8 T.P.I. Triple lead thread
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  1. #1
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    Default 8 T.P.I. Triple lead thread

    I had a friend ask me how to cut a triple lead thread and After explaining how I do it he tells me its 8 pitch. I dont see how I would set the machine up for that. The lathe is a Kingston HD 22 and the part is a custom neck for a diesel tank he’s building. The cap was purchased somewhere and it has triple lead 8 pitch threads inside. Shame is he already cut single lead before wondering why it wouldn’t fit. I guess its like Starrett is with their special threads on all their tools.

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    2.666666 lead, WOWSERS that will be hard on the gear train. I might be inclined to find another solution rather than strip the gears out of the lathe. Bore out the mating piece and rethread it. Maybe epoxy in a thread adapter.

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    Hey Fred-how did you calculate the lead without knowing the thread diameter?

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    Quote Originally Posted by Dan from Oakland View Post
    Hey Fred-how did you calculate the lead without knowing the thread diameter?
    That's lead per revolution, not pitch angle. Diameter doesn't enter into the equation.

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    3-start 8-pitch= .375 lead. Divide .375 into 1.000" and that will give you the T.P.I. to set your lead screw to. Like Fred said, 2.66666 or 2-2/3 tpi. Some of the old lathes out there are set up to cut that pitch. Couldn't tell you which ones.

    Ken

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    Error comes in the third decimal place if you can manage 9.5mm pitch, might be good enough for a cap if the threads don't need to seal.

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    Model N I Leblond has that selection on it's quick change.

    25" H D (30" swing)

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    If the lathe has 4TPI setting, then this can be accomplished by introducing 1.5:1 ratio into the gear train. eg. 30/20, 36/24, etc. compound. Then set QCGB to 4TPI. This is doable but will require care as that is a rather large lead. Hope this helps.

    Edit: If your lathe is a Kingston HD 22, threads go all the way down to 2 TPI. This shouldn't be any trouble for that machine. Question is whether you can change gears in the gear train. IF So 1.5:1 will work at 4TPI setting or you could use 1.333:1 at 2TPI setting eg. 40/30, 32/24, etc.

    Best Regards,
    Bob

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    I had a friend ask me how to cut a triple lead thread and After explaining how I do it he tells me its 8 pitch
    A picture tells the tale. You have three fairly small pitch but large LEAD threads all on the same piece of iron. I screwed this one up probably 30 years ago

    dcp_0268.jpg

    Creatures like this have to have the clearances on the cutting tool way more than one start threads

    The lathe of course is set up for the LEAD, not the pitch

    Then you have to arrange to INDEX the work 120 degrees between the three starts

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    The makers of special tank caps inevitably supply the matching weld-on spud for a few dollars......And dont fall for the old theory that you can weld on a diesel tank provided its full to the top.

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    yup- your right! sorry about that!

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    Quote Originally Posted by john.k View Post
    The makers of special tank caps inevitably supply the matching weld-on spud for a few dollars......And dont fall for the old theory that you can weld on a diesel tank provided its full to the top.
    I like the idea of a weld in premade spud.

    My experience with a Kingston 20 to 24 inch lathe 35 years ago was the spindle would slip under heavy load. I guess a pin or a key was sheared. The motor did not slow but I could stall the spindle. For me the name Kingston does not inspire confidence.

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    Thanks for the replies. though this is not my project I found it interesting. My take on it would be to run the 9.5mm pitch but I didn't notice if it is an option on this lathe and set the compound in line with axis and set each lead .125 as you cut them.I use H.S.S. on these jobs to accommodate that lead angle also. I like the machine it just has an annoying bump about 5 seconds after I start it. I just wait for it before jumping in. I ran a couple of new Clausings at another site and boy they just felt like junk. One didn't even have the ref. marks on the taper attachment, no tag or degree marks. The other one the lead screw wouldn't turn.They had springs on the chuck key and carriage hand wheel, pulled those right off.He tried to buy the neck piece too but it wasn't available. The material is aluminum but I have no pic.


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