Dual Dimensioned Drawings

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.
Reply to
tnik
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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
Reply to
jim
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.
Reply to
PrecisionmachinisT
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.
Reply to
Ned Simmons
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.
Reply to
Just Me
* Just Me :
Yeah, that's what they told us in high school shop class... in 1974.
Reply to
Kelly D. Grills
Physics class, too, in 1965. And science class, in 1960...
The drumbeat goes on. d8-)
Reply to
Ed Huntress
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. "
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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
Reply to
PrecisionmachinisT
I agree! I have to deal in both systems daily and it took years for me to be able to "think" in metric.
Reply to
Tom Gardner
The ability to deal with whatever comes your way is handy, most things = these days typically having been designed by Frankenstein or relative if = you catch my drift.
Reply to
PrecisionmachinisT
days typically having been designed by Frankenstein or relative if you catch my drift.
This has been one of my hobby horses for decades. I had to write an article about it in the late '70s, so I went first to Caterpillar. "What are you talking about?" they asked. "We're 100% metric."
Within a few years, most of the automobile industry was, too. Now they're 100% metric, as well. So is all of US science, medicine, and most of our other volume manufacturers.
I went to NIST. You'd think they'd be the biggest pro-metric folks on the continent. Their reaction? Officially, "Its a bad thing to be using inch measures." Unofficially, "Eh," accompanied by a shrug.
The conclusion is this: Where it matters, we're already 100% metric. Where it doesn't, we're *still* mostly converted to metric. Most of our use of inch/pound etc. ("customary units") is in consumer products for lengths and volumes. It really doesn't matter a whit for them. They aren't converting erg-seconds to femtowatts.
Job shops still get a mess of both measures, and they're the ones who have a right to be pissed off about it. Otherwise, it's not really a problem. And keep in mind that our dimensions for length are in decimal units for most technical things, anyway. It matters little whether you start with a meter or with an inch if you do that, as long as you don't have to keep converting. And with computer controls, you just push a button even for that.
For those reasons, it's mostly a tempest in a teapot. There are some good reasons to go all-metric, but there are few people who would even notice.
Reply to
Ed Huntress
1960 Military requires semiconductors to be good for 125 degrees C.
Engineer writing acceptance test is sure that the lab has only Fahrenheit thermometers so changes 125 C to 257 degrees F. Acceptance lab has only Centigrade thermometers. Tech heats semiconductors to 257 C. All semiconductors no longer work.
=20 Dan
Reply to
dcaster
Stupidity will always find a way.
Reply to
Ed Huntress
Yeah but if the U.S. went to Centigrade there would be one less way. And I would not remember that the melting point of copper is about 1100 degrees but not remember if that is C or F. Or that a oxy acet flame is 6300 degrees either C or F.
It is just one more way the U.S. is shooting itself in the foot as far as trade.
=20 Dan
Reply to
dcaster
Well, I feel for you Dan. FWIW, the melting point of copper that you remember is in C. And the O/A flame is in F.
There is no good reason to try to remember both. If those things are important to you, just get used to using and remembering one or the other. If you're relating them to highly technical information, I'd recommend C. Or K....damn that Kelvin...
Reply to
Ed Huntress
...
In all the ways US trade has a problem, this one is _not_ one any longer. I've disagreed w/ Ed a bunch, but on this one he's spot on--where it matters the US has been metric for a long time already; where it isn't, it just doesn't matter at all.
--
Reply to
dpb
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.
Reply to
Ned Simmons
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...
Reply to
PrecisionmachinisT
It does to me. I have inch and metric wrenches and sockets. I have inch and metric taps and dies. So it costs me time to find out which tool I need to use and money because I need two sets of tools. My lathe does inch threading with no problem, but needs manually changing gears to do metric threading. So more time gone. My dial calipers are inch. I do have digital calipers, but they are not as accurate as my inch micrometers. So it matters, Not that it can not be done, but it is a pain that is not necessary.
=20 Dan
Reply to
dcaster
Here's a suggestion, Dan: Throw your inch-measuring tools out. Why are you measuring in inch units to begin with?
Oh, you have some old plans or old devices that are made in inch dimensions? Well, throw them out, too!
Then you'll be all metric -- pure as driven snow. d8-)
Reply to
Ed Huntress

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