April 1963 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
It is understandable if,
based on this article's title, "Holes and the Service Technician," you thought
maybe it had to do with semiconductors. Silicon was beginning to overtake
germanium as the substrate of choice it appeared in a 1963 issue of
Radio−Electronics magazine. Electron conduction seemed intuitive to most
people involved in electronics; however, the concept of hole conduction caused a
lot of head scratching. But, I digress. This article discusses how to create
various types of holes in metal. It might seem like a no-brainer task, if you
have ever needed to make precisely shaped and dimensioned holes in metal, you
know it is not always such a simple task - especially in soft sheet metal.
Achieving a truly round hole - especially of large diameter - in an aluminum
chassis requires securely clamping the work piece to the table and using a
sturdy drill press. Otherwise, you almost always get an oblong hole. A machinist
at Westinghouse showed me one day back in the 1980's how he would get the
chassis secured and drill press positioned, then place a couple layers of paper
towel on the metal before lowering the drill bit onto the chassis. It helped
fill the space between drill bit flutes to prevent it from "walking" before both
sides of the bit had a bite on the metal. It works like magic, even when using a
hand drill. To this day I still do that. As someone who also does a lot of
woodwork, I can attest to the challenge of achieving a correctly located hole in
soft wood (like pine) because of the often extreme difference of density between
ring layers that causes the bit to "walk" toward the softer wood. If you've been
there, you know of what I speak. BYW, I have successfully used spade type wood
bits to drill large diameter holes in metal (steel) exterior house doors (for
deadbolt installation) and other thin metal.
Holes and the Service Technician
Fig. 2 - This handy backing block can be made from a short piece
of round metal shaft. Grind two flats on one end so it can be held in a vice.
Fig. 3 - Typical twist drill bit.
Fig. 4 - Correct angle of cutting edges gives best results.
Fig. 5 - Always use the tool rest when holding the bit against
the grinder. No one can hand-hold a bit steady enough to get the proper angle. Never
turn the bit while grinding. Both tip faces must be flat.
Fig. 6 - A twist bit that is properly ground (a) and improperly
ground (b). Side view of the improperly ground bit (c) shows how the tip is off-center
and the angles are unequal.
By Jack Darr
Hole - an open place. But making one in steel or concrete or wood isn't exactly
The title of this article might be misleading. Let's straighten this out right
now: the holes I'm referring to aren't the kind that wander around in transistors,
or that go "Now you see 'em - now you don't" in tunnel diodes, or even the ones
in the writer's head. The holes in this article are the plain old-fashioned kind
you have to make in TV chassis to hold things.
Cutting such holes is a job the service technician often runs into. I thought
it might be a good idea to run over some of the ways of making them.
Where to make the hole is determined by the part you're mounting. Mark the spot
with a soft pencil. Then use a center-punch to make a wee pit to keep the drill
from scooting off into nearby parts (Fig. 1). Set the chassis solidly on something
that will take the impact of the punch. A good trick here, if you can do it, is
to use a backing block of metal. It can be any kind of piece of solid metal. Fig.
2 shows one useful type. It's a short piece of metal shaft with two parallel fiats
ground on one end. Shove this end in a vise and place the other end below the place
you want to hit.
Drills and bits
There's some confusion about the terms drill and bit. They are often used interchangeably.
In this article, the bit is the part that does the cutting. The drill is the part
that holds the bit.
The standard twist-drill bit has two spiral flutes on its stem to clear the chips.
The tip of the bit is ground so the two cutting edges take off thin chips of metal
as they revolve (Fig. 3). These edges must be sharp. Standard bits turn clockwise,
looking down on the work.
Wood bit in chuck of electric drill. This is not the type bit
used with a hand brace.
Fly-cutter being used to cut 1-inch hole in car fender for mounting
Small burr and electric drill make job easier in tight places.
Rotary files are ideal for enlarging holes cut in sheet metal.
Fig. 7 - The correct way of holding an electric drill.
Fig. 8 - Temporary depth gage is made by wrapping tape around
Fig. 9 - A small burr is often better than a bit for removing
Fig. 10 - Holesaw made for use with an electric drill. They are
made in a variety of sizes.
Fig. 11 - Chassis punch uses a threaded bolt to pull the punch
through the metal to be cut.
Dull bits are standard equipment around the shop. However, they aren't hard to
resharpen if you use just the right twist of the wrist. You'll need a smooth, fine-grit
grinder. One very important point is the angle of the cutting edges. For sheet-metal
work, and this makes up most of our jobs and most of yours, it ought to be about
85° to 90° (Fig. 4).
To sharpen a bit right, you've got to hold it very steady. There's a special
holder for this purpose, but you can do it by hand, using the tool rest on the grinder
(Fig. 5). Hold the tip against the wheel, taking only a very small bite each time.
Keep a glass of water handy and dunk the tip in it each time you stop. Never let
the tip get too hot. This takes the temper out of the steel and the bit will become
dull again very quickly.
Touch one face then the other against the wheel. The angle at which the bit is
held determines the final tip angle. Be sure to keep the two faces equal (Fig. 6)
and flat. Never turn the bit while grinding. The faces, in Fig. 6, must be equal
in area and perfectly flat.
For drilling plastics, you can get better results with a flatter angle on the
bit. Most plastics tend to melt under the heat generated by the spinning bit, clogging
the hole. Drill very slowly. One trick used in drilling thin sheet plastic is to
sandwich it between scraps of Masonite, clamping tightly and drilling through all
three at once. Even then, take it slow and easy for the neatest results.
Electric hand drills
The most popular size electric hand drill for TV shop work is one with a 14-inch
chuck. There is only one right way to make holes with a hand drill. Hold the drill
firmly in both hands as in Fig. 7. Note the knuckle extended below the left hand
resting on the top of of the if transformer can. It keeps the bit from plunging
on through the sheet-metal chassis and wrecking numerous delicate components underneath.
Fig. 1 - Use a center punch to make a small pit in sheet metal. This holds the
point of the bit until it gets started and you put the hole right where you want
When drilling sheet metal, you'll soon learn to recognize the change in pitch
of the sound when the drill is just about ready to go all the way through. When
you hear that, get ready to catch the weight of the drill to keep it from going
too far. By the way, this should have been mentioned first, but always check the
chassis under any hole you're thinking of drilling! See what's under there
that could be damaged. If you must drill a hole where parts below might be damaged,
make a temporary depth gauge by wrapping tape tightly around the bit stem at the
proper place (Fig. 8). While it won't stop the drill entirely, it will slow it up
enough to let you gain control.
When drilling holes in thick metal, use lubricating oil on the tip of the bit
to reduce friction and speed cutting. As a further aid, use a small bit first, then
change to the final size of bit needed. This is particularly important when the
final bit is 1/4 inch or larger.
Drilling out rivets
Lots of components are riveted in modern chassis. Removing the rivets can be
a tough job sometimes. One way is to center-punch the head of the rivet and drill
it out (Fig. 9-a). This is all right, but it has its drawbacks. A rivet will often
come loose and simply spin instead of sitting tight and being drilled out. The easiest
way in the long run is to use a small burr or rotary file (Fig. 9-b). For very tiny
rivets a miniature burr is better. They can be used in a 1/4-inch drill. For even
better results in tight places, use one of the small electric drills known as a
For larger rivets, and also for finishing holes which must be made slightly larger,
the 1/4-inch shank rotary files are very handy. They have many other uses, which
you'll discover as you go along. (Cleaning rust, paint, etc.)
Once in a while you'll have to drill a very small hole. This can be done with
a hand drill if you're very careful. Be sure to keep the pressure off the bit as
much as possible. Let the weight of the drill take it into the metal. Hold the drill
very straight, so that there is no side pressure on the bit. That breaks small bits
You can make holes up to 2 inches in diameter with a 1/4-inch drill and the right
accessories. You can use either flycutters or holesaws. The flycutter gets its name
from the fact that the cutter bar, the small diagonal bar seen at an angle across
the shank, cuts as it "flies" around the circle. (The same action is found in a
lathe.) The holesaw is a circular hacksaw (Fig. 10).
Each of these tools uses a 1/4-inch pilot bit. It makes the starting hole and
holds the cutting tool steady after it has penetrated the surface. When using either
of these tools, be sure to keep a very tight grip on the drill and feed the cutter
into the work very slowly. Watch out for the moment when the cutter goes through
the work. Both tools have a tendency to "whip" at this point. Be prepared to cut
the power instantly and keep the tool from running wild.
Wood boring bits
Boring holes in wooden walls, making neat holes in phonograph mounting boards
and jobs like those have always been a headache for the TV technician. But a set
of electric-drill wood bits can make things much easier. The bits are actually just
flat metal shapings in a special holder, which in turn is chucked in the drill.
A nut is tightened to hold them in place. These will cut very smooth holes in any
kind of wood, in a fraction of the time needed to make a similar hole with a brace
and bit. They're real handy for boring through wooden walls when installing TV lead-ins,
About the final classification would be what I have always called shear punches
or chassis punches. These are made in two sections, the anvil and the shear (Fig.
11). In use, a pilot hole is drilled at the center of the larger hole, the punch
separated, the bolt passed through the hole, and the shear screwed on it. As the
bolt is tightened the two parts are drawn together.
The shear has two lips which penetrate the surface of the sheet metal. The anvil
is made so that the shear will just slip inside it. When the bolt draws the two
halves together, a very neat round hole is punched. They are also made in square,
D and keyed types, for making almost any kind of hole needed.
Such punches are much used to make holes for tube sockets and other components
in radio and TV work. In two-way radio work, the 1 1/4-inch hole needed for installing
a mobile whip antenna can be made very easily with one of these.
Another type of shear punch is shown in Fig. 12. It is actually a set of punches.
Matching shears and anvils are selected, placed into the pilot hole and hit with
Another place the electric drill is handy is with masonry bits. Any antenna technician
who has ever spent a miserable afternoon hanging on a ladder while he tried to make
holes in a brick or masonry wall with a star drill and hammer will certainly appreciate
these. They look just like ordinary drills, but usually have special silicon carbide
tips to withstand the terrible punishment of grinding through stone, mortar and
bricks. They come in several sizes from 1/4 inch on up to 1/2, 3/8 and so on. While
the tips are extremely tough, they can be resharpened by a machine shop with a diamond
wheel, used to resharpen lathe tools.
Masonry bits should not be used in a standard 1/4-inch drill. These drills rotate
too fast for them and will reduce their life. They should be used with a slow-speed
3/8- or 1/2-inch drill. If you must use a high-speed drill, apply minimum pressure
to prolong the life of the bit. When using these bits, be sure to hold the drill
perpendicular to the surface you are cutting the hole in. If this is not followed,
the silicon carbide tip will wear rapidly and form a bevel on the cutting edge,
making it necessary to resharpen the bit before it can be used again. This can happen
in a very few minutes, so be extremely careful.
Fig. 12 - This chassis punch is hit with a hammer to make chassis
holes. It comes with a variety of punches in different sizes.
Posted February 22, 2023