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BJC

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As
A friend who scratch built 5 or 6 homebuilts said that engineers never finish airplanes because they are always changing things to “improve” the design, and they don’t really know what details / dimensions are important.


BJC
As I work on my super- slow build project - I think about what he said as well as the pace of work at the Glasair Two Weeks To Taxi program.


BJC
 
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Pops

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Very true, some places its very important and some places not. Engine mount attach points, LG and wing attach brackets, etc. = important. Rigging= Important. etc. We built steel jigs for making engine mounts for the Lyc-360. But we also installed the 5th engine mount attach point for a Lyc- 540 engine mount. One of the builders wanted to use a 540 and ordered a engine mount from the factory. He was concerned about the engine mount fitting the fuselages that we built. I told him IF the factory built their engine mount to the plans, it will fit. Fit perfect. Bearhawk factory turns out very good parts and at a reasonable price.
 

Geraldc

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Engineers tend to define tolerances down to a few thousandths of an inch, while experienced production sewers know that symmetry within 1/2 inch is the best they can achieve in the long run.
We built some arms that were 6 metres long with a hole in each end .
Draughtsman had put a tight tolerance on the hole spacings so engineers had to get it bored in large cnc mill.
Turns out for the application it could have been an inch out and still worked.
When I was fabricating we would have to set graduate engineers right and then they would move on.
Some of the fabricators had engineering diplomas but would rather build stuff.
 

Jay Kempf

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There's a need for tight tolerances. Every time I specify a press fit on a smallish shaft for some bearings I get crap from someone. But it is absolutely how the game is played. When you can heat and cool and make a shrink fit in a highly loaded rotating assembly I always feel vindicated. Most of the time whatever the machine is qualified for is good enough. All the low end houses can hit .005 all day long on every feature so you only really need to worry about fits that matter. Landing gear parts can get interesting.
 

speedracer

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When I bought my first totaled, late model Corvette to rebuild (out of eight) years ago I was shocked to find that all the nuts and bolts were metric. What? In an American car? Now I have a full set of metric wrenches and the only thing I use my "Merican ones for is my airplane. In my building contractor career, I had a guy working for me, a really good guy and a hard worker, but somewhat mentally challenged and would work for 12 bucks an hour. We were building a high end composite deck. I would call out lengths of decking and he would cut. The problem was he couldn't quite understand the 1/16th and 1/8th marks on the tape measure, but was OK with 1/2's and 1/4s so I started calling the 1/16th "One little mark." (One thirty two and three quarters and two little marks). That worked amazingly smooth.
 

ragflyer

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Love all the comments and as the OP don’t mind at all the drift but would love an answer to the original question :
Of course theUS uses a mishmash of units but my understanding was Europe was uniformly SI but my two contemporary European design books use the old MKfS with kilo force. Is this common or very unusual within the European airplane design community?
 
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One thirty two and three quarters and two little marks
Rather common in the home construction industry in my part of the world. Workers can't seem to read a tape measure and 1/8ths are generally good enough. The call out gets translated to "32 and 3" for 32 3/8 or "32 and 4" for 32 1/2. Same with feet. Feet get translated to inches. "145 and 6" = 12' 1 3/8". Saves a lot of lumber and sheetrock.
 

wsimpso1

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In the parachute industry it is easy to see if a pattern was drawn by an engineer or an experienced sewer.
Engineers tend to define tolerances down to a few thousandths of an inch, while experienced production sewers know that symmetry within 1/2 inch is the best they can achieve in the long run.

That is funny. Sewer - a pipe for carriage of waste water, and something or somebody who sews. Looks like the same word.

I have changed industries a couple times in my career. The number of folks who set tolerance based on "How good do I think they can do?" is startling. Position of an air circulation blower on a submarine could be toleranced +/- 1", but the pattern of bolts for each blower had to be within about a 0.040" so that none of the rubber mounts had preload - the world's quietest machines needed that sort of thing. Then for small, there were the clearances, and thus tolerances in the multistage water pump housings and rotors.

In medical telescopes we used tight tolerances and on some of the stuff lapped them to fit. Other stuff was loose because no one thought it was important until they found out they could not focus a scope or worse, found out some of them would fill with body fluids while you were working on something...

In sporting firearms we had some stuff toleranced to whatever the worked sanding the gun stocks could manage to 0.010" for some parts to rifling toleranced to 0.0002" on diameters. What is important? What isn't? I have worked with folks who got a six digit dimension out of a calculation, so that went on the print. Ugh. And folks who would never tolerate a stackup derived upper and lower dimension... they always had to shift things to use one digit less on the print- like anyone else even cared.

Billski
 

Aesquire

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Reverse engineering from a sample isn't just rote copying. The tolerances are important. You don't know if your sample is on the high or low side on any given dimension. It's easy to just copy numbers off the measurements of a part, but it takes either experiments with multiple copies, ( the expensive way ) to make sure your guess work on specs functions, or the smarts and experience to replicate the work & reasoning of the original developer. Who might not be the "designer" or record.

John Moses Browning is one of the great inventors. Few would argue. But his invention was often a process from sketches and verbal instructions to his machinist, literally in some cases holding his fingers up & telling him, "make it about that big" . Then the prototype would be tested, patents filed, the sale to the customer made, and only then would the customer's team make the invention a thing that could be manufactured and work.

In other fields often the guy who gets the fame is the manager who writes the proposal, not the people who turn it into hardware.

In aviation an example of that would be Anthony Fokker. He knew what he wanted, in some cases, but it was his employees that did the translation of desire to airplanes or equipment.
 

Vigilant1

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My John Deere X320 lawn tractor.

Some things are metric and some Imperial.:confused:
Yup, mine too (X300) and some parts of it are a POS.


BJC
An interesting story below on JDs policies for its big equipment. Some farmers may think they own their JD tractors, but Deere kinda owns the farmer.
https://doctorow.medium.com/about-those-kill-switched-ukrainian-tractors-bc93f471b9c8

Orwell and Huxley foresaw the big stuff, just the details and mechanisms are novel.
 

BJC

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An interesting story below on JDs policies for its big equipment. Some farmers may think they own their JD tractors, but Deere kinda owns the farmer.
https://doctorow.medium.com/about-those-kill-switched-ukrainian-tractors-bc93f471b9c8

Orwell and Huxley foresaw the big stuff, just the details and mechanisms are novel.
Darn you, V1.

I was just getting settled down from accidentally hearing some news on TV, when I read that article.

Sad, scary, disappointing, infuriating, ....


BJC
 

Dana

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The number of folks who set tolerance based on "How good do I think they can do?" is startling.

I have to admit, I'm guilty of that. Standard title block tolerance is 3 place +/- .005, 2 place +/- .01, but the guy running the Bridgeport can hold .002 all day long so why not show 1/8" as .125 which it really is instead of .13 so they punch in .130?

An interesting story below on JDs policies for its big equipment. Some farmers may think they own their JD tractors, but Deere kinda owns the farmer.

The company I worked for in the late '90s put a one time disable code in all their machines sold overseas. A month after the final payment was due it would stop working. When they called for tech support, only if they'd paid the final bill it'd be, "What's the light flashing? Oh, that's a rare error... press the reset button five times while holding the stop button, then press the start button six times and it will fix it."
 

Vigilant1

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These ubiquitous VIN locks and kill switches are a factor worth considering as homebuilders convert future car, motorcycle, ATV, etc engines for aircraft use. If you are using any of the OEM "smart stuff," you might get a nasty surprise.

Lots of consumer, privacy, and security (infrastructure protection) aspects to this subject. I'm not confident we have the best minds working on it.

And don't get me started on inkjet/laser printer lockouts to force purchase of unneeded OEM supplies.:mad:
 

ScaleBirdsScott

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Haas machines financed thru them have lockouts after 180 days. Gotta call in and have the timer reset after they check account status. (They may have an online reset too, but I haven't connected ours to internet.)

I get it: some company finances 20 machines, after 2 months just stops paying... Meanwhile just keeps running the equipment hard for years while the courts figure out who owes what. Even if they repo them (at great expense) they're worn out. At least they are very up-front with the terms of the deal. The tradeoff is fairly decent financing from a US company, in an industry that can be tricky otherwise.
 

Riggerrob

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I got an inkling of why they add those software "kill switches."

This reminds me of an automotive engineer who worked for a small company that built automated manufacturing equipment for the Canadian automobile industry. His company designed and built some automated production tooling and sold it to a major manufacturer. After they built 100,000 widgets on the machines, the larger company refused to pay because some obscure reset function was slower than a specification. So the smaller company was forced to go broke after the larger company completed all their production run. My buddy got laid-off and left the automotive industry in disgust.

Wouldn't it have been wiser to add an obscure line of software .... After too many customers complain about silent automobiles, the complaint gets back to the company that made the production tooling. "Oh? He does not work here any more. Click!"
Cue evil laughter.
 

Geraldc

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We built some arms that were 6 metres long with a hole in each end .
Draughtsman had put a tight tolerance on the hole spacings so engineers had to get it bored in large cnc mill.
There's a need for tight tolerances.
In this case the tolerance was something like 5998 +- .02mm
Here is the actual machine.Pushing boxes of cold meat into freezer racks.Holes were in ends of yellow arms .
1653204798803.png
 

SeppoK

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Love all the comments and as the OP don’t mind at all the drift but would love an answer to the original question :
Of course theUS uses a mishmash of units but my understanding was Europe was uniformly SI but my two contemporary European design books use the old MKfS with kilo force. Is this common or very unusual within the European airplane design community?
I was trained in the 70's and already then we did all calculations in Newtons. We used some old material charts that used kp/mm2 for strength and kp/cm2 for pressure. No big deal as 1 kgf = 1 kp ≈ 10 N. I must say I haven't seen a new text book or a scientific paper for decades that uses kgf or kp.
 

Vigilant1

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Haas machines financed thru them have lockouts after 180 days. Gotta call in and have the timer reset after they check account status. ....
At least they are very up-front with the terms of the deal.
That seems fine to me. It is all above board, customers can choose instead to get other financing, and presumably the leash is off after the last payment is made.
The "DMCA 1201” revision held every 3 years to adjust the battle lines over copyright protections sounds way more important than most folks might think. I'm sure industries are well represented by the best lawyers money can buy. I'd sure like to know consumers are also represented and (especially) that the PTO isn't just a rubber stamp for industry.
 
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