Nose radius, feed rate and surface finish – an insert nose radius chart
The nose radius of the insert has a direct effect on the surface finish of the part in turning. Turning actually forms a helical groove on the part – a thread. The depth of the groove is the roughness height. The roughness height (Rmax) is directly proportional to the square of the feed rate, and inversely proportional to the nose radius of the tool. You can actually use a quick reference nose radius chart to determine the feed rate that you must use to get a specified surface finish for any nose radius.
This equation gives you a rough Ra value (the exact value is of course only got by actual measurement of the surface).
This nose radius chart shows the Rmax in microns for various combinations of nose radius and feed rate. These are theoretical values, and the actual finish that you get will depend on the tool wear, the quality of the machine, vibrations, etc. These values are however a good starting point.
To improve the surface finish you can either reduce the feed rate or increase the nose radius. Halving the feed rate improves the surface finish four times (reduces the Rmax value by 75 %). Doubling the nose radius improves the surface finish twice (reduces the Rmax value by 50 %). You can use the insert nose radius chart above to quickly decide the feedrate obtainable.
CADEM CAPSturn CNC lathe programming software automatically determines the cutting feed rate based on the surface finish required and the tool’s nose radius.
