Classic cars forum & vehicle restoration.

Kleftiwallah · Joined: 27 Oct 2016 Posts: 222 Location: North Wiltshire

I'm in the shed making a new brake rod from 5/16 th mild steel cutting a B.S.F. thread 5 inches long. The first two inches are spot on, but then the thread seems to form a long amplitude helix (although the threads along the whole length are acceptable).

Now, why is this happening (or has happened)? Question

Over to you knowledgeable people. Cheers, Tony.
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It may work in practice, but not necessarily in theory!

peter scott · Joined: 18 Dec 2007 Posts: 7118 Location: Edinburgh

Stretched!

Peter Question

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http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

Kleftiwallah · Joined: 27 Oct 2016 Posts: 222 Location: North Wiltshire

Nope, not stretched. Still the original length.

Cheers, Tony.
_________________
It may work in practice, but not necessarily in theory!

47Jag · Posted: Fri Nov 10, 2017 1:45 pm Post subject:

Could it be that the rod is twisting due to the resistance to the die cutting and in so doing reduces in diameter slightly Question

Art

alastairq · Joined: 14 Oct 2016 Posts: 1950 Location: East Yorkshire

Whilst I admit to inexperience when it comes to thread-cutting, a chum has suggested the thread form has 'stretched' because the thread itself was being cut, 'too quickly'...the die apparently can march down the rod improperly if turned too fast?

ukdave2002 · Joined: 23 Nov 2007 Posts: 4104 Location: South Cheshire

Uncle Alec · Joined: 14 Jan 2008 Posts: 734 Location: Manchester

Me too Dave. I convert surplus Riley RM radiator steady rods into clutch rods by doing just what you say; expand the die as far as by sending the V-pointed grub screw way in, then another run with it out and the locating grub screws tight in.

Kenham · Joined: 12 Mar 2012 Posts: 209 Location: Kent

I find it easier to turn threads with the lathe , quite a satisfying job I find. All well and good if you have a lathe of course. As to the original question yes to much too soon I suspect and turning the rod instead of cutting properly.

Kleftiwallah · Joined: 27 Oct 2016 Posts: 222 Location: North Wiltshire

Thanks to all your knowledgeable answers,

I think 47jag has hit the nail on the head. I did have the rod projecting quite a way up from the vice jaws. I cut another length with only 1 1/2 " out of the vice at any time dieing down until I barked my knuckles only then lifting the rod higher by the same amount and the thread is spot on all the way down..

Take a sweety out of the jar 47jag!

Cheers, Tony
_________________
It may work in practice, but not necessarily in theory!

Ray White · Joined: 02 Dec 2014 Posts: 6304 Location: Derby

Forgive me for casting aspersions but you say you are making up brake rods? . That sounds like you are using some pretty crummy steel to me. I would want something a bit tougher if it was my life depending on it. Shocked