thread calculation

[fermjohnp]

Hello,

In our shop we were cutting a small bar for it to be threaded into another threaded hole. I do not know the dimensions of the hole the threads fit into however we had to change the 1/2-28 to 0.497 -28. I'm not sure whether it was a UNC,UNF. Apprently it fits like a glove so its correct.

However,

How do i find this value(0.497) in the machinist handbook? i'm having problems locating it.

thanks,

 

[Jboggs]

Look at thread form data in Machinery's and you'll see that the actual major diameter of 1/2" threads is less than 1/2", in fact depending on the pitch its very close to your size. You have a 1/2-28 UNEF (Extra Fine) ANSI thread. Look it up.

 

[fermjohnp]

Jboggs,

i find it to be 0.4768 which is the basic Pitch Diameter. I looked under table 3c. extra fine thread series. It is 23tho less than 0.5. We cut it to 0.497. he said it was tight but did fit.

does that sound right?

 

[Jboggs]

Sounds like you are confusing the "pitch diameter" with the "major diameter". Those are two different things. Flip back a few pages for diagrams of the Unified Thread Form and explanation of each.

If 0.497 is the measured "O.D." of the thread that is only .003" under the theoretical major diameter, which is practically perfect.

 

[fermjohnp]

Jboggs,

The major dia of a 1/2-28 UNEF is 0.500.

we cut it to 0.497 however i don't see how we got that number. Why didn't we just cut it to 0.500?

Is there a rule?

thanks

 

[fermjohnp]

if i use another chart i find a External max b major diameter of 0.4989. Still not 0.497.
tolerance is 2A-allowance of 0.0011.

This screw did fit up into an ID threaded ring.
They are external threads so i think im in the right heading.
Internal designation is for ID threading.

comments?

thanks,

 

[powerhound]

fermjohnp,

There isn't a rule, so to speak. There are established standards and the numbers in the Machinery's Handbook reflect those standards. The major diameter of a 1/2-28 3A is .497-.500. For a class 2A fit the major diameter is .496-.499. There is no good reason to change the designation from 1/2-28 to .497-28. I have gone through this exact issue before with an inexperienced engineer who thought that the reason a 1-8 bolt didn't thread into a 1-8 hole was because both components were 1" so they were trying to occupy the same space. I had to explain to him that there was already clearance built into the specification and no adjustment to spec was necessary. I'm not saying that inexperience is the cause of your dilemma but it's more often that I've seen interfering pitch diameters than major or minor diameters.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[fermjohnp]

"The major diameter of a 1/2-28 3A is .497-.500. For a class 2A fit the major diameter is .496-.499"

how did you find this? i'm looking in the unified screw thread section

 

[Jboggs]

Frankly I'm having trouble seeing how.003" (.500 - .497) makes any difference in the real world.

 

[fermjohnp]

I see in my machinist handbook(21st ed) that 1/2-28UNEF class 2A major diameter is .4924-.4989.

how did he get 0.497?

 

[fermjohnp]

i think he just guessed between the two values.......

 

[powerhound]

You got it right. I read the wrong column. I just rechecked. Your numbers are exactly what I have I was just rounding to the nearest thousandth in my original reply.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[fermjohnp]

powerhound,

which column or row did you read from the first time?

fyi i have 21st ED.

 

[fermjohnp]

powerhound,

which column or row did you read from the first time?

fyi i have 21st ED.

Engineer and Programmer
MasterCAM X6
Solidworks 2012

 

[powerhound]

I use ME Thread Pal. It works really well. You just select the thread you want to cut and it spits out all the pertinent info, including a measurement over wires if you're using 3 wire to check pitch diameter.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[EMorel]

.497 falls within the allowable range of 0.4924 - 0.4989 major diameter for 1/2-28 UNEF 2A (Table 3, page 1740 of Machinery's Handboook, 27th ed.). It doesn't matter how they derived the 0.497, as long as it is within that allowable range and the mating threads are within their allowable minor diameter range then it will work (assuming the pitches are correct also).

 

[KENAT]

While you should always double check from an authoritative source

http://www.boltplanet.com

has a useful database/calculator.

Posting guidelines faq731-376

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

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