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Dual Dimensioning Standards

Dual Dimensioning Standards

Dual Dimensioning Standards

(OP)
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

When it comes to protecting yourself or your family it is better to be judged by 12 than carried by 6.

RE: Dual Dimensioning Standards

The secondary dimension is for reference only.  Whoever makes the part is accountable to manufacture to the primary dimension units, regardless of what the secondary says.

RE: Dual Dimensioning Standards

Why do you feel that the precision (number of decimal places) of a dimension on a drawing has any relation to the acceptable tolerance for that dimension?

RE: Dual Dimensioning Standards

MintJulep,
One of the first rules of drafting (using inches) is that the precision of the dimension determines the tolerance as noted on the drawing.  Maybe I don't understand your comment.

RE: Dual Dimensioning Standards

MintJulep

As far as I know the number of decimal places has inportance and 0.020 is not the same as 0.02. The explanation is that the dimension 0.020 should be checked with a measuring tool of 0.0001 accuracy reading while the 0.02 may be checked with 0.001 tool accuracy reading. Meaning, the measuring tool accuracy reading of the 0.020 should be ten times more accurate than the measuring tool for 0.02.

To my opinion this is also the answer to cygnas for the equivalent metric dimension. The number of decimal places on the secondary metric dimension should be such that it will require/force that the measuring tool accuracy will stay the same.

RE: Dual Dimensioning Standards

I agree with above answers.
MintJulep, it does matter. Would you dim a dwg with .02 or .0200 and the machinist assume it means the same tolerance?
Metric dims are more accurate in ref to inches. Therefore should not show the same decimal places when on the same dwg. Although one is ref to the other as TheTick states.

Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP2.0 / PDMWorks 05
ctopher's home site

RE: Dual Dimensioning Standards

If Y14.5 is applicable, you need to read par. 2.4.
It should clarify.

RE: Dual Dimensioning Standards

As ringman suggested, I read ANSI Y14.5, paragraph 2.4, which is titled INTERPRETATION OF LIMITS.  It specifies that tolerance limits are absolute and are assumed to have infinite trailing zeros.  It gives for example, 12.2 is the same as 12.200000, 12.01 is the same as 12.010000000000.

Keep in mind though that this is describing the tolerance LIMITS, not the applicable tolerance based on the number of digits (trailing zeros) in the DIMENSION itself.  And I believe the discussion was talking about the dimension itself with respect to trailing zeros as digits, am I right?

A dimension of 12.2 may have different limits than a dimension of 12.200 (and these limits are typically established based on digits after the decimal and defined on the drawing), but a tolerance LIMIT that is stated as 12.2 is assumed to be 12.20000000-->Infinity.

And yes my handle says electrical after it so I am asking, not criticizing.  

RE: Dual Dimensioning Standards

debodine and all,

   Most drawings have a note assigning tolerances to dimensions based on the number of trailing decimals.  If the number of trailing decimals corresponds to an entry in the tolerance note, then it has a tolerance.  This procedure is noted in ASME Y14.5M-1994.

   Obviously, this all assmes the absence of any other tolerance specification, including basic dimensions.  

                   JHG

RE: Dual Dimensioning Standards

Isrealkk,

Agreed on the precision of measurement.  However, precision of measurement is still different from acceptable deveation on a part.  Tolerances are not logically related to the number for decimal places in the nominal dimension.  It is simply lazy "standard practice" to relate tolerance to number of decimal places in a drawing title block.

Consider a part where the nominal dimension is 4 13/64 (perhaps and old part that hasn't been made in a long time.  Dust off the old hand-drawn print and convert it to CAD).  4 16/64 gets converted to 4.203125, and depending on the settings of the CAD program, and the diligence of the drafter and checker (if one exists) gets rounded of to 4.203, or 4.20, or just 4.2, or maybe 4.20313.  What is the proper tolerance for that dimension?

You cannot cause a non-conforming part to become conforming by measuring it with a more precise instrument.  If I have a part with a feature that needs to be 2 +/- .001 and a part where that feature measures 2.125 I can tell that the part is no good with a simple ruler, even though the precision of that ruler may be no better than 1/8.  I could measure the part with a more precise tool, and learn that the feature is 2.12408, but the part is still no good.

RE: Dual Dimensioning Standards

Tolerances are linked to number of decimal places only when explicitly stated as such in the title block or a note.  There is no such thing as a default tolerance.

RE: Dual Dimensioning Standards

Tick,

I will ammend your statement to say that there SHOULD BE no such thing as default tolerance.

Unfortunately, in many cases there are.

RE: Dual Dimensioning Standards

MintJulep

I agree with you completely. The part actual measurement has to be within the torerance. What I meant is that the minimum and maximum dimension of the part should be checked according to the number of decimals of the tolerance values. Therefore, the number of decimals in the tolerance has signicance. There is no point to give the the nominal dimension with more decimal places than the tolerance. Therefore, I always use decimal system when I give dimensions and if I want to use 4 16/64 +/- 0.001" I will write it as 4.203+/-0.001". If I will want a more precise tolerance I will use 4.2031 +/- 0.0010".

RE: Dual Dimensioning Standards

Israelkk,

I feel that there is an error in your statement about tolerances.  I believe that a tolerance of .00l0 is exactly equivalent to .001.  The trailing zeroes have no significance.

Mint Julep,

The precision required for the part should drive the requirements for the tolerance. ( Make as loose as possible and remain functional)  That being said, the dinension should then reflect the same number of significant decimal places.

Hopefully, but doubtful, that we are getting away from decimal equivalents for fractional dimensions.

16/64 ???????

RE: Dual Dimensioning Standards

When a dimension is really critical (bearing bores, fits), I use explicit tolerances, even if the title block tolerances work.  I've seen cases where a CAD titleblock changes, changing the title block tolerances and thus affecting critical dimensions.  Also, sometimes certain translations (DXF, etc.) lose title block information.

RE: Dual Dimensioning Standards

ringman

If you will read my first post you will see that from the dimension checking equipment point of view there is a difference.

Dimensions are checked by tools such as caliper micrometer etc. Each tool gives some error, for example if you specify 10+0.001mm diameter for a pin a caliper specified to 0.001mm may give an OK of 10.000+0.001 dimension reading while the actual dimension can be 9.9997 to 10.0013. Therefore, if one needs a more accurate pin with the same tolerance he sould specify the tolerance dimension to be 10+0.0010 that forces the measuring tool to have a more accurate reading such as 0.0001 resulting in actual dimension range of 9.99997 to 10.00103.

RE: Dual Dimensioning Standards

(OP)
Thanks very much for all the info.  I have what I need to go ahead and set the standard here for our new dual dimension drawings.

Cygnas

When it comes to protecting yourself or your family it is better to be judged by 12 than carried by 6.

RE: Dual Dimensioning Standards

israelkk,

We may be mixing apples and oranges.  I was addressing the issue of decimal places as they pertain to the drawing (part) and the significance of trailing zeroes.  

You, are addressing the issue of the accuracy of gaging to assess these dimensions.  Your point is well made.

RE: Dual Dimensioning Standards

All dims have a tolerance. So .001 should not be the same as .0010. The machinist may question you what is the tol if some dims are 3 places and some are 4, or 2. There should be a tol block or tol with the dim.

Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP2.0 / PDMWorks 05
ctopher's home site

RE: Dual Dimensioning Standards

   I am using SolidWorks 2003, SP5.0, and most of my drawings are metric.  Over the last year or so, I have been setting the trailing zeros to "Remove" so that my drawings conform to ASME Y14.5M-1994, which states that you remove trailing zeros on metric drawings.

   SolidWorks' support for this practice is not particularly good.  I would like to be able to explicitly turn on the trailing zeros on certain dimensions, but I cannot.  As a result, I am manually placing tolerances on everything.

   Observations...

1.  The GD&T profile tolerance saves a lot of time.

2.  Applying tolerances to everything is not particularly difficult or time consuming.  Having to think out the tolerance for each dimension is useful.  This is particularly true when you apply tolerances looser than the defaults.

                     JHG

RE: Dual Dimensioning Standards

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.

RE: Dual Dimensioning Standards

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.

RE: Dual Dimensioning Standards

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
Sr. Mechanical Designer, CAD
SolidWorks 05 SP2.0 / PDMWorks 05
ctopher's home site
FAQ371-376
FAQ559-1100
FAQ559-1091
FAQ559-716

RE: Dual Dimensioning Standards

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

RE: Dual Dimensioning Standards

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.

RE: Dual Dimensioning Standards

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.

RE: Dual Dimensioning Standards

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

RE: Dual Dimensioning Standards

israelkk,  

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

RE: Dual Dimensioning Standards

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
Sr. Mechanical Designer, CAD
SolidWorks 05 SP2.0 / PDMWorks 05
ctopher's home site
FAQ371-376
FAQ559-1100
FAQ559-1091
FAQ559-716

RE: Dual Dimensioning Standards

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

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RE: Dual Dimensioning Standards

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

RE: Dual Dimensioning Standards

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.

RE: Dual Dimensioning Standards

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?

RE: Dual Dimensioning Standards

drawoh,

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

RE: Dual Dimensioning Standards

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
Sr. Mechanical Designer, CAD
SolidWorks 05 SP2.0 / PDMWorks 05
ctopher's home site
FAQ371-376
FAQ559-1100
FAQ559-1091
FAQ559-716

RE: Dual Dimensioning Standards

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.

RE: Dual Dimensioning Standards

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?  

RE: Dual Dimensioning Standards

I would use the default/primary dimensioning units for any GD&T. The dual/secondary values are reference only.

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Ingersoll-Rand

RE: Dual Dimensioning Standards

Another standard we use is ANSI/IEEE STD 268 American National Standard for Metric Practices.  This standard has been adopted by the DOD for what it's worth.  See appendix C Rules for conversion and rounding subset C6 Conversion of Linear Dimensions of Interchangeable Parts.

Best Regards,

Heckler
Sr. Mechanical Engineer
SW2005 SP 2.0 & Pro/E 2001
Dell Precision 370
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RE: Dual Dimensioning Standards

The reference that ctopher gave is the one you need.  IEEE/ASTM SI 10 has very specific rules for conversion and rounding, which are only intuitive in hindsight.  Genium's Drafting Manual describes these rules in section 6.9 - Dimensioning & Tolerancing (Inch/Millimeter Conversion).

There are two rounding methods described, A & B.  Method A is simpler, but may result in converted limits which are outside the original limits.  Method B ensures that the converted limits are always inside the original limits.

Both methods use rounding tables to determine the precision of the converted limits.  The tables are based on the total tolerance spread of the original dimensions.  e.g., if the inch tolerance is at least .004, but less than .04, the millimeter limits are rounded to 2 decimal places.  Whether the last retained decimal place is rounded up or down depends on the method.

In method A, the rounding rules are: if the dropped digit is over 5, round up; under 5, round down; exactly 5, round to the nearest even number.

In method B, the last retained decimal place is rounded down for the upper limit, and rounded up for the lower limit, thereby ensuring that the original limits are maintained.

RE: Dual Dimensioning Standards

McLeod,

Thanks for specifying that for us.  I knew that the standard existed, but wasn't able to find out the specifics.  Most major CAD packages seem to follow method A.

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