Threads in Cast Iron

[Speedy Builder5Participant @speedybuilder5]

Whilst waiting for information on painting my SPEEDY loco painting locos I am machining up the buffers from cast iron. LBSC specifies the mounting boss to be threaded 1/2" x 26tpi. I was taught that threads in Cast Iron should be coarse, so why didn't he specify 1/2" Whitworth (12 tpi) or BSF (16 tpi) ?

I see that Juliet has a similar if smaller thread of 3/8" 32tpi Juliet

[Speedy Builder5Participant @speedybuilder5]

Selection of strongest thread

[Dave HalfordParticipant @davehalford22513]

I would use what ever I had it's not critical

[old martParticipant @oldmart]

I was surprised to find that the threads in the cast iron parts of the Smart & Brown model A were BSF rather than BSW. Tom Senior use BSW, except into the base which is a steel weldment. If there is room in the design and strength is important, then thread inserts like Helicoils are recommended.

[JasonBModerator @jasonb]

Would it have anything to do with the amount of thread sticking out the back of the buffer beam? Assuming a beam of 1/4" thickness then you are only going to have 1/4" length of the half inch thread for the nut to go onto which would not give many turns of thread if 12tpi were used.

[Speedy Builder5Participant @speedybuilder5]

Good point Jason, but I would have thought a thin BSF nut (buffer beam is 3/16" thick), and threaded boss is 1/2' long would have been stronger.

[Martin KyteParticipant @martinkyte99762]

How strong does it need to be?

All the force on a buffer pushes it against the shoulder on the boss. The thread really only has to stop the thing from falling off onto the track.

regards Martin

[ZanParticipant @zan]

Not so Martin. Most helpers when moving a loco use the buffers as handles. They need to be a solid fixing

[Clive Brown 1Participant @clivebrown1]

A 1/2" BSW thread has only about 70% of the core cross-sectional area of 1/2" brass thread. That's assuming a 3/16" hole through the centre. That means quite loss of strength to take sideways and bending loads.

[Bo’sunParticipant @bosun58570]

Fiatagri used a lot of fine threads in tractor castings. I just put it down to crap Italian engineering.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, I think it's more to do with application as to whether you use a course or fine thread, e.g. the studs that hold a cylinder head to the engine block would more likely be course rather than fine relatively, in most cases.

Regards Nick.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Bo'sun on 17/08/2020 10:27:22:

Fiatagri used a lot of fine threads in tractor castings. I just put it down to crap Italian engineering.

In which case you were wrong!

Coarse threads are recommended for soft materials were looseness and a bit of play don't matter and vibration is low. In wider use, they're good for quick assembly and disassembly provided strength and vibration proofing don't matter. Otherwise fine threads are stronger, much less likely to vibrate apart, and impact resistant. The main disadvantage of fine threads is slow assembly speed because they have to be accurately aligned.

Bad mistake to disparage foreign engineering. Failure to grip the reality that British engineering wasn't automatically best by virtue of being British led to massive resistance to change across manufacturing as the rest of the world caught up and then overtook. Many a British company went down the toilet due to persistently underestimating the competition. Not enough to believe Brits outperform foreigners, it has to be true in the real world. Patriotism is never an excuse for poor performance. To be the best it's necessary to deliver, not just talk a good show. Never believe your own propaganda, and above all tackle problems head-on. Very dangerous to justify making expensive products with time-honoured inefficiency in the belief that customers want 'quality', or that cheaper alternatives are somehow cheating. The customer is always right…

Dave

[duncan webster 1Participant @duncanwebster1]

Posted by SillyOldDuffer on 17/08/2020 12:02:17:

Posted by Bo'sun on 17/08/2020 10:27:22:

Fiatagri used a lot of fine threads in tractor castings. I just put it down to crap Italian engineering.

In which case you were wrong!

………

Oh no he wasn't. It all gets a bit complicated, but in brittle materials, most of the load is taken on the first couple of threads, whatever the pitch, and so you want big beefy threads. There was an article in ME which explained it very well

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