Countersunk
bolts are
bolts that are recessed with their heads flush to the surface. This
is done for several reasons, the most common being to gain clearance or
for aerodynamics. You have probably seen countersunk
fasteners in
wood decking or the runners on drawers. The steps are basically the
same no
matter what material you are assembling.
Whether it's for a HID mounting bracket or for an end-plate, at some
point you will probably need to countersink/spotface a fastener. Using
these steps, you can produce clean professional results. No
matter how tempting it is, do not partially drill a hole with a larger
bit to countersink a bolt head. Take the time and obtain the
correct countersink bit. When purchasing a bit you need to
know
two numbers. The diameter of the fasteners you are working
with
and how many degrees the bolt head is. 82°, 90°, and 100°
are all common. NAS fasteners used in aviation are 100
degrees
while most common machine screws are 90 degrees. Make sure
you
purchase the correct taper. A microstop cage like the one
shown
below is not needed, but it makes production faster and the results
cleaner.
Start by
drilling the
correct sized hole in your stock. If you are working with
metal
use a little cutting oil to speed drilling and preserve your bits.
For most thicknesses of stainless steel and inconel keep the
drill speeds under 300rpm. For other steels 500-800rpm, and
aluminum around 1000rpm.
Now comes
the
countersinking. If you are using a microstop pre-set the
depth so
that when tightened the bolt head is flush with the stock.
Drilling a few practice holes will assure the final piece is
correctly drilled. The countersink bit will cut the correct
taper
and debur the stock for you.
Below you
can see the
results. This photo is under magnification and you can see
there
was some slight chattering. More oil will help prevent this.
If you still experience low speed chatter, reface the hole at
a
higher speed. If you have a high speed chatter, reface at a
lower
speed. If the above methods do not work you may need to try a
different countersink. A tulip or a high speed seven flute
bit
would be an option for soft metals.
Below is
a close up of
the installed fastener. If correctly installed the head of
the
bolt will sit completely flush. If it does not, you need to
reset
your stop and reface the hole. If it is recessed, you set the
stop
too deep.
In this
guide I used Hi-Lok titanium bolts which have no grip on their heads.
They are tightened from the bottom with a hex socket and
wrench as shown below. These bolts produce a very smooth
unique finish. In addition to looks, they also offer the
weight savings of titanium. When used correctly on brakes,
rims, or other rotating assemblies, the pounds saved translate into
horsepower.
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