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I bought some 1" smurf tube so that I could run coax, ethernet, and speaker cable to various low voltage outlets in my house. I used a 1-1/4" speedbor bit to make a hole in the floor from my crawl space but discovered that the smurf tube was 1" inner diameter and 1-1/4" outer diameter. Oops. It turns out it is very difficult if not impossible to force the flex tube through the approximately 6" deep hole.

I tried using a file to widen the hole but it had little effect. Should I try widening the hole with a 1-3/8" auger bit, a 1-3/8" spade bit, a sanding drum bit, or something else? There's not a lot of room in the crawl space so I'm hoping to hit upon the solution the next time I go under.

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Somewhat related: diy.stackexchange.com/q/10391/2196 –  BMitch Jul 22 '12 at 0:51
    
I tried preparing a scrap 2x4" as @BMitch suggested but found it difficult. The bit would catch and kick back on the drill. It worked better when I repeatedly plunged the drill through the hole than when I applied constant down pressure. On the real thing, I ended up using a rasp bit to eat away at the hole then used the 1 3/8" bit to do the rest. Once I got the bit in the hole it went through quickly. I don't know why it was easier to drill through the floor than the scrap 2x4". I was able to push the tube through, but then ran into a strange diagonal support beam. But that's another story. –  Ryan Jul 26 '12 at 19:46
    
The bit catching and kicking back indicates you weren't lined up, and that's a hard thing to do in a crawl space. Increasing the size of the hole in the scrap wood, or even making it with a slightly larger bit, may have helped. –  BMitch Jul 26 '12 at 19:54

4 Answers 4

up vote 9 down vote accepted

Now you've learned the hard way, the same way the rest of us learned, why they say "measure twice."

A spade bit may work for this trick, but an auger bit may be easier to control. First, use the bit to cut a hole in center of some scrap wood. A piece of plywood that's 4"x6" would work well, and a 2x4 that's 6-8" long would also work. You need a few inches on each side of the hole, and enough so that it doesn't shatter apart when the drill bit is vibrating inside, but thickness isn't that important. Since you're working in a crawl space, start a few screws on each side of the hole that you drilled.

In the crawl space, line up the hole in the scrap wood with your existing hole and screw it into place so it doesn't move. This wood is now your guide to keep the drill bit from running all over the place. Put your drill bit inside this guide, make sure you're aligned, and drill your larger hole.

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exactly what I would do. –  shirlock homes Jul 22 '12 at 10:38
    
Chosen as the answer since it sounds like it would work and is also the easiest. :) Thanks! –  Ryan Jul 22 '12 at 16:59

It's hard to widen a wide hole by re-drilling because the bit will not stay centered, it will bounce around and not really cut. The easiest thing would be to plug the hole and drill a proper sized hole nearby. A sanding drum could work, but may be very time consuming. A fostner bit might work because the perimeter cutting edge would help keep it centered. It would still be hard to get it started, and buying one long enough could be fairly expensive.

You could use a sawzall to cut a bunch of kerfs around the hole perimeter, then cut a sort of steep starter funnel at the bottom. Then use the proper sized drill bit to chip out the remaining wood. Feed the bit very carefully, or it could wedge in the kerfs and violently wrench the drill in your hands, possibly causing injury.

Perhaps the best approach is to glue an appropriate sized dowel in the hole, then re-bore the hole once the glue is dry. I think carpenter's glue would less likely gum things up than other glue types. You'll need a very sharp drill bit to cut through the end grain of the dowel.

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I would try a 1 1/2 inch (or 1 3/4 inch) hole saw. While the standard approach is to use the center bit to start and guide the saw, if you carefully center the bit and go very slowly around the perimeter of the existing hole, you should be able to get a grip on the face of the boards. The larger saw will probably be easier to use if you can spare the cutout.

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Using a spade bit would be very difficult as others have stated unless you fill the hole. Auger bits are self feeding and need something to bite into. And sanding a six inch deep hole .... well I just can't imagine that being easy. A hole saw would work but you could only drill for an inch or two and then chisel out the material that you cut and then continue with the hole saw. I liked the idea of drilling a new hole if you have room. If not then my second choice is to fill the hole and re-drill it. I would take a scrap piece of wood and drill a sample hole the same size as the first hole, Then cut a square piece of wood that would fit tight into the sample block (this is so you don't have to make more than one trip under the house). Now you can take the new square wood under the house and drive it into the existing hole. A little glue might help but is not needed. Now re-drill the hole.

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