To perfectly match two parts, one of them must always be 0.1 mm smaller?

[dastructhm]

For instance, a screw and a nut, a housing/enclosure and a base.

  always?

[ataradov]

0.1 mm of difference is by definition not "perfect".

It all depends on the parts and the material and the fit you want.

There is the whole science behind this - https://en.wikipedia.org/wiki/Engineering_fit

[Mechatrommer]

0.1 mm of difference is by definition not "perfect".
It all depends on the parts and the material and the fit you want.
There is the whole science behind this - https://en.wikipedia.org/wiki/Engineering_fit

that link is imho very limited to hole and shaft connection type and i guess used as general rule of thumb for some type of steel. there are many many types of connection/pairing/mating that require different tolerancing, some are even negative and require heat/cooling process before mating them together. some will also need to consider thermal expansion or imperfection during manufaturing. for example you dont expect a cheap bolt has a perfect X distance between threads, if you do 0.1mm tolerance on the nut, probably it will get stucked on some threads along the bolt while screwing in. the real question for me is... can i afford the machine? that can produce consistently the required (small) tolerance design?

[jpanhalt]

There are numerous sources for fits and tolerances. Here's one for external threads: https://www.engineersedge.com/hardware/metric-external-thread-sizes1.htm

There needs to be some clearance for assembly.  Too much and the screw feels "loose."  Too little and you may never be able to get the pieces apart again.  Aluminum is really bad for that; brass will allow closer tolerances.

I suggest keeping the tables at hand.  Machinery's Handbook is the source I use most often, but as the link shows, the information is easily available on the Internet too.  0.1 mm (about 0.004") is about right many times for threads, but I would still go to a table.  For bushings and other applications needing a close fit, that may be too large.

[beanflying]

For instance, a screw and a nut, a housing/enclosure and a base.

  always?

The Engineers Blackbook is an OK reference without getting into the 3000+ pages Machinary's Handbook but buy both - I did 

For a fairly good FREE app or online I like this one as a sanity check https://app.fswizard.com/#/ISOTolerances It also will do speed calcs for milling and drilling in a range of materials too if you have a good look around. The Tolerance of Nuts and bolts as an example is well below H

[tszaboo]

Depends on the technology. It's larger if you want to fit an injection moulded enclosure to a PCB than a EDM machine cut metal piece to another one.

[ataradov]

that link is imho very limited to hole and shaft connection type and i guess used as general rule of thumb for some type of steel.

This is a very standard engineering table.

some are even negative and require heat/cooling process before mating them together.

Yes, those are called interference fits and they are listed in the table.

for example you dont expect a cheap bolt has a perfect X distance between threads, if you do 0.1mm tolerance on the nut

Those tables are for flat mating surfaces. Everything else is derived from those tables. And threads use a completely different system of tolerances.