I grew up in the US with our goofy units. I was told as a child that we were going to convert to the better way... but it never happened. With our tools (nuts and bolts) we have a mix, so I have to have both English and metric wrenches and socket sets, which is a pain. I prefer when the socket is metric and I don't have to figure out if 9/32" is bigger than 16/64" (as opposed to 7mm vs 6mm) when I rummaging for the next bigger or smaller socket. For nuts/bolts, it would be nice if things were all metric... except that I'm an RF guy and I use mostly 3.5mm (SMA) connectors which have 5/16" nuts. Aaagggghhhh!
What I never understood was kg, which is mass, but used as force (weight). Why was that done wrong? If you're going to make us switch to different units, don't make it wrong units. At least in our goofy units we didn't weigh is slugs. When I see pressure gauges with kg/cm2 scales I want to throw them across the room. I guess we can't expect to make people to change when they've done the wrong thing too long. And then those same people throw stones at those who are using old goofy units.
Yes, in metric countries engineers often (incorrectly) express force in "kgf" instead of Newtons.
In US customary units, the "lb av" is often used (incorrectly) as a unit of mass, although technically it should be a unit of force and weight.
Specifically: when the local consumer-department inspector checks the setting on a sales scale, he places a known artifact on the scale to check the reading. The marking on that calibration mass assumes a standard gravitational acceleration value (roughly that at sea level at 45
o latitude) to know the actual weight at that standard location. (NIST considers that acceleration value to be a defined constant, like 1 in = 2.54 cm exactly.)
However, he will use the same set of reference weights in Denver as in New York, even though a pound of steak transported to Denver weighs less at high altitude. (Don't go into your local grocery store and ask for slugs--they might have a weird imported food section.)
I find it hard to believe that you can't quickly determine that 9/32 = 18/64 is larger than 1/4 = 16/64. We live in a binary world.
I own two sets of sockets from Sears, one for inch and one for mm, that both have 1/4 inch drive. Each set fits nicely into its small rack.
Owning a full set of US number drills, letter drills, and fractional-inch drills means that I can get close enough to any mm size.