Types of dimensional Tolerance

Types of dimensional tolerances.

There are three types of dimensional tolerances. At this stage let us not think about codes like H7 and g6.

1.Bilateral 

2.Unilateral

3.Basic

1.In bilateral tolerance dimension is allowed to vary on plus as well as minus side of ideal condition.. 

2.In Unilateral tolerance dimension is allowed to vary on only positive or negative side of ideal condition

3.In Basic tolerances dimension is shown without tolerance and outlined by a rectangle. Allowed variation is shown in terms of geometric symbols.

What is basic dimension-Is it something with zero tolerance or something with open tolerance?

Both the statements are wrong because practically it is impossible to achieve dimension with zero tolerance and outlined box says that it is special kind of dimension, so open tolerance shall not be applicable at any time.

Observe that two drawings are same unless in first dim.20.00 is shown as basic and in second drawings it’s not shown. The second one  will produce component with more errors.

If I follow second drawing, I will assume that dim 40.00 as well as 20.00 can be manufactured with an error of 0.2 per side according to note, while maintaining geometric tolerance given to angle 27 degree. Angle 27 should maintain it’s angularity within 0.1 degrees. Is this possible without controlling height 20.00 mm?. Definitely It is possible as we can adjust dimension 40.00 mm accordingly.

What if I have to minimise errors?-Ok,

First drawing is telling me clearly that I have to control height 20.00 mm while controlling angle 27 within geometric tolerance.

Ok why I have been not given dim 20.00 mm with some close tolerance like 0.02 mm per side?

No it’s not proper way because angle is controlled by intersection point A and location of point A can not have tolerances mentioned two times. After maintaining 27 degrees within 0.1 with datum B and dim at 40.20,operator shall work on dim 20.00 to make it more accurate, as he has material in hand i.e. dimension 40.20 can be reduced up to 39.80

Giving a close tolerance to dim 20.00 is bad idea as operator may get confused between 2 tolerances.