ISO965-1 Thread Calculation

[dksoba]

The following is a "revoked" standard (which is probably why you can find it freely available here http://www.chinafastener.biz/gb/am/file_list.aspx?action=download&id=215&ext=pdf). I am going through the academic exercise on page 49 of the standard (Appendix B), and I am having a hard time understanding how they calculated the pitch diameter tolerance for tolerance grade 5. If I use table 10, I find that for the basic major diameter of 30mm, I have two pitch values which I interpolate between. For tolerance grade five, I interpolate between 100 and 118, to get 109 microns. If I use the formula in 6.12.5 (page 28), I calculate Td2(6) = 90*(1.25)^0.4*(30)^0.1 = 138.267 microns, which when converting to tolerance grade 5, I get Td2(5) = 0.8*Td2(6) = 110.613. If I round that value to the nearest R40 preferred number, I get 112 microns. If I round Td2(6) to the nearest R40 preferred number before converting to tolerance grade 5, I get 140 microns, which when converted, is 112 microns. I'm quite confused on how they arrived at 108 microns in the example. I really want to make sure I get this correct for no reason other than that this shouldn't be too hard to calculate otherwise it's usefulness would be limited.

Hopefully this was the right section to post this in, but if not please move it accordingly.

Cheers,
Matt

 

[Nick Mueller]

I have two pitch values which I interpolate between.

No interpolation on the pitch!
You can only take pitches that are listed. Others do not exist. Period.

Maybe you are more specific what thread you want to calculate.


Nick

 

[dksoba]

No interpolation on the pitch!
You can only take pitches that are listed. Others do not exist. Period.

Maybe you are more specific what thread you want to calculate.

Well in the example on P.49 they are calculating an M30 x 1.25mm 5g6g thread. I don't understand why the other pitches do not exist, it seems to me that you can have any size custom thread that you want. That is why they give you appendix B "Outline guide and examples for calculating limits of size of untabulated metric screw threads", right?

Matt

 

[Nick Mueller]

Read page 4, table 3. M30 x 1.25 is non existing.


Nick

 

[dksoba]

Read page 4, table 3. M30 x 1.25 is non existing.

I agree that this is not in table 3, however, why do they give an example of calculating an M30 x 1.25 5g6g external thread on Page 49? It seems like if it wasn't possible then they wouldn't give an example of the calculation of it.

 

[Nick Mueller]

however, why do they give an example of calculating an M30 x 1.25 5g6g external thread on Page 49?

I don't know.
My standard doesn't contain these examples. And it is from 1998. Theirs is from 1980. The BS also is a wild mix of standards (read foreword).

Maybe it even is a Chinese fake?


Nick

 

[dksoba]

Damn, so I basically need to purchase a copy of the ISO 68-1 standard. Also, I guess it wouldn't be possible to make an M18x0.5 6g thread to the ISO965-1 standard since it's not listed in table 3. However, an M16 and an M20 thread wouldn't be suitable for my customers application, so I guess we just have to make up the dimensions and call it good. Or is there a standard they could specify?

Matt

 

[The real Leigh]

Read page 4, table 3. M30 x 1.25 is non existing. Nick

Nick,

You apparently don't understand the term "untablated". That means sizes that do not appear in the table.

"Outline guide and examples for calculating limits of size of untabulated metric screw threads"

Variant threads can exist for any number of reasons in both English and Metric systems.

This is where theory meets reality. Reality wins every time.

- Leigh

 

[Nick Mueller]

Also, I guess it wouldn't be possible to make an M18x0.5 6g thread to the ISO965-1 standard since it's not listed in table 3.

No, you can make it. But you can't refer to ISO 965-1. So either state your own limits or state: "Tolerances like for a M30 x 1".

You apparently don't understand the term "untablated". That means sizes that do not appear in the table.

Did you look at that table? I'm very sure you did not.


Nick

 

[The real Leigh]

Did you look at that table? I'm very sure you did not.

No, I did not. Why should I?

An algorithm that's defined as producing proper values (tolerances) for threads that are "untabulated" means just that.

- Leigh

 

[Nick Mueller]

No, I did not. Why should I?

Maybe to enable you to make useful contributions?

An algorithm that's defined as producing proper values (tolerances) for threads that are "untabulated" means just that.

My standard doesn't have the example.
And it just lists 6 formulae for how they got to the values. It does not say to use them for pitches not defined in the table. It also says, that they are partially not valid for pitches of or less than 0.45 or 0.3 mm. It also expressis verbis explains that the values were obtained empirically. The formulae were developed later. And it clearly states, that the rules for rounding (described in the standard) were not always applied.

Conclusion is, that the formulae are just informative. Simply because they are incomplete and don't give proper results.
Or short: Useless.


Nick

 

[Gordon B. Clarke]

A person can make just about any thread (diameter and pitch and tolerance) they want to make. What is important is that the more "normal" (as in "can be found in a standard") the thread is the easier it is to state correctly in a specification and the cheaper it will probably be to make and inspect.

Those that make "weird threads" usually do so because they are making an entire product for a specific purpose and even then often to "force" customers to buy spare parts from them.

Henry Ford would turn in his grave if he saw how many "special" things are made today for no other reason than to be different.

 

[Gordon B. Clarke]

Well in the example on P.49 they are calculating an M30 x 1.25mm 5g6g thread. I don't understand why the other pitches do not exist, it seems to me that you can have any size custom thread that you want. That is why they give you appendix B "Outline guide and examples for calculating limits of size of untabulated metric screw threads", right?

Matt

There are a limited number of pitches and tolerances recommended for M30. OTOH that doesn't mean it's "against the law" to use/make any pitch (and tolerances) you want. However my advice to those that could or would do so would be "Fools rush in where angels fear to tread". In other words think carefully, and have a very good reason, for not just using the recommended pitches and tolerances.

Gordon

 

[The real Leigh]

Nick's fundamental error is in viewing a thread as a final product.

It's not. It's a component of an assembly, which may itself be a component of a larger assembly, etc.

Good engineers take many factors into account when selecting fasteners.
Primary among those is for the fastener to do its job properly.

If that requires using a non-standard thread, then a non-standard thread is specified.
I know of no standard that prohibits such selection. To do so would interfere with the engineer's ability to do his job.

The only possible problem is that such a spec may require more definition than is needed for a "standard" thread.

- Leigh

 

[Gordon B. Clarke]

Nick's fundamental error is in viewing a thread as a final product.

It's not. It's a component of an assembly, which may itself be a component of a larger assembly, etc.

Good engineers take many factors into account when selecting fasteners.
Primary among those is for the fastener to do its job properly.

If that requires using a non-standard thread, then a non-standard thread is specified.
I know of no standard that prohibits such selection. To do so would interfere with the engineer's ability to do his job.

The only possible problem is that such a spec may require more definition than is needed for a "standard" thread.

- Leigh

While I agree with you there's one point that has me thinking - ouch

The only possible problem is that such a spec may require more definition than is needed for a "standard" thread

Not all threads are specified in the same amount of detail but as far as "common" threads go I find it difficult to imagine what further specs could be necessary?

Maybe a special coating thickness or similar on the machined part? That would certainly give different (i.e. pitch) diameter tolerances.

At one company they had thread problems after electro polishing SS and seemed to have forgotten (or just hadn't thought about) the fact that that process removed metal and at the rate of twice as much (with a 60º flank angle) as on a simple diameter.

Gordon

Hmmmm I just gave an example that proved your point

 

[The real Leigh]

Not all threads are specified in the same amount of detail but as far as "common" threads go I find it difficult to imagine what further specs could be necessary?
Maybe a special coating thickness or similar on the machined part? That would certainly give different (i.e. pitch) diameter tolerances.
Hmmmm I just gave an example that proved your point

Hi Gordon,
Yes, that's one possibility, as you so kindly pointed out.

I was thinking more along the lines of simple tolerances, which can be done by simply referencing a standard for standard parts.

For example, there are alpha-numeric clearance codes for metric fasteners that might not be applicable verbatim to non-standard threads.

- Leigh

 

[Nick Mueller]

It's not. It's a component of an assembly, which may itself be a component of a larger assembly, etc.

Oh really! I didn't know that. I always thought that bolts are just bolts. And that they have no use at all.

Nick's fundamental error is in viewing a thread as a final product.

The problem is more that you think what I think. And you don't know that.

But back to the original question that you should re-read:
The OP doesn't want to make that thread (M30 x 1.25), he just wanted to understand how it *could* be calculated. And that's where his intent clash with reality. That diameter / pitch combination is not foreseen in the standard. So he simply can't refer to the standard. And he can't calculate the tolerances for reasons explained in the standard. Calculating might work, that's all the standard says.

That's all I said. I did not say that the thread is impossible or wrong. He might have reasons why he picked exactly this thread (yes, it was that dumb example that is not included in my newer version of the standard).


Nick

 

[The real Leigh]

That's all I said. I did not say that the thread is impossible or wrong.

Sorry, Nick, but you said the thread did not exist.

That equals impossible.

You can only take pitches that are listed. Others do not exist. Period.

- Leigh

 

[Nick Mueller]

Sorry, Nick, but you said the thread did not exist.

Yes, it is so easy to take answers out of context.
The discussion is about a standard.
The thread does not exist in the standard.
So you can't refer to the standard. I think that is easy enough to understand.

And continuing your way of thinking ("untabulated"), take the set of formulae and insist you want to have a pitch of 1.1 mm. And then argument that the formula did allow any pitch.

Now, you might say that a pitch of 1.1 mm does not exist and is "illegal". So why can't you accept that a pitch of 1.25 does not exist for M30?
If you still insist on using a M30 x 1.25, you have to define your own set of tolerances. I offered a simple solution for that.


Nick

 

[Clive603]

If you still insist on using a M30 x 1.25, you have to define your own set of tolerances.
Nick

Nope.
1.25 mm pitch is fully defined under the metric constant pitch series so you can apply it to any diameter you like. Hardly surprising as all ISO metric pitches are now fully defined in size and tolerance from first principles so they can be called out by name and tolerance class without further information whatever the application. Well theoretically at least but it would have to be pretty wild for directly formed thread to need to be outside any specified band. Different matter where surface coating is concerned. British threads used to have (maybe still do) standard size changes linked to chrome and other plating thickness as defined by British Standard processes. Not seen such linked standards for ISO metric.

Actually 1.25 pitch is an interesting one as its the only one in the constant pitch series whose general use isn't actually approved! The only approved application is for 14 mm spark plug threads. (The 10 and 12 mm 1.25 pitch threads are specific combinations not constant pitch.) As such its pretty common for applications where the OEM wants to lock you into their fasteners. For example last week I had to make some Husqvarna wheel spindles with 14 mm x 1.25 threads. Over the years I've had a scattering of other non approved uses too. Often found on lens fittings too where the approved jump from 1 mm is seen as too large.

Clive