Question about dimensioning and tolerancing in ASME Y14.5 standards?

Question

I'm not too familiar with GD&T standards, and really need some help. These could be simple questions but any help on any of the questions would be greatly appreciated.

Does a tolerance have to have both a positive and a negative? for example, is 3.000 +0.001 -0.000 ok?

What do parantheses mean around a number? 

Is there a specific gap size that needs to be incorporated to make sure an assembly is possible? For example, if a male part is 3.000 in diameter can it fit into a female part thats 2.999 in diameter? 

Is there a minimal radius size a machinist can perform? For fillets and such.

Anonym · Accepted Answer

1). Tolerances do not have to be both positive and negative.  Your example is just fine.

2). The parentheses mean that that is the size of the uncut material.  If a length is labeled as (6.000) that means that the piece you bought should be 6.000 already and you shouldn't need to cut it at all.

3). Usually you do put in a couple of thousandths in a gap size, but if you need to, you can label a hole to be reamed which should be accurate to the thousandth.  And no, a 3.000 male part will never fit into a 2.999 female part.  Even if the tolerances overlap, it's still very bad drafting practice.

4).  I'd say no, within limits.  As long as the fillet can be seen, a skilled machinist can cut it.

Scott H · Answer

First question, do tolerances have to be symmetric?  The answer is no.  It is perfectly acceptable to not allow any variation on the low side (i.e. -0.000) and have tolerance on the high side (i.e. +0.030), or vice versa.

Generally, it means that the dimension is a reference dimension (i.e. that it can be derived via other dimensions on the drawing).  But, sometimes parenthesis around a dimension means that it is given in the alternate set of units [i.e. 1.00 (2.54)], but both dimensions will be provided.

There are specified tolerances for shafts and holes depending on the specified type of fit (e.g. sliding, light interference, etc).  Check out ANSI B4.1 and B4.2 for the preferred dimensions.

Generally, the smallest radius that I would specify is 0.005-inch.

Question about dimensioning and tolerancing in ASME Y14.5 standards?

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