Dual Dimensioning Standards 1

[cygnas]

OK, here we go.....We are having a major battle here on how to go about a standard of dual dimensioning. Previously we have always dimensioned in english units only but we have decided to add dual (in/mm) dimensions to all of our drawings. Our biggest disagreement is with the conversion of the inches dimension to the "equivalent" millimeters dimension with accuracy. When we were using only inches we had a standard of the following: .xxx ± .005, .xx ± .020, and .x ± .030. Is there an "accepted" standard for dual dimensioning while not sacrificing accuracy in the conversion process? Should the metric tolerance hold the same amount of decimal places as the english? It seems like we are running into the "round up or round down" when it comes to the metric tolerance. If rounding is done it sacrifices accuracy. We need to keep the english dimension the "master" dimension and the metric would be the "slave". Thanks for any suggestions.
Cygnas

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[ringman]

Chris,

I believe that in accordance with Y14.5 .001 is the same as .0010 or .00100 for either a dimension or tolerance.

See Par 2.4.

 

[ewh]

It isn't if the dimension is 4.2031 as stated; acceptable range being 4.2041/4.2021 as opposed to 4.204/4.202. This can make a difference in very precise parts.

 

[ctopher]

So if you had a rectangle part with one side 1.001 and the other side 1.0010, they would have the same tolerance? If the tol block on the format is .XXX = .005, what tol would you use for 1.0010? You have to incorporate the tol with the dim. ASME Y14.5, 2.4 talkes about basics in the way numerals are displayed. If you use both inches and mm on a dwg, both tol blocks need to be on the dwg format because all dims need to display some form of tol, whither shown with the dim or referenced to the tol block.

Chris
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[drawoh]

ringman,

ASME Y14.5M-1994 contains nothing on the meaning of trailing zeroes. A note on your drawing describes what the trailing zeroes mean.

Most drawings do not define what four or five zeroes mean even on an inch drawing. If such is the case, the dimensions .0010 and .00100 have not been toleranced, and the drawing should not pass checking.

Another possibility is that the drawing is sent to a fabrication shop internal to the company. The company can have an internal standard stating what the trailing decimals mean. You just cannot conveniently send the drawing out to an external fabricator.

JHG

 

[ewh]

ctopher,
My example above was based on the post by israelkk, in which he stated a tolerance of +/-.0010. I agree that if 4 and 5 place dimensions do not have a tolerance with the dimension, and the default tolerances only go to 3 places, then the drawing should be sent back for clarification. If the dimension is basic, the geometric control block tolerance should be the same number of decimal places.

 

[ringman]

drawoh,

If you will check the examples in Y14.5 Par. 2.4, examples, I think it will clarify.

If of course you are inot involking Y14.5 on your drawing that is another thing.

 

[ctopher]

ewh, thanks

Chris
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[RedPen]

All,
In my experience, dual dimensions have always been displayed such that the secondary units are enclosed in brackets, [ ], regardless of the units chosen. Recently, one of our QA persons asked me if this was ANSI, ASME, industry or just a company standard. I don't know and haven't been able to find any reference to it anywhere I've looked.

Who or what established this display convention for dual dimensions?

Thanks in advance!

RedPen
Mech Designer/Lead Checker

 

[ringman]

israelkk,

If you are getting 4.203 when you are converting from 4 16/64, something is wrong.

 

[ctopher]

Check out IEEE/ASTM SI 10 for dual dim's.
Also, a very good book to have is DRAWING REQUIREMENTS MANUAL (ASME Y14.100-2000 & MIL-DTL-31000B). This book covers EVERYTHING.

Chris
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[MadMango]

Isn't 4 16/64 the same as 4 1/4... am I missing something?

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[ctopher]

you are correct. It can also be 4.25 or 4.250 or ...

Chris
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[drawoh]

ringman,

The whole context of this discussion is that an untoleranced dimension can have tolerances assigned to it by a note on the drawing, or by some internal corporate standard, or by some agreement with the fabricator. ASME Y14.5M-1994 allows this, but does not assign any additional meaning to the number of decimal places.

Paragraph 2.4 applies to the 4.250 dimension, and it applies to the note stating that X.XXX has a tolerance of +/-.005". The nominal dimension is exactly 4.25". The maximum allowable variance is exactly .005". If the part is measured out to 4.2504, it does not conform, as per ASME Y14.5M-1994.

JHG

 

[ajack1]

I have worked in both imperial and metric and the dreaded cross over, dual. If at all possible I would recommend not having dual dimensions.

Firstly you are primarily working in one or the other is for example a piece of stock 4” (101.6mm) or 100mm (3.937”) ? The same goes for stock items dowels and the like. Is an 8mm dowel a .315” reamed hole or a 5/16” dowel a 7.937mm reamed hole? Why try to use both?

Having said that adding up 3/16, 17/32, 29/64 and .764 would make my head spin these days, just move over to metric. ;-)

 

[ewh]

drawoh,
Didn't you mean to say that if the measurement is 4.2504 (based on 4.250 +/-.005 tol) it IS acceptable, but if it measured 4.2554 it would not be?

 

[ringman]

drawoh,

The standard states that the dimension and tolerance shall have the same number of decimal places.

 

[ctopher]

ajack1,
Dual dim's does not mean you mix in/mm. One is primary one is secondary. If you have a 8mm dowel, you would never use a inch hole, it would be sized for a 8mm dowel in mm.
In the real world these days both in/mm are used. We all need to get used to it and learn the conversions. I accept both units.

Chris
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[ajack1]

Ctopher, I do realise that you do not go out to mix up units, but in my experience that is exactly what happens.

When in the UK we changed over it was a real mess as much stock was still imperial but all fixings and dimensions were metric.

Basically you work in one or the other, if you have a simple shaft with a flange and a plate screwed to it whether you make the shaft 1” or 25 mm doesn’t matter if it is bolted on a 4” PCD or a 100mm PCD doesn’t matter but it looks strange and you get “silly” numbers in the other set of dimensions and even stranger with all dimensions in metric with ¼” tapped holes or all dimensions in imperial with 6mm tapped holes.

What benefit is there in dual dimensioning? You no longer see dual dimensioning in the UK these days I am glad to say. I would guess the US is one of a very few countries that uses them.

 

[ctopher]

Primary dims in inches so we can make the part, metric so Europe can read them

Chris
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[coolcad]

I have a different question on the same theme.

When you are dual dimensioning with GD&T, what do you put for a tolerance in the feature control frame?
ie: A hole, 1.00"/25.4mm is located with a positional tolerance. What should the tolerance be? inch? metric? both?