Metal Working Tips for First Time Builders - Part 2
Sport Aviation - 2/93
By Tony Bingelis
Working With Rivet Holes
It's
true, anyone can drill a hole. However, not everyone can drill a nice
round hole - in thin metal - where it needs to be - and do it right
every time. After all, with thousands of holes to be drilled in a
typical all-metal aircraft project, the "opportunity" exists for some
of those holes to be:
· Drilled with the wrong size drill bit.
· Drilled off your center mark.
· Drilled too close to the edge.
· Drilled sloppily and elongated.
· Drilled in the wrong place.
· Drilled O.K., but only after scratching the skin.
However, as if all that isn't bad enough - after the hole is drilled you can still create other problems by:
· Forgetting to deburr the holes in both skins.
· Forgetting to dimple the top and/or bottom skin for flush riveting.
· Making machine countersunk holes for flush rivets (where needed) too shallow or too deep.
Read on to see how you can avoid or at least limit your encounters with these, all too common, hole preparation problems.
A Bit About Drills and Bits
The type of drill bit you will probably use the most is the High Speed
Drill (jobbers length) because of its ready availability and low
initial cost. However, the tougher, more expensive, Cobalt bits should
be used to drill hardened steel parts like landing gear legs and engine
mounts.
In addition, you should acquire a couple of 12" long (#30 and #40) drill bits for drilling in hard to reach places.
Because
these long bits can be easily flexed with your fingers they can be made
to drill close to corners, and just about anywhere else that a 90
degree angle drill would otherwise have to be used.
I
also recommend the acquisition and use of a good lightweight air drill
because it is less tiring to use than the average electric drill and
has an easily teased trigger for precise speed control.
Your control (either air driven or electric) will very definitely have access limitations.
For
example, you will find that you cannot drill closer than, maybe, 3/4"
to a corner, or a perpendicular rib surface or bulkhead. At best, an
ordinary electric drill will allow you to get within 1" of a corner but
certainly no more.
In
addition, access to the area where the hole is to be drilled must be
accessible enough to allow you to maneuver and position the drill with
the bit installed. Even with a short bit installed, this means you will
need approximately 10" to 12".
It is in a situation like this that a right angle drill becomes necessary for making those almost inaccessible areas accessible.
You can count on it . . . a desperate need for some sort of an angle drill will arise from time to time.
Even
though you are building a metal airplane, you may have to drill holes
through fiberglass fairings, cowlings, and other component parts.
Drilling
holes through fiberglass dulls drills quickly. Expect this and count on
resharpening or replacing the dulled bits before using them again in
aluminum.
Before You Drill . . .
When a line of rivet holes is to be drilled, a centerline is normally
drawn first and then short cross marks are made to establish each
hole's location. Be alert. Before you mark a rivet location, assure
yourself that there is no obstruction (or flute) in the underlying
frame where you intend to drill a rivet hole. It is permissible to
alter the hole spacing slightly to avoid such problem traps.
Use a center punch and a light hammer tap to pinpoint and slightly emboss the marked location for every important hole.
Better still, use an automatic center punch to pinpoint every rivet location.
An automatic center punch is much easier to use and you may find it to be more accurate for your own use.
Although a punch mark indentation helps ensure that the drill will not stray off the mark, you can help, too.
Hold
the drill perpendicular to the surface, exert a little pressure, and
turn the drill chuck by hand a little before pulling the trigger. This
will almost guarantee that the drill bit will stay accurately seated in
the punch mark.
Ignore this technique and you will risk the chance that your drill will take a walk and scoot off the marked hole location.
When this happens, the drill will leave an ugly scarred trail across that nice smooth metal surface . . . what a shame.
Here's
a useful tip passed on from builder to builder. Lay masking tape over
the route where your rivet layout and spacing is to be drawn on the
aluminum skins. Instead, draw and mark the hole locations on the tape.
Drill the holes through the tape - then remove the tape.
Don't
leave the masking tape on more than a day or so because it becomes
increasingly difficult to remove with the passage of time.
But
why use the tape in the first place? Well, the tape protects the metal
from drill scratches and helps prevent the drill from walking.
Drill Bit Selection
Choosing the correct size bit for the hole to be drilled is extremely
important. Small drill sizes are very difficult to judge by eye, so use
a metal drill gauge to remove all doubt.
In
the typical flush riveted metal project most of the rivets used will be
either 3/32" (AN426-3) or 1/8" (AN426-4) in diameter.
You
cannot use a fractional size 3/32" drill bit for a 3/32" rivet . . .
the hole will be too small and the rivet simply will not go in. The
same applies to the 1/8" rivet and a 1/8" hole.
To
accommodate these rivet sizes you must use what are known as numbered
drill bits. A #40 drill bit is used for 3/32" rivets, and a #30 drill
bit is used with 1/8" rivets.
Notice
that the bigger numbered (#40) drill bit is for the smaller holes, and
the smaller numbered (#30) drill bit is for the bigger 1/8" rivet
holes. Who said anything has to be logical?
Drilling Techniques
Drill aluminum, especially the thinner sheets, at a fairly high speed.
You can improve your hole drilling accuracy and quality by making it a
practice to drill all your holes, initially with either a #41 or #40
drill bit.
You will find that drilling all holes initially with a small bit first is quicker and helps maintain greater accuracy.
Later,
if necessary, different holes can be redrilled to whatever larger size
is required. This is called "line drilling" and will ensure that the
final hole alignment on assembly is exact.
When
you can redrill a small hole to a specified larger diameter, you may
even be able to correct a slight hole misalignment . . . all the more
reason to drill your holes undersized first.
Drilling
accurate round holes in thin gauge metal skins is difficult. Unless
extra care is taken, you can end up with holes that tend to be
irregular and oversized (often taking on a hexagon appearance) instead
of neat and round like you want.
If the metal thickness is less than half the diameter of the drill bit, the described difficulty is almost sure to occur.
Improve
the quality of the critical holes you have to drill in thin metal by
clamping a scrap metal back-up block behind your work before you start
drilling. This will allow the body of the bit to be in solid material
at all times . . . before, during, and after penetration. The result -
a nice round hole.
When
drilling holes in thicker aluminum parts, withdraw the drill bit
frequently to clear the bit of chips. If you don't, the aluminum chips
inside the hole will build up and bind, preventing you from withdrawing
the bit. This is more likely to be a problem when drilling softer
6061-T6 bar stock than it is with the harder 2024-T4 bar aluminum.
When
using a hand held electric, or an air drill, to drill deep holes in
aluminum, take care when withdrawing the drill that you don't
accidentally tilt it sideways. This would be bad because it will cause
an elongation of the hole. If at all possible, do all your important
drilling in a drill press.
Never
hold small parts in your fingers when drilling holes on a drill press.
The drill bit will surely grab just as the bit exits the metal. Before
you realize what is happening, the drill will lock (jam) in the metal
and the part you are drilling will whip right out of your hands and
spin wildly out of control.
No need to go into the gory details of what would happen to your hands and fingers if contacted by that spinning cleaver.
Making Large Holes in Sheet Metal
Sometimes you will have to drill or make holes much larger than those needed for rivet installation.
For
example, rudder control cables are usually routed through holes drilled
in fuselage bulkheads. These holes must be large enough to allow the
passage of the rudder control cables and their formed terminals.
Because these holes may have to be 5/8" in diameter, or larger, a
regular drill would do a shoddy job in the thin metal.
The
solution? Use a step drill (UNIBIT®). This step tapered drill will
drill several sizes of holes and will do a beautiful job without
chattering or messing up the metal.
Holes in Stainless Steel Firewalls
A stainless steel firewall is mean to work with, and drilling larger
holes in one is a challenge. As always, start with a punch mark, use a
small bit, and apply steady pressure. Enlarging a hole with a regular
drill bit in stainless steel is something less than a precise operation.
A
UNIBIT works well if you enlarge the hole progressively from both
sides. The stainless is very ductile and tends to bend or deform at the
cut line.
To cut a very large
hole in the firewall for the installation of a cabin heat valve, or for
fresh air ducting, use a "nibbler." Use it to rough out a large opening
which can then be evened and trued with a file. You will need an
initial 7/16" hole or larger to insert the nibbler.
Rivet Spacing and Edge Distances
Drilling holes too close to the edges of the aluminum skins, or fittings
for that matter, is not acceptable. How close is too close?
The
basic rule is that you do not drill rivet holes closer to the edge than
two diameters of the rivet size used. For 1/8" rivets this would mean
that the centerline of any hole would be at least 1/4" away from the
edge of the skin (material). This is quite a critical factor as in-
sufficient edge distance reduces the strength of the riveted joint.
You should not reduce the edge distance from that shown by the designer on the plans.
Actually,
too great an edge distance for a line of rivets can be equally
undesirable as it may permit the edge of the sheet to curl up slightly
leaving a gap that will look lousy after the aircraft is finished and
painted.
You might be
interested to know that the minimum recommended pitch (spacing) between
rivets is 3 x the diameter of the rivet and the normal pitch is
accepted to be 8 to 10 x rivet diameter.
As
for rivet spacing for your project, use the nominal spacing called out
in the plans. However, you should realize the hole spacing may have to
be altered here and there to avoid placing a rivet where it cannot be
installed, or be properly driven, due to some structural interference
behind.
Skin Drilling Techniques
There are at least three basic procedures for marking and drilling
aluminum skins (fuselage, wing, tail). You should be familiar with each
so you can select the one (or a variation thereof) that would be the
easiest and best suited for a particular job.
Method One
Clamp and align the skin to the underlying structure. From inside, trace
around each rib or frame. Remove the skin and turn it over. Draw a
centerline on the skin within each of the traced frame outlines. Mark
off the specified rivet spacing and drill all the rivet holes marked on
the skin. Reinstall the skin and clamp it in place again. From the
outside, sight through each pre-drilled skin hole to verify that the
centerline (marked on the rib or frame) is visible before you drill.
Insert a Cleco clamp in each (or every other) hole as you proceed.
Method Two
With the skin clamped to the structure, work from behind with a long 12"
drill or an angle drill and back drill through both the frame and skin
at one time. Have a helper on the outside to press down on the skin
with a wood block as the holes are drilled. Your helper should install
Cleco clamps as the drilling proceeds. This method is awkward, and when
working from beneath you risk getting drilling debris into your eyes.
However, this method does make it easy to avoid flutes and other
structural interference. Wear protective glasses.
Method Three
Mark the centerlines and the rivet hole spacing on each frame (or rib).
Drill all the holes in the frames (or ribs). As you drill, it will be
easy to avoid flutes and places where the underlying structure would
make a rivet installation impossible. Position and clamp the skin in
place. From behind, or under the structure as the case may be, drill
the skin holes using the pre-drilled rivet holes in the frames as a
guide. Here again, you must protect your eyes from falling debris.
Whatever
procedure you use, proceed slowly and reassure youself that the
drilling will be centered on the frames or ribs; that the holes will
avoid flutes; and that each hole will be where a rivet can be inserted
and set. For the best results always try to drill from the center of a
section toward each end.
Dimpling and Countersinking
Aluminum skins that are to be flush riveted have to be prepared by first
machine countersinking, or dimpling each hole before riveting. Aluminum
skins that are less than .032" thick must be dimpled . . . not machine
countersunk. Dimpling is more time consuming an operation than is
machine countersinking. The reason being that both the surface skin and
the underlying structure (frames, ribs, etc.) have to be dimpled to
match the surface skin's bulged dimples.
You
do, however, have the option of machine countersinking the under- lying
frame instead of dimpling it - provided the frame material is at least
.032" when 3/32" rivets are used (.050" thick or more for 1/8" rivets).
Ma- chine countersinking is simpler than dimpling when the top skin is
.032" or thicker. That is because machine countersinking a rivet hole
does not cause a bulge under the skin as is the case with dimpling.
Therefore, no further preparation of the underlying rivet holes in the
frames is necessary. A real time saver.
A Few Parting Words of Wisdom
Finally, before drilling or cutting any metal . . . always stop and recheck! Make sure you didn't measure from the wrong point!
Although
you check and recheck your measured dimension and are convinced that it
is perfect, it may still be wrong if you measured from the hole instead
of from the edge . . . or vice versa.
In
other words, don't allow yourself to fall into the same mental trap
that continues to puzzle my favorite "advisor" and critic. He says he
can't understand what happened. He cut it twice and it is still too
short.
