QCGB question

markba633csi

Mark Silva
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I've noticed most lathes with quick change gearboxes don't offer a 27 tpi setting. Anyone know of a lathe brand that does? Many change gear lathes will do it.
How hard would it be to convert a qcgb-equipped lathe to do 27 tpi? Just one gear?
I can think of two things I ran across that use that thread- microphone stands and sink faucets.
Mark S.
 
It can definitely be done with a different change gear or two. I have a reference on how to do that on my lathe but can't find it at the moment. Regardless, it is just using simple math to achieve a ratio that will work on your lathe, hopefully, but not certainly, using the included change gears. The guy I got it from worked up a spread sheet showing the feed in inches and in millimeters per revolution of the spindle for every possible combination of gearing for his lathe, in order by decimal ratio of feed. Apparently that is not too difficult to do, but it is way above my pay grade for figuring it out for my lathe. Volunteers? Existing online shell program?
 
The thread and feed charts that are provided with a lathe do not necessarily include all possibilities.
My common QCGB lathe is capable of a total of 100 different ratios without changing gears.
A 27 tpi thread translates to a .037'' feed rate of the carriage. Does your feed rate chart for the carriage show that option? If so then use it to cut a 27 tpi thread.
 
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I've noticed most lathes with quick change gearboxes don't offer a 27 tpi setting. Anyone know of a lathe brand that does? Many change gear lathes will do it.
How hard would it be to convert a qcgb-equipped lathe to do 27 tpi? Just one gear?
I can think of two things I ran across that use that thread- microphone stands and sink faucets.
Mark S.

Hey Mark, I was wondering if this is just a casual query or if you need us to help you solve it. I only ask because we can certainly help you do the math if you actually need to cut this thread. We would need to know the tpi of your leadscrew, which gears you have in your change gear set, which gears are in your current drive train and whether or not your lathe will run in reverse.

Most lathes with a QCGB will cut a wide array of threads. This array can be greatly expanded with a simple change gear set. Spreadsheets and calculators are fine but they tell you what is mathematically possible; they do not tell you if the required gears to cut all those threads will actually fit in the lathe. Moreover, calculators will produce prime numbered threads but NO gear train will actually produce an exact prime thread; they can come close enough, though.

My point is that if you have to cut that thread then we will need a lot more info from you.
 
Oh I can cut it on my little MK2 Atlas, I was just window-shopping on Ebay, looking (and drooling) over bigger lathes and noticed the lack of 27 tpi on most 1340, 1440 and larger machines. But I'm sure it's no big deal to set up most any lathe to do it. I don't have a pile of broken mike stands to fix:big grin:
Mark
ps I didn't know that about prime threads- interesting
 
Moreover, calculators will produce prime numbered threads but NO gear train will actually produce an exact prime thread; they can come close enough, though.
Hmmmm ... My Grizzly G4000 (9x20) lathe claims to be able to cut both 11 and 13 TPI, both primes. Gonna have 'ta look at this situation carefully, maybe open up the QCGB and count teeth, so I can verify/disprove the lathe's ability to accurately cut these thread pitches. You've definitely offered up some food for thought.
 
Hmmmm ... My Grizzly G4000 (9x20) lathe claims to be able to cut both 11 and 13 TPI, both primes. Gonna have 'ta look at this situation carefully, maybe open up the QCGB and count teeth, so I can verify/disprove the lathe's ability to accurately cut these thread pitches. You've definitely offered up some food for thought.

Yeah, my Emco Super 11 is supposed to be able to cut primes, too, and my calculations also suggest it. Martin Cleeve, in his book Screwcutting in the Lathe, says that no gear train will generate a prime thread exactly.

I tried this with a Simple Gear Train calculation and a Compound Gear Train Calculation and while I can calculate it, I cannot generate an exact prime thread on the lathe. It is close enough but I doubt you could cut a class 3 fit. Dunno', maybe my lathe is weird, it being Austrian-made and all. I have a full change gear set and generated tables for all practical combinations of gear fits and then double-checked to see that each thread can be cut as calculated; it does, except the prime numbers.
 
Old school lathes, look for South Bend 10" 1" collet capacity (10L) lathes with the two drop lever QCGB. Most will cut 27 tpi. Also, most of the Sheldon line of lathes will cut 27 tpi also. On most lathes cutting English pitches, change the stud gear by two teeth, this will change the 26 or 28 pitch to 27 pitch depending on if you add or subtract from the stud gear.

Back in the day, if the drawing called for 1/8" NPT, you set the QCGB for 28 tpi and not look back. On a short length of 1/8" pipe thread, lead error is only about 0.000349" in the effective length of the thread. Or in 1" of length would be 0.001323" lead error. A.P.I. allows a +/-0.0015" per inch lead error in line pipe threads which includes NPT threads. This would allow a thread cut at 28 tpi to be acceptable under A.P.I. specifications. Ken
 
Mikey, do I understand properly that your gear calculations come out exactly to say 27.0000 tpi (for example) but that's not what comes out in practice?
 
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