# 57. Surface finish and nose radius

### CNC: Surface finish and tool nose radius in turning – the connection

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. This equation gives you a rough Ra value (the exact value is of course only got by actual measurement of the surface).

This table 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 %).

CADEM CAPSturn CNC lathe programming software automatically determines the cutting feed rate based on the surface finish required and the tool’s nose radius.

### Etc.

Wasps’ nests – marvels of civil engineering

I saw this huge wasps’ nest in Cubbon Park in Bangalore recently, wondered how it was made and what was in it.

Some scratching around on the www told me that the large fruit-like nest is actually made of paper, and the paper is manufactured by the wasps themselves. The wasps scrape wood from a tree or log, then use water and saliva to break down the fibers in the mouth to make soft paper pulp. They paste into the paper pulp into the required shape for the nest. The cellulose fibres in the wood are the equivalent of the steel reinforcement in buildings, and the saliva is the cement. The spherical structure is just the outer protective covering. Inside this are hexagonal cells where young wasps develop, one per cell. A whole lot of wasps live in a single nest in cooperation. There must also be a complex passage system so the wasps can navigate around the thousands of cells.

Some wasp facts

Wasps do not make honey. They are however closely related to honey bees, which do make honey.

Wasps are primarily predators and feed on other insects.

A Wasp can sting you and then retract its stinger, and stays alive after stinging you.

When a Bee stings you it cannot withdraw its stinger, loses its stinger and much of its abdomen in the process, and dies.