Espacio dedicado al tema de carrocerias de automoviles.
Similar to driving a nail with a claw hammer, always grip toward the end
of the handle for leverage. Unlike hammering a nail, you'll be doing
zero swinging with your arm; it's all in the wrist. The correct method
is to go with the natural rhythm of the hammer as it bounces off the
dolly. If your arm is getting tired, you're putting far too much effort
into it. Some prefer to extend their pointer fingers up the hammer's
handle for increased control, especially in delicate situations.
Remember that sheetmetal is actually fluid, and hammering on-dolly is
used for stretching. For correct technique, the face of the dolly and
hammer should sandwich the metal, making a sharp ting sound and bouncing
off the face of the metal.
Hammering off-dolly is used to equalize highs and lows. This is the technique used for working out dings and waves in sheetmetal
Forming an edge is one of the most valuable skills to have for adjusting
seams or gaps between panels. Note how the bend in the roll pan tapers
upward away from the guide tape (arrow). Bending the edge is simple, but
keeping it clean requires proper dollying.
After rolling over the bend, the hammering switches to the top side of the lip to flatten it out and sharpen the bend.
Note how the edge has moved in line with the guide tape without losing
the original crispness and without any deformation in the panel.
To fold the lip over farther, the dolly moves to the outside of the
panel. This special hammer is useful for getting close in corners, but
it's really about personal preference.
Resist the urge to bend too much at once; the best practice is to fold
the lip over a little at a time while working down the length. This
hammer is useful for tight spaces with little swinging room since it
lacks a second head.
We're not aware of any company that sells these T's, but they're simple
to make by using round bar and a piece of flat stock. Plus, making them
yourself allows custom tuning of the arc.
After deciding the size and arc needed, the T is clamped down in a vise.
The sheetmetal is laid across the top according to where the roll is to
be started. Mike Romero taps along the edge of the sheetmetal that
hangs past the T to begin the roll. Hammering on top of the T will
result in stretching and deformation.
After just a couple of minutes, we have a smooth and fluid roll. This
technique is handy for numerous projects, such as custom motorcycle gas
tanks or making panels where a rolled edge would be more aesthetically
appropriate than a sharp fold.
The basic principle of shrinking and stretching relies on squeezing the
metal together or pulling it apart to stretch. Shrinker/stretchers
accomplish this with a pair of ribbed jaws that bite into the metal then
slightly move either inward or outward. Hand-operated tools are always
the cheapest, but unless you've got a helper, foot-activated ones will
speed your work and control since both hands are free to guide the
metal.
The result in this case is a piece of 90-degree-angle sheetmetal that
curves fluidly. Note that the marks are only on one side of the
90-degree stock; this technique is handy for making custom wheelwell
parts and patches as well as flanges for joining panels and templates
for cloning existing parts. If the arc is too great, stretching along
the same path will open it back up.
Often called the poor man's Pullmax, tuck shrinking is another way of
shrinking metal to gain shape. It's ideal when lots of shrinking needs
to be done and you don't have access to a machine. Shrinker/stretchers
can only do so much, while tuck shrinking folds lots of metal together
quickly. It's an old method that is easy to build a tool for. Blake
Tomlinson used a scrap piece of heavy-wall square tube with a couple of
tips from cheap pry bars welded on to it. The sheetmetal is slid between
the two bars and twisted back and forth to create a fold. In this case,
the sheet of flat steel will be formed into a wheel disc.
The number of folds necessary depends on the shape needed. To capture
the tuck, Tomlinson uses a leather hammer to pound the bulge flat. The
leather is perfect for tapping without marring the surface of the metal.
This particular hammer has a lead core to give it more swinging mass.
Once you've captured enough tucks to create the basic shape you're
after, planishing will smooth out the ripples. Using a pole-mounted
dolly clamped in a vise is a wise choice since it keeps the working
surface consistent while allowing you to move the metal. Tomlinson opted
to use a spoon since it spreads the impact over a large area, but
hammers will work as well.
Here's the before (left) and after (right) of tuck shrinking and
planishing. After only a few minutes, the sheetmetal is already starting
to smooth out.
For a quick reality check, a Pullmax and a pneumatic planishing hammer
were used on the upper right side of the disc. Though the machine shows
fewer impact marks, the two sides are remarkably similar. It's
definitely a slower process, but tuck shrinking can yield the same
results without the expense and logistics of having two massive machines
in your garage.
The edges were tuck-shrunk to create the basic contour on this piece of
sheet, but to complete this wheel disc shape, we needed the sandbag.
Bags are filled with various media depending on the company and intent
for the bag, but all are used for stretching. Note the spiral; this is
the basic guide for hammering with the wood mallet.
Begin at the center of the spiral and work outward, but realize that not
all areas receive equal attention. Most of the hammering will be
concentrated toward the center of the spiral since it needs to be
stretched the most.
After roughing in the shape, planishing was required to smooth out the
inherent dents. Our quickie disc isn't symmetrical because it didn't
receive the same amount of work on all sides. To make it perfect, an arc
template made in the shrinker/stretcher would show us where more work
was needed.
Occasionally referred to as the old Italian method, since it's been
rumored that early Ferrari race cars were built this way, stump forming
holds a great deal of possibilities with very little expense. To begin,
find a solid stump from a hardwood tree that doesn't hold a lot of sap
and dig a hole in it. There's no strict guideline on how large or deep
the hole needs to be, and in truth it doesn't really need to be
symmetrical, either. Whereas sandbags work more slowly and subtly,
stumps can create a lot of shape quickly.
Place the metal over the hole and strike in roughly the center of the
hole. Wood mallets are often the weapon of choice, but Tomlinson prefers
the high crown and striking force his custom hammer creates-it's
actually the top of an acetylene bottle mounted on a pipe that's wrapped
with rubber gripping tape.
The buckling occurs because Tomlinson is stretching the center of the
sheetmetal into a concave shape while the edge remains the same.
After working the sheet into roughly the shape you want, lift it out of
the hole and use the top of the stump to trap the buckles, similar to
the method for tuck shrinking.
After several repeats of the above steps, we had a pan shape that is an
excellent start for a custom cycle fender. Of course, just as with the
sandbag, there will be some dimples to planish out.
Although you can start with as many or as few tools as you want, Walden
recommends this setup as an excellent starting point for occasional
custom work. What we have is a mix of the most common hammers, dollies,
left and right sheetmetal sheers, a sandbag, a spoon, a hand
shrinker/stretcher, and a handmade tuck shrinker and T's.
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