Thread Note

[TXspace]

Once upon a time, back in the days of board drafting, we old folks were taught this is how to read a thread note:

1/4-20 UNC

1/4 = Major Diameter in inches
20 = Threads per inch

Everybody in agreement so far?

How about this one:

1-8 UNC
1 = Major Diameter in inches
8 = Threads per inch

Still all on the same page, are we?

Well, how about this one:

4-40 UNC

Hmmm..., we have a situation here. Does the "4" mean "Major Diameter in inches" or does it mean "#4", which translates to a .112 Major Diameter in inches?

Without the "pound sign" (#), wouldn't the 4 mean a "4-inch diameter bolt"?

But perhaps the "UNC" defines the "4" as meaning "#4". After all, UNC is a standard unto itself.

Thoughts?

It has been my habit to include the pound sign (#) for good measure, but I do not find that requirement in the ASME drafting standards.

Can anybody find the requirement to use the "pound sign" (#) in a standard? If so, please post.

 

[fcsuper]

The # symbol is not used for thread callouts. It is only used as short hand for the actual drill. However, even that isn't clear because the question can be whether one is requesting a #4 clearance hole drill, or a #4 drill (two completely different bits).

4-40 UNC is ID'd by the number of threads. Standard 4" threads do not have 40 threads per inch, nor would that be UNC...it would be UNS.

Nowadays, it is recommended that we use the decimal equivilent to the old numbers, but many of us still use the traditional numbering system.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
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[KENAT]

Numbered sizes may be shown because of established practices. The decimal equivalent to threee decimal places, should be shown in parenthesis. Examples are as follows:

No. 10 (.190)-32 UNF-2A
or
10 (.190)-32 UNF-2A
or
.190-32 UNF-2A

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[KENAT]

Sorry, hit submit instead of edit.

What I was going to say was that sadly ASME B1.1-2003 doesn't quite agree with 14.6.

If you look in table 2 or 3 it lists

4-40 or .112-40 UNC

and if you look at section 6, Screw Thread Designation it essentially says you can use just the screw number and has examples showing this.

If memory serves yet another inconsistent method is shown in one of the specs I mentioned but I can't find it now.

As you suggest in your case it is effectively governed by the fact that there are only certain specific sizes that are UNC and 4.000-40 isn't one of them.

I don't see the '#' sign used for screw size anywhere in either standard. That said we do use it on our standard thread callouts such as "#10 (.190)-32 UNF-2A" - a real belt & braces approach.

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[ctopher]

Also:

http://www.draftingzone.com/standards/standard5/

http://www.inserta.com/PDF/AI-STR-THD-PORT-DIM.pdf

Look in the Machinery Handbook for more info.

Chris
SolidWorks 08, CATIA V5
ctopher's home (updated Aug 5, 2008)
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[CheckerRon]

What are "braces"? Is that British for suspenders?

It seems that usage of the options in Y14.6 depends on the company or industry. Government jobs pretty much use the decimal callout exclusively, and ignore the letter callouts for the small machine screw threads. i.e.
.190-32 UNF-3B.

 

[KENAT]

Yeah Ron, it's English for suspenders.

Suspenders are a garter belt.

Well off topic though.

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[ctopher]

They straighten teeth.

Chris
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ctopher's home (updated Aug 5, 2008)
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[cadman1964]

We here use the # sign for the smaller thread callouts. (#4-40 UNC)It's standard in our cad system, which is supposedly follows ASME standards. We build and design deep drawn stamping die's. Plus if i saw a callout like 4-40, i would double look and double check my reference books. You never know these days

Solid Edge V20

 

[fcsuper]

Just to be clear, no common standards are pound signs used as part of any thread callout.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[ctopher]

I agree, remove the #.

Chris
SolidWorks 08, CATIA V5
ctopher's home (updated Aug 5, 2008)
ctopher's blog
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[cadman1964]

Well for use to remove the # would be a waist of time since anytime i get a new cad version, which is a bout once a year, i would have to go through and remove it from the .txt file. I have enough to do due to them not having tap drill sizes right.

Solid Edge V20

 

[fcsuper]

Regarding CAD applications, having standards not met is a common problem because the software CAD companies will often rely on persons inexperienced in the field to come up with detailing defaults. SolidWorks used to be pretty bad at this too, but is making a lot of progress towards meeting current standards.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[KENAT]

cadman, tap drill sizes aren't normally specified on drawings. The minor diameters in the holes.txt file are based on the actual thread minor diameter for male or female threads, although I know they used to be the tap drill till a few versions ago. This is actually more correct to my mind and works better with fit checks and the like. I know Rick may not like it but to me it's more useful & more correct.

You can normally copy the old holes.txt file over to the location required for the new version of SE, I think V19 was an exception because they added a new parameter but generally it's not a problem as far as I know.

If you want to delete the # then just open the holes.txt in word or similar and just do find replace, it wont take that long. However, we have the # and I can't get excited about it.

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[cadman1964]

Yea i understand what your saying. but here at MFC we use tap drills for E-pin head clearance when e-pin is in the same spot as the tap. So we do rely on tap drill size's for certain things. a few times even use it for a gas spring clearance hole.

We have been copying the Hole.txt file over for except V19.

For the # sign, it's one those things that actually at time works to our benefit. So least at this time, until i have to redo hole.txt cause they change something and they removed the # sign, were going to stay on the same course.

Solid Edge V20

 

[cadman1964]

I'm new to this forum so not sure how to edit a post so I'm going to reply again...

Forgot to mention, we do not specify Tap drill sizes on drawings unless we want to make sure there going to use the correct tap specified in the mach. handbook. normal thread call out would be something like 5/16-18 UNC .750 THD DP

Solid Edge V20

 

[KENAT]

We occasionally use the tap drill for a through hole on manifolds and the like with the thread and it can be awkward to handle so I get what you're saying.

I agree on the #, technically it should probably be "No." per 14.6-2001 or blank but while normally a stickler for the rules I can't get excited by it.

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[drawoh]

cadman1964,

Tap drills are specified systematically by SolidWorks, and by designers who do not know better. I do not know abou the other CAD packages.

Machinists know what size tap drill to use. When they see your instruction, they probably will shrug their shoulders and select the drill they think appropriate. Tap drills are uninspectable. All your inspector can do is test the hole with a thread gauge.

If the drilled hole mattered to me, I would make a section view, and explictly apply diameters and depths, with the appropriate tolerances.

JHG

 

[cadman1964]

I do see what you mean but, in our industries mistakes are costly. We use to not spec them out 5-6 years ago. But when machinist would use a smaller drill than what spec out by the Machinery Handbook it caused problems. I was always taught never to let them guess what your thinking. Plus this way, since %90 of our tools are built outside, we can go back to them and have them fix it with out cost to use.

If it's a critical hole than a section view is created and hole is spec out with proper tolerance. Tap drill's are more used for none critical item that go to in a tap hole.

"Machinists know what size tap drill to use. When they see your instruction, they probably will shrug their shoulders and select the drill they think appropriate."

Well we just had a die plate where a machinists most of not been properly trained, or didn't remember his drill sizes. they used to big a tap drill so now when we threaded a bolt in, it pulled right out. We ended up sending this back for them to fix.


Solid Edge V20

 

[KENAT]

I think maybe there's a little confusion.

If the 'tap hole' is truely that, just the start hole for the tap to follow and no part of the tap hole is left/relevant to function after machining then it shouldn't normally be detailed on the drawing per ASME Y14.5M-1994 1.4.

However, if the 'tap hole' is desired to be longer than the thread length to serve some purpose such as in a manifilold, or to hold a pin or similar then this portion of the hole should be properly specified, preferably with a lose enough dia tol to allow the machinist some lee-way in picking his preffered tap dril. I typically dimension this as a counterdrilled hole similar to figure 1-38 & section 1.8.12.

The inability of a machinist or vendor to meet the requirements of the drawing isn't inherently a drawing issue.

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