Threading issue

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Hukshawn

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I need to cut a thread bigger than my lathe has an option for...
I need to cut a 6 tpi acme thread but my lathe only goes to 8.

Am I up the creek?
 
It is all a matter of gear train ratios. See if you can cobble one together that advances the lead screw twice as much as for a 12 pitch thread... It might well be possible with what you have.
 
If you're cutting coarser than the lathe's designed to do, take *light cuts* to avoid extra strain on the leadscrew, half-nuts, gears etc. - try to take off as little as you would for each pass of 8 tpi. I have some 3 ipt (yep, inches per turn) to cut at some point, I may have to threadmill on the lathe at the 45* helix angle...

Dave H. (the other one)
 
If you have external change gears, you may be able to come up with a gear combination that will give you 6 tpi.
 
It is all a matter of gear train ratios. See if you can cobble one together that advances the lead screw twice as much as for a 12 pitch thread... It might well be possible with what you have.

It's a gear box with a small banjo to switch between imperial or metric. How do you calculate all the gear ratios?
All my lathe gearing has been posted here before when I was sorting out my first threading issue. But I'll post the pictures again in a minute.
 
divide the drive by the driven to find the ratio
you may need to change to a larger headset gear and your threading chart will not be the same.
 
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So, the 34 is the input from the fwd/rev selectror
the 74/51 gear train is the imperial output to the gear box.
The 34/62 is metric.

IMG_20170909_233214.jpg
IMG_20170909_233153.jpg

Does this help at all?
 
The 74 is meaningless for the imperial setup, it is only acting as an idler and does not change the ratio at all, only the direction. The gear ratio is from 34 to 51. You would need to use the AZ4 gear box setting (12 TPI), and then double the change gearing ratio to double the gearbox input speed, for instance, change the first drive gear from 34 to 68 teeth, if that is indeed possible on that lathe. You could also use the AZ2 gear setting and increase the existing change gearing by a ratio of 3:2, a 150% increase. That could be done by replacing the 34 tooth drive gear with a 51 tooth gear and leaving the existing 51 tooth gear for a 1:1 ratio. You could also use any two gears with the same tooth count to achieve that 1:1 ratio. Again, the center idler does not change anything unless set up for compound gearing. I assume you only have the four change gears shown in the two charts, and they alone will not do the job for a 6 pitch thread.
 
Correct. I have no other gears. I would have to make them, which I have no method of doing.

So, I'm up the creek then.
I have to make screw legs for an oven. I did a kitchen reno and the counter tops are higher than the flat top stove the client bought. The damn legs aren't tall enough. I have the stove propped up on pieces of plywood, to add insult to injury, yesterday when I was caulking the backsplash, I slid the stove out slightly to get behind it and snapped the two back legs off like twigs nudging the stove back in place. The legs were cheap plastic... Useless...

Monday is my last day there. I have one of the broken plastic legs in the garage. I was reaaaaaaaly hoping I could just make some new longer ones from aluminum bar.
I may try to make a thread at 8tpi and try it. The thread in the bottom of the stove is just a piece of formed sheet metal, maybe engages 1-1 1/4 threads, tops. Maybe I can just force it...
Uhhg...


Edit: I should mention, I spent a combined 3.5 hours on the phone Friday trying to find longer legs. I can find replacements, at $30 a pop... But not longer ones.
 
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Drill and tap them all to accept standard 5/16" or 3/8" appliance leveller feet? Kind of like a telescoping thread.

-frank
 
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