Dual dimensioning rounding ?s / is it legal to say "dims in brackets are for reference only&quo 6

[ingallspw]

Is it "legal" to call out your secondary dual dimension as "REFERENCE ONLY" ?

If not is there a standard that forces you to use a set amount of decimal places in your dual dimension?

We have metric dimensions that are technically tighter than the English and vice versa that may cause someone to reject a part due to rounding of the two.

I realize dual dimensioning is not everyone's favorite so thanks for bearing with me.

 

[CheckerHater]

I'd say it's "better" to call your secondary dim's "reference only"
If you are looking for "legal" way, keep in mind that ASME/ANSI never really supported dual dimensions.
There were several discussions in this forum related to this matter.
Quick search for "dual dimensions" will return many threads like this:
thread1103-354286

 

[dgallup]

It is impossible to have exactly the same value for all dimensions and tolerances in both unit systems so the general practice is to have one fewer decimal place for millimeter dimensions than inch dimensions. Also, which ever set of dimensions is the secondary units should be rounded such that you can never pass a part that would be outside the limits in the primary dimensions. This does result in smaller tolerances for the secondary units. It is much better to reject a good part than to accept a bad part. If you don't like it, just inspect in the primary units.

I avoid using dual dimensioning when ever possible because of the problems but what is even worse is when a vendor supplies me inspection data in units that are not on the drawing. Then I have to double check every one of their conversions to make sure there are no discrepancies. You would be amazed how many people think that that a +/-1 mm tolerance gives them +/-.040". It is a never ending battle.



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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[drawoh]

Is it "legal" to call out your secondary dual dimension as "REFERENCE ONLY" ?

The smart thing is to not have dual units. The next smartest thing is to make it clear that the alternate units are for reference only. Everything else is crazy.

I am not aware of any standards explicitly banning dual units, so just about anything is "legal". Let us ponder the nature of a purchase requisition.

[ol]
[li]You issue a PO to your fabricator. [/li]
[li]Your PO states that your part is to be fabricated as per your drawing 123-456.[/li]
[li]Your drawing 123-456 states that the dimensions and tolerances are to be interpreted as per some standard, perhaps ASME Y14.5-2009.[/li]
[li]You have dual units on the drawings, the dimensions and units do not convert exactly.[/li]
[/ol]

Does y0ur CAD software convert all tolerance units accurately? I am familiar with SolidWorks. SolidWorks converts [±][ ]tolerances, but the contents of any feature control frame is dumb text. Is it really worth all the effort?

--
JHG

 

[ingallspw]

Thank you for the input!

I would love to go to one unit of measure but it's not my battle. I think in the notes I will just put "DIMENSIONS IN BRACKETS ARE METRIC (MM) AND ARE FOR REFERENCE ONLY." It's just messed up here because we have parts that can be trimmed to meet metric dimensions but all other details are English. Plus the entire plant has tools designed using English values.

We are using NX so it updates on its own.

Thanks again!

 

[ewh]

Since imperial dimensions and their metric "equivalents" never return the same exact value, I would recommend doing as you ask and state that any dimensions in brackets are reference. Otherwise you will have two different (however slight the difference) values wherever you use dual dimensioning. Drawings are supposed to have only one interpretation, and that is impossible if the dimensions aren't exact equivalents.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[looslib]

PTC states that when metric are the dual value, it is rounded down, so you can never produce a part outside the tolerance band of the primary dimensioning.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[juergenwt]

You already know the answer: Why do it!!
If it is in metric - keep it that way.
If it is in inches - keep it that way.
Why open a can of worms?
Never ever try to work both systems on one job at the same time.
If you are exporting jobs to another company - it is their problem.
You asked for metric with metric Tol. - that is what you should get.
You asked for inch with inch Tol. - than that is what you should get.

 

[ingallspw]

juergenwt,
Are you suggesting that the English values of the part only have English units and the Metric Values only have metric units (all on the same print)?

Let's just use a pulley as an example. Let's say the pilot and bolt circle were designed using English measurements but the outside diameter is 100mm. How would this be handled?

 

[CheckerHater]

Why would someone in he right mind design the pulley this way?

 

[ingallspw]

Available Tooling / casting where the outer diameter can be turned down / the company is stuck with English tools but being moved in to a metric world.

 

[CheckerHater]

So, I imagine your formerly all-inch pulley is now machined to accommodate metric belt or something like that.

In this case juergenwt’s suggestion is actually in line with the standard:

From ASME Y14.5-2009:

“1.5.3 Identification of linear units
On drawings where all dimensions are in millimeters or all dimensions are in inches, individual identification of linear units is not required. However, the drawing shall contain a note stating “UNLESS OTHERWISE SPECIFIED, ALL DIMENSIONS ARE IN MILLIMETERS (or IN INCHES, as applicable).

1.5.4 Combination SI (Metric) and U.S. Customary Linear Units
Where some inch dimensions are shown on a millimeter-dimensioned drawing, the abbreviation IN shall follow the inch values. Where some millimeter dimensions are shown on an inch-dimensioned drawing, the symbol mm shall follow the millimeter values.”

Still, the sooner the rest of the part will be converted to metric the better. But I understand it is not your call.

 

[ingallspw]

Thanks for the standards reference. That is very interesting. I did not know both could be on the print like that. I assumed it was all or nothing (or dual dims)

The thing that really complicates things is not all are customers are requesting metric parts. Some still use English. Which makes it hard to have a template that can easily be updated on the fly. Which is possibly another reason why we are using dual dimensions.

Also, our tooling has all been validated in English. Our routings are in English. Our laser, CNC's ware all programmed in English.

I get it. It's easy to say, "Let's make the switch to all ______" but it affects so much more than just the print. As a designer/drafter, it'd make my life easier to just say "lets use all ______" but it would cause a lot of work and eat up time / money and for what? We still have customers who are requesting English and others metric.

The parts should technically come out the same.

So now to an equally controversial topic...


ROUNDING!!!!! (in dual dimensioning)


Lets assume in the examples below both are tightly & clearly toleranced (and ignore how ugly and dumb looking this exaggeration is):

If we were to put 0.141" as the English and 3.5814mm as the metric (instead of the more common 3 decimal English and 2 decimal Metric)
or
If we were to put 0.55mm as the Metric and 0.02165" as the English (instead of the more common 3 decimal English and 2 decimal Metric)

is there anything that causes this to be "illegal" in ASME standards? I realize both would be incredibly hard to measure at that level of accuracy but that's kind of the point. At least someone couldn't say your conversions don't match.

 

[ewh]

Y14.5 Fundamental Rule (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.

Unless one of the values is reference, by including both imperial and metric values you are creating two different interpretations of that dimension (regardless of how you express the tolerance). If one is reference, I don't think it would matter.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[TheTick]

Don't know if it's "legal", but I think it should be mandatory.

 

[ctopher]

I don't see the point of showing them as reference if they are not the exact rounding dim anyway.
It just creates more questions and adds confusion.

If you absolutely have to show them, I suggest adding "REF" next to each dim. Putting a note is good too, but may not be read during machining.
Some people don't pay attention to the difference between brackets and parenthesis around secondary dims.

Chris, CSWA
SolidWorks 13
ctopher's home
SolidWorks Legion

 

[dgallup]

I have been making and using all metric dimensioned drawings in the US for 30 years now. It really is not a problem except for the suppliers who want to submit PPAP documentation in English. I try to eliminate them at the first opportunity. It is not that hard to measure things in the stated units, except for hard gauges like plug gauges, almost any measuring instrument today has a nice little switch conveniently labeled in/mm. The same goes for CNC programing.

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[dgallup]

So now to an equally controversial topic...


ROUNDING!!!!! (in dual dimensioning)


Lets assume in the examples below both are tightly & clearly toleranced (and ignore how ugly and dumb looking this exaggeration is):

If we were to put 0.141" as the English and 3.5814mm as the metric (instead of the more common 3 decimal English and 2 decimal Metric)
or
If we were to put 0.55mm as the Metric and 0.02165" as the English (instead of the more common 3 decimal English and 2 decimal Metric)

is there anything that causes this to be "illegal" in ASME standards? I realize both would be incredibly hard to measure at that level of accuracy but that's kind of the point. At least someone couldn't say your conversions don't match.

No that is incorrect practice. The secondary units are SUPPOSED to be rounded so that here is no chance of accepting a part that would be rejected in the primary units. Remember, QA is supposed to use a gauge that is accurate to one more decimal place than your dimension. Putting 5 decimal place English dimensions means it is now impossible to inspect your part.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[ingallspw]

dgallup,

I've heard that a lot but some times it does not work out that way.

Take
+/-0.06in
[+/-1.5mm]

If we are inspecting it internally and the metric is in spec... Great!

But what if the customer's PPAP-er (I like to make up words) uses the metric values (assuming we did not call the secondary unit out as REF ONLY). The metric tolerance when converted is now +/-0.059" which yea would be anal to reject but none the less, using the metric value could give you a rejection where the English would not have been out.

So if the secondary unit was called out as reference only and had an accuracy of 12 digits where the primary had 2 digits then technically they should only measure to the next primary unit's decimal. And if just by chance they used the secondary value and missed the ref only note, at least then they couldn't argue about a rejection due to a rounded digit, which would save us the time of setting them straight.

At any rate dgallup I do appreciate your input and opinion.

Just battling things out in my own mind.

 

[looslib]

One other thing like with the pully. Some things are designed and dimensioned BY THE STANDARD to be in one unit or the other. Wheels and tire diameters are 2 that are always dimensioned in english units, and maybe width of wheels, too.



"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli