81. End mills – Helical vs. Straight
CNC – Why a helical end mill is preferable to a straight one
Each time that the edge of an end mill comes in contact with the part, there is an impact and a sudden shock load. If the end mill has 4 edges and is rotating at 1000 RPM, that’s 4000 impacts and vibration of 66 Hz (4000 / 60, to convert to seconds).
With a helical end mill, some edge or the other always in contact with the part. There is no periodic impact, and cutting is therefore smoother. Helical end mills give you better part quality, and are kinder to the machine’s spinde bearings.
Pics. and text source: Cadem NCyclopedia multimedia CNC training software.
Peacocks – not as stupid as a I thought
I saw a peacock doing its ritual of trying to attract a mate near Delhi some time ago. The ritual consisted of spreading his stunning feathers, and shaking them around. I didn’t see any peahens around, and thought that the guy was wasting his time. Turns out I was wrong – he wasn’t wasting his time.
The quivering feathers emit a low-frequency sound inaudible to human ears. Depending on whether the peacock wants to attract females from far away or nearby, it changes the sound by shaking different parts of the feathers. So this peacock of mine knew that there were no hens around, but was using the sound to attract any that were hanging around out of sight. The peahen presumably comes over to check out the hot guy who’s making all that noise. Peahens judge the suitability of a peacock as a mate by the length, width and color of the tail.
The tail is only there during the mating season, and is shed every year after the season. So if you see anyone selling peacock feathers in markets (common all over India), rest assured that the feathers were donated voluntarily by some peacocks, which were not bumped off. By the way, killing a peacock (it being India’s national bird and all) attracts a jail term of upto 7 years.