Old Machinist Trick

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Lawless

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For drilling holes in sheet steel. Pretty cool trick i intend to try on my drill press.

[video=youtube;4HOxfZKIZQk]https://www.youtube.com/watch?v=4HOxfZKIZQk[/video]
 
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Cool trick and will have to give it a try it but now it has me curious how it works exactly to prevent holes from being torn out or oblong.
 
I've clamped wood around the hole (as close as possible) and drilled holes in shim stock. Holes looked good
 
Jp8819;n65947 said:
Cool trick and will have to give it a try it but now it has me curious how it works exactly to prevent holes from being torn out or oblong.
Click to expand...

I suspect the cloth acts as a kind of self-centering grommet, keeping the bit centered as it makes its final penetrating cut through the metal.

If you'll go back and look at the point where he was making his first hole, he backed the bit out when he was about halfway through the metal. You'll note at this time that the hole was round.

What likely happened as he continued drilling, when the tip bit finally cut through the metal the cutting edges of the side of the bit were now slicing into the edges of the hole. The bit "flexed" a small amount as this happened, cutting deeper into the metal as it did. Either that, or the bit "tore" a small bit of the metal loose as the sides made their final cut. Or a bit of both. This produced the oblong hole.

You don't see this in thicker pieces of metal because the hole itself acts as a centering guide for the bit, keeping the bit straight as it makes its final cut through the metal.
Likewise, you don't see this happening if you first clamp the thin piece between two other pieces...because the piece above helps keep the bit shaft from wandering while the piece below helps keep the bit centered as the tip cuts into it.
 
Thin sheet flexes. When the drill is being pushed through, this causes the sheet to flex. The flex causes the side edges of the bit to dig into the sides of the hole making it out of round. The cloth keeps the drill shaft edges from digging in.
 
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