Rolled vs. cut threads (was about brake bolt modification...)

[jtaylor]

Just remembered I had another set of brakes - and other parts - in a
different box in the dark places of the basement...

Zeus 2001 - look very much like cut threads, even though there is a
reduced-shank at the end. That section also looks turned - under a loupe
you can see the toolbit marks, same as those on the full-dimension section.
I suspect that they were just making sure that the thread was less than 100%
diameter, which is common engineering practice.

Now, what is perhaps more interesting is that the Zeus QR skewers ALSO have
cut threads, contrary to the following assertion:

"...qr skewers, like brake bolts are rolled for fatigue resistance, NOT
cut."

The cut nature of the threads is unmistakable - you can see the steps at the
end where the toolbit was withdrawn for each threading pass.

Beside it was another skewer (M.M. Atom). This one also looked cut - though
there was a reduced diameter shank at the end of the threaded portion.
There were no visible "steps" in the end of the thread (this will happen if
the toolbit is withdrawn just a little later on the last pass), but there
was a shallow, long, very steep-angled helical cut which extended from the
last part of the cut for the last thread along the main shank. I doubt that
these were turned by hand, more likely an semi-automatic machine did the
work, and the part was being ejected as the lathe was nearly stopped.

It may be that short-sighted readers will think "well, these are old parts,
they don't make them like that anymore, the science of metallurgy has
advanced so much further than what the old bearded ones knew, and all
threads are rolled nowadays". For all I know that may be true - but
consider

a) if the metallurgy is better, why would not older parts have used a
stronger method of making threads to compensate?
b) if there really was an issue with fatigue, would not old parts show this
before new ones?

The answer is simple. Yes rolled threads are stronger, all else being
equal. For brake bolts, it does _not_ matter - and it may be that neither
does it matter for skewers.


Further, there is a confused poster who said:

"...but this UTS-don't buy it. with 6mm grade "8" from the usual hardware
store supplier as rack support bolts shimmed with nuts/washers thru nthe
rear dropouts i get a failure rate of 2-3 a year with 40 pound loads. "

This is a different situation altogether, just like the broken bolt example
at which jim beam wanted us to look (but perhaps not read). Using a bolt to
"support" a rack as described loads it in shear, not tension.

It is sometimes said that bolts are not designed for shear stress (or, for
that matter, bending stress), but the truth is, they are just designed.
They will be USED for whatever we choose. Loaded in tension, they will have
a certain load which can be carried. Loaded in shear, or under some other
stress, they will have (usually) less. As rack support bolts, considerably
less, especially if the shear plane involves the threaded section.

In all cases, whatever the load, there will be a question of failure. If
the bolt as chosen does not fail, then there is not a problem. If it (or a
certain proportion of bolts so designed, manufactured, and used) does fail,
then there _is_ a problem. The severity of the problem depends on the
probability and the consequence of failure.

For the rack bolt situation, it appears that a different method of fixing
the rack would be worth investigation. For brake-bolts, all that is
required is a refusal to join the Idiots' Club - or the Alarmists'; there
may well be some overlap.

[jim beam]

jtaylor wrote:
<snip stuff>

the most authoritave way of differentiating between the two is
metallography. if rolled, the work history makes permanent changes to
the grain structure of the material. if cut, those are not apparent -
grain structure is almost entirely uninterrupted. but assuming that
metallography's not an option for the average home mechanic, there are
several other easily identified tell-tales when differentiating between
rolled & cut threads.

*rolled threads*:
1. the threads "rise" above the shank of the unrolled section. if it's
a shouldered shank, that may be less apparent as the threads are up
against the shoulder, but that's a matter of experience and knowing what
you're looking at.

2. the threaded end of the rod usually has a concave shape. the threads
cause quite severe deformation of the shank's skin, and that deformation
"overflows" beyond the original end like this:

http://www.flickr.com/photos/38636024@N00/66536041/

sometimes that "overflow" is ground off if it's a fastener that is going
to be frequently removed and re-engagement without cross-threading is
considered to be an issue.

3. there is almost always a circumferential ring where the threading
stops on the shank. the threads are [obviously] pitched, but where they
end, the /unpitched/ ring is formed where the die stops.

http://www.flickr.com/photos/38636024@N00/66536039/

dead center in this pic is the ring which is clearly not pitched like
the thread. this is impossible to create with a cutting die, only a roller.

*cut threads*:
1. threads wholly intrude on the diameter of the shank.

2. end of the rod is square, not concave. some are rounded out to allow
easier start of the die.

3. there is no circumferential ring.

4. there are radial cut marks at the end of the thread where the die
teeth end. the number of marks corresponds with the number of tooth
sets on the die - tyically 3 or 4 in a piece this size.

[jtaylor]

"jim beam" > wrote in message
...
> jtaylor wrote:
> <snip stuff>

You snipped ALL of the stuff that contradicted your assertions - funny,
that...

Then posted a bunch of stuff about the visual differences between rolled and
die-cut threads, which sounds good, until one realises it's a smoke-screen.

p.s. the Zeus skewer threads were clearly cut on a lathe.

[Joe Riel]

jim beam > writes:

> 3. there is almost always a circumferential ring where the threading
> stops on the shank. the threads are [obviously] pitched, but where they
> end, the /unpitched/ ring is formed where the die stops.
>
> http://www.flickr.com/photos/38636024@N00/66536039/
>
> dead center in this pic is the ring which is clearly not pitched like
> the thread. this is impossible to create with a cutting die, only a roller.

Nice pictures and descriptions, thanks for posting. In the one above,
what would you say about the threads on the other (right) end? I
don't see a circumferential ring.

Joe

[jim beam]

jtaylor wrote:
> "jim beam" > wrote in message
> ...
>
>>jtaylor wrote:
>><snip stuff>
>
>
> You snipped ALL of the stuff that contradicted your assertions - funny,
> that...
>
> Then posted a bunch of stuff about the visual differences between rolled and
> die-cut threads, which sounds good, until one realises it's a smoke-screen.

??? wow! that's the most exacting technical rebuttal i've ever seen!

>
> p.s. the Zeus skewer threads were clearly cut on a lathe.

if you say so...

[jim beam]

Joe Riel wrote:
> jim beam > writes:
>
>
>>3. there is almost always a circumferential ring where the threading
>>stops on the shank. the threads are [obviously] pitched, but where they
>>end, the /unpitched/ ring is formed where the die stops.
>>
>>http://www.flickr.com/photos/38636024@N00/66536039/
>>
>>dead center in this pic is the ring which is clearly not pitched like
>>the thread. this is impossible to create with a cutting die, only a roller.
>
>
> Nice pictures and descriptions, thanks for posting. In the one above,
> what would you say about the threads on the other (right) end? I
> don't see a circumferential ring.
>
> Joe

that's because they're not up against a shoulder - that's typically when
you see it. where that second set of threads end, there is another
[smaller] ring as they are up against a second shoulder.

[Andy B.]

"jtaylor" > wrote in message
t.ca...
> a) if the metallurgy is better, why would not older parts have used a
> stronger method of making threads to compensate?
> b) if there really was an issue with fatigue, would not old parts show
> this
> before new ones?
>
> The answer is simple. Yes rolled threads are stronger, all else being
> equal. For brake bolts, it does _not_ matter - and it may be that neither
> does it matter for skewers.

It's simpler than that. Investigate the tooling investment for rolled
threads.

-Andy B.