1. What is pitch

The distance from the center of one hole to the center of the adjoining hole is known as the pitch.
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2. Can the hole size be less than the thickness of the sheet

For conventional perforation , the size of the hole has to equal to or more than the thickness of the sheet. Other processes like laser cutting can be applied in cases where the hole size has to be less than the thickness.

3. What can the minimum wall thickness for perforation.

As a thumb rule, the thickness of the sheet is prescribed as the minimum wall thickness between two holes.