Tap drill for 3/8-28 tap
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    Default Tap drill for 3/8-28 tap

    I have to tap 2 holes 3/8-28. All the drill charts I've found do not have this size. Per the ghuring site it recommends .340 for the hole size at 75%, which is basically a letter R (.339) drill. Does this sound correct? This is for a sensor not load bearing.

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    Yes that sounds about right.
    You don't have a copy of the bible? (Machinery's Handbook)

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    I do, I didn't even think to look there...way to make me feel like a dumbass lol


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    The book shows 3/8-27 and R drill...how much you wanna bet the engineer wrote the wrong number lol.

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    Quote Originally Posted by plastikdreams View Post
    I have to tap 2 holes 3/8-28. All the drill charts I've found do not have this size. Per the ghuring site it recommends .340 for the hole size at 75%, which is basically a letter R (.339) drill. Does this sound correct? This is for a sensor not load bearing.
    For standard taps (75%). Your tap drill will be the major diameter (.375) minus the pitch (1/28)
    .375-(1/28)
    .375-.03571=.3393
    Or, as close as you can come to than size.

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    My 28th Ed. MH has lines for 3/8"-27 and 3/8"-28

    Anyway, for 3/8"-28, .336" - .345" minor for 2A, .336" - .3426" for 3A.

    Regards.

    Mike

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    Thanks guys...I'm waiting for some clarification on a couple things...

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    My Machinery's has 3/8-27, 28, 32, 36, & 40 plus the more common ones.

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    I didn't see 28 in my mh v28. But looks like the numbers match up. Looks like they are going to use a nut to tighten up some threaded suction cups to hold vacuum. I guess npt wasn't good enough

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    Well it's always easy to do 3/8 minus 1/28 even if you have to do with a pencil and paper. :-)
    ...lewie...

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    Ended up being 1/8 bsp thread lol.

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    Quote Originally Posted by plastikdreams View Post
    Ended up being 1/8 bsp thread lol.
    At least it wasn't "LH".....

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    It can be a shitshow here one engineer from dept a specifies something being made for dept b but then an engineer from dept b says its supposed to be something else...then we can't get the 2 of them to qualify what is right because neither are sure...we've had to rework brand new molds because of this lol.

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    Quote Originally Posted by plastikdreams View Post
    Ended up being 1/8 bsp thread lol.
    I was sorta 1/2 expecting that ........but having had my ass kicked before (for merely suggesting a check was made - as they were so close) decided to stay out of it

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    1/8 BSP or 1/8 NPT?

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    Quote Originally Posted by john.k View Post
    1/8 BSP or 1/8 NPT?
    1/8 bsp straight.

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    And 55 degree thread.....of course all of our first questions should be 'why' rather than just rushing for the solution

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    Quote Originally Posted by gustafson View Post
    And 55 degree thread.....of course all of our first questions should be 'why' rather than just rushing for the solution
    Well You just answer the question. It's the "questioner's responsibility to ask the right question. :-)
    ...lewie...

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    Bsp wasn't even remotely in the equation...I've never even threaded anything bsp. Funny thing is, they brought me 2 taps and a similar part that would thread into the block. The nut on the part threaded onto the 3/8-28 a little loose but wouldn't even start on the bsp tap. Tapped a hole using the 3/8 tap and the part was very tight threading in, tapped a hole bsp and the part was just a little lose and the nut could be used to tighten it up. Crazyness...

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    Quote Originally Posted by plastikdreams View Post
    It can be a shitshow here one engineer from dept a specifies something being made for dept b but then an engineer from dept b says its supposed to be something else...then we can't get the 2 of them to qualify what is right because neither are sure...we've had to rework brand new molds because of this lol.
    and to cap it all a machinist who can't figure out the right diameter drill for a tap........


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