On 2015-04-16, Jim Wilkins wrote:
wrote in message
...
On Wed, 15 Apr 2015 17:15:10 -0400, "Jim Wilkins"
wrote:
[ ... ]
"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.
Have you ever gotten a Microstop countersinking cage? The shaft
is spring loaded in the cage, with provisions to set the depth by 0.001"
intervals, and a nice bearing between the shaft and the cage. You can
find a lot of them on eBay by searching just on "Microstop". Here is an
example:
ebay # 361220396427
or:
http://www.ebay.com/itm/Nylon-Tip-Microstop-Countersink-Kit-NEW-/361220396427?pt=LH_DefaultDomain_0&hash=item541a69 d58b
I prefer the auction number search, personally. Less to cut-and-paste. :-)
Anyway, the countersinks screw into the shaft, and come with
various size pilots. (Hmm ... this one says bronze bearings, but the
ones which I have feel like ball bearings. :-)
Anyway -- it looks like at least one of those countersinks will
fit your tap hole for the 6-32.
So -- drill and tap first, then come back with this in the drill
press chuck, use the pilot to align it, and countersink to the depth
which you preset.
There are some with a smaller footpiece if you need to get close
to flanges on the heat sink.
These are normally used for countersinking sheet metal for
aircraft use to accept countersunk rivets, but work nicely for many
other things. The #30 pilot is pretty close to the tap drill size for a
6-32. It should work with roll tapped aluminum in 6-32 as well.
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.
The piloted MicroStop countersinks are less likely to deflect,
even if you are using a hand-held drill motor instead of a drill press.
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.
Ever look into the drill/tap bits? Useless for blind holes, but
nice for through holes -- especially if you have a tapping head for your
drill press. I've even used some down in the 4-40 size range. Cutting
lube for aluminum was WD-40, though others would work well too. It just
happens to be one of the few things which WD-40 works well for, unlike
the things they *claim* it to be good for. :-)
Enjoy,
DoN.
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