# Dual Dimensioned Drawings

• posted

I'm just wondering what everyone else does, or what is actually the standard. I get drawings that are dimensioned MM [IN] eg. 57.2[2.250 in]

What are you actually supposed to do. I always thought that the first dimension is the driving dimension. but like the above example, their conversion is actually .002 off.

• posted

I would call it a mistake. It should be either 57.2[2.25] or 57.2[2.252]

What are the tolerance specs?

-jim

• posted

I see this from some customers, where the parts were originally designed in metric. They convert it all to inches, as best as it fits... (for that customer) you make it all to the inch dimensions. They have fudged the inch dimensions to fit the other inch dimensioned parts they have.

Bottom line, ask the customer what they want. It is pretty easy to ask. If they don't like to answer questions, get new customers.

• posted

If the drafter wasn't such an idiot, there would be no need for = questioning his decision in the first place...

He is introducing conflicting manufacturing data into the process stream = instead of providing clear and concise information which can be used to = determine with absolutely certainty whether or not the part meets = specification.

• posted

decision in the first place...

instead of providing clear and concise information which can be used to determine with absolutely certainty whether or not the part meets specification.

And you figger the best plan is to guess what someone you've determined is an idiot may have been getting at by working to a reference dimension ("I use the 2-1/4 in dimension...") instead of the plainly called out controlling dimension.

• posted

I get a lot of drawings dimensioned in mm, it's not a big deal to convert them. Thinking in mm takes a bit of practice, a number is just a number.. If the US would get of it's arse and switch to metric like the rest of the world it wouldn't be such a big thing,it's going to have to eventually so why not just do it.

• posted
• Just Me :

Yeah, that's what they told us in high school shop class... in 1974.

• posted

questioning his decision in the first place...

stream instead of providing clear and concise information which can be = used to determine with absolutely certainty whether or not the part = meets specification.

The term "controlling dimension" is meaningless so far as determining = primary measurement unit unless it is clearly stated on the drawing = notes.

=20

57.2 MM ?=20

Think about it...perhaps the guy used random number generator, = otherwise...

And when you're done thinking about the above, then I suggest compare = the 184T motor frame dimensions to the 112m ( metric frame ) and then = ask yourself where in the hell did all those weird numbers come from on = the metric motors....

...AND...

Just in case anybody want's to be a real stickler about it, there's = always ASME Y14.5.....

"Dual dimensioning has never really ever been allowed by any incarnation = of Y14.5. This is because of very specific wording under the standard's = Fundamental Rules. The wording may vary between versions, but carries = the same meaning in all versions. In ASME Y14.5M, that wording is as = such in 1.4(d), "Dimensions shall be selected and arranged to suit the = function and mating relationship of a part and shall not be subject to = more than one interpretation." (Support for dual dimensions in pre-1982 = versions was a mistake that was likely political in nature.)

General practice in the use of dual dimensions is that they are of equal = importance to the primary dimension. This creates issues in that it = allows for more than one interpretation of the dimension. It is nearly = impossible for nominals and tolerance ranges to be identical between = units of measure. This means that the dual dimension tolerance range is = usually resized to fit within the tolerance range of the primary unit of = measure. This creates a situation where the dimension has more than one = interpretation, which is specifically prohibited by 1.4(d). The = conclusion that can be drawn from this is that dual dimensions are = actually not allowed by ASME Y14.5M-1994. "

Again, if someone REALLY wants to use dual dimensioning, then using = brackets is probably fine but to avoid individual interepretation it is = still is necessary to add a flag note stating something like " = Dimensions are given in inch unit, metric values are approximate"=20

• posted

I agree! I have to deal in both systems daily and it took years for me to be able to "think" in metric.

• posted

his decision in the first place...

instead of providing clear and concise information which can be used to determine with absolutely certainty whether or not the part meets specification.

measurement unit unless it is clearly stated on the drawing notes.

Since you invoked Y14.5 below, you should know that it requires that,

************* "On drawings where all dimensions are in millimeters or all dimensions are in inches, individual identifica- on of linear units is not required. However, the draw- ng shall contain a note stating ?UNLESS OTHERWISE SPECIFIED, ALL DIMENSIONS ARE IN MILLIMETERS (or IN INCHES, as applicable).? ****************

Your cite goes on to say,

****************** "ASME Y14.5M-1994 defines a reference dimension as such,

?A dimension usually without tolerance, used for information purposes only. A reference dim is a repeat of a dimension or is derived from other values shown on the drawing or on related drawings. It is considered auxiliary information and does not govern production or inspection operations.?

By definition of reference dimensions, dual dimensions must be treated as reference dimensions. However, anyone who uses them knows this is generally not their intent. As generally intended, dual dimensions are disallowed unless they are considered reference only."

****************

In other words, in the example in the original post, the inch dimension in brackets is a reference dimension, to be "used for information purposes only," not, as you suggested, an invitation to ignore the 57.2mm figure.

I don't know what you're getting at. The .002 difference between

57.2mm and 2.250 inch won't make a bit of difference in many cases.

motor frame dimensions to the 112m ( metric frame ) and then ask yourself where in the hell did all those weird numbers come from on the metric motors....

Y14.5.....

Y14.5. This is because of very specific wording under the standard's Fundamental Rules. The wording may vary between versions, but carries the same meaning in all versions. In ASME Y14.5M, that wording is as such in 1.4(d), "Dimensions shall be selected and arranged to suit the function and mating relationship of a part and shall not be subject to more than one interpretation." (Support for dual dimensions in pre-1982 versions was a mistake that was likely political in nature.)

importance to the primary dimension. This creates issues in that it allows for more than one interpretation of the dimension. It is nearly impossible for nominals and tolerance ranges to be identical between units of measure. This means that the dual dimension tolerance range is usually resized to fit within the tolerance range of the primary unit of measure. This creates a situation where the dimension has more than one interpretation, which is specifically prohibited by 1.4(d). The conclusion that can be drawn from this is that dual dimensions are actually not allowed by ASME Y14.5M-1994. "

He's overreaching when he says Y14.5 does not allow dual dimensioning. It certainly doesn't encourage it, and neither would I, but I can think of at least one example of when I feel its use is justified: when a feature may be conveniently made with a tool sized in the other unit system.

For example, about a year ago I had to drill a couple thousand holes with assembly clearance thru several 12ft long bars for 1/2" shafts. Turns out a 13mm drill was just right. Luckily my son was home from college and I paid him to do the tedious work, but if I had sent a print out for quotes, I probably would have called out the holes something like .512+/-.005[13mm].

probably fine but to avoid individual interepretation it is still is necessary to add a flag note stating something like " Dimensions are given in inch unit, metric values are approximate"

I agree dual dimensioning should be used sparingly. My objection is calling the drafter an idiot without actually seeing the drawing in question.

• posted

So which one exactly are you talking about here ?

The problem lies with the use of brackets--unless there is some note stating that "dimensions in brackets" are to be used as "reference only" then there still exists two possible interpretations....

And for this reason, ref dimensions are typically followed in text with the letters "ref"

Selecting proper tool size is generally the shops responsibility.

And I'm okay with that....though even better would be .512+/-.005[Drill

13mm].

Granted, since there might even be a company directive REQUIRING him to use dual dimensioning.

Takes all kinds to make a world you know...

• posted

With a drawing like that if I would would pick either metric or inch our quality control would squeeze the tolerance band to cover both.

2.250"=57.15mm 2.251"=57.175mm 2.249"=57.1246mm

57.2mm=2.2519685"

So it looks like the driving size is in inch and the metric conversion is lousy. Having 2 sizes is daft enough, having them in the wrong order makes it worse.

DanP

• posted

I'd say tell them having 2 sizes makes the tolerance tighter therefore the price has to go up. Pick a system or pay a premium.

DanP

• posted

get the answer in writting though.... a verbal instruction over the phone can always be denied

• posted

The dimension in brackets is a reference, as the guy you quoted reasoned.

I agree, the ambiguity is unfortunate, but it seems to me treating the bracketed dimensions as a reference is the reasonable interpretation.

By the way, Y14.5 doesn't support the use of "REF", parentheses only. Nor "TYP". I can live the parens, but prefer "TYP" to "4X" in some cases.

Agreed, I don't care if the machinist chews the part out with his teeth as long as it matches the print. But if I had an easy out in mind, as in the example below, I don't want the shop to miss it on account of some non-obvious dimension conversion.

Then I don't think we have much to disagree about.

• posted

I beg to differ...

• posted

quality control would squeeze the tolerance band to cover both.

• posted

I'm the reverse. I think in mm and have to do some mental arithmetic to convert from inches. I have 25.4 set into the memory of the mant calculators I use.

Another odd thing is standard collet sizes. I have a spindle that has 1/4",

6mm and 8mm collets, with sleeves taking it to 1/8". I can only just distinguish the 1/4" collet from the 6mm one.

Personally I prefer mm as I find 1 base 10 system much easier to work with, but then again I live in a metric country.

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