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  1. #1
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    Default single point threading help

    20190103_005440.jpg

    strange happenings when cutting a thread on 416 barrel steel.
    shown is a 10x magnification of a 1/2-28 thread.
    Any thoughts why the right side looks like a 45 degree angle while the left is a nice 30 degree cut?
    Anyone ever experience this.
    1. Compound set at 29.5
    2. .005" per pass / .002" for final pass
    3. Cutting oil was used / (brushed on)
    4. AR Warner T-15 insert used with 10 rake on top.
    Looks like deflection except the load theoretically:
    Toolbit moving to the left, pushing into the left side should prevent any right side deflection.
    Correct?
    Any thoughts or tips would be appreciated.
    IE: Should I set the compound to 20 degrees?
    Used carbide instead?
    Use flat top insert with no backrake?
    FYI I cut and threaded a piece of aluminum to double check my setup and it cuts proper and the thread looks great with a 30 degree angle on both sides.

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    Have you used the comparator to blow up the cutting tool to verify it has a 60 degrees included angle?

    "AR Warner T-15 insert used with 10 rake on top" only specifies the brand and type of HSS, not the shape of the insert.

    Larry

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    Which way is your compound numbered.. Looks to me like you are cutting 60.5 degrees.

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    QT: Any thoughts or tips would be appreciated.

    You want to feel your bit/insert into your fish gauge, to the side of the part.
    also feel the bit /insert into the fish gauge to know it's angle is correct....


    LATHE fish gauGeCHECKIG a pART. PHOTO - Google Search

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    Your compound is set to the WRONG 29.5 degree angle. You want 29.5 degrees from the setting that is perpendicular to the lathe's long axis, not from the axis of the cross slide.

    IGNORE the numbers on the scale.

    Set the compound perpendicular to the long axis (parallel to the cross slide axis).

    Now move it 29.5 degrees from there.

    As for the numbers on the angular scale, some lathe makers do them one way and others do it another. So you can not just read the numbers. In your case you will probably wind up at 60.5 degrees on the scale and this will be correct for YOUR lathe.

    Life if tough. Deal with it. And don't fret over it: you are far from the first to make this mistake.

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    I agree, the machines at the community college I went to and was a classroom assistant at had the numbering laid out so you set the compound to 59 degrees. I saw a lot of threads that looked just like that from people who ignored the teachers instructions and set their compound to 29 degrees like it said in the book.

    In hindsight it was a very good learning experience for everyone involved, if not rather frustrating.

    EDIT: already covered, I'm a slow typist.

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    Good to have a hand protractor and know what number to use.. Most lathe hands just eyeball swing to 30/60 or what ever, then look at the dial..and pull back a half degree or so.
    I used to run at exactly 30 then pull another/last .002 (or so) straight in with the cross.

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    Very common mistake, compound angle error.

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    Thanks for the look see and all the replies.
    I checked the setup over and decided to set the compound to 25 degrees and take .003" per pass on some A2 drill rod and things went back to normal.
    Cleared up the problem.
    Also am threading at 70 rpm.
    Appears to be too much deflection with only a half degree cut on right side. (angle must not have been less than 30 as you all suggested)
    Then again my 29.5 may not have been a perfect 29.5 but...all the double checking in the world doesn't matter I suppose unless I set the compound to what the material likes to cooperate with.
    I used a digital bevel square to check the angle of the compound against the head of the 4 jaw, which I believe to be more accurate than using the protractor stamped into the cross slide.
    Resolution is within 1/10 of a degree. Although its accuracy may only be within 3/10 of a degree.
    The inserts are for 8 to 36 pitch 60 degree threading.
    Thanks again for all you fellas insight.

    20190110_130813.jpg

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    Just wondering ,,Do you have a fish gauge?..and a simple hand protractor?

    Do you know 1 / 28 = about .036 and /8 = is about .005... and what this represents..it is basic..

    70 rpm.a good threading speed.

    QT: [BTW the inserts are for 8 to 36 pitch 60 degree threading.] that seems a very wide spread...I would not use the same nose for 8 to 36 thread..

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    I would suggest just leaving the compound at 0° and threading from there if you have a DRO. Use the face of the jaws to get the tool perpendicular to the work and go from there. The only use for the compound at a 29.5° angle is on larger threads which you're not cutting.

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    A very good read....anyone single-pointing threads should have this all down pat...
    Absolute Beginners Guide to Lathe Threading | Screw | Metalworking

    But the above information does not tell how to figure the pitch and the feed depth or the nose flat.
    so one needs to download the south bend book
    How to Run a lathe.

    Better to just buy a copy..
    How To Run A Lathe by South Bend Lathe Works | NOOK Book (eBook) | Barnes & Noble(R)

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    you should nit have to set the compound at "whatever the material likes".

    As others have said, your compound is likely numbered for degrees in such a way that you need to
    Quote Originally Posted by Dugan View Post
    Thanks for the look see and all the replies.
    I checked the setup over and decided to set the compound to 25 degrees and take .003" per pass on some A2 drill rod and things went back to normal.
    Cleared up the problem.
    Also am threading at 70 rpm.
    Appears to be too much deflection with only a half degree cut on right side. (angle must not have been less than 30 as you all suggested)
    Then again my 29.5 may not have been a perfect 29.5 but...all the double checking in the world doesn't matter I suppose unless I set the compound to what the material likes to cooperate with.
    I used a digital bevel square to check the angle of the compound against the head of the 4 jaw which I believe to be more accurate than the protractor stamped into the cross slide.
    Resolution is within 1/10 of a degree. Although its accuracy may only be within 3/10 of a degree.
    The inserts are for 8 to 36 pitch 60 degree threading.
    Thanks again for all you fellas insight.

    20190110_130813.jpg

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    If 25* gets the result you want, something is wrong with your machine or your set up. I feed straight from the carriage in until maybe the last spring cut or two where I then use the compound to get a tiny little burnishing cut by moving the tool maybe .0005 or so to just finish it off in each direction. You can calculate the compound angle needed to translate the angle setting and the actual tool movement. But on 416 barrel steel I don’t need to because it usually cuts so clean.

    You should get a fishtail gage as suggested above if only to help check visually your set up before you start. I think the way you are doing it you’d never be able to pick up an existing thread. Something you might need to do some day.

    Start with big cuts and as the whole tool gets engaged in the work take smaller bites. The first two or three can be pretty big, then let up and finesse it some when the tool really starts getting a full load.

    Calculate the thread depth before you start and think about how many and what combination of cut depths add up to the correct number. Then follow your numbers.

    Faster spindle speed is better and a chicken groove makes it easier. No shame in that especially if you do it artfully.

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    Nobody should even think about turning a thread on a manual lathe with not having a fish gauge.
    Part could be in a V block or made square to the shadow graph...
    And understanding about 29.5 -30* and such should be down pat for barrel work...
    Last edited by michiganbuck; 01-11-2019 at 12:14 AM.

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    Quote Originally Posted by michiganbuck View Post
    Nobody should even think about turning a thread on a manual lathe with not having a fish gauge.
    What is so amazing about this piece of sheet metal exactly?

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    Quote Originally Posted by Hazzert View Post
    What is so amazing about this piece of sheet metal exactly?
    You lay it on the work and then set the tool into the v notch. Using a piece of white paper background and a bright light you can see that your tool is cutting 30* on each side for a straight in 60* thread angle. No math needed. Don’t even need to be able to read.

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    What math are you talking about exactly?

    Use the chuck jaws or the face of the part to square the tool perpendicular and cut the damn thread. If you're talking chasing threads where is any math going to come into the equation that a fishtail is going to help you?

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    More good replies thanks.
    When I was a full time machinist many years back I used to simply plunge cut threads.
    I recently decided to employ the 29 degree compound method for threading.
    Never saw the point before when my threads always came out fine, including making scope caps and such with very fine threads up to the shoulder.
    BUT- I do see merit in having the leading side load against the tool to reduce any wander due to backlash in the leadscrew halfnut engagement.
    Also to reduce chipload on both sides of the cutter face which reduces deflection and possible chatter.
    I also mostly used to use carbide.

    Recently though, when upon recommendations of certain tool steels doing a great job such as momax and T-15 I decided to try them.
    Just didn't know if my tool steel was up to the task I was performing or if it may have been something else.
    I agree faster spindle speed would likely help although, with low tolerance for an oops moment and sometimes having to thread with no relief groove....I am limiting myself to kicking out and using 70 rpm for now.
    I also agree that I should not HAVE TO set my compound at 25 deg. but as shown 29.5 deg made for a sub par thread job.
    I didn't use a fish gauge.
    The toolpost/toolholder was squared to the chuck by bringing it up to it and kissing it, then locked down.
    I'm not seeing how the fish gauge would get things any more squared to the chuck than by mating them together with contact. (IE: Toolpost against chuck - squares toolpost to chuck)
    After lock down I doubled checked the toolpost did not turn which I figured was the most likely, but not so.
    My tool is centered and double checked by taking a face cut after centering to ensure. I am well versed in centering having in the past had to center tools on CNC lathes to hold tolerances within .0002" for Govt. Contract Communication parts. The spiral to the center tells the tale.
    So, if my tool holder is squared to the chuck,
    My Toolpost is squared to the chuck.
    The part is dialed in to within .0005" TIR
    My digital protractor reads 29 deg. between chuck and compound...
    All I can conclude is the steel being turned is tough enough to cause deflection on the right side unless I make the angle more acute and take a bigger cut on that right side. (IE: the closer I get to a plunge cut the better)
    Yes?

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    I give up.. Just way too much to explain from here.
    Absolute Beginners Guide to Lathe Threading | Screw | Metalworking

    How to Run a lathe.

    Better to just buy a copy..$5
    How To Run A Lathe by South Bend Lathe Works | NOOK Book (eBook) | Barnes & Noble(R)

    ****Copy this to your computer and read it cover to cover..****
    How To Cut Screw Threads by South Bend.
    http://vintagemachinery.org/pubs/1617/3537.pdf


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