double depth of thread

Discussion in 'General Engine Discussion' started by Don Huseman, Feb 6, 2008.

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  1. Feb 6, 2008 #1
    I have a gage that is used to set up your threading tool bit and on it it has double depth of the thread pitches . What the hell is it. I have asked the smartest machinist I know and he doesn't know. I thought it was how much you wind in on your compound to go to max depth for your thread. But it does not work.
    If you don't help me then I will turn over on the shaper table and you will be machining my butt.
    don
     
  2. Feb 6, 2008 #2

    Davyboy

    Davyboy

    Davyboy

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    Hi Don, if you are looking at what I call a 'fishtail', or thread gage, the double depth is a theoretical depth on Diameter of various threads. That is perpendicular to the thread, to a Sharp V. Don't get too hung up on the exact number, it will get you close, but it's not what matters. for instance, IIRC, 12 threads per inch (i must'a cut a million), double depth is .108 that means .054 per side. Need to correct for compound at 29-12 degrees. Need to allow for tool nose radius (I hone it with a stone.) Also need to allow for variation in OD of the shaft at the start. If you start with a correct sized turn, and a correctly ground tool, then you should be able to see when you are getting close, use your mating part for a gage, or measure with wires or a thread mic if you have them avalilable. Again, don't get hung up on the exact number, it will only get you close.
     
  3. Feb 6, 2008 #3
    Thank. I use thread mikes.
     
  4. Feb 6, 2008 #4

    mklotz

    mklotz

    mklotz

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    Don,

    If you read the articles on our club web page, you would have encountered this one that I wrote on that very subject.

    Like most folks who thread on the lathe, I have a fishtail gauge (also called a
    center gauge) which I use to ensure that the thread cutting tool is
    perpendicular to the work.

    On said gauge (and most others that I've seen) is a set of numbers labeled
    "double depth of sharp thread". Specifically, the numbers on mine are those
    given in columns A (tpi) and B (double depth of sharp thread) in the chart
    below. I've always guessed that these numbers were somehow useful in deciding
    how much to feed in when cutting a thread but I never took the time to sort out
    how to use them. (For me, it's always been easier to draw a picture of the
    thread and derive the depth I need using mathematics.)

    A couple of questions at the meeting made me decide to puzzle out, once and for
    all, what those numbers really are and how to make use of them.

    Mathematically, the height of a thread, measured perpendicular to the thread
    axis from sharp root to sharp crest is given by the following equation.

    h = .5*pitch/tan(30)

    where:

    h = height of thread
    pitch = 1/tpi

    The little program I wrote prints out two times 'h' in column C in the table
    below. As you can see, 2*h agrees perfectly with the numbers printed on the
    fishtail gauge.

    So the numbers on the gauge are indeed as described - the "double depth of
    sharp thread".

    So now the question becomes, "Why are those numbers on the fishtail gauge?"
    Those numbers aren't particularly useful when cutting the thread. Most of the
    time the question is, "How much do I need to feed in the compound when it is
    set to angle 'ca' (compound angle)?" Mathematically, the answer to that
    question is:

    cin = h/cos(ca)

    where:

    cin = compound infeed

    I've printed out cin for ca=30 deg in column D. For this case, we have:

    cin = .5*pitch/[tan(30)*cos(30)] = .5*pitch/sin(30) = .5*pitch/.5 = pitch

    and you'll note that the numbers in column D are exactly equal to the pitch of
    the thread with tpi as given in column A.

    So, the bottom line here is that I still don't know why those numbers are
    there. Perhaps an old school machinist can explain how to use them but I
    don't see any immediate value to them. (I can't imagine a machinist
    multiplying the number in column B by .5/cos(ca) to get the compound feed
    depth he needs.) If you didn't angle the compound at all when cutting threads
    (i.e., feed straight in with the cross feed) and your crossfeed was calibrated
    in diameter reduction (a .001 feed reduces diameter by .001), then the numbers
    in column B would be your infeed to cut that thread. But what competent
    machinist wouldn't angle the compound?

    I don't know the answer but I do know this...I'm going to continue to ignore
    the numbers on my fishtail gauge and base my calculations on what I understand.

    A B C D

    4 0.433 0.433 0.250
    5 0.346 0.346 0.200
    6 0.289 0.289 0.167
    7 0.247 0.247 0.143
    8 0.217 0.217 0.125
    9 0.192 0.192 0.111
    10 0.173 0.173 0.100
    11 0.157 0.157 0.091
    12 0.144 0.144 0.083
    14 0.124 0.124 0.071
    16 0.108 0.108 0.062
    18 0.096 0.096 0.056
    20 0.087 0.087 0.050
    22 0.079 0.079 0.045
    24 0.072 0.072 0.042
    26 0.067 0.067 0.038

     
  5. Feb 6, 2008 #5

    Stan

    Stan

    Stan

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    The direct crossfeed numbers are probably a holdover from when lathes didn't have a compound.

    That reminds me of a question that came up during the Korean War. Someone questioned the purpose of one man on the gun crew who appeared to do nothing. Research showed that his job was to hold the horses.
     

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