September - October - The Recreational Aircraft Association

Sheet Metal Hole Cutting
By Don Sinclair / We have all used standard drill bits for creating holes
in our sheet metal, but as the holes get larger, standard drill bits are
not always the best choice. When cutting larger holes there are few
items that become more important than when making smaller holes.
These considerations include creating a round hole, keeping the centre
where wanted, having the finished hole the diameter we intended,
preventing the material from being distorted in the work area, and of
course safety, both of the operator and the surroundings be it other
people, tools, or aircraft structure.
One of the least expensive ways to create large holes in sheet aluminum
is the single blade fly cutter. There is a reason this is the least
expensive (under $20) way to cut holes of various sizes, and while
it can create a good quality hole up to several inches in diameter,
extreme care must be taken not to injure the operator or damage the
work. The material needs to be securely clamped and the tool used in
a very slow turning drill press to obtain good results. Attempts to use
a fly cutter without taking these precautions or using it in a hand drill
can easily become disastrous.
A slightly safer approach is to use a single size bi-metal hole saw
(under $10 for each size). This type of tool can create an acceptable
hole, but again, there is a reason they are relatively inexpensive. Hole
saws are easily dulled after a small number of holes, often resulting in
the bit wandering and therefore leaving enlarged or oval holes.
A better quality and significantly safer result from the first two mentioned
methods can be obtained using a step drill ($15 each to $75 for
a set of good quality bits). Like any type of tool, there are a variety of
qualities out there, but versions up to about an inch are readily available.
Once you have used this kind of tool, you will not want to go back
to a fly cutter or hole saw when making holes of up to about an inch in
diameter in sheet metal.
A superior tool that doesn’t require much force applied to the sheet
metal, thus virtually eliminating the chance of distorting the work area,
especially on very thin material, and one that produces a high quality
hole with very little burring, are ‘Annular Cutters’. One manufacturer
of such annular cutters is Hougen (www.hougen.com, picture 1) and
is available from a number of sources including the homebuilder tool
supplier Avery Tools in Texas (www.averytools.com). Product names
from Hougen include Rotabroach, RotaCut, and Holcutters. These
cutters safely create accurate and clean holes in sheet metal up to a
couple of inches in diameter with little chance of hurtling your work
across the shop. Material thickness of up to 1/8” or ¼” can be cut
depending on the tool diameter. The smaller sizes (up to ¾” in diam-
Top Down: Hougen's Annular cutter,
Rotacut kit; Holcutter kit;
and example of a hole cut by the Rotacut kit.
continues
September - October 2007 Recreational Flyer 29