What you describe sounds like a nightmare, but I may be able to help with one aspect. To line up long screws I have taken longer screws and cut the heads off leaving me with a stud. Thread the studs into the member that is tapped and stack everything up in the right order. Once you have all of the parts in position all the holes should be aligned. Replace one of the studs with a standard screw. then just go around and relace each stud with a screw one by one. It is still a royal pain, but it does make the job easier.
Now if the mounting holes are in a symmetrical pattern, but the parts must be mounted in an irregular orientation life has really gotten interesting and you will learn the value of a scribed line before disassembly.
But if you really wish to see a really hard to service appliance, one that I wound up using plain brute force on, check out those newer Keurig coffee brewing machines. Besides that they are incredibly complex. Two DC motor driven pumps, two solenoid valves, a pressure transducer, heaters, a check valve, a microcontroller plus an LCD screen controller, a triac, and a multi-element inline noise filter. And over 50 self tapping screws, in addition to quite a few threaded ones. And it seems that taking it apart must need to start with the serving head instead of simply opening the bottom by removing a few screws. And replacing the air pump requires almost complete disassembly.
I would really like to see a teardown of one of those, it would probably be at least 75 pages. Unfortunately I disposed of most of the one that I disassembled because of all the broken plastic.
One can see, possibly, how the PCB screws ended up the way they did.
Each of the 3 metric screws were sized for their particular location or function. The shorter one is probably a result of something behind the screw location which prevents using the same length screw as its mate.
The larger metric screw may be the screw that grounds the PCB.
The US screw may be threading into a purchased component which was not available in metric threads.
It's possible to see the design intent in that light.
Seriously - US manufacturers making a machine might choose to standardize on US fasteners, but if they need a photoelectric sensor, they'll end up needed metric fasteners which thread into the sensor.
It doesn't mean I'd support or approve such a mish-mash, just that one could see the intent.
It seems from the description that the package was not intended to ever be serviced or repaired. I have come across a number of products like that. And the choice of different threading may have been driven by a desire to use materials already in stock at the factory. Using parts that are already sourced for other products is a fairly common and smart cost reduction method used in many area.
So are the different screws for different uses, like the PCB screws are metric, the chassis screws are sheet metal and the mounting screws are ASME? I'd hate to think that two chassis screws would be metric, then one is sheet metal, and the fourth is ASME.
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