by Matt Joseph
(Taken from Automotive Sheet Metal Forming & Fabrication by Matt Joseph)
Creating crown is basic to many sheet-metal fabrications.
While many structures can be constructed in flats and simple bends, such as items as simple as battery boxes or as tricky as some rocker panels, most auto body panels have some crown.
Crown describes a format in which metal falls away from a point in more than two planes or directions. Think of it as a three-dimensional compound curve. Reverse crown is the opposite, or other side, of a crowned structure. If a closed end of a bowl faces you, this is an example of a crowned structure; the other end, which is open, is reverse crowned, as opposed to an eaves trough or downspout, both of which are composed of several simple bends and curves, but that have no crown.
One way to impart crown to metal is by bending it, and causing it to stretch at the same time. This is done, with decreasing efficiency, by striking the metal between two hard surfaces (think of a hammer and an anvil), or one hard and one soft surface, or two soft surfaces. The reason that efficiency decreases in that progression is that while striking metal with soft surfaces deforms it, it lacks the stretching potential of hits between two hard surfaces, and that limits the ability to create crown. Hitting metal against a dolly with a hammer is an easy way to create crown, sometimes more than you want to create.
The part mounted upside down on this wooden hammer form is having its crown adjusted—sounds a bit like psychiatry for royalty—by stretching and closing its corners by hammering the metal against a tube dolly. Crowned areas of metal come in sizes from tiny to huge.
Crown can also be created by shrinking (upsetting), or stretching metal around part or all of the area to be crowned, causing the metal to bulge. In both approaches (stretching and shrinking), the creation of crown depends on an exchange of thickness for lateral panel dimension in the area being crowned. Understanding this exchange of dimensions is crucial to working with crown, no matter what method you employ to create it.
While there is big, sophisticated equipment that can move and shape a lot of metal into crowned formats in a hurry, you can accomplish the same end with simple hand tools and some skill, albeit much more slowly. In most cases, for most people, hammering metal remains the mainstay of shaping it.
These Cook forming heads on a Pullmax make one mean shrinking, crown-forming machine. This setup shrinks metal and creates a basis for forming crown almost as fast as you would pull your thumb out of its dies, if you suffered the misfortune of getting it in there.
Not all hammering to create crown is done with steel or iron hammers. Wooden and plastic mallets are often useful for fine tuning the shape of crown, as are lead-, copper-, and brass-faced mallets. Hammer backups of rubber, corrugated cardboard, leather, wood, copper, and aluminum are often useful. In fact, almost any non-brittle material that you can think of may have some application as a hammering backup for steel and aluminum panel materials.
Non-ferrous metal hammers are great for moving metal, while stretching it less than conventional iron hammers would. At the top from left to right are two lead hammers, an aluminum mallet, and a copper mallet. The hammers in the bottom row are all brass.
These wood (three top left), rawhide (four top right), and plastic (bottom) mallets are very useful for modifying crown. The three teardrop plastic mallets are particularly handy for this purpose. The rawhide mallet on the far right is shot filled, which makes for very decisive persuasion.
Shot bags are particularly helpful in some crown-forming operations. Steel or lead shot can be used to fill these leather bags. Shot bags allow metal to move and deform incrementally under impact with less than maximum stretching. Steel and iron dollies of many different shapes and facings are essential in numerous operations, as are hammers and hammering backups.
These lead-shot-filled shot bags weigh from 21⁄2 to 120 pounds. The two in the center are hand held. The piece of cardboard mounted on wood (bottom left) is a terrific backup for many jobs, like picking high spots out of flat and crowned panel areas.
At the low end of power forming metal to create crown is the planishing hammer, a pneumatically driven percussion device. These hammers tend to be fast, violent, and noisy. While they lack the precision of more senior and sophisticated power hammers, they can move metal quickly. They are relatively inexpensive compared to those machines and are very useful if you want to stretch and deform a lot of metal quickly.
One sure way to create very specific crowned shapes is to hammer them out over, or into, bucks that are detailed to, or close to, the exact shape that is desired. Wood is often the material of choice for creating forms and bucks. This approach to fabrication is different from hammering over dollies or into shot bags to attain specific shapes. With rigid, general hammering backups, like tear dollies, you attain desired shapes by generally working toward them. With a hammering buck, there is nothing general about your backup; it is roughly or exactly the shape that you are seeking to create. In some cases, complex parts may require transitional hammering bucks before they can be formed into one that reflects their exact, final shape.
You can’t own too many dollies. The ones shown here are pretty basic for metal forming operations. The plastic-clad dolly (bottom, right) makes a terrific semi-rigid backup when you want to move metal a little bit, without stretching it much.
The main advantage of hammering bucks or forms is that they guide you exactly toward the finished shape, so there is less need to calibrate what you are doing as you work. Consequently, there is less room for error that will require time-consuming correction(s) later. The main drawback is that it can be time consuming and skill intensive to create the form to hammer metal over, or into.
This old planishing hammer came with six floating dollies and three hammer heads. When it was new, it was sold as a “fender bender.” It is capable of smoothing metal, at the expense of stretching it considerably.
Excerpt from Automotive Sheet Metal Forming & Fabrication