Sonex Bending Flat Parts - Manual

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Builder's Guide Bending Flat Parts
Some airframe parts require bending, whether bending a
flat part to a given angle, or reforming an existing angle (rib
flange, channel flange, etc.) to a proper fit.
Measuring Angles
Angles can be measured a number of ways - an adjustable
protractor, an angle finder, lines drawn on your workbench,
a piece of cardboard cut to the proper angle, etc.. There is no
need for expensive or "machinist" grade tools.
These photos show how a piece of .062" or .090" can be bent
in a vise. Scraps of aluminum protect the part from the jaws
of the vise and the part was easily formed with the palm of a
hand. When using this method, one bend tangency line is even
with the top of the vise jaws, and the other is above that. An
acceptable bend radius usually occurs automatically.
Tools and Techniques
The tools and techniques mentioned here are only suggestions.
Every builder has different skills and different tools. The goal
is to achieve proper results and usable parts no matter what
tools and techniques you use. Take pride in your work.
When trying a tool or technique for the first time it is best to
practice on scrap before bending the part.
Parts Up to .032" Thick
Aluminum up to .032" thick can be easily bent with a hand
seamer or small bending brake. Irregular shaped objects,
such as ribs being built from scratch, are best shaped on form
blocks. Long bends, such as the bends in the seat pan, need
to be done on a bending brake. If you don't have access to a
bending brake your local heating and cooling shop may make
these simple bends for you for a nominal cost.
Parts .063" to .090" Thick
Aluminum from .063 to .090" thick is best bent on a small
brake. Narrow parts can also be clamped in a vise and bent
with the palm of your hand or rubber mallet. Be sure to pad
the vise jaws so you do not mar the surface of the part. Keep
in mind the bend will occur just above the jaws. If using a vise
it is best to practice on a piece of scrap before committing to
the actual part.
Accuracy
Angles are often noted very precisely on the plans (e.g. 93.7
degrees). However, there is NO NEED for an individual
builder to be nearly that precise. Often being within a degree
or two is more than enough.
Bend Radius
The plans identify the minimum bend radius for each bend.
For material .032 and less this is often a .0625" radius. This
radius occurs naturally when using a form block or most
other tools, however, if you are using a bending brake or hand
seamer with a particularly "sharp" edge you will need to be
careful not to form a sharp bend, which, in time, can crack.
Thicker parts require a larger bend radius. As with bend
accuracy, there is no need to be precise on the bend radius,
just close. If you use the V-block method described here for
bending your thick parts the proper radius will be achieved by
using the proper diameter rod.
Part Preparation
It is good practice to debur the edges of a part before it is bent.
Removing the scratches near the bend will prevent the part
from cracking at the edge while it is being formed.
Bend Tangency Lines
The plans identify "Bend Tangency Lines". These lines
indicate the beginning and the end of a bend radius. When
forming parts in a vise or bending brake, one of these lines
will typically be at the edge of the vise jaw or bending brake.
When forming parts using a V-block, the steel rod is centered
between the bend tangency lines.
Parts .125" Thick
Aluminum .125" thick should be bent on a brake, or with the
V-block method.
Parts .1875" and Thicker
Material over .125" thick can be easily and accurately formed
by the average builder with some simple, homemade "V"
blocks, rods, dowels or pipes of the proper diameters, and a
small arbor press, large vise, or car jack. This method is de-
scribed in detail on the following page.
Finishing Up
Inspect the bend area for signs of cracking - both the surfce
and the edge. Seldom will cracks form using these methods. If
cracks do form the part must be discarded.
Orange peel is common. Orange peel, as its name implies, is
an uneven texture at the bend which is restricted to the surface.
Orange peel should be polished off to make sure it is not hid-
ing an underlying crack.