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Interesting thread for me...
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QCGB question
- Thread starter markba633csi
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It is possible to calculate anything but if you lack the proper gear then you cannot cut it precisely. If you have a gear that has a tooth count of a prime or multiples of that prime number then you can calculate and cut it, right? But unless you have a special gear made for cutting these prime threads then I don't see how its possible. Cleeve had this to say: "... In the example just given, 19 (or 13) is, of course, a prime number, and the impossibility of exactly gearing a lathe for primes or multiple of primes should be noted."
EDIT: Mike (@Tozguy ), I responded to Hman and did not respond properly to your query - my apologies. 27 tpi is not a prime number so the gears to cut it can be calculated and loaded on the lathe if available and the thread can be cut. It is only prime numbered threads that give you the fits.
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A prime number is a number greater than 1 that can only be divided evenly by itself or 1.
In truth, you can find a gear set up that will come close. It will be off a little bit and in most cases it will be close enough. For my lathe, I can calculate the gears for 13 tpi but when I set it up on the lathe and actually cut a thread you can see that it is off by a small amount. I also scribed the pattern with a fine point Sharpie and checked it with a thread gauge under 10X magnification and it is indeed a tiny bit off.
Thread calculations can be confusing because you have to factor in the influence of the gears in the QCGB. It is much simpler to do this for a simple change gear lathe. With that said, once you know what the gears in the QCGB do, you can create matrices that are very handy to have because what you see in the matrix can actually be cut; they are not theoretical.
In truth, you can find a gear set up that will come close. It will be off a little bit and in most cases it will be close enough. For my lathe, I can calculate the gears for 13 tpi but when I set it up on the lathe and actually cut a thread you can see that it is off by a small amount. I also scribed the pattern with a fine point Sharpie and checked it with a thread gauge under 10X magnification and it is indeed a tiny bit off.
Thread calculations can be confusing because you have to factor in the influence of the gears in the QCGB. It is much simpler to do this for a simple change gear lathe. With that said, once you know what the gears in the QCGB do, you can create matrices that are very handy to have because what you see in the matrix can actually be cut; they are not theoretical.
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A 27 tpi thread is usually a pipe thread, unless it is a straight thread for lighting or other old technology hand me downs. If it is truly for a pipe thread, the fit needs to be very close if you intend it to seal liquids or gases, even if you plan on using pipe dope or Teflon tape. Why not use a tap or a die to do the threading?
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I have a PM1236 & it also doesn't list 27 TPI or 11.5 TPI. A while back I wanted to figure out how to cut 11.5 TPI. Looking at the chart on my lathe I figured that I simply needed a 23T gear which was not included with my lathe. I never ended up trying to locate one cause I found something else.
Member ftl on HSM (I wanted to mention to give him credit) created 2 documents that listed every single possible thread that could be cut on a PM1236 with the stock gears. Sadly the DL links were expired so scouring the net for those 2 PDFs, to my surprise someone had uploaded them here on H-M in a post.
I haven't verified if the PDFs are accurate but they seem to be. And if they are (hopefully) they are absolutely invaluable to anyone with a PM1236. In the "All_Pitches" PDFs it shows change gears & QCGB combinations that will allow you to cut pretty much any thread pitch you could ever want...and then some. In it shows 27 TPI & most importantly for me 11.5 TPI (if I ever want to make some garden hose fittings or large pipe thread but I don't have a taper attachment).
The docs are in the post that I linked to but I'll upload them here too (I renamed the files though to better describe them)
Member ftl on HSM (I wanted to mention to give him credit) created 2 documents that listed every single possible thread that could be cut on a PM1236 with the stock gears. Sadly the DL links were expired so scouring the net for those 2 PDFs, to my surprise someone had uploaded them here on H-M in a post.
I haven't verified if the PDFs are accurate but they seem to be. And if they are (hopefully) they are absolutely invaluable to anyone with a PM1236. In the "All_Pitches" PDFs it shows change gears & QCGB combinations that will allow you to cut pretty much any thread pitch you could ever want...and then some. In it shows 27 TPI & most importantly for me 11.5 TPI (if I ever want to make some garden hose fittings or large pipe thread but I don't have a taper attachment).
The docs are in the post that I linked to but I'll upload them here too (I renamed the files though to better describe them)
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Regarding the issue of being/not being able to cut "prime" thread pitches ...
As I mentioned previously, my Grizzly G4000 (9x20) lathe claims to be able to cut both 11 pitch and 13 pitch threads. Just for fun, I took the QCGB apart and counted gear teeth (OK, coulda looked in the manual. I did afterward, just to verify my counts.) The following diagram shows all the gears involved in creating the two thread pitches in question. The "A" and "B" gears are replaceable; 60T and 30T are ones specified on the threading chart, as are the 22T and 26T QCGB gears, for 11 and 13 pitch, respectively.
Let's do the math:
Thread pitch (when set for 11 TPI) = 1 spindle turn X 40/40 X 60/30 X 16/22 X .0625"
= 0.09090909(repeating)"
= 1/11" per turn EXACTLY
Thread pitch (when set for 13 TPI) = 1 spindle turn X 40/40 X 60/30 X 16/26 X .0625"
= 0.076923(etc.)"
= 1/13" per turn EXACTLY
So, as far as I can tell, the claim that you can't get "prime" tread pitches with a QCGB is DISPROVEN. Mikey, I have not read the author (Cleeve) that you cited, so I don't know how he arrived at his claim**. But given the gearing involved, there's on way (other than maybe inaccuracy in the leadscrew) that you can arrive at anything other than the results I've calculated above. It's in the hardware!
**PS - I'm wondering if Cleeve might have been referring to producing gears with prime tooth counts(?) Despite having a goodly number of rows of holes on the dividing plates, my own rotary table is incapable of producing any prime number gears from 67 on up. It can produce prime interval numbers (gears or whatever) with values between 2 and 61, because the index plates include all these numbers or multiples thereof. And yes, you can produce pseudo-prime gears by cheating a little bit. For instance, I figured out that if absolutely necessary, I could probably produce an acceptable 127 tooth gear. (use the 62 hole plate, advance 44 holes for 15 teeth, then 43 holes for the 16th tooth. Rinse and repeat until done. Maximum error 0.065º)
As I mentioned previously, my Grizzly G4000 (9x20) lathe claims to be able to cut both 11 pitch and 13 pitch threads. Just for fun, I took the QCGB apart and counted gear teeth (OK, coulda looked in the manual. I did afterward, just to verify my counts.) The following diagram shows all the gears involved in creating the two thread pitches in question. The "A" and "B" gears are replaceable; 60T and 30T are ones specified on the threading chart, as are the 22T and 26T QCGB gears, for 11 and 13 pitch, respectively.
Let's do the math:
Thread pitch (when set for 11 TPI) = 1 spindle turn X 40/40 X 60/30 X 16/22 X .0625"
= 0.09090909(repeating)"
= 1/11" per turn EXACTLY
Thread pitch (when set for 13 TPI) = 1 spindle turn X 40/40 X 60/30 X 16/26 X .0625"
= 0.076923(etc.)"
= 1/13" per turn EXACTLY
So, as far as I can tell, the claim that you can't get "prime" tread pitches with a QCGB is DISPROVEN. Mikey, I have not read the author (Cleeve) that you cited, so I don't know how he arrived at his claim**. But given the gearing involved, there's on way (other than maybe inaccuracy in the leadscrew) that you can arrive at anything other than the results I've calculated above. It's in the hardware!
**PS - I'm wondering if Cleeve might have been referring to producing gears with prime tooth counts(?) Despite having a goodly number of rows of holes on the dividing plates, my own rotary table is incapable of producing any prime number gears from 67 on up. It can produce prime interval numbers (gears or whatever) with values between 2 and 61, because the index plates include all these numbers or multiples thereof. And yes, you can produce pseudo-prime gears by cheating a little bit. For instance, I figured out that if absolutely necessary, I could probably produce an acceptable 127 tooth gear. (use the 62 hole plate, advance 44 holes for 15 teeth, then 43 holes for the 16th tooth. Rinse and repeat until done. Maximum error 0.065º)
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John, I can't fault the math - I got the same results. Can you put a Sharpie in your tool holder and run the 11 and 13 tpi threads and check it with a thread gauge? I'd like to see if the lathe will actually cut those threads. If so, then Cleve is wrong (and so am I).
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The 22 and 26 teeth gears are what are making the 11 and 13 tpi threads possible. No magic there, just gear ratios...
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Getting back to the original post ...
My G4000 is another one of those that does NOT include 27 TPI on the threading chart. But looking at the diagram in my previous post, since I can get 11 TPI from the 22 tooth QCGB gear by using a 2:1 A:B ratio in the head, I think the following will work:
One of the QCGB change gears has 18 teeth. So by using a 2:3 A:B ratio, I should be able to get to 27 TPI with it. Digging through the available change gears, the 30 tooth and 45 tooth gears will produce the requisite 2:3 ratio.
When I have some spare time, I'll verify that setup (as well as the 11TPI ... just to please Mikey)
My G4000 is another one of those that does NOT include 27 TPI on the threading chart. But looking at the diagram in my previous post, since I can get 11 TPI from the 22 tooth QCGB gear by using a 2:1 A:B ratio in the head, I think the following will work:
One of the QCGB change gears has 18 teeth. So by using a 2:3 A:B ratio, I should be able to get to 27 TPI with it. Digging through the available change gears, the 30 tooth and 45 tooth gears will produce the requisite 2:3 ratio.
When I have some spare time, I'll verify that setup (as well as the 11TPI ... just to please Mikey)