Re: Dumb question on the drilling of holes

--- In CAD_CAM_EDM_DRO@y..., Marv Frankel <dcdziner@p...> wrote:

> Tim,
> I'm pretty familiar with reaming holes for proper fit, but

the holes

> I'm talking about, are sometimes so misshapen, they almost look

triangular.

> They're 1/4" holes, and I've even tried cutting them through with

an end

> mill, and that wasn't great either.

Because the machine frames that we can afford to use at home are
sometimes not real stiff and solid, it is possible to get a situation
in which the tool and workpiece interact in such a manner that the
cutting forces exceed the ability of the spindle and or quill to
resist. Things start to flex. This is a major concern when
designing small machines that can use large tools. What I have seen
happen is that when drilling thin metal sheet, the drill point will
cut through the material, before the full diameter starts to cut.
This results in a chisel shaped tool cutting with a sort of helical
path and the cutting forces are not balanced. Careless hand
sharpening of drills makes it worse as the point may not be
symmetrical and centered.

If you don't have a rock solid spindle and frame, you can improve the
situation by drilling a small pre-drill or pilot hole through. It
should be at least as large as the chisel point on your larger
drill. This way the second drill, to size, will have even, balanced
cutting forces on both flutes and a little offcenter sharpen will not
have as much impact. Sometimes it is useful to poke an end mill
through, to straighten and position the hole before the final size
drill is used.

Best Regards, Fred Smith- IMService

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