On Thursday, April 20, 2017 at 11:30:03 PM UTC-7, James wrote:
On 21/04/17 13:35, wrote:
On Fri, 21 Apr 2017 10:21:56 +0700, John B Slocomb
wrote:


And it depends what you are threading the bolt into. Using fine
threads in coarse grained cast iron is generally NOT a good idea.

As far as the actual "thread area" there is very little difference. If
you double the TPI the threads are only half as deep, but there are
twice as many threads so the total load bearing area is not much
different.


Thread area isn't the issue that John brought up.

That's what I was getting at with the discussion on course and fine threads.. If you use course threads the bolt is not as strong as a fine thread because the area of the metal inside of the threads is smaller.

It used to be said that you could tighten fine threads to a higher torque because there was more "leverage" from the fine thread but that isn't the case at all.

So if you are designing a bolt into an iron casting you use course threads but have to use one size larger than you would if it were a finer grain metal and a fine threaded bolt.

Engineering isn't a case of choices as the Germans insist. Designing something to the lightest by making an design to the irreducible minimum while expecting the highest performance is simply asking for troubles. The English are another example. When I had a local mechanic troubleshoot my electrical system to find out why my battery was dying it turned out to be the battery had bitten the dust. It would only hold a charge for a couple of hours. While we were talking he pointed to a Jaguar coupe. That car had a four speed manual transmission that had failed. Anyone knows that manual transmissions do not fail. The replacement cost? $12,000 on a ten year old car. Jaguar would not stand behind their car. If some cheesy plastic part had failed that would be one thing - but manual transmissions DO NOT FAIL. Particularly when it's a little old lady that drives it like she's at Le Mans. She double shifts down better than I can.