Preventing Drill Bit Tearout on Irregular Pieces

[Steve H Graham]

Yesterday and today, I put sling studs on a Marlin Model 60 .22 rifle. The front stud was no problem, although it's the one that usually bugs people. The rear one, which should have been easy, gave me trouble.

The instructions for the stud, which had a wood screw as its base, said to use two drill bits: 5/32" and 7/32". You make a 1"-deep hole with the 5/32" bit, and you counterbore the first 1/4" with the 7/32" bit.

I made the 5/32" hole. Knowing the stock's wood loved to chip and tear, and that it didn't like countersinks, I took a round Dremel-type stone and opened up the top of the hole to keep it from tearing when the 7/32" bit hit it. I didn't know what else to do.

I still got a little tearout. I am highly annoyed. It was only a tiny bit, and I can make it go away with filler and whatever, but I thought I took extraordinary measures to prevent it, and it still happened.

I know I could have avoided this by doing the entire counterbore with the stone, but what's the best way to keep irregularly-shaped pieces of wood from tearing when a drill bit hits them?05 15 18 marlin 60 sling stud rear installed with chipping visible small.jpg

[michael langman]

Counterbores are made with a pilot to guide them so they do not run out. Using a drill for a counterbore is not good practice, especially on an irregular surface.

[Mike Cutler]

It's after the fact, and definitely some arm chair quarterbacking, but it seems as if a step drill might have been a better solution.
The WL Fuller SD1208 would probably do this. It's about 8/1000ths. smaller in diameter than the 5/32nd pilot, but that could easily been opened up a bit more.
Whenever I have to drill holes in already finished material, I tape the area really well with blue painters tape and fiberglass packing tape. The size drill you're using would have been at about 3000rpm, so it would have left a clean hole in the tape. Definitely a soft vise and drill press operation.

[Steve H Graham]

I was afraid a step drill would be even worse!

[Peter Christensen]

I would have drilled with a brad point 7/32 and then finished with the 5/32 regular bit. The brad point would have started clean and the regular would have centred in the point made by the brad point. Lee Valley sell brad points in 64th inch increments.

[Steve H Graham]

Do brad points generally tear less?

[Robert Hayward]

I would have drilled exactly as Peter outlined. My brad points make an extremely clean entry hole. When I need a precise clean entry my brad points are my go to bits.

[Jim Becker]

I would have drilled with a brad point 7/32 and then finished with the 5/32 regular bit. The brad point would have started clean and the regular would have centred in the point made by the brad point. Lee Valley sell brad points in 64th inch increments.

I would also have drilled the larger hole first.

[Brian Behrens]

What about drilling the larger hole first with a good forstner bit. This would leave an indent in the bottom of the hole to continue on with your smaller bit. I would use a drill press with the stock held in a vice.

[Peter Christensen]

The Forster bit would be great if they came that small.

[Tim Bueler]

I would have drilled with a brad point 7/32 and then finished with the 5/32 regular bit. The brad point would have started clean and the regular would have centred in the point made by the brad point. Lee Valley sell brad points in 64th inch increments.

+1 on the larger hole w/ brad point first.

[Steve H Graham]

What about going through a series of bits instead of jumping from a small bit to a large one?

[andy bessette]

Twist drills are entirely inappropriate. Brad points and Forstners make a clean hole on flat stock, but possibly not on a shaped gun stock. A Fuller taper drill does make a clean hole in a shaped workpiece.

[Randy Viellenave]

Peter,
I have had some success using a split point bit in reverse at high speed. Burn your way past the surface a little, then flip it forward and drill. Drill a tiny pilot hole to keep it centered first. If the wood is not mega hard, it is more likely to work.