Cutting a square hole in sheet metal can be a difficult process. If you are not equipped with the correct tools, the task will most likely take twice as long than is necessary, and the results will be far less than ideal. The most effective tool for cutting a square hole in sheet metal is, without doubt, a punch. Square punches are available in a wide range of sizes and make this task simple. The hole made from using a square punch will be crisp, clean and uniform.
Measure and mark the outline of the square hole you wish to cut on the surface of the sheet metal using a scribe and steel rule. Find the centre of the square by measuring halfway in from two adjoining sides, and mark this point.
Center-punch the mark in the middle of the square; this will stop the drill from slipping. Put on leather gloves and safety glasses. Using the drill at a slow speed, drill a 1/8-inch pilot hole.
Open the hole out using a drill that is large enough to accept the mandrel of the square punch. The mandrel is basically the bolt that holds the two halves of the punch together. The user manual for the punch will detail what size drill bit is necessary.
Disassemble the square punch; remove the lower half and leave the cutting side attached to the mandrel. Place the base of the punch at the back of the hole. Pass the mandrel with the cutter head attached through the hole, into the base of the punch, and finger-tighten.
Ensure that the blades on the punch are lined up with the square marked on the sheet. Hold the cutting side in the correct position. Use a wrench of the appropriate size to turn the hex head on the end of the mandrel clockwise, as if tightening any normal bolt. Continue to tighten the punch until it cuts completely through the sheet.
Disassemble the punch and remove from the hole. Use a fine file to remove any burrs from the edges of the hole. Apply a coating of light oil to the punch before storage to protect it from damp.
Use a light oil to lubricate drills and reduce overheating.
Take care around the blade of the punch as it is extremely sharp.