GD&T & Dual Dimensions

[Randy1111]

We have a couple customers who insist on dual dimensions when we send them drawings for approval. Where im confused is what one doies with geometric tolerancing control boxes.

Do you put both dimensions, with the secondary in [ ] inside the box, do 2 boxes, or just show geo tolerances in the primary units only ?

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Randy

 

[MechNorth]

Unfortunately Y14.5M-1994 doesn't explicitly cover this. Per section 1.5.3 & 1.5.3.1, first indicate on the drawing your primary units, then add IN or mm following the occasional dual dimension. In your case, I'd suggest the following. Add a note such as

DIMENSIONS ARE IN MILLIMETRES
DIMENSIONS IN SQUARE BRACKETS
ARE IN [INCHES]

Most CAD systems seem to put the secondary dimension in the square brackets. If at all possible, I wouldn't put the tolerance in the secondary units due to conversion errors (if you're working in the 0.005mm / .0002 IN range, it can be critical). If you have to put the secondary unit tolerance into a FCF, then I'd suggest putting the square brackets around the FCF as well.

Hope that helps.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[ctopher]

All dims have tolerances. Have the customers indicate what dims they want, ie in[mm]. The dwg needs to specify in a note which ones they are and what the tolerances are.
For example:
Show as .123 [.456]
.122 [.455]
Or .123 [.456]
Note: All dimension are in inches. Dims in [] are in mm.
The dwg format will have a tol block indicating what tolerances that are not inidicated at the dim.

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)

 

[MechNorth]

Chris, how are you proposing to deal with the secondary-unit FCFs? My preference is to only put an FCF with the primary unit, essentially making the secondary unit-dimension a reference. Thoughts?

Jim

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[powerhound]

All of my drawings are dual dimensioned. I include the secondary units in the feature control frames too. As far as tolerancing goes, I only have inches in the tolerance block, no mm.

Powerhound
Production Supervisor
Inventor 11
Mastercam X
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[dingy2]

It sure sounds like your customer does not have an understanding of GD & T and sometimes dual dimensiong will not resolve the problem.

I would love to see how one would double dimension a feature of size that has a cylindricity, as an example, which value is inside the size tolerance range.

How about dual dimensioning on a positional diametrical tolerances to a pattern of round holes at MMC. One cannot do it. We can place a +/- value in brackets (reference) under the basic dimensions but it is not the same. Tolerance depends upon the size of the individual features in the pattern.

I am not a Designer but here is what I would do.

Only place GD & T on features where there is a FUNCTION & RELATIONSHIP between features. In most cases, there would be less GD & T on the drawing. Now you would have to explain to the Customer the meaning of each one and why you could not double dimension it.

I would always suggest that if there is a GD & T seminar in your plant (and there should be), please invite the your Customers at N/C.

Most people in industry are sooooooooooooooooo messed up in this subject.

Hope this helps.

 

[aardvarkdw]

Randy,

When you are applying dual dimensions you are just taking the primary units and extrapolating the other units. There is a level of error inherent in this that may negate any tolerence you apply to the dimensions. To minimize this I would only tolerence in the primary units. By giving them two different tolerences, you are allowing them to pick and choose which dimension set they want to use on any given feature. ASME Y14.5M-1994 does not address dual dimensioning. I believe the ANSI standards used to (I may be wrong on that). I feel that anything that can cause this much ambiguity should not be allowed on a drawing. If the customer want to see it, fine, but don't give them the option of choosing which tolerence they want to apply to the part. Give them one set of tolernences and if they want to convert it for use in the other units fine but it better meet the spec of the tolerence given not the secondary equivelent. Just my two cents.

David

 

[drawoh]

Randy1111,

The reason you should not do dual dimensioning on fabrication drawings is that you might do the conversion wrong. This would be a risk with everything you do on a drafting board.

With any kind of CAD software, you can apply dimensions with automatic dual dimensions and get it right. On SolidWorks, you are entering your GD&T values manually, and doing the conversion. The converted figure is not reliable. I do not know about the other CAD packages.

Is the customer insisting on dual dimensions, or is he insisting you work in his units? There is a difference.

JHG

 

[meintsi]

Dual dimensions were developed by several early metrification standards a dimensional system "allowing faster and more accurate dimensional interpretation." One could view them as a reference dimension for a group of people who may not be familiar with the primary units.

The notion that their inclusion would allow one to pick and choose between them its completely wrong.

DOD standard MIL-STD-962C discourages their use.

DOD standard MIL-STD-1476C revoked it as an obsolete practice.

Many companies still allow them when thier usage is deemed beneficial. But good drafting practice would then be if the primary dimension unit is 3 places, the dual dimension should be limited to two places. Same as for incleded tolerances.

5.13 ±.15 inches is then shown as 130.2 ±3.8 millimeters.





Remember...
[navy]"If you don't use your head,[/navy]
[navy]your going to have to use your feet."[/navy]

 

[ctopher]

I see the primary and secondary dims as the same dim. The FCF would go with the dim and used both for prim and sec. The FCF would also indicate a prim/sec tol within the FCF "in [mm]".

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)

 

[ajack1]

Of course if America would try and keep up with the rest of the world this would not even be an issue. ;-)

 

[ImnotfromMars]

Use dual dimensioning in the GD & T box. Be careful because the rounding of the numbers can produce two different limits. When the dual dimensioning does not give exactly the same results Design Department should be consulted.

Robert

 

[JohnNabors]

I would determine what system of units is the actual, ultimate design requirement and then specifically show the converted dimensions as being 'for reference only'in the drawing notes.

 

[drawoh]

JohnNabors,

I agree with you. After they threaten to fire me for not providing alternate units on my fabrication drawings, my next step would be a note stating that the dimensions in brackets are for reference only.

Unfortunately, CAD software does not always support this. I would prefer that the alternate units not show tolerances. AudoCAD and SolidWorks both show them.

JHG

 

[Randy1111]

Thanks for the replies all.

Yes i'd prefer to not dual dimension,in addition to the confusion, it takes up a lot of room. Accuracy is not a concern since we dont manualy do the tolerance conversions, its cad, not a drawing board

Unfortunatly, i cant tell customers who do millions of dollars of business with us that "sorry i can no longer sell you services in the manor you requested". If dual dimensioned drawings are in the contract, thats what i have to provide, regardless if its proper or not. And as Meinsti stated, we do secondary at 1 dec place less than primary.

So looks like adding both dimensions inside a feature control box, like concentricity or total runout is the way to go in those cases. Just makes it so messy

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Randy