The Po' Boy Bending Brake

Scratch-build an airplane out of aluminum, you gotta bend some tin. Narrow
bend, short piece, you can use a vise. Longer pieces you generally use a
bending brake, even if you have to make one up. Make it out of metal, you call
it a brake and it tends to hang around the shop. I got several like that; even
made one out of aluminum, ‘way back when. But make it outta wood you
generally call it a Poor Boy bending brake, dismantle it once the job is
finished.

Over on one of them Yahoo Groups a feller asked about scratch-building a
CH-701. Seeing as I've been doing exactly that for a couple of years now I
posted a message saying he'd probably need a fixture to do the leading-edge
slats and maybe a Po' Boy bending brake for some of the angles but that
otherwise it was just basic tin-bending, nothing exotic at all.

"What's a Po' Boy Bending Brake?" someone asks.

Coupla boards & hinges plus whatever it takes as a clamping bar.

"Don't work." (From several sources. Several as in a bunch. Mebbe even two
bunches.)

"That ain't right! Works fine for 90's but won't do nothing else."

"You can't get a square bend. Comes out looking like a banana!"

"Works fine," sez another expert. "But not for acute angles."

"Whatdaya mean, a cute angle?"

"Not a banana."

"Well, Mr. Dumb ****, MY home-made brake makes the cutest angles I've ever
seed!" ...and generally ran downhill from there.

Truth is, there's all kinds of locally fabricated bending brakes. How well a
Po' Boy brake does a particular angle depends on which kind we're talking
about. But the bottom line is that you can bend any angle you want on one
basic type of home-made brake. And without the banana.

I think the main reason for the confusion stems from the fact that on a Po' Boy
brake - - and most other kinds of locally fabricated brakes - - the pivot point
of the hinge is offset from the plane of the bed and the leaf. You all know
what I'm talking about here? Bending brake has a bed, a leaf, a clamp and a
pivot. The bed is the fixed part, the leaf is the part that moves, the clamp
holds the work to the bed and the pivot is precisely aligned with the plane of
the bed and the leaf. If any of this is new to you, you'll find drawings &
photos illustrating these points in the BASIC BENDING folder, in the Sheet
Metal folder, in the files archive of the Fly5kfiles Group over on Yahoo. (The
drawings are in DeltaCAD, the photos .jpg. Midnight Software (see their site)
offers a free trial copy of DeltaCAD that will allow you to read the drawings.)

A real bending brake is hinged only on the ENDS of the bed & leaf. The center
of the pivot point is perfectly aligned with the edge between the leaf & the
bed. With just two pivot points the leaf must be extremely rigid to resist
bowing. The longer the brake, the more massive it must be. More expensive,
too. Which is why most shops make do with a four-footer... and make up a Po'
Boy brake when they need something longer.

When you use a regular hinge to make up a bending brake the outer edge of the
knuckle of the hinge is usually installed level with the bed. That means the
pivot point - - the center-line of the hinge-pin - - is going to be offset by
some amount from the plane of the bed and may be even farther from the bending
surface of the leaf. (See the drawings.) Because of the offset the geometry
between the bed and the leaf is going to change according to the angle of the
bend. Try to work out the set-back and set-FORWARD allowances for every
angle, radius, thickness and temper, you'll end up crazier than a Kraft Foods
recipe.

Tuna Fish - Marshmallow - Sauerkraut Surprise

Go lookit the drawings. Figger out the geometry for yourself. Go crazy on
your own time. The key point you want to keep in front of you is that the
silly thing really does work (and rather well, too). The reason for the
differing opinions as to how well it works probably stems from differences
between the various types of brake and the nature of the work being done.

Didja lookit the drawings? Depending on how you install the hinge and shape
the leaf, you can end up with a brake that is only good for making right-angle
bends - - which is all that some folks want. But you can also build a Po' Boy
brake that gives you perfect acute angles - - but leaves a banana-nose on
everything else. Ditto for shallow bends; thirty degrees or less, it gives you
a perfect corner but try making any greater angle of bend and you've got a real
problem on your hands. Fortunately, it only takes half an hour or so to
fabricate the leaf & bed of a Po' Boy bending brake. (The real work goes into
the clamping bar and hold-down fixtures.) Since they are so easy to make most
guys whip one out that is specific to their needs. Then someone comes along
later and tries to use it for a different type of bend... and the controversy
begins :-)

Wanna do acute angles, like a bar-folder? Then you're going to have provide a
deep offset. But doing so means you must also displace the bending-line by a
significant amount of set-back. Need only a shallow angle? Then you don't
need as much offset. In fact, you may even cut an angle into the face of the
leaf to ensure 100% contact with the work. And your bending line will probably
fall FORWARD of the hinge axis; how far depending on the diameter of the
knuckle and the desired radius of the bend. Of course, having optimized the
brake for doing open angles, it will not close far enough to do acute angles.


The Natural Order of Things

Even though the skins go on last they are your first consideration when cutting
metal since they are usually done as unspliced panels. Once the skins have
been cut out, marked and put aside you focus on the material for your spars,
stringers, longerons and so forth. Metal for the remaining components, mostly
your ribs and frames, is pieced out of the residue. Following this sequence
usually results in the least amount of scrap.

After cutting the stock for your long skinny parts - - your spars, stringers
and whatever - - the next step is to do any bending they may require. (One
reason for following this sequence is to allow you to make any adjustments that
may be required in the patterns and form-blocks for the ribs. ) This is the
point when you make up your Po' Boy bending brake. Or brakes. Many homebuilt
designs call for several hundred feet (!) of self-bent angle (what I call
stringer stock). At seven to eight feet each, that could mean bending more
than thirty pieces. Identical pieces. With that amount of bending to be done
it makes good sense to build a brake specific to the task.

Unfortunately most novice builders aren't aware of the distinction between the
different types of Po' Boy brakes. They know they need something but when they
ask for help they're liable to get half a dozen conflicting versions of
reality, all valid because they are based on that individual's personal
experience... with ONE TYPE of home-made brake.

In the drawings I've shown a Po' Boy brake that can be used for bending any
angle. It is laid out with sufficient offset to handle acute folds but it will
also do shallow angles if you add a shim to the face of the leaf.

General Brake Poop

‘Brake' of course is an archaic term for a vise and the ability to clamp and
hold the material is fundamental to the proper operation of any bending brake.
Other than the example in the photos I've not shown any of the many methods of
clamping the work. My ‘real' Po' Boy brake was eight feet long and used the
top of a workbench as the bed. Clamping for long, wide sheets is provided by
threaded inserts installed on the underside of the table-top, capable of
putting an heroic amount of pressure on the clamping bar. Alas, the workbench
is now occupied by a wing and the lumber used in the brake has gone on to serve
other purposes. (So what if I need it again? Then I'll make another. But I
have a regular 48" brake. I made the thing mostly for doing 8' stringers, a
couple of skins (for the slats) and making the nested doublers for the elevator
spars.)

One of the handiest methods of clamping work to the bed is to simply run a deck
screw through it and the clamping bar, right into the bed. (Always keep in
mind that your goal is to build an AIRPLANE rather than a shop-full of tools.)
In many cases you can position such holes in an area that will be later cut
away to form a lightening hole but so long as the hole is on the neutral axis -
- and you don't drill too many of them - - they usually have no impact on the
final strength of the panel. (The slats on the -701 are up to six feet long,
their skins about 22" wide with a ninety degree bend about 13" in from one
edge. Since the ninety degree bend gets a doubler riveted inside I used what
would become rivet holes to screw the thing down.)

For narrow stock, such as when making stringers, it's common practice to make
up a clamping bar having a strip of metal fastened to the underside that serves
to locate the workpiece. With the projection of the work under the clamp both
known and fixed, you may then install dowels to locate the clamping bar and a
few deck screws to provide clamping pressure.

You don't find this sort of thing in manuals on sheetmetal work. The manuals
give you the principles. Their application to any particular job is more a
reflection of your experience than anything else. Nor will you find such
construction details in the typical set of plans. Nowadays, they're only
interested in selling you a kit. (The metal for the -701 cost about a thousand
dollars. I'll leave you to do the math :-)

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I haven't shown any arms or levers on the leaf. I assume everyone understands
that the greater the depth of the leaf, the stronger and more rigid it will be.
If you must bend some seriously strong material you will probably be using a
leaf having a depth of at least twelve inches and suitably reenforced.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Locally fabricated tools, from simple jigs and fixtures to an eight foot brake,
are a fact of life in the construction and repair of airplanes. When it comes
to bending, a real brake should always be used if available since it skews the
odds of success in your favor. But if you need to make a few bends that are
longer than your available tools, a Po' Boy brake can do the job, although you
will have to work out the proper settings empirically.

-R.S.Hoover