Thread Call-outs

[MadMango]

I have a vendor requesting we take off the metric reference dims on our thread call-outs. I can't find the paragraph to cite the practice one way or the other. Our drawings are in INCHES, and everything gets a metric reference.

"Art without engineering is dreaming; Engineering without art is calculating."

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[swertel]

How can you have inch and metric threads? The two are not compatible. You can't do a soft conversion. The thread tolerances between an ASME 1/4-20 and an ISO M6.35 aren't the same. And that's not including thread class.

--Scott

http://www.wertel.pro

 

[MadMango]

Example:
.250[6.35] -20 UNC-2B

Our vendor wants .250-20 UNC-2B.

Sorry for the confusion.

"Art without engineering is dreaming; Engineering without art is calculating."

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[MintJulep]

Well, it really doesn't make much sense to have metric dimensions on things that are inherently inch based. It's called a 1/4-20 thread for a reason.

But to have a vendor ask for a drawing revision just to take them off?

 

[CheckerHater]

I think your vendor is right.
It is not like you have .250 dimension, but rather the whole .250-20 designation calling for certain Major diameter, Minor diameter, Pitch diameter, Pitch. Why not convert pitch .250[6.35]-20[1.27]?
.250-20 UNC-2B looks perfectly fine even on full-metric drawing.
Does your vendor have authority to demand drawing to be revised? – That’s another story.
But if I was him I would charge you extra for measuring the same major diameter with two different micrometers.

 

[KENAT]

I'm with your vendor on this one.

In fact I thought dual dimensioning (except on data sheets, interface drawings etc.) had come to be frowned on? Plus aren't ref dims usually in parenthesis not square brackets?

I certainly don't believe your current call out is supported by ASME Y14.6 which is I believe the relevant spec for thread call outs. Or in B1.1 which is the actual thread spec.

When you say 'paragraph' what spec are you looking in?

Posting guidelines faq731-376

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(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[MechNorth]

The vendor is correct. Threads are either inch or metric, not both.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[MadMango]

The vendor is right, it's just trying to get co-workers to "buy-off" on doing things the correct way. We don't have any Checkers here, and people follow the standards as time allows (to be nice). I can refer folks to Fig. 15 of ASME Y14.6, and that should be that. CheckHater's comments on the ".250-20" being a designation and not just a dimension will also have legs to run.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?

 

[ctopher]

I also agree, don't mix them.

Chris
SolidWorks 11
ctopher's home
SolidWorks Legion

 

[juergenwt]

The vendor is correct. If the print is solid metric - the Thread call out should be solid metric. Same the other way around. A MIX is a ticket for trouble. Also, if the call is for a metric print, all fit's and tolerances should be standard DIN/ISO 286.

 

[dgallup]

Juergenwt - I don't believe it is ever correct to use metric callouts on a UN thread. Even if the drawing is metric a UN thread should be called out in inch values. The callout specifies a designation, it's not really a dimension. I came across this while working in France. The drawings were of course metric but we used a good old NPT tapered pipe thread.


From Machinery's Handbook:
Screw Thread Designation.—The basic method of designating a screw thread is used where the standard tolerances or limits of size based on the standard length of engagement are applicable. The designation specifies in sequence the nominal size, number of threads per inch, thread series symbol, thread class symbol, and the gaging system number per ASME/ANSI B1.3M. 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 interpreted as being nominal size designations only and shall have no dimensional significance beyond the fractional size or number designation. The symbol LH is placed after the thread class symbol to indicate a left-hand thread:
Examples:
1⁄4–20 UNC-2A (21) or 0.250–20 UNC-2A (21)

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[MechNorth]

Juergen, Metric does not automatically mean ISO/DIN; in fact, ASME Y14.5 is a metric standard which is equally applicable for US Customary Units. Also, it is very common to mix inch/metric threads on drawings with the opposite unit system, at least in North America; I've experienced very few issues with this here or in Europe. And as has been debated a number of times in this forum ISO 286 is useful as a design basis, but really shouldn't be put on the drawings as it leads to interpretation mistakes downstream.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[CheckerHater]

Speaking of applying ISO to UNC threads; does anyone have a copy of ISO 68-2?

 

[drawoh]

MadMango,

One more vote for your vendor. The machinist will never find a tap labeled 6.35-20UNC.

At least, I hope he won't.

JHG

 

[MechNorth]

Drawoh ... for whatever reason, they were available at one time; I saw them in Europe maybe a decade ago. Our company prohibited them after we found them being used in our shop.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[juergenwt]

MechNorth - Thank you. You are correct. I was trying to give MadMango a "heads up" on some very common problems when working with "mixed" prints. True, metric threads can not be stated in inches and the other way around. The two systems do not mix. Yes - you can make a part to inch dim's and list solid metric threads or a part produced in metric using inch threads - if that is what the customer requires. China does it all the time. Even the screws have inch threads (sometimes) and the head is metric. However that is not a good way of manufacturing. Stick to one system - or you will regret it later when you have a real mess on your hands.
I have gone the whole route on this during the last 25 years and we learned the hard way.
I have re-read MadMango's post and he should list his threads in inches - not converting inch thread dim's to metric.

 

[KENAT]

When interfacing with lots of off the shelf items as we do it's almost impossible to completely standardize on either inch or metric fasteners.

In that case to my mind the significant thing becomes to not have screws of similar size in the different systems that could get mixed up.

On newer designs we're trying to go mostly metric for fasteners to help field service out when in other parts of the world.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[flash3780]

You should be referencing ASME B1.1 somewhere in your com-docs if you're using UN threads. The actual definition of the threads is specified in the standard, so the drawing callout should match the appropriate threadform in the standard.

 

[fcsuper]

For all the reasons about, the vendor is correct. My own point to this is that the thread callout is not a dimension. It is a reference to a particular specification. In this, it is a NAME, not an actual toleranced number. The tolerance is specific by what the name represents. Adding metric doesn't making any sense at all. It's a bit like translating the name "Ford" with the word "Mercedes" like "SUPPLY ONE FORD [MERCEDES] SEDAN".

Matt Lorono, CSWP
Product Definition Specialist, DS SolidWorks Corp
Personal sites:
Lorono's SolidWorks Resources & SolidWorks Legion