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I'm trying to put a 16mm hole through the centre of some 35x35x2.0mm square hollow section (SHS) gal steel. The holes need to be fairly accurate, because they're going to be holding an axle that needs to be straight. I put some 3mm pilot holes through both layers with a drill press, but the press table flexed, and now my holes on the back side are out by about 1-2mm. That's really gonna screw up the axle.

I can't easily re-drill the holes, because the correct centre is inside the existing pilot hole. Is there a simple way to fix this mistake, and get a hole centered in the correct position?

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Get some flat bar and drill the hole size that you require. clamp the flat bar into position over the existing hole (in the correct position). Then drill or file into the flat bar hole (acting as a guide/template) to your desired position.

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You should be able to "pull" the hole back to centred as you step up the drill sizes. Just aim for (or with small drills towards) the correct centre and each step up in size will get closer and closer to being correctly centred.

  • This will work if his press table is now stable (not flexing). Otherwise he will have the same or worse problem. – bib Oct 9 '14 at 19:49
  • This seems like a good way to break drill bits. – naught101 Oct 9 '14 at 23:00
  • @naught101 As long as you go slowly and let the drill do the work, rather than trying to force it, it works fine. – John Oct 10 '14 at 7:32
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I have used (carefully - they are brittle - wear saftey glasses and work slowly) solid carbide diamond-pattern "tile-cutting-bits" (for Roto-zip®, Dremel® or similar tools) as a side-grinding tool in a drill to correct holes in metal. I generally lubricate them with oil. Don't waste a diamond bit on this (diamond, when grinding steel, unless very carefully controlled often just gets destroyed - tungsten carbide will be more economical and work better in nearly all cases.)

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the only way I think you can be truly accurate is to tack-weld or rivet a new piece layer of flat steel over the existing pilot hole. Then drill a new pilot hole, then drill final size. Then remove the extra steel layer.

The extra layer of steel will basically act as a drill bit guide to keep your bit centered in your new pilot hole.

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Tap the existing hole if it is not already tapped. Clean the hole and a bolt or screw. Use JB weld and screw in the bolt tightly. After all is set up, cut the bolt off flush. Then center punch the new location and drill the new hole. Just don't get things too hot when you cut the bolt off or you the epoxy may fail. You want the bolt plug to stay stationary and not spin while you are drilling or tapping.

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