Double Lead 12-28 Thread Question

That said, if the need is great enough to overcome the challenges, a shop made tap and die are possible. It would involr machining and hardening and tempering to produce a cutting tool. It the material to be threaded is softer as in brass or aluminum, you could get by without the hardening and tempering for limited use.
 
Janderso said:
That’s a very fine thread for a double start/lead imho
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That may be, but you see these or similar sets of threads on depth adjustment posts used in several different block planes. The one I am working on is from an Ohio O220 block plane made around 1910. Best I can tell, using a pitch gauge, the larger threads are 1/4-28 the smaller are 3/16-28. The 3/16 threads appear to be double start. I have already come up with a fix for the plane I am working on, using a 12-20 as the smaller and 1/4-28 at the top. I get just enough movement for the mechanism to work but was looking for a faster thread at the bottom for future fixes. (BTW I know 12-20 is unusual but I have the taps and dies for those. Attached is the component. The smaller threads are too worn to hold
 

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RJSakowski said:
That said, if the need is great enough to overcome the challenges, a shop made tap and die are possible. It would involr machining and hardening and tempering to produce a cutting tool. It the material to be threaded is softer as in brass or aluminum, you could get by without the hardening and tempering for limited use.
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See my reply to Janderso. I show the part that has the original threads
 
RJSakowski said:
That said, if the need is great enough to overcome the challenges, a shop made tap and die are possible. It would involr machining and hardening and tempering to produce a cutting tool. It the material to be threaded is softer as in brass or aluminum, you could get by without the hardening and tempering for limited use.
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I don't think the need is great enough. These block planes are not worth all that much. I also don't normally work on planes that size. I was just hoping for a tap and die solution for the rare need.
 
RJSakowski said:
A double start thread requires a double start tap and die. The thread depth of the double start differs from that of a single start so even though you could in theory cut a double start thread by clocking the tap or die 180º out of phase witha second thread, the thread geometry would be wrong and unusable.
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The thread depth stays the same whatever the number of starts, just the lead angle differs.
Starting a standard tap or die 180 degrees out of phase will not produce a double start thread as the lead is wrong.
 
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