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Thread: How to I fix a crooked hole?

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    Aluminum Professor's Avatar
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    How to I fix a crooked hole?

    In the "Welcome" section of this forum Nikon Ron wrote:
    Soooooo, feeling a little frisky I decided to drill and tap the compound rest on my Compact 8 lathe for a QCTP, which many of you helped me with. Well to make a long story very short, I drilled the hole crooked, very crooked. Which brings up a question and subject for a new post in the group that I haven't joined yet. The best way to right the wrong, or make the wrong right whichever you prefer, should I try repairing the compound or should I try, with your help, to find another one? It seems that if I found another compound I would have the same problem, drilling it correctly. I squared everything up on my drill press before I started but I "still" got it wrong.
    He indicated he would start a thread in a different group so I thought I'd give my answer here instead of continuing on in the Welcome section. So here goes.

    Bob replied with one good approach:
    My thought on the mis-drilled part is to install a threaded stud to completely fill the hole and mill it off so the it is level and square. We used to do this on diesel engine blocks that cracked between the liners and it is a tough and durable fix. When you install the stud, use a bit of green loctite and it is nearly permanent as any other repair.
    I have used stud plugs as Bob describes for many different applications. They work particularly well when further machining is not going to occur at the same location as where the hole is being plugged. Of course, a threaded stud requires a threaded hole (which is Nikon Ron's case). In some cases when I just had a round hole, I simply threaded it so I could stud plug it.

    Stud plugs become a bit more problematic if they are plugging a hole that was drilled close to the correct location, and which will be corrected with a new threaded hole. The problem arises when the new hole drills out a significant portion of the plug. The remaining bits of threads from the stud plug may not hold securely enough to enable clean threads to be made in the new hole. (In this case, the more the first hole missed the mark, the better.)

    For holes that have to be moved only slightly (or realigned slightly) filling the hole with weld may be an alternative. Use soft filler material and let the entire piece cool slowly to anneal it. After it is cool, grind off the excess filler to get a smooth surface. Of course, this may not be practical for hardened or precision pieces (such as Nikon Ron's QCTP).

    So, in Nikon Ron's situation:
    1) Can you stud plug the errant hole and drill and tap in an undisturbed location?
    2) Could you disassemble all except the base, weld the hole shut, anneal, re-drill and tap it? (I don't think it has to be hard to function well. You would want to do a little clean-up surface finishing to make it look better and work well.)
    3) Can you, as Bob suggests, re manufacture just the base?

    Then, Ron, you need to figure out just what went wrong when you drilled and tapped the first time. Why did it come out crooked?

    Other comments/suggestions?

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    Global Moderator Rbeckett's Avatar
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    Re: How to I fix a crooked hole?

    Ron Nikon and Prof,
    You are absolutely right about the location of the original hole being an issue. If it cant be moved far enough, then maybe using a helicoil in the newly drilled hole will give the QCTP stud a stable and durable base. If you could move it completely off of the original hole location that would be best but the stud repair coupled with a stainless helicoil should be plenty strong enough for even the tightest tool post. If it is Iron it is going to be difficult to weld and get good results with unless you have a welder freind who is willing to devote the time to do it properly. Cast Iron and cast steel are pourous and problematic, but can be worked nicely with silicon bronze alloys among others. Just some more thoughts from the peanut gallery.
    Bob
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    Plastic Nikon Ron's Avatar
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    Fixing a crooked hole

    I believe that the compound is made of cast steel. I could do a spark test on it today to determine what it is.
    I was using a tap drill for a 14mm stud that came with the QCTP. I think that I was turning the drill bit at too high an RPM and it walked a little on me.
    I might be able to move the location of the hole, ?, not sure. I have a friend who has a home hobby machine shop who may have a milling machine. Could I perhaps bore a bigger hole in it and then press in a threaded bushing?
    One would think that after 41 years working as a millwright that I could do a better job than this. I know, age and senility have set in.
    Regards, Ron

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    Administrator Tony Wells's Avatar
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    If you can get it on a mill, drill and tap to the next size that will fit. The make a step-stud for it, one end to fit the new hole, the rest stepped down to fit the tool post.

    A pressed in sleeve will be too easy to pull out, unless you make one that has a "head" on it and can get to the backside for a matching counterbore.
    Late to bed, early to rise puts big dark circles under your eyes! Author - Me!

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    Tool Steel Hawkeye's Avatar
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    If the stepped stud Tony suggests isn't practical for some reason, it might be worth drilling and tapping a fair bit larger than you need. Then screw in the threaded plug that size, with green Lock-tite. You'll need a mill to ensure straight drilling and tapping, after milling or grinding the plug flush. That way, the new hole isn't too close to the other threads.

    Overall, I like Tony's stepped stud threaded in from the back with a shoulder, if that can be done.
    Mike

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    Crooked hole fix

    I agree that Tony Wells' solution is best if the crooked hole is in the right location. A new piece of raw stock would be needed and you would get some practice turning and threading in your lathe.


    Benny
    Last edited by bcall2043; 02-03-12 at 09:56 PM. Reason: correct spelling

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