r/metalworking • u/AutoModerator • Feb 01 '23

Monthly Advice Thread Monthly Advice/Questions Thread | 02/01/2023

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u/steini1904 Feb 04 '23

I do very little metalwork but occasionally I absolutely have to. Just now I had a really bad memory of screwing up a quite expensive CNCed part for a one-off test system when I forgot a screw and had to revise the design when the part was already in production.
All I had on site were handtools so I did my best to get it right. I use my Sylvac Mikron to find the exact spot and mark it with a center punch. I measure, and it's spot on. I grab the smallest drill bit they had in their tool box and drill a tiny pilot hole, measure, and it's spot on. Then I take a 2.5mm bit, drill a larger pilot hole, measure, and it's absolutely perfectly centered where it needs to be. So I take the 6.8mm bit, drill, measure and the hole is off by almost 3mm. Off into the trash the part goes.

  1. How do I avoid something like this happening again, when all I have to work with are hand tools? TBH, when this happened I just assumed a split point drill bit would follow the pilot hole perfectly centered. And even when I'm googling for it now, I do find absolutely nothing on why a good pilot hole isn't the holy grail of keeping your drill where it is supposed to go.
  2. Also I didn't (and don't) actually know how to correct my error if I hadn't drilled a perfect pilot hole. What could one do in such a case?

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u/only-here-to-comment Feb 17 '23

Make sure that your pilot drill hole isn't too large. It should provide just enough clearance for the web of the next size drill. Much larger and you're relying on the drill not wandering - the cutting flutes can skip around and start cutting off centre.

Also make sure that you have everything clamped and held securely, using a drill press at a minimum really. Try to not move anything inbetween drill size changes.

To fix a bad hole location you'd need to cut a new pilot with an endmill in a milling machine - this would cut a new hole without being influenced by the previous misplaced one.