A formula is just a way of conveying the method to someone.
I'm obviously not making myself clear. In fact I left this rathole because jaguar36 is a dishonest piece of shit but it's 4:00 a.m. and I'm sick of being locked up, even tired of watching hot girls do exercise routines on wechat but the wifi is good ...
No, what I do is not a formula. All I do is take the desired speed in distance over time, divide by distance over one revolution (same as multiply by the inverse) to get revolutions over time. If this is a formula, then so is filling your water glass 3/4 full at the sink. Or taking a recipe for an 8" pie and multiplying it by relative volumes for a 9" crust. It's just simple logic.
Why care ? In fact, people are welcome to do whatever they like. They don't actually have to know a damn thing about machine tools these days to make parts. They can just look up a recipe in sinha's book and now they are a machinist.
there are also other ways. I was "inspired" by the fact that this thread got to be pages long with ridiculous descriptions of a simple problem taking up a whole page from a certain party who couldn't even turn on a damn lathe. Then along comes a guy who thinks that 22/7 is a troll and throwback to the ancient Egyptians, when it comes out with the exact same answer as his "accurate method" and better than his quick and dirty. But, you know, he's cool and everyone else is a troll or fool or "wrong".
My position is simple. On a mill there's three axes of motion and complicated contours and all the hassles of programming by hand justify crappy programs. Just watch a complicated program some day and the 20% air time will have an actual machinist cringing. But it's worth it because the programming is too complex to do in a reasonable amount of time by hand.
Lathes are not generally like that. The op's example should have been answered in three posts. "Take your desired speed, divide it by the distance per rev, and that's your answer." Simple. Instead we got pages of fanuc-reasoning.
If people want to use canned cycles and cookbook recipes for blowing their nose, it's up to them. But there is a better way. Sinha's way sucks if you've ever actually run a lathe. Same part, let him program it and me program it and my parts will be better and come out faster and be easier on the machine. For two or three pieces, fine, do it the quick and shitty way. But if you make 50 or more, you're losing time and quality. There is a better way.