Building the Po' Boy Bending Brake

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 :-)

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

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

Thanks for another good one, Bob. I built a wooden brake years ago (three
planks of wood and three hardware store hinges (countersunk the one in the
middle). Used it just a couple of months back, bending up the edges of a
3' long inspection panel for the belly of my airplane. Worked fine.

Ron Wanttaja

Ron Wanttaja wrote:
Thanks for another good one, Bob. I built a wooden brake years ago (three
planks of wood and three hardware store hinges (countersunk the one in the
middle). Used it just a couple of months back, bending up the edges of a
3' long inspection panel for the belly of my airplane. Worked fine.

Ron Wanttaja

Before I go cobbling together some 2x4s and hingesg, will this type of
brake have enough 'authority' to bend a 3/8" flange on a 2ft piece for
..035 4130? Just in case I didn't have enough numbers, I need a 90
degree bend. Most of the applications I've seen seem to apply to aluminum.

--
http://www.ernest.isa-geek.org/
"Ignorance is mankinds normal state,
alleviated by information and experience."
Veeduber

Before I go cobbling together some 2x4s and hingesg, will this type of
brake have enough 'authority' to bend a 3/8" flange on a 2ft piece for
.035 4130? Just in case I didn't hav

---------------------------------------------------------
Dear Ernest,

I suggest you file the required bend-radius onto a piece of angle iron, clamp
the work to it and tap it over with a hammer. You'll be done long before
you've built a wooden brake and figured out how to clamp the work to it.

Wooden brakes work best with aluminum & copper. They can do thin gauge steel
stock but the typically tighter bending radius tends to crush the leading
corner of the clamp, forcing you to use hardwood or steel. Indeed, If you
want to bend steel it's generally best to start with steel -- using angle iron
or CRS for your leaf & bed, with a heavy duty steel piano hinge welded to them.

-R.S.Hoover

In Veeduber wrote:

Wooden brakes work best with aluminum & copper. They can do thin
gauge steel stock but the typically tighter bending radius tends to
crush the leading corner of the clamp, forcing you to use hardwood or
steel. Indeed, If you want to bend steel it's generally best to
start with steel -- using angle iron or CRS for your leaf & bed, with
a heavy duty steel piano hinge welded to them.

How do you think the wooden brake would do on 8 foot lengths of .032
2024t3? I suggested something almost identical a while back on the
Bearhawk list for bending the spar webs and was met with much skepticism.

----------------------------------------------------
Del Rawlins-
Remove _kills_spammers_ to reply via email.
Unofficial Bearhawk FAQ website:
http://www.rawlinsbrothers.org/bhfaq/

For small sections, 18-24" wide, don't overlook the ultra-cheapie bending
brakes from HF. I used the 18" model to bend 16ga. 1018 mild steel and it did
just fine... even though it was only rated for 18ga. They work even better for
aluminum. The only problem that I've evperienced it that they cannot make
multiple reverse bends, i.e. "hat-section" when the sides of the section are
less than about 2 1/2" wide. The leaf only drops to level with the bed,
instead of an additional 90* like the larger brakes. I got my 18-incher on
um... "sale" from HF for $19.95.

Just my $0.02 worth,
Harry

How do you think the wooden brake would do on 8 foot lengths of .032
2024t3? I suggested something almost identical a while back on the
Bearhawk list for bending the spar webs and was met with much skepticism.

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

Dear Del,

'Think' or know for a fact? :-)

As a general rule the upper limit for wooden brakes is about .063 for tempered
stock but a lot depends on the quality of the wood and workmanship.

Although the load per unit of length remains the same the overall magnitude of
the load naturally increases with the length of the bend. It also becomes more
difficult to keep the leaf in the same plane as its length increases. There
are fairly simple solutions to each of these problems and I would have no
qualms about making up a long brake for .032 myself but I've got a lifetime of
mistakes behind me :-)

I couldn't find any .032 scrap but I did come across some .040 under the bench
and put a 1" flange on a piece just to give you some idea what a wooden brake
will do. (See the pictures WOODEN_BRAKE 05 thru 08 in the file previously
mentioned.) Other than adding a bar and a couple of clamps this is the same
set-up for .016 meaning the radius is all wrong. But what I wanted to
illustrate was the fact that a wooden brake is more than adequate
strength-wise. The real problem is maintaining the uniformity of the bend as
the length increases. It can be done but the solution will depend on your
tools and experience, the quality of the wood you use and how well the brake is
secured. For an eight footer you're talking some loads that will throw the
typical workbench all over the shop; it needs to be secured to the deck.

-R.S.Hoover

In Veeduber wrote:

Dear Del,

'Think' or know for a fact? :-)

As a general rule the upper limit for wooden brakes is about .063 for
tempered stock but a lot depends on the quality of the wood and
workmanship.

Although the load per unit of length remains the same the overall
magnitude of the load naturally increases with the length of the bend.
It also becomes more difficult to keep the leaf in the same plane as
its length increases. There are fairly simple solutions to each of
these problems and I would have no qualms about making up a long brake
for .032 myself but I've got a lifetime of mistakes behind me :-)

Thanks for posting the info and the additional pictures. Since this
would be for the wing spar webs, the dimensional uniformity is kind of
critical so I am still going to try to pursue other options before
trying a wooden brake. I may have a line on a real 8' brake I can
possibly use, and I'm seriously considering just taking the plans across
town to Atlee Dodge and having the spars bent there. It will cost me
some labor but at least any mistakes won't be on my nickel.

If I go with the wooden brake idea, I'm thinking along the lines of a
4x6 treated beam for the leaf if I can find one straight enough.
Treated only because they seem to be less warped by the time I get my
hands on them.

----------------------------------------------------
Del Rawlins-
Remove _kills_spammers_ to reply via email.
Unofficial Bearhawk FAQ website:
http://www.rawlinsbrothers.org/bhfaq/

If I go with the wooden brake idea, I'm thinking along the lines of a
4x6 treated beam for the leaf if I can find one straight enough.

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

Dear Del,

Consider a couple of different lines :-)

The point of contact with the metal is seldom more than half an inch wide and
usually less. The only thickness consideration in the leaf is to accommodate
the fasteners.

The leaf is a LEVER. You should be more concered with its depth; 2x10 or
2x12... something along those lines.

You only need one square edge to accomplish the bend. Doesn't really matter if
its warped, bowed or BOTH... so long as you square it up before attaching the
hinge.

Drawing from the knot-hole collection at the local Home Depot, the best leaf
material is often 1x12 pine shelving... albeit cupped, bowed, twisted & warped.
Simply pick a pair of complimentary boards, REVERSE their defects, slather on
a good coat of urethane glue on both surfaces, clamp them together and secure
with one deck screw for about every 9 square inches of surface. Then leave the
thing to cure, well supported and out of the weather. True up one edge and
there's your leaf.

There's a lot less to accurately bending tin than most folks realize. The
problem is that most homebuilders have no experience doing repairs in the
field, where your only option is to use whatever is available to accomplish the
task. Bending one airplane's-worth of tin isn't anything to get excited about
and even a rather casually built Po' Boy brake is capable of producing better
accuracy than the average homebuilder can utilize.

-R.S.Hoover

"Veeduber" wrote
Dear Del,

Consider a couple of different lines :-)

The point of contact with the metal is seldom more than half an inch wide
and
usually less. The only thickness consideration in the leaf is to
accommodate
the fasteners.

The leaf is a LEVER. You should be more concered with its depth; 2x10 or
2x12... something along those lines.

-R.S.Hoover
????????????????????????????

Perhaps I missed it. Where are the pictures?
--
Jim in NC


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