Logic behind ASME 14.5 1.6.2(a). No Zero before decimal point. 2

[MCRodg]

Can someone explain the logic behind ASME 14.5 1.6.2(a) which states:
1.6.2(a) A zero is not used before the decimal point for values of less than 1 in.

To the contrary 1.6.1(a) for metric units (millimeters) states the opposite:
1.6.1(a) Where the dimension is less than one millimeter, a zero precedes the decimal point.

Seems to me the leading zero before the decimal point should be required always, regardless of the units.
Otherwise, 0.25 inches could look like 25 inches on a poor quality reproduction of the drawing. That decimal point is tiny and can end up not being printed clearly or at all.

Conversely, a random spec or printing artifact could look like a decimal point. If it was unfortunately close to a 25-inch dimension or worse drawing note it would look like 1/4 inch instead.
That is; "Is that a spec or a decimal point" is totally remedied by a leading zero before the decimal point.
In a drawing note, it would not be as apparently wrong as it would be in a dimension. In a dimension, the scale of other part is a clue. So is the relative size of other dimensions. Not the case in a drawing note.

So why a zero before a mm dimension and not before the inch dimension?

If it's just to distinguish between mm and inch dimensions that seems like a poor way to do it.
Generally, all dimensions are in either SI or Imperial units and stated so in the title block or notes. That's the best place to check.
Dual dimensions on a drawing only emphasize the difference. Lack of or presence of a zero before the decimal isn't so necessary. The drawing will have a note somewhere explaining which is which, top or bottom dimension, side by side with alternate units in brackets or parenthesis.

What am I missing?

 

[fel3]

I say the standard is wrong. USE the zero in both cases. Only batting averages are OK without the leading zero.

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[jackk]

MCRodg,

Any suggestions on the trailing zeros? ANSI/ASME keeps them, but ISO/SI/DIN/JSI drops trailing zeros.

And what do you think about decimal points: period or comma?

 

[DraftingMan]

MCRodg,

What you are missing is the time frame in which this standard was first developed.

Back in the days when manual drafting was done, the less you lettered the faster a drawing was finished.​

John H. Dunten, CD
Certified Drafter

 

[IFRs]

Trailing zeros indicate precision, stick to the number of significant digits that are appropriate for the process.

 

[Manifolddesigner]

IMHO,
It's there to quickly differentiate if you are in fact, looking at a metric drawing or one done in stupid units.
Does it have a leading zero? that is 25mm, not 25". So holy crap this thing just got smaller.
It also helps get rid of trailing zeros and the idiocy of block tolerances. If it's a y14.5 drawing, you should be thinking about each and every tolerance.

Ignore it at your peril. Shops will look at your drawing and assume you are an amateur that has never read the standard.
Might as well put lowercase lettering on there too, just because you like it better. (Amateur)

 

[looslib]

Trailing zero's are only on imperial units, not metric units in both ASME and ISO standards.

As for leading zero's, Manifolddesigner hit it on the head. It differentiates the drawings so you can tell at a glance what the units are.

If your title block says you follow ASME Y14.5, then follow the standard. Do NOT change it just because you don't think it is right.

When I went to work for a global company about 23 years ago, they had started designing all their machines in metric, but still used trailing zero's. I changed the settings on the CAD system to not use trailing zero's and the engineers screamed! I went to management and pointed out what the standard said, that our drawings say they are drawn to that standard and we needed to comply. Trailing zero's were history and dimensions had to be tolerance since we had a 0/1 decimal place default tolerance of +/-1.5.


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