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How to determine angle tolerance on UN, UNC, UNF threads?

wbstadeli

Plastic
Joined
Mar 24, 2009
Location
Oregon, USA
Hi All,

I have a question i hope someone can help me out with, or point me in the right direction. We machine a lot of standard threads, UN, UNC, UNF. However I have never known the angle tolerance that would be allowed on these? I am talking about the 30 degrees from the axis of the thread. Another way to put it, we are machining these using a single point in a CNC lathe, how angled(out/or in) can the thread tool holder be from the turret wall, assuming the turret is a perfect 90 degrees... Hopefully this makes sense. Thanks for any help!
 
Gordon's document is useful, as the actual specs (at least for ANSI/ASME, can't say for ISO/DIN) do not actually provide an angular tolerance. What the specs define is an envelope, based on the nominal thread flank angle, that the surfaces of the as-cut threads must fall within. You have to figure out what that might mean for thread flank angle tolerance (and how that interacts with the various diametral tolerances) by working out the trigonometry.

As Gordon's tables show, you get a lot more angular leeway on fine threads than on coarse ones. And for almost all threads, you'd better be pretty accurate with your tool alignment.

Now, whether a typical pair of GO/NOGO thread gages will catch bad thread form is a very different story. But a decent inspection on an optical comparator should catch such errors pretty easily.
 
Hi All,

I have a question i hope someone can help me out with, or point me in the right direction. We machine a lot of standard threads, UN, UNC, UNF. However I have never known the angle tolerance that would be allowed on these? I am talking about the 30 degrees from the axis of the thread. Another way to put it, we are machining these using a single point in a CNC lathe, how angled(out/or in) can the thread tool holder be from the turret wall, assuming the turret is a perfect 90 degrees... Hopefully this makes sense. Thanks for any help!

If you make "a lot" why not use manufactured thread cutting inserts? With "home made" stuff I can't even imagine you get the nose radius right.

Threading inserts and grades

http://f-m-s.dk/1.05.pdf
 
Now, whether a typical pair of GO/NOGO thread gages will catch bad thread form is a very different story. But a decent inspection on an optical comparator should catch such errors pretty easily.

It'd probably shock most at how little thread gauges do say "good" for. They have the virtually impossible task of verifying umpteen variables at the same time.

I certainly agree with using an optical comparator to see what is what.

Even something as simple as measuring external OD and internal ID isn't always done and yet that isn't something "caught" by thread gauges. If the external thread OD is below tolerance or the internal ID above tolerance that isn't caught by the gauge.

I'm not saying thread gauges aren't good but don't rely on "blind faith".
 
Van Keuren Catalog / Handbook #35 has a chapter on the two sizes of wires method and provides tabulated data for 58 to 62 degrees included
 
The way the original post was phrased suggestd the concern is correct alignment of a (presumably correctly shaped) tool. My intuition suggests the two-size-of-wire method is not very sensitive to thread spaces of the correct shape but incorrect orientation. Presumably, there would be a weak interaction between measured/computed PD and orientation, but this would be a lab-grade procedure, I think, rather than a routine inspection. Would have to play with the geometry to know if intuition is correct or not.
 
Van Keuren Catalog / Handbook #35 has a chapter on the two sizes of wires method and provides tabulated data for 58 to 62 degrees included

I'm trying to figure out how that would work. It could only be used on one V and require a very accurate measuring indicator or similar. What's the finest pitch the handbook gives?

I'm still going with the optical comparator suggestion.
 
Looks straight forward. Probably less intended for fine single start threads and more for multi start threads as per example

Its easy to appreciate Van Keuren looking for ways to sell more "wires" - that was their bread and butter

I'm trying to figure out how that would work. It could only be used on one V and require a very accurate measuring indicator or similar. What's the finest pitch the handbook gives?

I'm still going with the optical comparator suggestion.
 

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Looks straight forward. Probably less intended for fine single start threads and more for multi start threads as per example

Its easy to appreciate Van Keuren looking for ways to sell more "wires" - that was their bread and butter

I'm guessing your book is at least 60 years old, maybe close on antique :cheers:
 
What sfriedberg said in post #3. We use Johnson Tri-Rolls, one station to measure pitch diameter, one station to measure form error.

Johnson Precision Tri-Roll Thread Gage

This'll make me unpopular with some and I have bought and used tri rolls years ago.

Apart fom the fact that they show a measurement (and that's good) they have the same draw-backs as thread ring and pug gauges. They try to do too much at one time.

I've never as yet come across the "perfect" inspection system but the closest I can come up with is:

1. Measure the thread OD or ID depending on external or internal.

2. Inspect with a GO thread ring gauge or GO thread plug gauge.

3. Measure pitch diameter.

Of course you've got to know your tolerances. If you use a GO thread gauge and measure pitch diameter a NO-GO gauge is a waste of money.
 
This'll make me unpopular with some and I have bought and used tri rolls years ago.

Apart fom the fact that they show a measurement (and that's good) they have the same draw-backs as thread ring and pug gauges. They try to do too much at one time.

I've never as yet come across the "perfect" inspection system but the closest I can come up with is:

1. Measure the thread OD or ID depending on external or internal.

2. Inspect with a GO thread ring gauge or GO thread plug gauge.

3. Measure pitch diameter.

Of course you've got to know your tolerances. If you use a GO thread gauge and measure pitch diameter a NO-GO gauge is a waste of money.

Well...
While I agree that a tri-roll isn't the end-all be-all, it most certainly is far, far more comprehensive than your 1-2-3 process.

http://www.johnsongage.com/technical.shtml

And take a note of this excerpt from the above link:

A quantitative measure of the entire effect of thread form and profile error, the DIFFERENTIAL BETWEEN THE FUNCTIONAL AND PITCH DIAMETER SIZE is the key to both efficient production and ultimate performance.

That part in red DOES in fact indicate form truncation as well as flank angle error!
 
Well...
While I agree that a tri-roll isn't the end-all be-all, it most certainly is far, far more comprehensive than your 1-2-3 process.

http://www.johnsongage.com/technical.shtml

And take a note of this excerpt from the above link:

That part in red DOES in fact indicate form truncation as well as flank angle error!

I don't know how often you make or measure threads but there is an important fact you've omitted. COST.

Tri rollers can't be used for threads with different diameters even if they have the same thread profile and pitch. For companies mass producing specific threads they're certainly better than thread gauges.

They are also limited as to how small an internal thread they can be used for. I've also never seen them used on large diameter threads. Oh yea, they require a master. It all mounts up so for most places my 1, 2, 3 is probably the least expensive.

Just out of curiosity, have you set up and used tri rollers? I have. I also remember how shocked I was at the price for everything I needed.

TRI-ROLL Thread Gage Frames

Anyone know the complete price if you wanted to measure a 1-8 UNC? I'm guessing between $1,500 and $2,000. Am I too low?

Slightly off track but for over 2 years now the aerospace industry (in 6 different countries - including the USA) has been buying my system and most know that are very careful with what they make and do.

Here's how I made measuring Buttress threads easy.

http://f-m-s.dk/Buttress Thread Measurement.pdf
 
I don't know how often you make or measure threads but there is an important fact you've omitted. COST.

Tri rollers can't be used for threads with different diameters even if they have the same thread profile and pitch.

Just out of curiosity, have you set up and used tri rollers? I have. I also remember how shocked I was at the price for everything I needed.



Anyone know the complete price if you wanted to measure a 1-8 UNC? I'm guessing between $1,500 and $2,000. Am I too low?

Gordon

Yes, I have used tri-rolls.
In fact there is one of these: http://www.johnsongage.com/pr_is_external3.shtml set up for .5000-20-UNF right now,
and one of these: http://www.johnsongage.com/cat_cx_cx.shtml ( albeit dial indicators ) is set for .7500-16-UNJF on another machine.
The first part is a standard consumable item out of TI that is to fit mating units all over the world in the field, while the other is an aerospace
part.

For the first, I absolutely have to make sure that the thread is correct to mate with everything, the other I don't have a choice.
Aerospace absolutely REQUIRES quantitative measurement, no exception ( well there is an exception for small threads but I digress )

Now, a couple of things:
Thread rolls ( at least from Johnson Gage ) are pitch specific only. A 20 pitch PD and Functional roll can be used on everything from 1/4-20 and up.
IOW you buy the roll set ( approx $600 as I remember the last time around ) you are good to go on that specific pitch of any size. ( more on that in a bit )

Adjustable Tri-roll frames have a minimum and a maximum size they're capable of measuring.
The ones I've linked to can measure from .250 to 3" by design, but there are ones from 3" and up ( to a point of course )

Now all you need to purchase is:
1: A set-master that is specific to the size and pitch you're making. Cost is approx $250-350 depending on size and commonness.
2: A set block specific to the size-only ( such as 1" or 3/4 or .5625 etc ) to adjust the frame properly. ( approx $80 )

If you're trying to get away on the cheap, you may forego the last two items and use a jo-blocks to set the frame and plug gage to set the rolls.
Not as accurate, but as long as you've got the rolls you're good-to-go for free.


Now lastly, I am not advocating that everyone here go out and plunk down $4K or more for a frame set immediately and then buy all other
necessary accessories to measure threads. I certainly do understand that it is NOT a cheap way to measure.

At the same time I do take exception to the notion that a ring or plug gage coupled with a micrometer and an optical comparator is equivalent
to a tri-roll setup.

Sidenote: All of the above BS I've spewed applies to external threads only!
For internal measurements one could easily triple or quadruple the prices, and they ARE pitch and size specific.
I've only got 5 sizes, and it's $2K and up for each.
So, we do the Rube Goldberg method and swap back and forth within a few shops as-needed.
 
Gordon's document is useful, as the actual specs (at least for ANSI/ASME, can't say for ISO/DIN) do not actually provide an angular tolerance. What the specs define is an envelope, based on the nominal thread flank angle, that the surfaces of the as-cut threads must fall within. You have to figure out what that might mean for thread flank angle tolerance (and how that interacts with the various diametral tolerances) by working out the trigonometry.

As Gordon's tables show, you get a lot more angular leeway on fine threads than on coarse ones. And for almost all threads, you'd better be pretty accurate with your tool alignment.

Now, whether a typical pair of GO/NOGO thread gages will catch bad thread form is a very different story. But a decent inspection on an optical comparator should catch such errors pretty easily.

GO NOGO gauges can catch some types of form error, that is why the NOGO is truncated (major of the GO plug is greater than major of the NOGO plug)
Conditions of form error exist where a NOGO plug will go before the GO plug does for example, a wide thread form with an undersize major can let NOGO pass before the GO does.
The GO is a full form check, while the NOGO is more of a p.d. check because of the truncated crest and the extra root clearance.
 
Hi All,

I have a question i hope someone can help me out with, or point me in the right direction. We machine a lot of standard threads, UN, UNC, UNF. However I have never known the angle tolerance that would be allowed on these? I am talking about the 30 degrees from the axis of the thread. Another way to put it, we are machining these using a single point in a CNC lathe, how angled(out/or in) can the thread tool holder be from the turret wall, assuming the turret is a perfect 90 degrees... Hopefully this makes sense. Thanks for any help!

in my experience, if you use a good quality tool holder and insert and dial in the tool shank within a couple thou you are good to go.
In gauging class 2 UN profiles and class 2G acme profiles, you can visually see profile errors large enough to negate the use of GO NOGO gauges. Thread rolls etc have more value in high production than accuracy in these looser thread classes.
For class 2 threads a simple major and minor diameter check along with go and nogo gauges and profile gauge is adequate to meet spec. As well as you can see profile on a profile gauge is better than your profile needs to be to meet spec.

I also agree with sfreidberg that the tolerance on the flank angle is not specified, the tolerance zone or envelope is specified. As long as the entire profile lies within the tolerance zone it meets spec. A step on the flank that you can see with the naked eye and also feel with a pick might actually meet thread form spec.
 
Gordon

At the same time I do take exception to the notion that a ring or plug gage coupled with a micrometer and an optical comparator is equivalent to a tri-roll setup.

Sidenote: All of the above BS I've spewed applies to external threads only!
For internal measurements one could easily triple or quadruple the prices, and they ARE pitch and size specific.

I don't know who claimed that notion. I didn't.

If you prefer the way you do things (and can afford it) I have no problem with that.
 








 
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