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  • How Do I...?

    I want to drill a hole through a piece of 4" round stock at top dead center of the piece. My edge finder will not work on a piece of larger round stock so I cannot use it in this case. What do I use to find TDC?

  • #2
    Pat, I suspect you've gotten no response due to the fact that the ordering on the posts has gotten all fubar'ed since 26 May.

    But one way to find the TDC (I assume we're talking about a vertical mill here) is to stick a thin piece of paper (e.g. cigarette paper) to the side of the part, then bring a rotating endmill (or drill) up against it slowly until contact is made. Then lift the drill/EM up and move over 1/2 the dia of drill/EM Plus 1/2 the dia of the workpiece Plus the thickness of the paper.
    That should put you on center ...or pur't near.


    • #3
      If it's not immediately obvious, you'd stick the paper on the workpiece with some oil or something like that.

      Another way, if you're holding it in a vise, would be to indicate the vise jaw as a reference, then move from there. One problem with this technique is that the feedscrew backlash ends up going the wrong way, but that's correctable. (If that makes no sense, it may if you actually try to do it.)

      Or get a different edgefinder. Starrett makes a "wiggler" edgefinder with a selection of interchangeable points: an 0.100" disk, a 0.250" ball, and some others. They're on extended shafts, so they can get at things the more common edgefinder can't.
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      • #4
        A very simple method is to install an end mill, and with the machine running, just raise the table a few thousandths at a time and move the part under the mill until it just kisses the part. It will leave a line at contact that you can pick the center off of.
        Jim H.