I got a handful of microstop countersink cages off of ebay a while back.
The ones that had cutters were all 100 degree style for aircraft rivets &
screws. I want to get a set of cutters for 82 degree screws in standard
machine screw sizes, basically #6, #8, #10 & 1/4".

The question is: what sized pilots should I get? I tyically use the
"close fit" clearance drill sizes when I make stuff:

#6 0.138" OD (close fit clearance drill is #27 = 0.144")
#8 0.164" OD (close fit clearance drill is #18 = 0.1695")
#10 0.190" OD (close fit clearance drill is #9 = 0.196")
1/4" 0.250" OD (close fit clearance drill is #F = 0.257")

There are a bunch of outfits that sell these. One that was recommended
was: http://www.californiaaircrafttool.com/21.htm

In looking at the standard pilot sizes, the only thing that makes vague
sense is for 1/4". The pilots are 0.250" in diameter. I would think
that going up to 0.253" or so would center things better in the hole,
while still leaving clearance for the pilot, but that is small poptatoes.
I suspect lots of folks use a 1/4" drill for 1/4" hardware, in which
case the 0.250" pilot still works.

For a #6 screw, it looks like they expect you to use a #21 drill sized
pilot, which is 0.159", or almost 15 thou bigger than a close fit
clearance drill! #21 is several thou smaller than a #8 screw, so it
doesn't make sense for that either. The next size up is way too big for a
#8, and so on.

I suspect the "standard" pilots are for rivets or something other than
common screw sizes. The folks who make the cutters will grind them with
custom pilot diameters, so it looks like that is what I will do. An
alternative is to get a common cutter size, but with replaceable pilots.
That way if I wear out a cutter, I can get one sparethat will work for
all sizes.

If I'm getting custom pilots, what size should I get? I'm thinking 4
thou less than the close fit clearance drill size. That should be easy
to pop in the holes, but still keep things very well centered.

Thoughts? Comments?

Thanks!

Doug White

On 2008-12-31, Doug White wrote:
I got a handful of microstop countersink cages off of ebay a while back.
The ones that had cutters were all 100 degree style for aircraft rivets &
screws. I want to get a set of cutters for 82 degree screws in standard
machine screw sizes, basically #6, #8, #10 & 1/4".

The question is: what sized pilots should I get? I tyically use the
"close fit" clearance drill sizes when I make stuff:

#6 0.138" OD (close fit clearance drill is #27 = 0.144")
#8 0.164" OD (close fit clearance drill is #18 = 0.1695")
#10 0.190" OD (close fit clearance drill is #9 = 0.196")
1/4" 0.250" OD (close fit clearance drill is #F = 0.257")

There are a bunch of outfits that sell these. One that was recommended
was: http://www.californiaaircrafttool.com/21.htm

What I would suggest for you is to get ones with interchangeable
pilots, and you can make your own pilots to the size which you need at
the moment. I know that quite a few of my countersinks for the
Microstop cages do have that -- a setscrew which holds a reduced
diameter extension behind the pilot.

Enjoy,
DoN.

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