Decimal Dimensioning/Fractional Equivalents

[ringman]

ONCE UPON A A TIME,

I believe there existed a standard/spec which recommended the appliication of decimal/inch dimensioning for designs where allowed, verses fractional/inch dimensioning as much as possible.

That is to say for example, use 6.20 X 4.80 for the sheet size instead of 6.25 X 4.875.

Does anyone on this forum have a handle or recall?

 

[CheckerRon]

I have seen such examples in company DRM's, for machined parts but not any published standard.

There was a push for such round offs when decimal inch measurement began replacing fractions for programming and NC machining purposes. Often it was metric based people who didn't know or like fractions anyway. Don't know or any industry spec like that, however.

 

[CheckerRon]

What bugs me is not dimensions like 4.312 or 4.413 for
4-5/16", but CAD generated dimensions like 4.179. Why not 4.800?

 

[drawoh]

ringman,

I do not worry about the standards here. I worry about what my fabricator understands. If they have to take a pencil and a calculator and convert all your dimensions into units they understand, you are driving up costs and errors.

I don't think machine tools work in fractional inches. The displays all are in decimals. If it were welding, I would call the shop and ask.

JHG

 

[GBor]

I don't fully understand your example. Are you asking why don't people round to the nearest 10th of an inch? Why do they carry out to the thousandths place? We have applications that are machined to within tolerances that require 3 decimal place accuracy so that they line up with the adjacent part, which is often in another country.

Or are you asking why 0.25" instead of 1/4"? That, I think, has much more to do with QA measuring device read-out and CNC-driven machines.

As for a "standard"...doubt you will find one because the construction industry would probably go nuts if you didn't use architectural dimensioning of 0'-1/4" in the US, while the aerospace industry likes decimal or metric units.

 

[ringman]

Let me try again.

I am not suggesting that the dimensions need not be controlled to 3 or more decimal places....

I am looking for OLD information that I thought directed us AWAY from decimal equivalents to fractions as much as possible. Such as .3125 when most likely .3 would do the job.

Does that clarify my position? and question?.

 

[MechEng2005]

Try telling the mechanical installation contractor to use a eight-tenths bolt sometime instead of a 3/4"... Let me know how that goes...

Seriously though, I think that unless a part for form/fit/function requires decimal, I would much prefer fractional. If nothing else, the decision to use a 5/16" hole for a 1/4" bolt is simple. If I were using decimal and not driven by some standard/code would I put a 0.31" or 0.32" hole? Maybe 0.30" or 0.35" because I like multiples of 5...

-- MechEng2005

 

[KENAT]

ASME Y14.5M-1994 directs the use of decimals and does not address fractional dimensions.

However it doesn't as far as I know say anything about using 'true decimal' rather than the decimal equivalent of fractional sizes.

I've seen similar somewhere ringman but I don't think it was in a standard.

KENAT, probably the least qualified checker you'll ever meet...

 

[ringman]

Thank you Kenat.

Im with you and your answer assures that I am not totally bonkers perhaps but still looking for the lost spec. I do believe it existed at one time. Thanks again.

 

[TheFeds]

It sounds like you're looking for guidelines with regard to "preferred numbers" (

http://en.wikipedia.org/wiki/Preferred_number).

If so, it looks like ISO 3, ISO 17 and ANSI B17.1 address them as they apply to mechanical design.

I seem to recall one of the mechanical engineering handbooks having a list of numbers and descriptions of when to use them. It was probably Machinery's or Marks'.

I'm curious as well whether current ASME, ISO or CSA standards call for any particular scheme.

 

[TheFeds]

The previous post should read "ANSI Z17.1", not "ASME B17.1"....

 

[CheckerRon]

I just queried Global IHS to view the ANSI Z17.1 spec on Preferred Numbers and they say it has been withdrawn. Maybe there is an old copy out there somewhere, but it doesn't sound like it is current.

 

[btrueblood]

Kenat,

From an earlier post of yours regarding fastener/thread callouts, does not the relevant ASME spec. talk about using decimal and not fractional sizes?

 

[ctopher]

Our company does not use fractions (major aerospace co).
NC does not read them and QA, inspection, etc do not accept them.

Also...our industry standard:

The dimension and the tolerance shall contain the same number of decimal places; for example,
- "5000 ‰ .0005, 1.50 ‰ .06, or 4.000 ‰ .010. All decimal dimensions shall contain two or more
decimal places; for example, 1.00 or 1.60 not 1 or 1.6."
- "...parts should be dimensioned to the exact values required, rather than to decimal equivalents of fractional dimensions; for example, ".250" should not be used when ".246" satisfies the design requirements."

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Jul 13, 2008)

 

[KENAT]

btrue, I think it's a bit off topic but how to call out threads is given in Y14.6-2001 3.2.1.3 & it allows use of fractions (this part matches B1.1-2003 6.1) for fractional unified threads.

KENAT, probably the least qualified checker you'll ever meet...