[Kenneth]

I feel like an idiot confessing this but I seem to have forgotten how to drill a hole.

While drilling the rear spar doubler plates, using the spar as a template per the instructions, I somehow ovaled a few of the holes in the spar. The holes in the doubler plates are all round, so it appears the drill bit wandered before starting through the doubler material.

After noticing a few ovaled holes, I made sure I was holding the drill square to the work, tried high speed, low speed, high pressure, low pressure, all with varying results. There aren't an overwhelming number of ovaled holes, and they're only slightly so, so I'm not worried about it, but I'd like to get this under control before continuing on with the project.

Any hints by the gurus of the drill out there?

Thanks!
--Ken

 

[Mel]

Soft materials...high speed, low pressure.
Hard materials...low speed, high pressure.
This is why air drills work great for aluminum but not for steel.
Sharp drills...Hold perpendicular (sp?)
Out-of-round holes most likely caused by either dull drill or cheap drill with off center point.

 

[MTBehnke]

I had the same problem with a couple of the holes in the rear spar as well just a couple of weeks ago. Equally puzzled where I went wrong, but decided it wasn't something I was going to worry about.

 

[FlyerJumper]

Some other causes

We've had a couple of oval holes also. A couple of the things we noticed:

• Drill perpendicular to piece at start, but allowed to tilt slightly during drilling
• Holes not lined up exactly. This can result in one hole being round while the other is oval (This appears to be most common on flange tabs that have only one hole. After the cleco is removed the tab is free to move.)
• After the drill bit "pushes" through the hole, depending on how the (excessive?) pressure is applied to the drill the bit may lurch through the hole an inch or so. Due to this lurching and the operators reaction to stop it the bit may be allowed to tilt for a split second.