6 ms·

I think the idea is that learning to use a slide rule results in a deeper "intuitive understanding" of what the results of calculation should look like. Using

I think the idea is that learning to use a slide rule results in a deeper "intuitive understanding" of what the results of calculation should look like. Using a slide rule is also explicitly imprecise, so the user isn't expecting that the result to accurate to n decimal places. They're aware of the imprecision and are likely at least considering whether the level of precision is enough to answer the question they're asking. If I'm looking at a blueprint as see a dimension listed as "1.5mm", my instinct is that anything from 1.47mm to 1.53mm "-ish" is likely to suffice. I'm going to want to understand how that part interfaces with others to make sure it won't cause an issue if it's slightly different. If on the same drawing that dimension is marked as "1.5125mm", my assumption would be that the person who drew it out was specific for a reason. I'm going to be much less likely to try to consider the interface with other parts because I assume that level of precision indicates that it's already been considered. Note that the above is just a conceptual example. I'm not a draftsman, machinist, or an engineer - I've just done enough amateur machining and design work for 3D printing that it popped to mind. Yes, I'm aware that there are implicit and explicit tolerances based on the number of significant digits in a measurement. :)

WalterBright 4y ago
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. The drawings got approved :-)
- robocat 4y ago
  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?
  - Filligree 4y ago
    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.
  - WalterBright 4y ago
    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.
- rdtwo 4y ago
  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?
  - WalterBright 4y ago
    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.
    - jdougan 4y ago
      You wouldn't happen to know if they also did the jackscrew work for McDonnell Douglas?
      - WalterBright 4y ago
        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.
operator-name 4y ago
I think the digital vs analogue clock is an adjacent, everyday example. On the human scale precise clock maths is rarely necessary, and conceptually thinking of time as a base 60 number can be more trouble than good. Technology Connections has a very good video on this, and completely changed thinking: https://youtu.be/NeopkvAP-ag https://youtu.be/NeopkvAP-ag
jacobr1 4y ago
Further, you need to understand fits and tolerances. Maybe even things like thermal expansion properties. I learned this the hardway in my freshman intro-to-engineering class when I 3d-milled parts for a basic mechanical clock ... and had the whole system freeze up with friction because I didn't take it into account that you don't have exact fit of parts.
Dylan16807 4y ago
If you're serious enough to make a blueprint then you'll have explicit tolerances and it won't matter why they wrote out a specific number of digits. If the tolerances are fewer digits then treat it like a decimal seventh or thirteenth or whatever, not a deeply engineered result.

WalterBright 4y ago

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. The drawings got approved :-)

robocat 4y ago
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?
- Filligree 4y ago
  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.
- WalterBright 4y ago
  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.
rdtwo 4y ago
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?
- WalterBright 4y ago
  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.
  - jdougan 4y ago
    You wouldn't happen to know if they also did the jackscrew work for McDonnell Douglas?
    - WalterBright 4y ago
      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.

operator-name 4y ago

I think the digital vs analogue clock is an adjacent, everyday example. On the human scale precise clock maths is rarely necessary, and conceptually thinking of time as a base 60 number can be more trouble than good. Technology Connections has a very good video on this, and completely changed thinking: https://youtu.be/NeopkvAP-ag https://youtu.be/NeopkvAP-ag

jacobr1 4y ago

Further, you need to understand fits and tolerances. Maybe even things like thermal expansion properties. I learned this the hardway in my freshman intro-to-engineering class when I 3d-milled parts for a basic mechanical clock ... and had the whole system freeze up with friction because I didn't take it into account that you don't have exact fit of parts.

Dylan16807 4y ago

If you're serious enough to make a blueprint then you'll have explicit tolerances and it won't matter why they wrote out a specific number of digits. If the tolerances are fewer digits then treat it like a decimal seventh or thirteenth or whatever, not a deeply engineered result.