Getting a hole correct on the first attempt are an important task in a sheet metal shop; getting a hole correct on the first attempt will save the shop more time and money than any other task that has to be completed within a sheet metal shop. A clean cut will be different than a cut that creates burrs and tears on the sheet metal. The difference between a clean cut and a torn cut of the sheet metal is the diameter of the punch, the diameter of die opening, and the type of clearance that is used between those two components.
By ensuring that those three components are correctly matched, the sheet metal press will be able to complete the punching operation without any problems. The various metals will behave differently within the press when the punch is dropped into the metal sheet. In order to understand which metal will create what results in the sheet metal, the shop should have an understanding of the behavior of the different metals within the shop so that it doesnt have to guess at whether metals like stainless steel are punched in the same manner as metals like aluminum.
How to make clean holes in sheet metal
Metals that is harder to punch, like stainless steel, will spring back more than softer metals like aluminum. Therefore, harder metals requires more clearance on the sides of the punch than softer metals do, since softer metals will allow for a shearing action to occur. The shape of the punch is another variable that is important in determining whether the punch will successfully cut the sheet metal.
Round punches will result in a clean slug being removed from the sheet metal, and the punch will create less stress upon the die than a square punch. Square punch tools will require more tonnage to complete the punching operation than round punches, since the corners of the punch will create more stress upon the sheet metal during the punching process. Many sheet metal shops will purchase tooling that allows punches to be formed according to the shape of the sheet metal, so that there is no need for filing the punched sheet metal to match the desired shape.
The tonnage that the sheet metal press provides will impact the ability of the punch to successfully cut the sheet metal. The tonnage that is required to make a hole of a given size will vary with the length of the cut that must be made, as well as the strength of the metal sheet being cut. Thin sheets of metals like aluminum may require very little tonnage to punch a hole, while sheets of metals as strong as stainless steel will require more tonnage.
As such, tonnage calculations must include a safety margin for the expected wear upon the sheet metal press and punch tools. Springback of the sheet metal is a phenomena that will impact the size of the punched hole. As the sheet metal is punched, the final diameter of the punched sheet metal will not have the same diameter as the punch tool.
The difference in diameter between the punch tool and the sheet metal hole will be small for sheet metal holes of less than a half inch in diameter, but will increase in size with the size of the punch tool. In this situation, then, the sheet metal job will require that the hole sizes be measured after a few strokes of the punch tool, to ensure that the sheet metal meets the requirements for that specific part. The size of the punch tool will need to be adjusted as necessary to ensure that the punched sheet metal has holes of the required size.
Clearance between the punch and die will have the most influence upon the quality of the punched sheet metal. Using too little clearance with the sheet metal will cause the punch tool to tear the sheet metal rather than shear it. Using too much clearance will result in the punch tool creating a large rollover zone of the punched sheet metal.
To avoid these problems, a clearance table can be used to find the proper clearance for the thickness of the sheet metal being punched. Using the wrong clearance relative to the metal that is being punched will reduce the life of the punch tool. For example, rubber and gasket sheet metal products will require no clearance between punch and die, but hard sheet metal like steel will require at least one-third the thickness of the sheet metal to be provided for clearance.
Regular maintenance of the punch tools will ensure the longevity of the punch tools. To extend the life of the punch and die tools, they should be cleaned each day that the sheet metal shop is in operation. Each tool should have a very thin and even application of oil to the tooling to prevent the metals from sticking to one another.
Additionally, each punch tool should be inspected every week for chips along the edges of the tooling, since those chips will create burrs in the sheet metal if they are not removed weekly. Monthly, the sheet metal shop should perform another inspection of the punch tools to check alignment and stripper pressure so that the sheet metal parts will not become oversized or develop cracked slugs. Punching operations can be used instead of drilling operations on the sheet metal.
Punching operations are faster than drilling operations. Additionally, punching operations will not create chips on the sheet metal, while drilling operations will. Punching operations are best used in situations where large quantities of sheet metal products are to be produced.
Drilling operations will create a smoother edge on the sheet metal product. Additionally drilling operations can be used on thick metals that would damage the punch tool during punching operations. Both punching and drilling operations can be used by sheet metal shops, as different sheet metal jobs may require one process or the other.
An understanding of which sheet metal variables are adjustable and which are fixed by the type of metal that will be punched will allow a sheet metal shop to make decisions about the sheet metal punching process. For example, once it is understood that the punch clearance percentage, punch tooling shape and tonnage are adjustable variables within the punching process, a punch shop can use a clearance chart for that specific metal being punched to make decisions regarding the process. Its also important to realize that many shops might of made mistakes with the wrong clearance.
Youll find that actualy getting it right the first time is alot harder than it looks.
