Universal thread cutting

[Anonymous]

Posted by DC31k on 05/07/2021 19:09:50:

In an alternative universe, we might have a definition of the inch where 5 inches equals 126mm. There, 126t change gears might be rather more common.

You wouldn't need 126, you could use 63, 21, 7 or even 3!

It's a fundamental theorum of mathematics that any positive integer can be factored into primes, and only primes. So whatever the conversion factor there will be a need for a gear with a prime in it somewhere. It may have to be a multiple of a prime as small numbers of teeth wouldn't be practical.

Andrew

[John HaineParticipant @johnhaine32865]

From Wikipedia:

"As a result of the definitions above, the U.S. inch was effectively defined as 25.4000508 mm (with a reference temperature of 68 degrees Fahrenheit) and the UK inch at 25.399977 mm (with a reference temperature of 62 degrees Fahrenheit). When Carl Edvard Johansson started manufacturing gauge blocks in inch sizes in 1912, Johansson's compromise was to manufacture gauge blocks with a nominal size of 25.4mm, with a reference temperature of 20 degrees Celsius, accurate to within a few parts per million of both official definitions. Because Johansson's blocks were so popular, his blocks became the de facto standard for manufacturers internationally,^[27][28] with other manufacturers of gauge blocks following Johansson's definition by producing blocks designed to be equivalent to his.^[29]

In 1930, the British Standards Institution adopted an inch of exactly 25.4 mm. The American Standards Association followed suit in 1933. By 1935, industry in 16 countries had adopted the "industrial inch" as it came to be known,^[30][31] effectively endorsing Johansson's pragmatic choice of conversion ratio.^[27]

In 1946, the Commonwealth Science Congress recommended a yard of exactly 0.9144 metres for adoption throughout the British Commonwealth. This was adopted by Canada in 1951;^[32][33] the United States on 1 July 1959;^[34][35][36] Australia in 1961,^[37] effective 1 January 1964;^[38] and the United Kingdom in 1963,^[39] effective on 1 January 1964.^[40] The new standards gave an inch of exactly 25.4 mm, 1.7 millionths of an inch longer than the old imperial inch and 2 millionths of an inch shorter than the old US inch.^[41][42]"

[Nigel Graham 2Participant @nigelgraham2]

Millionths….. I'm lucky to find a thousandth!

For practical purposes I cut threads to near-depth then finish with a die, to form the profile as much as anything.

[brian jones 11Participant @brianjones11]

growing up in the days when we still had an empire, I know what a thou looks like and know what tenths are and how the undamaged finger can feel a ridge of a few tenths

I struggle to force myself to think in microns = eg 100mu ca 1/2 thou, 10mu ca 5 tenths

CNC goes to 1mu and as for finishing well ……………

repeatability on a Myford 1 thou if you are skilled with a light touch.

As for mils thats for the cousins

modern machining techniques leave me totally in awe

[John HaineParticipant @johnhaine32865]

I wonder if it crossed Johansen's mind that defining the inch as 25.4mm would result in a fairly low integer ratio for change gears?

[Martin ConnellyParticipant @martinconnelly55370]

Johansson's choice was similar to what Whitworth did for his thread standard. Collated information about what the various threads being made in the area were like with regard to angle and TPI and based his new standard on what seemed closest to the average. It is probably impossible to know what drove their choices one way or the other when they made them but they were both businessmen so were probably thinking about the bottom line not the future ramifications of their choices. I know that when I have been creating a drawing of something at work I often tweaked designs to get nice round figures on the dimensions, not because it was necessary or better for the part, more for the fact it just made it look like I had put a lot of thought into the part (CAD has made this easy). I often had discussions with the design departments about why they had done something one way or another and often they did things vertical, horizontal or at 45° just because everyone else did and so it all looked well thought out. It was rarely about the engineering needs or the built in cost of their design.

Martin C

[duncan webster 1Participant @duncanwebster1]

From my days working on a real live drawing board, angles were as far as possible multiples of 15 because there was a quick release button that locked the rules in 15 degree increments.

And yes on occasions I did have to draw in Indian ink on linen to modify old drawings, but not with a quill pen. Spring bows were a nightmare, Rotring pens much more forgiving

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