Pete,
A lot has to do with the lathe manufacturers and how the gears are set up for
thread turning. This includes the screw machines too. Sometimes, there's just
not any way to put all those different ratios in a gearbox and make them work.
Then you'd end up maybe with one special machine just for 1-2 types of thread
where nobody would invest in it. The one nobody hardly ever fools with and
tries to change is the acme thread. The reason why is most inch measurement
gear teeth is formed around this for rack gears having a 14-1/2 degree pressure
angle. It is also one of the best designs for load lifting in screw jacks. Now
there are the metric gears too that use the module system, and another termed
stub tooth gearing. This goes back into lathes as the same acme screw is used
for the feed screw in the lathe. So the gear set has to match the feed screw
for the ratios. That's my guess on why some have dropped out of being.
Best,
Will
*********** REPLY SEPARATOR ***********
On 8/5/05 at 10:07 AM Peter Chadwick wrote:
>Getting back to something to do with amplifier building, to my mind, one
>of the abominations of the metric system is the ISO metric thread. Even
>the ISO metric coarse is far too fine a thread for many applications,
>especially tapping into aluminium, or heavens forbid, that horror to work,
>copper. UNF and UNC or BSW and BSF are far better in this respect, while
>for small screws, the US threads are good. Incidentally, does anyone know
>why the standards apparently changed from something like 4-36 to 4-40 in
>about 1950 or so? I also do not understand why we had to stop using the BA
>thread, which is actually a metric thread anyway, developed originally by
>the Swiss! And because of the 47-1/2 degree, somewhat stronger than 60
>degree threads.
>I know you can use coil inserts, but that seems to me to be adding
>unnecessary complication for the sake of using something that is not
>right for the job. Interestingly, the model engineering people are firmly
>wedded to BA: one advantage is apparently that the odd numbers tend to
>have heads that are relatively close to scale.
>One day, I must get around to getting a die to match my 5/16 left hand
>Whitworth tap......
>
>
>73
>
>Peter GW3RZP/P
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