Threading problem model 9 a
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  1. #1
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    Default Threading problem model 9 a

    Set gear box to cut a 20 tpi and its cutting 15tpi. Why is this? It's my first lathe trying to learn as I go thanks

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    Quote Originally Posted by Jordan429 View Post
    Set gear box to cut a 20 tpi and its cutting 15tpi. Why is this? It's my first lathe trying to learn as I go thanks
    Probably something to do with the gear chain at the head of your lathe. Give us a complete list of the gears in the chain and clear photos of the gearing. Most of the gears are stamped with the number of teeth, but if you can't find a number, COUNT THEM. All this assumes that there aren't any problems with your gearbox.

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    Further to that - do you have the right stud gear installed? There are two of them to choose from

    sb-model-end-gears.jpg

    A look at your chart on your quick change gear box in the first column on left will tell you that there are 20 tooth and 40 tooth stud gears

    Here are 12 pages of way more info on the 9A (no open? means I need your email address - which you can private message me - with something useful in the subject line - like SB 9A gearing)

    http://bnordgren.org/files/model-a-gears.pdf

    have fun
    Last edited by johnoder; 06-14-2021 at 04:22 PM.

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    Jordan, the 40 tooth stud gear cuts the tpi in half for the top row of the chart. You’d notice that large of a gear in there, and there is no 30 tpi on the chart anyway, so nothing to cut in half to get 15. Just a guess but the inner gear on the stud gear is 24 tooth (the one that contacts the reverse tumbler gears) while the outer is 20 tooth (the one that contacts the lead screw gear.) They both have the same shaft size and key way. If they were reversed it would alter the feed rate. Make sure the smaller gear is out.

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    Are you using the half nuts , or the power feed ?
    animal

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    Thanks. half nuts trying to get some pics not shure how to post them. Got 20 tooth on top 40 on bottom

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    Quote Originally Posted by Jordan429 View Post
    Thanks. half nuts trying to get some pics not shure how to post them. Got 20 tooth on top 40 on bottom

    One of the how-to threads on posting photos

    Posting Pictures on the South Bend Forum

    have fun

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    Got 20 tooth on top 40 on bottom
    Note behind the stored 40 should be this gearbox gear

    This is always 56T in the model A with the English Lead Screw

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    If your screw gear is 42 teeth instead of 56 teeth you could get 15tpi on 20. That’s what I do to get 30tpi… I use a 42 tooth for the screw gear and set the box for 40 tip. If your lathe doesn’t have a 56 tooth gear they are always available on eBay cheap.

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    The 40 tooth is running in line now with a 56 tooth on outside of the 40 as a spare are we saying swap the to to correct the issue

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    20t to an 80t to a 40t

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    Quote Originally Posted by Jordan429 View Post
    20t to an 80t to a 40t

    Need the 56 working and the 40 stored doing nothing - if you want the threading (20 TPI) to be right

    You can fairly plainly see this in the photo in Post #3

    Or better yet from the link in Post #3

    9a-end-gearing.jpg
    Last edited by johnoder; 06-21-2021 at 12:15 PM.

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    Quote Originally Posted by johnoder View Post
    Need the 56 working and the 40 stored doing nothing - if you want the threading (20 TPI) to be right

    You can fairly plainly see this in the photo in Post #3

    Or better yet from the link in Post #3

    9a-end-gearing.jpg
    Yes. And just as an aside, the 80 tooth gear is an idler. It can be any size that reaches and the ratio will be the same.

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    Going to try it thank for the help

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    Ok I switched the gears around and now I tried cutting a 20 tpi and it cut a 22 tpi alot closer. thought I had it, anyone have any suggestions?

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    anyone have any suggestions?
    I'll assume you actually got the 20 stud, 80 idler and 56 tooth gear box gears all on and meshing properly

    You can assure yourself it is either doing what its supposed to or not.

    Put it in 20 TPI, engage half nut and turn chuck exactly one turn after removing all lost motion - like one turn before the measuring turn

    A dial indicator set against the carriage and zeroed will move exactly .050" in that measuring turn - this is because one inch divided by 20 = .050"
    Last edited by johnoder; 06-24-2021 at 07:28 AM.

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    It's real easy to misread the numbers on the chart on the quick change gear box on the 9A lathe. Especially if there is any wear and tear to the plate. I've made that mistake a time or two on my 9" SBL. Older age and eye sight along with delxsus, makes it real easy to make mistakes with the placement of the lever. Ken

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    Sounds like you had two problems in the beginning. The gear train should be 20 - 80 - 56 which you have corrected and got you closer. I think the second problem might be the gear attached to the end of the lead screw in the gearbox being flipped over. This thread from a little over a year ago describes the problem better than I can.

    Strange threading problem

    Hope this helps.

    Ben

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    leadscrew-gear.jpg

    I made this model of a 9A QCGB a couple years ago. I think you can clearly see the consequence of having the leadscrew gear reversed.

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    Great I'll take it apart and check out the gear👍


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