Cutting a 27TPI thread on an Atlas 10F
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  1. #1
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    Default Cutting a 27TPI thread on an Atlas 10F

    Hi all;

    I have a 27TPI thread to cut on an Atlas 10F ... I have a quick change gear box installed on the lathe, but it has no setting for 27 TPI

    I have a quick change gear set and a bracket (I have always called it a banjo...) BUT am missing some of the gears required to get the correct ratio (2x32 tooth gears) that and the bracket position A (the one to be used of course) has been welded several times & I can see a crack behind one of the welds - I don't have much faith in it holding together.

    There are already about 6-8 gears on the bracket for the quick change box to be able to engage the lead screw forward/reverse control and engage the spindle. (of course none of them were 32 teeth!!)


    Does anyone know if I can swap out the gears in place now (to engage the quick change box) with different sizes to be able to change the ratio and get that 27TPI?

    -thanks
    -sean

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    Two suggestions here. First, look at the Yahoo group covering Atlas/Craftsman lathes for thread chart showing how to set up for 27 TPI. There's a way to do it if you've got the change gears.

    The second suggestion is to read the rules for the forum. The forum owner prohibits discussion of lower end machine tools and specifically the Atlas/Craftsman machines so this thread might get locked any time.

    Good luck.

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    thanks for the pointers... ! looks like the thread got killed already

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    27 TPI is one of those oddball pitches. Setting up a lathe for threading is all about prime numbers. 27 has prime factors of 3, 3, and 3 (3 x 3 = 9 and 9 x 3 = 27). Prime factors are unique so that's the ONLY way to get there. I believe your lathe has an 8 TPI lead screw (2 x 2 x 2 = 8) so you have no threes there.

    Atlas lathes

    The standard succession of threads for a lathe that is set up in a QC gear box for English measure (inches) lathes is 8, 9, 10, 11, 11.5, 12, 13, 14. Additional threads are usually provided with a second lever that multiplies or divides those numbers by a factor of 2. The 9 TPI position provides two of the needed 9s, but nothing in the lead screw or the gear box can provide that third factor of three. For that you must use additional gears on the bracket/banjo. I don't know what gears you have on the banjo now, but if you can let us know, some possible changes may be suggested. Also let us know what multiples of 9 TPI your gear box has: probably 4.5, 9, 18, 36, 72 but there is no sense in working this out for a setting that may not be on yours.

    Here is another page I found while searching for your lathe.

    http://www.vintagemachinery.org/pubs/51/3454.pdf

    There were some references for a user manual but most of them wanted $$$$$s. I also saw a reference for a threading supplement for it.

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    Sounds like you are trying to cut pipe threads or lamp parts. If so buy a die, and get after it.

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    Quote Originally Posted by EPAIII View Post
    27 TPI is one of those oddball pitches.
    27 TPI is the standard pitch for 1/16" US pipe threads (NPS and NPT) so I'm not sure how "odd ball" it is.

    Within US pipe threads then 11½ TPI is what I would call "odd ball".

    http://f-m-s.dk/3.07.pdf

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    Quote Originally Posted by true temper View Post
    Sounds like you are trying to cut pipe threads or lamp parts. If so buy a die, and get after it.
    it's a 100 year old screw for an 1873 Springfield Trapdoor.... odd size - odd thread

    But following tgtools advice I found links to a manual that does have tables for using the change gears in combination with the QCGB to get the feed I need for the 27tpi

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    I know that 27 TPI is used on a lot of pipe threads. That was not my point. MOST Quick Change Gear Boxes on lathes with English (inch) lead screws do NOT have it. They multiply the 9 TPI setting by twos giving 9, 18, 36, and 72; skipping over 27. IMHO, that makes it an "odd ball" thread. Cutting it has been discussed many times before and extra change gears are almost always needed.

    Now, on my SB with MANUAL change gears, it is just one more gear combination (16t & 54t). And those gears were part of the set that came with the lathe when I purchased it. No problem.

    On the other hand, 11.5 TPI IS on most QCGBs and most English system lathes can cut it. I don't know what it is used for and I suspect that the gears are in the box for one of the other threads in that column; perhaps 23 or 46.

    The most common design for a QCGB operates with two stages of gearing: one stage provides a stepped sequence of thread pitches and the other stage provides multiples of those pitches. So there is a grid of values and you can not change one value in that grid without also changing the others in it's row or column. So there are a bunch of uncommon TPIs in that grid: they are not very useful, but they are there in order to have the ones that are needed.



    Quote Originally Posted by Gordon B. Clarke View Post
    27 TPI is the standard pitch for 1/16" US pipe threads (NPS and NPT) so I'm not sure how "odd ball" it is.

    Within US pipe threads then 11½ TPI is what I would call "odd ball".

    http://f-m-s.dk/3.07.pdf


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