You will notice from the Print Reading Exercises answer file

You will notice from the Print Reading Exercises answer file located back in Module 1 that dimensional tolerances can appear multiple places on a drawing. Some tolerances are tied directly to the dimension. Others require us to reference the title block. You will also notice that some tolerances are larger than others. Some features are held to a very tight tolerance while others have a very large tolerance. Please provide some actual or expected tolerance examples why some areas of a drawings might be controlled differently. Please include why some features are controlled more tightly than others

Solution

The tolerance we give depends on the purpose of the part and where it has to fit in.
Normally tolerance is provided in title block but in some special cases where we need difference tolerance (generally tighter tolerance it is attached to the dimension). Now by providing a general tolerance in title block we are skipping the work of attaching the tolerance to each and every dimension and we can\'t give general tolerance very small as lower the tolerance, higher will be the cost. We need to have a tradeoff between cost and tolerance, due to this every feature can\'t have same tolerance.

Now why some features need to be tightly controlled?

It depends on the part with which they are assembled and how critical they are for the process. Lets say we have a hollow cylinder and a pin to fit in that. Now the outer diameter of cylinder can have a relaxed tolerance but ineer diameter needs to have a tighter tolerance to match with outer diameter of the pin.