One part (the stabilizer trim jackscrew) I designed at Boeing had a tolerance expressed as 4 digits after the decimal point. This was bounced back at me, suggesting I round it to a tighter tolerance with fewer digits.
I replied that I had calculated the max and min values based on the rest of the assembly. When a part is delivered, if it is out of tolerance it gets bounced to the engineers to see if it can be salvaged. As the jackscrew was an extremely expensive part, I reasoned that giving it the max possible tolerance meant cost savings on parts that wouldn't have to get diverted to engineering for evaluation.
Walter, I’m curious how you imagined that would all get implemented at the shop floor? Did you think it would actually be built as designed or did you always assume there would be some additional degrees of freedom or out of tolerance build you didn’t account for?
You ever go out and shoot the shit with the guys on the shop floor?
I wish I could have. The jackscrew was made by Saginaw Gear, a rather awesome company that did all Boeing's jackscrew work. I would have really liked to see that forging made. Probably the only better metalwork would be that on a turbine blade.
> Did you think it would actually be built as designed
Of course. You can't build modern airplanes any other way.
> did you always assume there would be some additional degrees of freedom or out of tolerance build you didn’t account for?
Nope. I accounted for the tolerances specified for all the parts it was to be connected to. When the airplane #1 was built, the jackscrew fit perfectly on the first try, which surprised the old mechanics working on it :-) It was my job to account for everything anyone could think of. It really wasn't a miracle or anything, just arithmetic.
Your response really surprises me. There may still be pockets of folks that work that way but by and and large the most I’d expect to see now out of the big aerospace manufacturers is a Monte Carlo simulation of the tolerance stack with a assumed normal distribution centered around nominal. Very unlikely to account for all the tolerance possibilities or even skewed distribution. Even that would be unusually detailed amount of engineering that you might only see on something as critical as the jack screw you worked on.
No, but I wouldn't be surprised if they did. If you're referring to the Alaska Air crash, the design of the jackscrew assembly was much older. The accident wasn't caused by a manufacturing fault in it. The design had problems, the maintenance on it was difficult, and pilot should have stopped trying to move it when it showed signs of trouble.
The other crash involving jackscrew failure (on a 747) was when an unsecured armored personnel carrier slid back and fell on it, snapping it. You can't really blame the jackscrew for that. No airplane is designed to handle heavy iron cargo flopping about in the hold.
Do you start having to specify measurement temperature? 10°C change can change length measurement in the 4th decimal place for say steel? Or is measurement temperature standardised?
Temperature had to be accounted for, as steel and aluminum expand and contract at different rates. There was a max and min temperature. The measurements were to be at room temperature. It was surprising (to me) how much the metal would move across the temp range. It'll also bend and compress and expand from the tremendous loads on it. I got so used to thinking of it as being like rubber, that I was a bit shocked when I got to handle the real thing, and how solid it was.
Temperature matters even for something as wildly variable as 3d printer build plate measurements. (Aka, always do it at 40C. Or any other fixed number. Americans tend to say "room temperature", but that only works for americans, who seem to have HVAC.)
Given that, I can only assume that every other branch of engineering has long since fully accounted for it.
I replied that I had calculated the max and min values based on the rest of the assembly. When a part is delivered, if it is out of tolerance it gets bounced to the engineers to see if it can be salvaged. As the jackscrew was an extremely expensive part, I reasoned that giving it the max possible tolerance meant cost savings on parts that wouldn't have to get diverted to engineering for evaluation.
The drawings got approved :-)