Merits of various screw threads - L'escargot
I understand that Nitroburner wants a discussion on screw threads, so here goes.....

As part of the metrification policy, UNF threads (and in fact most threads other than metric) have been superseded by metric threads in the UK. I believe that the US of A still uses UNF (amongst other things), but most of the civilised world has gone over to metric in the interests of global standardisation. And a good thing, too. When I first started my career in engineering, there were numerous threads ~ BA, BSF, BSP, BSW, UNF, UNC, NPTF, etc etc etc. If you had a bolt without a nut, finding the right nut could be a nightmare. With metric threads you just measure the thread o.d. and the thread pitch and you then know exactly what the thread is ~ perhaps a slight exaggeration, but only just.
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L'escargot by name, but not by nature.
Merits of various screw threads - Ivor E Tower
Yes but even in Europe, ISTR that seatbelt fixings are an "odd" size, something like 5/16 UNC, to stop people from using any old bolts that they have lying around fromn being used to hold items that have to take several tons of load in the event of an accident.
Merits of various screw threads - THe Growler
This is where you can take the proverbial out of the auto-censor by, quite legimately, referring to a b****** thread.
Merits of various screw threads - Clanger
"b****** thread"

Is that what the moderators call your prose, G?


Hawkeye
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Stranger in a strange land
Merits of various screw threads - Ivor E Tower
ROFL!!!
Merits of various screw threads - L'escargot
This is where you can take the proverbial out of the
auto-censor by, quite legimately, referring to a b****** thread.


You can also legitimately refer to b****** files!
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L'escargot by name, but not by nature.
Merits of various screw threads - NitroBurner
L'escargot;

Well, thanx 4 starting this off old chap.

Totally agree with you regarding the damned metric THREADS.

I have worked in the grubby world of engineering for many a year & think that British Standard Whitworth is the best!

A proper man's (or woman's) thread...
Merits of various screw threads - Ian (Cape Town)
What I hate is those really crazy 'heads' you get on various domestic appliances and automotive bits and bobs, where ytou need a 'key' (for want of a better word) shaped like a witches hat, or a pyramid, or a tricorn hat etc. Apparently this is to stop people opening said equipment up, and electrocuting themselves.
OR in my cynical view, to stop people opening it up, and realising the equipment can be fixed with a modic of technical knowledge, and a blob of solder...
On the motoring side, i remember a story (probably apocryphal) that certain British cars built in the 70s had many bolts, nuts etc which were metric, and many which were imperial, neccesitating TWO sets of spanners!
Merits of various screw threads - nick
It's true, Ian. I've come across mixtures of imperial and metric on the same 70's car. Can't remember what it was though.

I still think whitworth is the best, if only for the name. I remember fondly a 1962 Rover 100 P4 I owned a few years ago. The fasteners on that were superb quality, they never seemed to rust seroisly and always came apart easily even after 30 years. They were all 'parkerised', presumably a coating process. Can anyone enlighten me as to what 'parkerising' actually is?
Merits of various screw threads - Leon on Derv
IIRC Parkerising is a chemical treatment process where steel or ferrous metals are coated with manganese phosphate. There is a further process which can be carried out referred to as prkolubing where an oil coating is applied to the parkerised finish.

Commonly used on long and short barrelled weapons to reduce glare and prevent corossion.

Open to correction on this one.....

Leon
Merits of various screw threads - Phil I
On the ball exactly Leon. Process was invented and developed by Birmingham gunsmiths Parker-Hale Ltd. Is a chemical bath. I have no idea what the constituent chemicals are - was considered super trade secret by people who ran PH at the time (early 50's) PH were still in business on the site of the old BSA factory in Small Heath a few years ago. They were taken over by some firm of wholesalers and are now at Petersfield just as a marketing firm since 2000.
Merits of various screw threads - Dizzy {P}
The car having a mixture of threads was the later version of the Ford Cortina. This had imperial, metric and imperial-with-metric-heads. Removing and replacing the gearbox brought you into contact with all of these.

There is a major difference between UNF and Metric threads that hasn't been mentioned. The ISO (International Standards Organisation) specifies that the Metric standard thread is Coarse, with fine threads being approved only for special purposes, whereas the standard Unified thread was UNF (fine-thread). The drawback with Metric Coarse bolts is that they vibrate loose more easily than UNF. On the other hand, Metric Coarse doesn't cross-thread as readily as UNF and it needs less turns with the spanner.

Whitworth was fine (no it wasn't - it was coarse!) and was reckoned to be the first attempt at standardisation. However the heads are quite large so they take up more space. They also tend to be less strong than modern bolts which are usually made from a higher grade steel. Modern bolts also usually have cold-rolled threads which are a bit stronger than machined threads.

I reckon to handle Whitworth bolts at least once a week (on railway locomotives). I worked for many years in an engine specifications position with Metric threads, and one of my cars has UNF threads. There isn't much in it but I think Metric is the overall best.
Merits of various screw threads - Stargazer {P}
Funny you should mention the Ford Cortina Dizzy, the Mk1 Escort of 1968-1974 vintage also had a mixture of imperial and metric threads. It was Imperial (AF) everywhere with a smattering of Whitworth EXCEPT for the gearbox and bell housing which were metric. (Large warnings in the \ford workshop manuals).

Having started car maintenance late in life (first car at 25 in 1990) I acquired all my tools to work on the Escort....all Imperial then at a critical point one weekend I had to splash out on a set of Metric spanners for that one job.

regards

Ian L.
Merits of various screw threads - L'escargot
Metric standard thread is Coarse, with fine threads being approved only
for special purposes, whereas the standard Unified thread was UNF (fine-thread).
The drawback with Metric Coarse bolts is that they vibrate loose
more easily than UNF.


The factors taken into consideration by the designer when he is choosing between UNC and UNF are just the same as when he is choosing between "standard" (coarse) metric and metric fine, namely the required bolt load, the tensile strength of the material of the bolt and that of the corresponding internal thread, and any special factors. If vibration is a factor, then this is taken into account accordingly.

The torque specified also takes into account the surface finish of the mating items, i.e. plain, zinc plated, Parkerised, Sheradised, etc, etc, and whether lubrication is present. (It is not universally known by the hoi polloi that the surface finish affects how much of the applied torque is expended in overcoming friction and that the use of items that have a different surface finish from the original should be avoided.)

The use of "shakeproof" and serrated washers (intended to prevent loosening) has largely been abandonded because at best they are ineffective, and at worst they are detrimental. Modern practice is to have (at most) a plain (flat) washer and to specify a torque that is just sufficient to produce "yield" or permanent stretch of the bolt. Incidentally (although I personally would never condone the practice) it is said that an experienced fitter can tell when yield has just started to occur without using a torque wrench.
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L'escargot by name, but not by nature.
Merits of various screw threads - SjB {P}
I quite agree, Ian.

By chance one day whilst out shopping, I was however able, for the princely sum of a fiver to purchase a set of tool heads designed to undo all commonly used 'tamperproof' screws and bolts, so prompted by your post, have just added up the value of replacing devices repaired through use of the tool since purchased last year. I'm sure there are more, but these are the ones I recall:

Household iron
Replacement cost saved: £40
Cost to fix: Forty five minutes plus a piece of emery paper to clean the bi-metallic switch.

Food mixer
Replacement cost saved: £60
Cost to fix: Thirty minutes plus some hot water to wash encrusted flour from the plastic gear train.

Motorcycle intercom
Replacement cost saved: £75
Cost to fix: Thirty minutes with a multimeter, plus a blob of solder on a dry joint.

Plastic laminator
Replacement cost saved: £80
Cost to fix: Twenty minutes of my time, and hot water to clean the rollers.

VCR
Replacement cost saved: £149
Cost to fix: Ninety minutes and a blob of graphite grease.

So, spending that fiver saved five otherwise sound items from the scrap head, and my wallet £404. I don't count my time as cost, because I repaired all the items whilst enjoying my passion for music. The best fiver I ever spent, I think!
Merits of various screw threads - Clanger
I've got a set of bits to unlock these "puzzle screws" and the ability to undo them and use the absolute minimum of repair skills and a blob of solder is now contributing to my new career as Home Facilities Management - General Repairer.


Hawkeye
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Stranger in a strange land
Merits of various screw threads - L'escargot
What I hate is those really crazy 'heads' you get on
various domestic appliances and automotive bits and bobs, where ytou need
a 'key' (for want of a better word) shaped like a
witches hat, or a pyramid, or a tricorn hat etc.


Then of course you get "Torx" socket heads which require a special "Torx" key.
Anyone know the advantage of these over the ubiquitous hexagon socket heads which require an equally ubiquitous Allen key?
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L'escargot by name, but not by nature.
Merits of various screw threads - Dizzy {P}
Then of course you get "Torx" socket heads which require a special "Torx" key. Anyone know the advantage of these over the ubiquitous hexagon socket heads which require an equally ubiquitous Allen key? >>


Torx has huge advantages over Allan socket heads. The following is a repeat of what I said on the subject in an earlier thread:

Allen screws are becoming obsolescent now, mainly because of their high cost. This is due to the fact that they need to be made of a high grade steel (at least 10,9) because of the tendency for the heads to crack across the corners. A high grade steel is more expensive to buy and forming the head is also costlier.

Torx screws are now far more popular because the heads are inherently stronger, so the screws can be made from a cheaper steel (typically 8,8) and the heads are easier to form. However, because Torx has a stronger head, the socket depth has been reduced in comparison with Allen screws (to ease head forming) so it is equally important to make sure that the socket is clean and the tool fully seated.

I'll just add that a respected maker of Torx screws claims that the production tooling used for tightening Torx screws can last 1200 times longer than for the equivalent Allan screws.

Merits of various screw threads - L'escargot
L'escargot;
Well, thanx 4 starting this off old chap.


No probs, NitroBurner.
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L'escargot by name, but not by nature.
Merits of various screw threads - andy n
im pleased to be able to report however that the good old unified thread form is alive and well in the aircraft industry even the european airbus is held together 100 per cent with unf fastners!
Merits of various screw threads - Health N Safety
Going off at a slight tangent, why are combination spanners the same size at both ends so that you have to buy two sets to undo a nut and bolt?

Oh, I've just answered my own question!
Merits of various screw threads - helicopter
Talking of coatings , does anyone remember a coating called sherardising , I think it was a bit like galvanising. It was certainly around in the seventies but not sure if it was ever used in the motor industry.
Merits of various screw threads - Dizzy {P}
Wasn't sheradising a method of zinc plating steel components by tumbling them in a mixture of zinc powder and ball bearings or the like, so that the zinc was hammered into the surface of the component? Or perhaps I've got the name wrong.

Mechanical zinc plating is in fairly wide use, especially for high tensile threaded fasteners. The electro-chemical zinc plating process is not really suitable for anything above 'S' range (imperial) or 8.8 (metric) as it can cause something called 'hydrogen embrittlement'. This can't be seen but can cause bolt heads to fly off when under load, either as they are tightened or some time later.
Merits of various screw threads - Leon on Derv
Yip - A mixture of Zinc, Flux and Sand are used the parts are 'tumbled' in the powder. Heat is applied followed by a quench. Part Sheridised - Job done.....

There is a newer process similar to sheradising. It is called ZN Three Quarters. Only ZN powder is used (no sand). Process times are reduced and the results are better.

Leon


Merits of various screw threads - Garethj
The reason that Torx head screws are used is not really to do with stopping people from taking things apart - more to do with assembly on the production line. Because there is more surface contact area the automatic screwdrivers are less likely to jump out of the screws and damage the cosmetic parts. The operators also don't have to be so careful with orientatio of the screewdriver into the screw, if it was a flat head screw you could only fit the driver in 2 positions, a crosshead screw can be in 4 positions etc.

The other useful thing is that you *have* to use the correct size screwdriver bit with a torx head, you can sometimes get away with using the wrong size bit with a flat head or crosshead but this can damage the head and leave bits of swarf around the delicate electronic equipment you've just sealed. This can happen on production lines!

All useful stuff when you're building hundreds (or thousands) of units per day with several screws in each with semi-skilled labour.

Gareth
Merits of various screw threads - joe
Can't you undo those Trox heads with an Allen key anyway?

I would be really interested in getting one of those kits with the keys in. What are they called and where can you get them?
Merits of various screw threads - CQ
Security bits,9.99 @ Screwfix. Tel.0500 414141
Merits of various screw threads - L'escargot
Also see www.screwfix.com for full internet catalogue, or to order a hard copy. You'll get regular e-mails from them, though, if you do this.
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L'escargot by name, but not by nature.
 

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