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Tapping Chinese Cast Iron for DRO-- help :)

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  • Tapping Chinese Cast Iron for DRO-- help :)

    Hey Folks thanks for all the tapping help on my previous post. Now I have enough confidence to try and mount my DRO on my sq. column mill.

    I was able to tap the column 1/4-20 and mount the support arm for the DRO using an aluminum block I made. It is similiar to this picture. I used a # 7 drill and red thread locker. It seems to have come out OK.

    The next one is the y-axis scale and head I plan on mounting it just like this picture. The ones in the base should be like the column. IE if I screw up I will just put a bolt straight through with a nut.

    The thing I am worried about are the blind hole taps in the saddle.
    Is 75% the right thread depth for Cast Iron ? Any tips for the drilling the saddle looks like it will be a pain to get it straight and it's blind hole.

    Any suggestions greatly appreciated.


  • #2
    Originally posted by kben77
    The thing I am worried about are the blind hole taps in the saddle. Is 75% the right thread depth for Cast Iron ?
    Yes, you want as much thread engagement as you can (with a coarse thread), because cast iron is so brittle.

    It's straight-forward drilling/tapping the blind holes. I rough-out the hole with a cheap carbide masonry bit that I've resharpened with positive rake (just stick it in a Drill Doctor). It taps really easy -- the cast iron crumbles off, so make sure you back off a quarter turn or so each full turn of the tap to clear the little cast iron chunks.
    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."


    • #3
      GOD made helicoil inserts for putting a permanent thread into crap metal.

      You insert the larger tap, turn the stainless thread in with locktite, allow to dry, then you can insert a bolt and actually torque it down.

      If you don't believe in divine assistance in holding threads in, you have not tapped crap metal much. I swear, I am tempted to put them into the white plastic stuff I build at times.
      Excuse me, I farted.


      • #4
        If you are worried about getting it square, you could make a guide block for the drill. Say, a piece of 3/4" scrap stock at least 2" X 6" and drill it square with the same drill you will be using, in the drill press or mill. Then hold it against the mill's base and use it to align the drill. I would suggest a smaller locating hole drilled to a shallow depth, perhaps 1/16" or 1/8" first to help find the location when you use the guide block.

        You may also want to use a guide block for the tapping operation. Here's a picture of some I use:

        They are made from mild steel and are drilled to match the tap's diameters. Two of these are for English sized taps and the other is for metrics. I find they are very helpful for getting the taps started square. After a few threads I usually remove them.

        Don't worry about a slight misalignment. If it is only a few degrees off, it will work just fine for attaching a bracket.

        And don't worry about the "blind" nature of the hole. The casting is probably about 1/4" to 1/2" thick there. You could probably still drill through and use a nut, but it would be a PITA to tighten as the mill would have to be tilted. But with a little care and patience, I am sure you can drill and tap it.
        Paul A.

        Make it fit.
        You can't win and there is a penalty for trying!


        • #5
          The blind tap holes sound like the perfect time to try the trick of completely filling tap drill holes with crisco and then when you run the tap in the chips will get pushed out the tap flutes.