A chassis punch is a tool used to make shaped holes in sheet metal. In many ways, these hand tools work like a cookie cutter. The tool is placed against the sheet of metal and pressure is applied on the sheet. As the pressure increases, the sheet deforms until the chassis punch removes the metal. There are three basic styles of punches: manual, ratcheting and hydraulic. In most cases, each works the same way with the exception of the way the pressure is applied to the sheet.
The design of a chassis punch is quite simple, usually consisting of three parts. The punch creates the actual shape in the sheet metal. These can be of any design, although larger punches require far more force than smaller ones. The die provides a solid surface for the punch to push into, assuring the pressure is equal across the entire surface of the punch. Lastly, a screw holds the two pieces together on either side of the metal sheet.
Using a chassis punch is just as simple as the design. First, a hole is drilled into the sheet metal. This hole is generally in the exact center of the punched shape and needs to be precisely measured to avoid mishaps. The die is placed on one side of the sheet and the punch is placed on the other. A screw is threaded through the die, then through the hole in the metal and then into the punch. As the screw is tightened, the punch is pulled into the sheet and eventually punches a hole.
The three main types of chassis punch are more variations than separate tools. The most simple type is the manual punch. It works just as described above. A person using the screw provides the power used to pull the punch through the metal. There are no additional factors involved.
The other two types use additional systems to help the worker or amplify the turning power. A ratcheting chassis punch has a special screw head and wrench specifically designed to work with the punch. The entire system ratchets so the worker can put as much force as possible on the screw. A hydraulic punch has a pressure system built into it so the worker only needs to pull a trigger to apply a very great force.
The tool used is important since the more force applied to the screw, the greater the maximum punch size. Manual punches have the smallest potential hole size and metal thickness. Ratcheting punches are a step up and hydraulic systems can make the largest punches and use the thickest metal.