Problem with half nuts on circa 1940's S B 9A

joebiplane

joebiplane

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my friend just bought a SB 9A lathe and all was well for a few days. Then he wanted to try threading .
The half nuts seemed fine when inspecting the machine at purchase. But when he flipped the half nut engaging lever the unit " jumped and banged for anfew seconds then quieted down but couldn't engage
We removed the apron and the half nuts were stripped out ! I asked if he had the carriage against the head or has tightened the carriage lock and he said NO.... the lead screw is like New ...no dirt or grease just light oile ( the machine was refurbished perform purchase was made)
The lock-out for the half nut/power feed is working properly ... and I am at a loss to explain it !
We have ordered a new set of brass or bronze rebuilt half nuts but We sure don't want to lunch them at $ 150 a pair. Can anyone steer us in the right direction to find out what might have happened
Thanks, as always for any ideas
Joebiplane
 
The half nuts are likely just wore out from use. If you have access to another lathe with a working threading mechanism, you could possibly make a replacement yourself...
 
Joe,
Just engage the half nuts with the lathe not running, and pull it over by hand. If everything is right it will turn easily and the carriage will traverse. No chance of stripping the new nuts. But it sure sounds like the power feed and screw threading engaged at the same time.

Cheers Phil
 
joebiplane said:
my friend just bought a SB 9A lathe and all was well for a few days. Then he wanted to try threading .
The half nuts seemed fine when inspecting the machine at purchase. But when he flipped the half nut engaging lever the unit " jumped and banged for anfew seconds then quieted down but couldn't engage
We removed the apron and the half nuts were stripped out ! I asked if he had the carriage against the head or has tightened the carriage lock and he said NO.... the lead screw is like New ...no dirt or grease just light oile ( the machine was refurbished perform purchase was made)
The lock-out for the half nut/power feed is working properly ... and I am at a loss to explain it !
We have ordered a new set of brass or bronze rebuilt half nuts but We sure don't want to lunch them at $ 150 a pair. Can anyone steer us in the right direction to find out what might have happened
Thanks, as always for any ideas
Joebiplane
Click to expand...
I wonder if he had the carriage feed engaged. There is a rod that locks out both from happening, but the rod maybe worn and not fully engaging. It's in the apron. If the carriage feed was engaged at the same time as the threading half nuts, the half nuts will strip.As they are not traveling at the same rate as the feed. Save your old half nuts, you can braze a new sleeve on, after cutting threads, and slitting it. Since he is new, it might be a good idea to make sure the rod locks out the half nuts if the feed is engaged.
 
Sounds to me like some one swapped out the half nuts with a worn out set to get a good unworn out set for use on another lathe before the lathe was sold. I hate admit to it, I'm guilty of doing this on a 9" SBL I sold may years ago. Hope that's not the lathe.:( Is it a 4 foot bed lathe?
 
You might also try making the half-nut yourself out of Delrin. Google "making half nut from delrin"
 
Good thoughts guys. He ordered a set of "re manufactured" half nuts from an add on Flea bay. I suspected the lock out, myself and will check it carefully after New nuts are installed


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