I've read in the list a rather interesting discussion (though
short in the number of messages) about
the merits of one thread type over another on bolts and studs,
i.e. rolled, cut, ground.
One of the messages stated: "The process of making rolled threads
often creates some anomalies in the root and tip of the thread.
These can seriously compromise the strength of the fastener."
After some research here is a summary and general consensus of
folks at NASA, standards testing labs, manufactures, and
university engineering professors.
Advantages of rolled threads:
No scrap
Faster on a production basis
The thread itself is stronger in fatigue
The theoretical stronger threads are rolled thread
Dimensional tolerance is consistent
Dies may be chosen as expanded or contracted lead for
special performance results
Heat may be applied during the process so very hard
materials can be thread rolled
Fatigue rated
Consistent thread form and finish
Suitable for aerospace fasteners
Easily produced in large quantities
Disadvantages of rolled threads:
Need specialized equipment to manufacture
Dies are expensive
Need specially trained operators
Most of the time it is a separate operation (production)
May create laps on the crest of the thread (acceptable to a
certain depth)
Advantages of cut threads:
Inexpensive to produce in small quantities if the
requirements are not stringent
Any good machinist can cut threads on a lathe or similar
equipment
No special or expensive tools needed
Can be cut as part of another process such as screw
machining, usually on soft materials
No thread length restrictions
The steel diameter is the same as major diameter of the
thread
Used in instrumentation components where a very high number
of tpi are used
Used for very large threads
Used for 'specials'
Disadvantages of cut threads:
Not suited for aerospace fasteners
Not fatigue rated
May have hidden flaws such as burrs, tears, or bad finish
Dimensional variability from part to part
Need expensive CNC equipment if threads are to be consistent
in finish, dimensions and if
variability is to be controlled
Accumulated lead deviation can vary depending on the type of
equipment used
Threads must be cut before heat treat on hard materials
During manufacture the grain fibers/flow lines of the steel
are cut
Long thread lengths are very expensive to make on a
production basis
Advantages of ground threads:
Used on instrumentation, positioners, or high-precision
components
Used on larger parts
Very fine finish on the thread flanks, root radius
Good thread form control
Can be done after heat treat for better finish
Absence of burrs
Disadvantages of ground threads:
Not fatigue rated
Not suited for aerospace fasteners
Need specialized equipment to manufacture
Need specially dressed grinding wheels
Wheels do not last very long
Wheels need to be dressed often
Need specialized operators
Expensive to produce
Here is the bottom line dudes and dudetes:
Rolled threads are not necessarily stronger in tension! But
they are much stronger in fatigue strength. And this is what you
want in your expensive British iron engine. Cyclic stress loads
that are imposed on studs/bolts cause general weakening and
failure of most threaded fasteners so you want a fastener that
will take that kind of stress, i.e. fatigue rated rolled thread
fasteners.
Let's look at this another way. Would you prefer a cut thread
fastener with low fatigue strength to hold that engine on the 747
you just got on to fly to London or one with high fatigue
strength?
As you can see each and every type of thread has it's place but
the top performer is the rolled threaded fastener.
For those of you who want to dispute what I have to say then I
would suggest you read the following material first.
FATIGUE STRENGTH OF STEEL SCREW THREADS WITH LARGE ROOT RADI
UNDER AXIAL LOADING ESDU No. 67020 ESDU, March 1967 (5 p, 3 figs,
2 refs)
NEW DATA ON FASTENER FATIGUE
C. Crispell
Machine Design, April 1982, pp71-74 (3 p, 4 figs)
EFFECT OF SCREW THREADS ON FATIGUE
S.M. Arnold
Mechanical Engineering, July 1943, p 497 (9 p, 6 figs, 167 refs)
NASA Tech Brief
MFS-28518 REPAIRING A SHAFT PRONE TO FATIGUE
If you want some more interesting reading on fasteners try these
NASA articles:
NPO-18318 PROOF TESTING OF STAINLESS-STEEL BOLTS
MSC-16172 PROCESSING HIGH-STRENGTH STEEL ALLOYS
MFS-29878 REPEATED TORQUE CYCLES REDUCE BOLT PRELOAD
MFS-28975 TURNING THREADED FASTENERS WITHOUT INDUCING SIDE LOADS
MFS-27325 EXPERIMENTS ON VIBRATION-INDUCED LOOSENING OF BOLTS
LAR-13009 FASTENING PARTS HAVING MISMATCHED THERMAL COEFFICIENTS
LAR-12015 ROUS SYSTEM
Other items of note:
1)Bullwinkle and Zandor are not the same person. We get into
arguments on a regular basis and sometimes Bullwinkle wins.
2)When talking about technical items be prepared to back up what
you say with some research as you are likely to be called up on
the carpet. The information should be from books, texts,
publications, or noted authors of the subject, be it on fasteners
or what colour pattern MG's were painted. So if you are asked
what you base your statements on you can answer with something
other than "I heard it from a friend of a friend of a friend".
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