7/25/2019 _How to Drill a Round Hole
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How to drill a round holeFrom the workbench
by Frank Hofmann,
COPA VP/EAA Technical Counsellor
If you are going to install a one-quarter inch bolt somewhere,
the last thing you should do is grab
the nearest one-quarter inch drill bit. There is much more to
the process.
The strength and durability of a bolted joint is partially
dependent on the closeness of fit between
bolt and hole; the closer the match the stronger the joint.
A loose joint will allow relative motion wear and fatigue!, will
allow moisture corrosion!, willcause hardening of the bearing
surface eventual crac"ing!, will transmit loads unevenly to the
material it is supposed to hold stress crac"s! and will allow
the bolt to cant sideways in a single
shear application crac"ing under the head of the bolt or
thinning a softer material, wea"ening it!.
#ow long that new airplane will stay $new$ is somewhat dependent
on how good the joints are
by which it is held together.
The craftsman has two problems% one is that A& bolts are
nominally undersi'e by about .(()-
inch; the other problem is that a drill bit creates a slightly
triangular hole. Aspiring builders
should chec" both of these facts out for themselves. *easure a
typical A& bolt and then insertthat bolt into a hole drilled
into a scrap of sheet aluminum. +areful inspection will show
light
coming through on three sides around the bolt.
#ow can this poor fit be avoided and a stronger joint be
created
erhaps you should chec" the different "inds of rill Inde/es you
can buy. 0tandard is the
fractional set - usually from 12)3-inch to 123-inch in 12)3-inch
increments. Then there arenumbered sets - from no. 1 - 4( .335-inch
- .(6(-inch!. &umbered bits are .(()-inch oversi'e
relative to their nominal fractional equivalent.
7or e/ample, a no. )( bit is .135-inch and if you use it the
hole will be .(()-inch larger than the125-inch .138-inch! it is
meant to replace. This bit is used when you use 125-inch rivets
for
e/ample, where you want a bit of clearance so the rivet can be
installed easily and where the
rivet will e/pand to ta"e up the slac". This clearance is not a
good idea for bolted joints. 9astlythere are the lettered sets,
from A - : .3)6-inch - .61)- inch!. 9ettered bits are .(()-inch
undersi'e relative to their nearest fractional counterparts.
Thus a $$ bit is .36-inch, or .(()-inch smaller than the 126inch
.38(-inch! fractional bit.
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?. 0tep rill. 0tep drilling is the process whereby you wor" up
to the final si'e of bit for that
hole. 7or e/ample, if you want a 126-inch hole, drill through
with a )2)3-inch bit, then a 2)3-
inch, and finally the 126-inch bit. ?etter drill inde/es have
bits in 1246-inch increments but therewill be many bits in a
123-inch set that you might never use. The idea is to come up to
the final
dimension in small increments, with an end result which will be
well worth the e/tra effort. This
is a low cost method of obtaining good round holes.
+. @/pansion or Taper eam. A tapered reamer is used much li"e a
tap but its gradual taperallows you to open up a hole gradually and
enough to fit your bolt e/actly. 7or e/ample, a 126-
inch taper reamer goes from 2)3-inch to 126-inch over a length
of about three inches, giving it a
very gradual change in diameter. =ith a bit of practice you will
be able to judge just how far youneed to wor" the reamer into the
hole to get a tight fit. Taper reamers can be obtained from
tool
supply houses.
A fancier version of a taper reamer is an e/pansion reamer. The
cutting edges of this tool can be
adjusted so that you can create a clean, parallel-faced circular
hole to fit a particular bolt e/actly.
