That last 1/8"

April 22, 2013 7:47 AM

How do you drill a 3/8" hole into a 1/2" hole having the same center without a drill press?

A friend is installing an engine lift on his boat and I've loaned him some tools for the job and a hand with some of the lifting. Yesterday he drilled two 3/8" holes at particular, precise locations in the hull of the boat. Now it seems as though those holes need to be 1/2" in diameter. The boat is a fiberglass hull and the holes go through both wood and fiberglass. When we put the 1/2" bit to the hole, the bit grips the wood and causes the drill to wobble such that we're not confident we can drill 1/2" holes on center. We only get one go at this and therefore aren't comfortable pushing through in the hope that it will work. The holes are on the side of the boat.

What can we do? How would you drill these holes out?

posted by gauche to Home & Garden (16 answers total)

Either a step drill or an "Oops Arbor" with two hole saws, one 3/8" and one 1/2".
posted by mrrisotto at 7:51 AM on April 22, 2013

I would use a unibit. It will keep the same center for you and increase the hole to 1/2".
posted by wile e at 7:52 AM on April 22, 2013 [1 favorite]

Best answer: Are these thru holes? Use a 1/2" step bit.

If they aren't thru holes you can grind a step bit down till you just have the two steps you need though you still need a certian amount of depth.
posted by Mitheral at 7:55 AM on April 22, 2013

The other ideas posted so far are probably better, but you could also fill the holes with epoxy and drill them out again.
posted by tylerkaraszewski at 7:55 AM on April 22, 2013 [2 favorites]

Response by poster: They are through-holes going through about 2" of material. I've never used a step-bit. It looks to me like I'd have to get one that maxes out at 1/2" at the widest point so I could push it all the way through. Is that right?
posted by gauche at 8:00 AM on April 22, 2013

Best answer: Yeah, that's right. Might have to drill through from both sides to get the hole right unless the bit has a long enough shank.
posted by exogenous at 8:03 AM on April 22, 2013

Response by poster: Yeah, that was my next concern but I think I can work with it. Thanks everybody!
posted by gauche at 8:05 AM on April 22, 2013

Best answer: The step drill won't take you the full 2" thickness of the material, but if you get a good solid 1/2" hole/indentation started using the step drill, you should be able to use your 1/2" drill in the 3/8" hole, and the existing 1/2" indent will keep your drill bit centered.
posted by aimedwander at 8:23 AM on April 22, 2013 [4 favorites]

aimedwander has it right. The step drill will get you from a 3/8" hole to starting a concentric 1/2" hole, and then your normal 1/2" bit should follow the hole with no problem.
posted by Bruce H. at 8:33 AM on April 22, 2013 [1 favorite]

You could drill a 1/2" hole in a block of wood (with a drillpress) and then clamp it on top of the 3/8", this would keep the 1/2" drill from wobbling. The trick would be to clamp it exactly over the current hole, but you should be able to look inside and align it.
posted by 445supermag at 8:33 AM on April 22, 2013 [2 favorites]

Best answer: Also, you can get step drills with longer or shorter steps. You want the one with the longer steps for this.
posted by Bruce H. at 8:39 AM on April 22, 2013

You use a transfer punch. They come in indexes like drill bits, and are like a smooth rod with a nipple on the end that is obviously in the center of the hole. Fixture as appropriate after you mark your center and drill away.
posted by FauxScot at 10:06 AM on April 22, 2013

Transfer punches are used to mark the center of a hole from an existing hole. gauche's friend already has a 3/8ths hole all the way through both pieces so there isn't anything for the punch to mark.
posted by Mitheral at 10:48 AM on April 22, 2013 [1 favorite]

Speaking from similar experience. I stongly recommend you use some kind of template to accurately draw (or scribe) the outline of the bigger holes, centred around the old ones (on the inside and outside) Then use a rattail file to widen the holes from each side.

It's just two holes in relatively soft material and if you try to use a power tool, the chances are HUGE that your bit will wander to some degree.
posted by bonobothegreat at 12:31 PM on April 22, 2013 [1 favorite]

Response by poster: We did this! Or at least, my friend did with a step-bit and it worked out great. Thank you all for your help here.
posted by gauche at 9:49 AM on May 9, 2013

And thank you for the followup.
posted by Bruce H. at 12:27 AM on May 24, 2013