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Necessary to tap for knife thread inserts?

Discussion in 'Luthier's Corner' started by silky smoove, Jun 21, 2016.


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  1. silky smoove

    silky smoove Supporting Member

    May 19, 2004
    Seattle, WA
    I picked up some inserts for use in a Warmoth neck. I've used these before on basses (this build is a guitar), but did them when I was younger, more brash and without much forethought. They both turned out well, but here's what I'm wondering: With a knife thread insert, is it better to simply drill and allow the threads on the insert to cut into the wood, or is it better to tap the hole first so that the threads don't have to cut into as much wood?

    These are the inserts in question:
    10-24 Stainless Wood Knife Thread Insert | Fastenal

    There's a misprint on the specs. It says that the external thread size is 0.35" and the drill size is 0.375". It doesn't take a rocket scientist to understand that the drill should probably not be larger than the external thread diameter. I checked with EZ-Lok (the manufacturer) and the external thread size is actually 0.453" (29/64"), but they couldn't offer me much on which tap would work well with these, which lead me to my question here.

    So luthiers, or woodworking pros: Would you pre-tap the 0.375" hole, or would you simply drill it and allow the threads themselves to do the tapping as they were inserted. If you would pre-tap, which specific tap would you use, and would you go along with Fastenal's suggestion of a 0.375" drill as well?

    Thanks!
     
  2. HaMMerHeD

    HaMMerHeD

    May 20, 2005
    Norman, OK, USA
    I don't always use threaded inserts, but when I do, I don't tap.

    I spread some System 3 T-88 epoxy into the whole, then drive the insert in with a 1/2" socket-cap screw and a T-handle Allen wrench.
     
  3. silky smoove

    silky smoove Supporting Member

    May 19, 2004
    Seattle, WA
    Do you drop the epoxy into the hole, or apply it to the threads?
     
  4. Callused Finger

    Callused Finger

    Feb 22, 2007
    New York
    This but I suggest you try it in a spare piece of wood preferably with the same hardness as the neck.
     
  5. silky smoove

    silky smoove Supporting Member

    May 19, 2004
    Seattle, WA
    That's a good idea. The neck is birdseye maple. I've got a couple extra inserts. I'll track down a board of maple and give it a shot first.
     
  6. HaMMerHeD

    HaMMerHeD

    May 20, 2005
    Norman, OK, USA
    I smear it on the side-walls of the hole, into which the threads will cut. The wet epoxy seems to have a lubricant effect, as it is markedly easier to drive them in with epoxy than without. Of course, once it cures, it isn't coming out, for love or money.
     
  7. silky smoove

    silky smoove Supporting Member

    May 19, 2004
    Seattle, WA
    Great! Thank you very much. I'm going to pick up some supplies tonight and get to this tomorrow night.
     
  8. BeeTL

    BeeTL Commercial User

    Sep 26, 2006
    Oldsmar, FL
    Brad Lowe, Lowe Custom Guitars
    On the original question, I don't think there is an off-the-shelf tap made that would work with those inserts because they have knife threads.

    Knife thread = Wood Screw (not ever tapped)

    Machine thread = Machine Screw (always tapped)

    If you're interested in seeing the process for using a machine-threaded insert tapped into a hardwood neck, I posted a thread a while back:

    Guitar and Bass Neck Threaded Inserts
     
    okcrum likes this.
  9. Callused Finger

    Callused Finger

    Feb 22, 2007
    New York
    I did something like hammerhed mentioned.
    I was worried about it being square in the neck so I held the screw in a chuck on a mill and started threading the insert into the neck by turning the chuck by hand. Once it was started and had a few threads created I used a wrench.

    When I was testing it on another piece of wood I tried the suggested hole size and increased it until it screwed in comfortably.

    insert1.

    Most important part of this besides taking my time was making sure the neck was clamped firm when drilling.
     
  10. T_Bone_TL

    T_Bone_TL

    Jan 10, 2013
    NW Mass/SW VT
    If you really wanted to, you could convert (grind) one of the spare inserts to be a sort of insert-tap. But that's not the way they are designed to work, as hammerhed and beetl have said.
     
  11. silky smoove

    silky smoove Supporting Member

    May 19, 2004
    Seattle, WA
    Did some test runs in a scrap of maple last night. The manufacturer's suggested 3/8" diameter drill size was considerably too small. I could get the insert in, but not without significantly high torque that caused the wood to creak more than I would think safe when I transition over to the actual neck. I'm going to try some different bits tonight to try and find the "right" one.

    Three cheers for scrap maple and spare inserts :p
     

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