wrote in message
...
On Wed, 15 Apr 2015 17:15:10 -0400, "Jim Wilkins"
wrote:

"whit3rd" wrote in message
...
On Wednesday, April 15, 2015 at 6:26:25 AM UTC-7, Joe Gwinn wrote:
In article ,
Ignoramus18077 wrote:

On 2015-04-13, Don Foreman
wrote:
On Sun, 05 Apr 2015 07:47:14 -0500, Ignoramus10114
wrote:

I have an aluminum heatsink that I need to tap , I wanted to
know if I
should use 6-32 or some less coarse thread like 8-32 would be
better?

I go the other way with aluminum -- I use forming taps, not
cutting
taps, and lubricate with soft wax made for the purpose.

But, a heatsink might require a very flat top surface; a forming
tap
will raise
the surface slightly as it deforms the material, so it might not
be
suitable
for this particular application (unless a second operation
follows,
to
flatten the mating surface you've just disturbed).

http://www.katonet.com/article/mindrilltapping.html
"Good machining practice dictates that a countersink be provided at
the top of the tapped hole. Countersinking is recommended before
tapping to create a good starting thread."

Easier said than done, though.

-jsw

easier said than done? How? Hit the top of the hole with a
countersink, or a drill a few sizes bigger than the hole if you
can't
afford (or can't find ) a countersink. Best done in the drill press,
but for crying out loud it's "just" aluminum - you can do it with
the
drill bit in your bare hand if you have to.

My Clausing 8525 mill lacks a DRO to easily repeat a pattern so
countersinking after drilling is a tool change at each location that
loses precise depth on blind holes, since the chuck jaws move down as
they tighten. I don't have countersinks with shanks as long as drills,
to cut before hitting the depth stop, and there isn't enough quill
travel to allow hand countersinking or clearance to swap chucks and
arbors.

Maybe I should buy a set of intermediate length center drills to use
as countersinks and spotting bits. I have only the long ones. Burrs on
steel deflected larger drill bits.

I do swap the drill, countersink and tap on through holes.
Hand-tightening the chuck is enough to start a tap straight by pulling
on the drive belt.

-jsw