Metric threads

A while ago a gentleman , cannot remember who, gave details of Metric threads that where very similar to UNF etc ,and that they may be used in an emergency when you have not got the correct bolt or nut, can someone please tell me how to find the information . thanks, Ken.

There are thread sizes which are almost a good fit, but my advice is to avoid them. Any thread relies on several turns in good contact to give strength to the fastening. Anything which is 'nearly the same' will, in effect, only match the thread for perhaps a turn, if that. All the load goes on that turn, and of course, when loaded it yields. This puts all the load on the next turn, and guess what, that yields too. And sometimes it is the bolt that fails, sometimes the nut. Now try to undo it, and what happens? Both parts are jammed, you break the bolt and seize the rest of it in the hole.

Thus your 'emergency' fix turns into a disaster. You cannot complete the job, and the cost and wait for new parts is a real emergency ...

Of course, as you will realise, there are close matches in size (such as 5/16" = 8mm), but they should only be used when you are renewing both parts - a M8 nut and bolt will do instead of a 5/16 UNF nut and bolt or a 5/16 BSF nut and bolt, but only if you change both bits together.

And finally, if you choose to ignore this advice, please let me have your address details so I will know not to buy anything you may have worked on (!)

Best wishes

Tim

I can't remember which comedian it was but it's reminiscent of a joke about using "crinkly" nails (screws)!

Crinkly nais put in with a Glasgow screw driver(hammer), reminds me of a story told to me by my grandfather when I was a little chap, long ago.

ian S C

Where I live (East Midlands) they are known as Birmingham screwdrivers!

Brian

Ken,

Like Tim Stevens ... I would never condone such practice

But; this may be the list that you were think of: **LINK**

http://pages.cs.wisc.edu/~bolo/workshop/thread.html

[ ... if not, it's a very useful reference anyway ... ]

MichaelG.

In God's Country they are Sheffield screwdrivers of course!!!

My grandfather was telling me, or more likely dad about the building of the Queen Mary, the orders were that in fitting out the cabins, the woodwork was to be screwed in place(no electric screw drivers in those days), and the screws were not to be hammered into place, thus a Glasgow screw driver.

I saw the QM in 1984, and the screws seem to have survived until then.

Mum a Paisely girl saw the launching of both the Queens.

Ian S C

When Marx recorded that "In Birmingham alone, 500 varieties of hammers are produced..." did that include different "Brummagen screwdrivers" for different screw heads?

Brian

In the aircraft industry (at least in the UK) fasteners with Unified threads were specially marked to prevent safety issues with mixing different thread types such as the OP suggested. This was three touching circles . Example about half way down the page **LINK**

I have an old friend, now in his nineties, who worked all his life as a joiner, who maintains that screws should be put in with a hammer and removed with a turnscrew ( his terminology ) he also always refers to spanners as nutkeys

Roy

Well I thought that this Forum was to help and educate other people that are interested in engineering not to make crass and insulting remarks as put forward by Tim Stevens about my request for information on Metric threads. I will not follow in the same vein as his but just to thank a other members for their replies. Ken.

2BA, M5x0.8 and 10x32UNF are hard to tell apart by eye and some will fit each other in a fashion but none make a really decent exchange, in an emergency you would have to weigh the consequences of failure against the need to get going again. I like a good bodge as it exercises the mind to make the best from the materials available, I have quite a few to my name and most have worked well until a proper fix can be made.

Mike

In an emergency At one time one part or the other was threaded 3/8" UNF, while the other part was 10 mm, It was the brass bush on a windscreen wiper shaft, It was probably the bush that I renewed, and opened the die a bit more than usual and took it down until the nut fitted (Toyota van with about 150,000 k on the clock).

Ian S C

I can only apologise to Kenneth as he clearly found my last sentence crass and insulting. It was meant in humorous vein.

All I can say in my defence is that I have spent quite a lot of my life trying to overcome the effects of other people's bodges. Old machinery is hard enough to cope with when the effects of corrosion, fatigue, and so on, accumulate, and I am sure I am not alone in wishing to avoid additional complications.

Regards, Tim Stevens

Apology accepted. Ken.

This is what can happen if the wrong bolts are used:

**LINK**

More than enough to ruin ones day.

Andrew

Cannot recall which way round, but as a bodge, BSW and UNC can be interchanged, (But only one way round)

Bad practice since even when they do screw together, the thread forms are different, so that the load is not properly spread over the flanks.

Metric threads tend to be a bit larger than Imperial, so even if the pitches do approximate, an Imperial bolt in a Metric nut will be a loose fit, with the risks associated with both those conditions.

You wouldn't use a Mini for ploughing, anymore than you would make a habit of driving long distances on Fergusson TE20 tractor.

Unless an absolute emergency, DON'T do it

Howard

In the real world BSW & UNC will fit together [not 1/2" though as TPI is different], there is normally enough slop in bolts to allow this. Obviously not technically correct.

Tony

only one way round)

And not half inch because they are of different pitches! Whitworth are 55 degrees and UNC are 60 degrees, so UNC are difficult to screw into whit - but not impossible, or at least can be eased with a die.or nut runner.