Fractional vs Decimal Screw Thread Callout

[flash3780]

Interestingly enough, one of our draftsmen recently put together a print that had fractional screw thread dimensions (ex. 1/4-20 UNC-2B). I didn't think much of it until our customer questioned the callout. I often see threads called out with decimals, but do any of the ASME standards give guidance on which is more appropriate?

 

[KENAT]

"…The nominal size is the basic major diameter and is specified as the fractional diameter, screw number, or their decimal equivalent. Where decimal equivalents are used for size callout, they shall be shown in foru place decimals (omitting the cipher in the fourth place) for fractional sizes, and in three place decimals for number sizes. ...”

So one might argue fractional is slightly preffered in that standards.

In ASME Y14.6-2001 Screw Thread Representation says similar, though if using number sizes it says you should add the decimal in parenthesis, which isn't quite what B1.1 says.

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[flash3780]

Well, I guess that answers it. I found the paragraph that you're referring to. What do you suppose that they mean by:

Where decimal equivalents are used for size callout, they shall be shown in four place decimals (omitting the cipher in the fourth place) for fractional sizes, and in three place decimals for number sizes.

I've always invoked number sizes as "6-32 UNC-2B". Are they recommending "6.000-32 UNC-2B"?

 

[KENAT]

No. They mean ".138-32 UNC-2B".

.138 is the decimal equivalent of #6.

Actually, in B1.1 they appear to use leading 0's which is odd for an inch standard.

Per 14.6 it would actually be "No. 6 (.138)-32 UNC-2B" or 6 (.138)-32 UNC-2B".

As I've mentioned before the 2 specs don't quite match on some details.

What you put would I believe be a 6" thread.

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[flash3780]

6" Extra, extra, extra, extra fine.

It sounded a bit odd when I read it yesterday, but now that I've reread it, it's clear what they're talking about.

 

[ctopher]

Also, check out the Machinery Handbook for some info.

More links:

http://www.engineersedge.com/screw_threads_chart.htm

http://mdmetric.com/tech/tict.htm

http://www.carrlane.com/Catalog/ind...C3B285357474A2D020609090C0015312A36515F554A5B

http://www.carrlane.com/TrigBook/TrigBook1.cfm

http://www.wikihow.com/Read-a-Screw-Thread-Callout

Chris
SolidWorks 09 SP4.1
ctopher's home
SolidWorks Legion

 

[fcsuper]

I think there was a battle about this in the past and the fractionists kinda won for the decimalians. For my money, fractions are harder to read when designing, but easier to interpret because they don't accidentally invoke standard decimal place drawing tolerances.

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

http://groups.yahoo.com/group/solidworks & http://twitter.com/fcsuper

 

[ewh]

I also think this has been addressed before, but I don't remember giving in. ;-)

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[Tunalover]

You can't go wrong specifying the decimal size. It would be awfully hard to incorrectly transcribe .250-20 UNC-2B.

Tunalover

 

[drawoh]

I think there was a battle about this in the past and the fractionists kinda won for the decimalians. For my money, fractions are harder to read when designing, but easier to interpret because they don't accidentally invoke standard decimal place drawing tolerances.

The 2B specification as per Tunalover's post above, supersedes the tolerance block. Most English tolerance blocks don't define four decimal places, anyway.

I prefer the number (6-32UNC-2B) and fraction (1/4-20UNC-2B) myself because usually, that is what is written on the taps and on the inspection gauges.

JHG

 

[fcsuper]

We use the fractions in callouts on drawings. But for hardware descriptions in our PLM, we use decimal for 1/4 and above, and the ## for 0-12. Sorting is not perfect this way, but still better than if we used fractions.

Matt Lorono
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