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Comment by beerandt | Hacker News Reader

>They said they'd feel more comfortable with 10% extra strength.

Did they want the extra 10% buffer for any particular reason?

Reason I ask, I always agreed with your take on not over-designing, philosophically. I had a couple of professors who made a big deal about being conservative/ round-up on calculating design elements, but once you tally up the different loading cases, you shouldn't ever bump the total required strength (again) beyond whatever the required multiplier is.

However, when faced with real life projects, I learned fairly early to ignore that advise, mostly because of the risk of having to redesign due to a changed design requirement. Of course I've also never worked for a company as structured as one like Boeing, and I've also encountered LRFD 99% of the time over ASD.

It's one of those life lessons that's never really sat right with me, but I still consider to be the rational choice.

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> Did they want the extra 10% buffer for any particular reason?

They said they just were more comfortable with more margin. The stress group was an independent organization to avoid conflicts of interest with the design group. Our job was to design, their job was to verify. If parts broke, they got the black eye. If the airplane was overweight, design got the black eye. We both had to agree, and it was a system that worked well.

Most design engineers didn't work out the stresses themselves, they just hoped to pass the stress group. It was pass/fail for them. In my not so humble opinion, they were making overweight parts. I was interested in using math to sculpt perfect parts :-)

For example, the inside diameter of the jackscrew was specified to the ten-thousandth of an inch, something like 2.1834..2.2096. So I'd get the "why not make it a nice round 2.19..2.20?" I'd reply because if it came in at 2.205, it would get rejected, even though it was perfectly usable. And I got my way, because the jackscrew forging was a very, very expensive part and rejecting a usable part did not make anyone happy.

(Rejected expensive parts often got bounced back to engineering to find a way to salvage them. I just was doing the math in advance so they wouldn't have to.)

That specification is to 2.5 micron accuracy. Surely the forging can't be made to that precision?

> Surely the forging can't be made to that precision?

It's forged slightly oversized, then machined down to spec.

Forgings are about 3x stronger than a casting, and are of a more consistent quality, which is why when I increased the HP for my Dodge, I spent the extra bucks to get forged spinning parts (usually they are cast).

The range is much larger: OP was just being very precise in expressing the range so the manufacturing process had some extra leeway.

I wonder what was measuring to that precision with such accuracy! It’s when I think about tolerances on the equipment that’s checking tolerances that I start getting dizzy and retreat to software for a bit.

2.5 micron is basically the metric value for a 'tenth', i.e. one ten-thousandths of an inch. A typical machine shop will usually work in tolerances of a 'thou' or two, but most are equipped to measure to tenths without too much fuss. Standard micrometers have a vernier that reads tenths, and tenths indicators aren't uncommon.

Going beyond that, to tens of millionths or sub-micron, is where things get nutty with special climate controlled rooms. Digital indicators can easily read with such precision, but controlling for factors like radiated body heat become important.

Reference measurement is usually done with gauge blocks and pins. The development and popularization of these sets is basically how mechanical parts became standardized.

If you're interested in that sort of thing - bootstrapping precision and the like - check out "The Foundations of Mechanical Accuracy".

The precision (ability) of the machine rarely matches the precision of the measuring apparatus, if it's even part of the same equipment. And that's usually ok, even preferred, and cheaper.

If it's a rabbit hole you're interested in, then check out the history of geodesy / geodetic surveying & the design of mechanical / optical surveying equipment- especially theodolites & auto-levels (and even chronometers and astronomic observatories, if you want to go 4-dimensional).

It was the "industry" that had a large hand in innovating this technology, which led to the smaller versions used in manufacturing (made even more interesting due to the fact it was all funded to better artillery, and ultimately ballistic missle targeting, sans-GPS.)

I'd be happy to steer you in the right direction, if interested. It's a personal favorite.

Possibly a coordinate measuring machine (CMM) in an environmentally controlled environment, but even then it's coming awfully close to the capability limits

The spec is almost certainly for the machining after it's been forged.

I would have loved to take a field trip to Saginaw Gear and see all the various steps in its manufacture.

Like this but bigger.

https://www.youtube.com/watch?v=dzIsR4Mg158

"ball screw" and "lead screw" are your search terms. For aerospace it's gonna also be center drilled to save weight as well.

The forged parts are likely machined (for better finish and tolerance).

I wonder if you guys all went on a field trip to watch The Wind Rises haha

Sometimes these factors of safety are matters of convention (like the discretion of a company of certifying engineer) and sometimes they are grounded in some safety standard.

For example, if you're testing the integrity of a pressure vessel, the standard is to test it to 110% the rated maximum allowable working pressure if the medium is a compressible fluid (like air) or 150% if it's a 'non-compressible' fluid (like water, although everything is compressible to a certain extent). This is because the compression of a fluid stores a lot more internal energy if suddenly released. (Don't quote me exactly on these numbers because it's been a looooong time since I've had my nose in ASME standards)

I get factor of safety- (and phi etc for LRFD) what we're talking about is beyond the required factor of safety. OP designed the part using required multipliers, but the department checking the design wanted an extra 10% strength.

The point I was trying to get at is that in many cases (e.g., those not covered by an industry standard) there is no explicit “required” factor of safety. It’s left up to the discretion of the authority having jurisdiction. So while the design engineer may feel like a factor of safety of 2.0 is sufficient, the AHJ may “require” a 2.2 FOS to sign off

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