Living through this change over in Australia, here are some observations I noticed. Some things changed over very quickly, others still haven't changed.
Metric dimensions on machining was pretty easy, because the imperial measurements were usually decimal anyway. Our lathes had new collars engraved for the handwheels, in metric. This was fine unless you needed to traverse more than one rev of the feed screw, when the metric changeover point occured at an odd number, because the feed screw was still imperial. DRO's fixed that.
BA threads finally made sense, but no one used them anymore!
If you need a nicely rounded thread form for fatigue resistance, you have to go to something exotic, which is a pity, because we already had whitworth threads.
Pipe threads stayed put, and so did the pipe, but with really odd metric sizes on them.
Fractional inch measurements went almost instantly, can't recall even hearing a negative comment from the works carpenters ( more like cabinet makers) who were glad to stop jiggling 32 nds around in their heads. Ask them to hand you a pice of 50 by 25, and they would hand you a bit of 2 by 1 though. Some of them would hand it to you faster than others.
Road speed signs, kilometers were accepted pretty quick, don't hear many talking mph these days, unless they have been watching drag races, where metric doesn't seem to fit at all. This also gave more speeds, used to be
35, 45, 55 mph, now 60, 70, 80, 90 and 110 kmh. The 70, in particular, was a welcome addition, lifting some 35mph sections up a bit over their initial 60 kph conversion..Number and letter drills were replaced by metric drills at 0.1 mm steps, thats 4 though a step, no one in our shop seemed to miss those. If you go to the local hardware store today, you will find plenty of fractional imperial drill sets, and just lately, some metric ones. The metric ones are priced higher, because they don't sell many to the average householder.
Tire pressures are still in psi, and show no signs of ever changing. The local garage has a new electronic gizmo that you set the pressure on. It's in psi all the time. I believe this is due to the large difference in the numbers. Pascals are such a small unit, you need too many kPa's in your tire, just don't seem right for some reason???
I regularly mix metric and imperial measurements in spoken conversation, sometimes even in the the same sentence. Drives my kids nuts, they only get taught metric at school.
I notice they still give height measurements for bank robbers in both metric and imperial on the news, because more than half the population can't relate to the metric height, me included I must admit.
Temperature measurement in celcius seems to have been accepted, I guess the zero at water freezing and 100 at water boiling gave the cooks something to visualise, made the old 32 212 measure look a bit silly.
Just this morning on the weather report, I was surprised to hear the (sea) swell measured in feet only, no metric. I knew what she meant, but my kids can't visualise what a foot is, apart from the one on the end of their leg.
Designing a new work shed, had to review my blocks survey plans, they're all in chains and perches, ackk! First time I have wished to see feet and inches for a long while.
I did electrical engineering, my dad gave me his old ee text books for a reference. How he ever made sense of it is beyond me. Anyone who has actively worked in both systems will prefer the coordinated nature of SI units. In fact, if they are initially trained in SI units, it's very difficult to 'think' in the old units. I believe it's easier to change from imperial to SI, than to go the other way.
I believe it was worth the change, and I'm happy to put up with all the issues that it brings, because I know in one or two generations it will settle down to a much more coherent system. The problem I see in the US is that because there has not been an official change, the transition period will be much longer, and more painfull and costly, if the next generation is still getting their fundamental awareness in imperial units.
The official change started here around thirty years ago, I reckon it's got another thirty to settle properly. Probably more like one hundred years in the US, at the present rate, and being caught between the two systems for any longer than necessary is a worry.
regards,
John