) and Rz = f squared / (8 R), with f as feed per revolution and R as the nose radius. Feed enters as a square, so halving it improves the roughness figure by roughly a factor of four. Running 0.004 IPR with a 1/32 in nose radius gives a theoretical Ra near 16 microinch.
Metric prints state the same quantity in micrometers. One micrometer equals about 39.37 microinch, so Ra 1.6 micrometer is roughly 63 microinch and Ra 0.8 micrometer is roughly 32 microinch. Both units appear in the results so you can read either drawing without converting by hand.
Keep in mind this is a purely geometric model of the tool nose path. Built-up edge, chatter, tool wear and ductile smearing typically push the measured value to 1.2 to 2 times the calculation, so leave margin when planning the finish pass. If a finish still will not come in, a larger nose radius or a wiper insert usually beats simply dropping the feed further.
Frequently asked questions
For turning, Ra is about feed squared divided by 32 times the nose radius, and Rz is feed squared divided by 8 times the nose radius. Because feed enters as a square, halving the feed cuts the roughness value to roughly a quarter.
The formula only covers the geometric marks the tool nose leaves behind. Built-up edge, chatter, tool wear and ductile smearing usually push the measured value to somewhere between 1.2 and 2 times the theoretical figure.