Centimeters defeat one of the primary benefits of metric over inch. With mm, almost everything you do is whole units, not decimals. Needing less than 1mm is un-common, yet 1mm works out nice as a base thickness. Even in engineering work it's nice as most all dimensions are whole mm numbers. Not many things get over 1m, and when they do mm still work fine up thru 10m. 1mm is ~.040" which is a classic big tolerance, and 0.1mm is a good tighter tolerance limit .004" (aka about 1 sheet of paper).

Just re-read this, odd understanding of engineering you have, I must say.

I did the first year of engineering school, didn't finish my whole study because I got side tracked, but what we learned is that you have different tolerances for different applications.

And for working with metal, the standard tolerance is measured in thousands of a millimeter, 0,001 mm or micrometer. Engineers don't mind decimals, there is nothing wrong with decimals. They are very used to decimals.

Working with decimals is not the same as working with fractions. Want to add 0,123 + 0,456 ? That's simply 0,579.

But want to add 3/8 + 5/6? You'll have to convert to 1/24th! Very complicated to work with and you e.

"Not many things get over 1m, and when they do mm still work fine up thru 10m" - You know bridges and skyscrapers are also made by engineers, right? Trucks, tables, even a fridge is easily over 1 meter. Engineers don't mind, they simply convert.

I'm not sure where you are going with this. This isn't about fractions v decimal. We don't use fractions, but sadly when working with inch stuff, even though you work with decimals, the values used are generally decimal fractions (.125, .375, .750) The point I'm making along with others is that metric gives you a great system to work with, and jamming cm in there along with mm at the same time is just asking for mistakes, see my example above. I know from previous discussions you use cm, that doesn't change the matter that globally when working in metric cm are not generally used. The cm-kg-s system is obsoleted for m-kg-s and or mm-kg-s.

On tolerances, I'm not sure where you were taking issues. Some of this may be a difference in how metric prints and inch based prints are generally toleranced. Inch prints typically have a sheet tolerance block. Tolerance in inch based world is based on places after the decimal. So in the lower corner of a drawing there will be a block that looks something like this : .X +-.1 , .XX +-.05 .XXX +-.025 . (the actual values are up to the company to decided, and is generally based on what they build). In the field of a drawing, if there is a dimension that is 12.5" then it has a tolerance of +-.1 if it says 12.50" then it has a tolerance of +-.05". if .125 then +- .025". The number of decimal places dictates the tolerance. If you want to be different from the default, then you call it out in the field. 12.500 +- .004, this trumps the block. You can do the same with metric, and update the values of things. Of course in metric trailing zeros don't matter. 12.500 is the same as 12.5 also there are now leading zeros 0.5, not .5". Metric drawings generally use range tolerances. Something like 0-10mm +- something. 10-100mm +- something. I'm not sure if that plays into your comment on tolerances. Handling of leading and trailing zeros in metric vs inch systems is completely different. With inch based stuff you almost always have decimals places in play because an inch is a big unit of size. If you were to work in cm, you will still have the same annoyance as working in inches. But by working in mm, especially for doing something like house construction, you are never going to go into decimal places. Everything will be whole numbers, life is great.

With modern machining we can expect to hold sub .004" (.1mm) all day. So this is where working in mm is very nice. Just call out dimensions in mm, and have sheet tolerance for no place to be something like +-1mm (.039", nice loose easy tolerance), and when you need tighter, you have .X tolerance as +-.1mm. So it's a very convenient setup. Of course GD&T changes it all, stuff is now basic, but that works great with mm too, again all whole numbers and you only see decimals in the Gtol block.

It comes down to things being the right scale, and that's why mm are so nice. Far as when you shift to m from mm, no idea what various industries do. I know we stick to mm all around, and stuff we build gets rather large. I've worked on stuff near 10m long, everything was still mm. As needed a jump to meter from mm is easy enough. Certainly no need to jump to meter on a house, only extremes of the house get very big.

But there is no reason to use cm, we don't talk in cm, we don't design in cm, standards/specs for things aren't in cm. So it comes down to why inject something in there for no reason. Stuff is generally listed/talked about in mm and m. Long distances we will make the jump to km, and for things like surface finish um, why toss cm in there? If someone is making cabinets, it's 32mm cabinetry, the material is in mm thickness, the hardware/slides are in mm, Festool marks their tools in mm (parallel guides as a good example, and the tracks are designated in mm too), everything being done is in mm, so why would someone want to use a cm tape measure and risk the mistakes like I showed above?

switch to mass for a moment, we work in grams and kilograms. Sure kg may be a bit big of a unit for some things, and gram may be a bit to small for things, but we stick to gram and kg, we don't don't randomly decide to use centigrams. If you bought a scale and it read out in centigrams, sure it works and yes you can convert easy enough. But will a person make mistakes, you bet and often.