Drilling Hole Near Wall Without "Drift"?

I need to drill a clearance hole for a #8 bolt (0.165" dia) through ~ 1

1/4" of 6061-T6 aluminum. The first half is well inside the material, but the last 3/4" needs to be close to one outside wall of the material.

I know that if I go too close to the wall, the material can give a little, and the drill bit will drift towards the outside wall. What I don't know is how close "close" is, and any techniques to minimize the drift. A carbide drill bit will help, but I don't have time to go shopping.

I was thinking 1/16" of an inch of material between the wall of the hole & the outside wall of the block would support the drill reasonably well. If I could go thinner, that would be good. I figure a 1/32" is probably asking for trouble.

I can also clamp the offending wall in a vice when I drill, so the metal will be supported. I don't want to risk drilling into the side of my vice, so I will use a block of something less expensive. I figure steel will provide even more support than additional aluminum, except that the interface won't be perfect.

Another thought was to drill a smaller pilot hole that would be better supported by the material, and then finish with the final clearance drill which should follow the pilot hole. If this is prudent, how small a pilot drill? Bigger would be stiffer & less prone to drift, but thinner keeps it away from the hole (& is therefore less prone to drift).

For example, if I drill a 0.100" inch hole with a 1/16" wall thickness, and I follow up with the #8 clearance drill, I will have ~ 1/32" wall left, but the first hole should be well supported & straight, and then guide the final drill pretty well.

This isn't a super critical application, but it brings up an interesting machining/design problem that I'd love to hear the groups thoughts on.


Doug White

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Doug White
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When I was making RF enclosures I found that 0.020" to 0.030" was a reasonable minimum wall thickness for small tapped holes. 0.015" was usually enough for small drilled holes, if a pro machinist drilled them in a Bridgeport. I never could get that close in a drill press.

Try a parabolic drill bit close to or at the finished size. If it's important I'd practice on scrap and measure the deflection.


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Jim Wilkins

Do you have a mill?

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The issue is deformation of the work when the (large) drill-tip forces get near the sidewall. The 'pilot drill' technique seems unlikely to work out well, unless you use a stepped drill (i.e. a drill with a pilot section) to do the finish cut; after the first hole is in, presumably straight (because it isn't close to the wall), your second drill is ONLY guided by the sidewall forces.

I'd try a short drill (for stiffness) from the thin side, and use suitable lubricant. TiN bits in aluminum are somewhat self- lubricating, but you can't grind a standard one into a shortie. Finish with a full-size twist drill, it clears chips better.

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"ATP*" wrote in news:4b33b48d$0$4987$ snipped-for-privacy@cv.net:

Yes, so I can drill reasonably precisely, with the work clamped & located well.

Doug White

Reply to
Doug White

Clamp work.

Spot drill.

Use a new drill bit. Examine cutting edges and symmetry of drill point.

Make sure your drill chuck doesn't have run out.

Is your work square to spindle?

Don't over feed drill.


-- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller

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