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First try at thread cutting

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  • Commander_Chaos
    replied
    Figured I'd throw this up here for the sake of completeness:

    Many videos and a lathe purchase later, I've cut threads. The tool wasn't the right shape or even sharp enough, the lathe has a ton of slop in it and the lack of a thread dial and a clutch makes the process difficult, but those are all problems I can solve or live with.

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  • Mr Fixit
    replied
    Commander,

    This Yahoo site is a wealth of knowledge as well as the practical machinist site under the south bend subgroup, watch out for the rest of the site they frown on hobbiests with the simple questions.
    Google is your friend when it comes to a topic like adding a QC box to a SB lathe. I have not done an add to a machine but there are others that have.

    TX
    Mr Fixit for the family
    Chris

    https://www.google.com/url?sa=t&sour...FI3emJuRXwniPA

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  • janvanruth
    replied
    if you can find one, no problem

    Leave a comment:


  • Commander_Chaos
    replied
    Originally posted by J Tiers View Post
    the threading chart that should be riveted to the gear cover.
    There ain't no gear cover.

    I'm scouring the internet for parts as we speak. When I get what I need, I'll be back asking lots more questions.

    Is it possible to retrofit a gear box to my lathe? By "possible" I mean bolt together, not heroic craftsmanship.

    Leave a comment:


  • J Tiers
    replied
    I still suggest you first figure out what the gears you have will cut, select as coarse a thread as you can, and set up for that per the threading chart that should be riveted to the gear cover.

    Then go ahead and cut threads, assuming that the coarsest thread is something no worse than 30 or 40 threads per inch,. You DO want to SEE the results, and also a coarser thread is less affected by slop in the system.

    Actually, slop should be canceled out by just letting the carriage move a bit before it starts cutting. That should take up the slop, and ensure that you cut accurately.

    That taking up of the slop is the reason for advancing the compound and not the crossfeed, as well as the 29 1/2 deg angle for the compound. The idea is to always have "pushback" via the cutting edge forces. Without that, on a lightweight lathe, there is a chance of the carriage being pulled forward, so that it is no longer following the leadscrew, the partly cut thread has "taken charge". The leadscrew should always push the carriage.

    On a lathe with a heavy carriage, thatis much less of a problem, and consequently many folks who have never used an Atlas or other light lathe will tell you that the compound stuff is all hogwash.... because for them it is!

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  • Don Young
    replied
    You can cut threads that are multiples of the leadscrew pitch without a thread dial but not those which are a factor of the pitch. If you have an 8 pitch leadscrew and try to cut a 4 TPI thread, you will likely split it and cut a 2 start thread since you can engage the half-nuts every 1/8" along the screw. 1/8" off on multiple threads does not matter as you just skip one or more whole threads in your positioning.
    Originally posted by becksmachine View Post
    For purposes of practicing, if you pick a thread pitch that is an even multiple or divisor of the leadscrew pitch, you don't need to pay any attention to the thread dial.

    If the lathe leadscrew is 6 tpi, you should be able to cut 6, 12, 24 and maybe 2, 3 threads per inch without even having a thread dial.

    Dave

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  • Mr Fixit
    replied
    Thread cutting info

    Commander,
    These links have SB manuals that have charts for gears for the A, B, C model lathes. This can help in determining what you have and what your wanting.
    The lead screw is 8tpi unless it is thread setup for metric.

    TX
    Mr Fixit for the family
    Chris



    https://www.google.com/url?sa=t&sour...bFgw3GDvbF6t2g

    https://www.google.com/url?sa=t&sour...GVDmuY4znOcf2A

    Leave a comment:


  • Richard P Wilson
    replied
    Originally posted by Commander_Chaos View Post
    The videos I've seen so far have said even number threads can be picked up on any line or number and odd number threads have to be picked up on just the number.
    Nearly but not quite. Threads which are a multiple of the leadscrew (which I'm guessing is 8TPI) such as 8, 16, 24,etc can be picked up on any line or number. Threads such as 4, 12, 20 can be picked up on any number or any line, but you have to stick to lines or numbers, you can't mix them. After that, its simplest to stick to the same line or number.

    Those gears on ebay. Assuming the seller is correct that they are for a 9" Southbend, and he will confirm that they have all their teeth with little or no damage, then yes, they should fit your 9" Southbend. Would be handy if you could access a Southbend screwcutting chart to see what gears make a full set first though.

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  • becksmachine
    replied
    Originally posted by Commander_Chaos View Post
    The videos I've seen so far have said even number threads can be picked up on any line or number and odd number threads have to be picked up on just the number.
    For purposes of practicing, if you pick a thread pitch that is an even multiple or divisor of the leadscrew pitch, you don't need to pay any attention to the thread dial.

    If the lathe leadscrew is 6 tpi, you should be able to cut 6, 12, 24 and maybe 2, 3 threads per inch without even having a thread dial.

    Dave

    Leave a comment:


  • Commander_Chaos
    replied
    Originally posted by quadrod View Post
    Also depending on what change gears you have, you need to figure out what thread per inch you are cutting. You may want to make sure your thread dial lines up with the same number before engaging the half nuts so that you know for sure you are cutting the same thread on each pass.
    The videos I've seen so far have said even number threads can be picked up on any line or number and odd number threads have to be picked up on just the number.

    Leave a comment:


  • quadrod
    replied
    Also depending on what change gears you have, you need to figure out what thread per inch you are cutting. You may want to make sure your thread dial lines up with the same number before engaging the half nuts so that you know for sure you are cutting the same thread on each pass.

    Leave a comment:


  • Commander_Chaos
    replied
    Thanks for the replies, everyone. Sounds like my initial suspicion was correct, I need some change gears.

    Saw this on eBay. Anyone know if they'd work my lathe?

    http://m.ebay.com/itm/121866210679
    Last edited by Commander_Chaos; 01-17-2016, 09:42 AM.

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  • danlb
    replied
    Looking at the picture in post #9, you have a large gear attached to a small one on the same shaft, with the small one driving a big one. That's a typical step down arrangement used for a power feed (very fine feed to make a smooth cut). Many people leave it in this configuration whenever they are not threading.

    I suspect the seller did not bother to look for the change gears used for threading.


    Dan

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  • J Harp
    replied
    Download this PDF, it should give you the info you need. If you study this it will help you get to know your lathe. The section on lathe tool grinding is a must read, it applies to small lathes in general not just to South Bend.

    http://campkahler.com/files/How_to_R...he_SB_1of2.pdf

    Leave a comment:


  • oxford
    replied
    I am not real familiar with the SB9 but from what I am seeing you will need different change gears to cut threads. It looks like the half nuts on the lead screw is the same for cutting threads and turning metal. Putting it in backgear doesn't do anything other than slow everything down, you still have the same feed per rev. until you change gears.

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