Why are the punched holes I see in sheet metal not the same diameter on the top and bottom side of the sheet?
When a hole is punched in sheet metal work either with CNC punch press tooling, fly presses or power presses the action on the tooling process is the same within the material. The diameter of the hole on the top side of the metal sheet is created by the punch and will be the same diameter as the punch diameter. The diameter of the hole on the underneath of the metal sheet is created by the die which is slightly larger than the punch having a “die clearance” thereby producing a partly tapered hole. The die clearance ideally increases as thicker sheet metal is punched but in practice some standard dies can be used for a range of materials. You will see from our CNC punch tooling tables that we have a range of die clearances to cover sheet metal materials from 0.4mm thick up to 5.0mm thick. The cross section of the punched hole is not a perfect taper as approximately the first 25% to 33% of the sheet material is sheared by the punch and is a parallel hole. The next 75% to 67% then “breaks” and tapers out to the die clearance diameter creating a 2 stage cross section to the hole. So, for example a diameter 5mm punch with a 0.2mm die clearance die will produce a hole that is diameter 5mm on the punch side of the sheet and diameter 5.2mm on the die side of the sheet. If the holes are in thicker sheet metal and are going to be tapped out with a thread after punching a smaller pilot hole may need to be punched and then drilled out to achieve a parallel sided hole which will produce a uniform thread after tapping.
If you want to achieve a constant diameter through the sheet metal component then drilling and reaming may be needed which is a slower and far more costly operation.
Is there a minimum hole diameter that can be punched for a given sheet metal thickness?
For most common materials we use a simple rule of thumb that the diameter of the hole being punched should be no smaller that the gauge of sheet metal being punched. As the tensile strength of the material increases we would recommend that the punch diameter increases from the thickness of the gauge to 1.5 times the thickness of the sheet metal gauge being punched.
How close can I place a punched hole to the edge of a sheet of metal or to the edge of a cut-out within a sheet metal component?
As with the minimum punch diameter being 1 gauge thick a similar rule can be applied to most component cases in that the material left from the outside of a hole diameter should be no less than the sheet metal material gauge thickness. If the edge of the hole is closer to the sheet metal component edge the material will bulge out as it’s not supported by enough material when being punched. With CNC punching we can sometimes punch the hole first and then CNC punch the outer profile after but this may then push the sheet metal material left back into the hole. It’s always worth asking us first if your sheet metal component design needs a hole close to the edge profile so we can experiment for you to see what will be acceptable before you commit to a design.
For more examples of CNC punched holes please take a look at the following web pages on our site:
Trumpf 200/3000R CNC punching
What is CNC punching?