The most common hole shapes are: round, square, equilateral triangle. While the triangular hole is rarely used for specific purposes, round and square are undoubtedly the most requested types.
Along with the just mentioned hole shapes, the oblong and rectangular holes are also very used.
The round hole is determined by the measure of its diameter “d”, while for the square hole by the measure of the side “a”. It is possibile to have diameter “d” and side “a” up to 120 mm.
The DIN 24042 norm allows a maximum corner radius of the square hole equal to 0,15a.
The oblong and rectangular holes are determined by their “a” and “I” measures. Respectively, “a” can reach a maximum value of 120 mm, while “l” a maximum of 200 mm.
Besides those common shapes there are many other special shapes, e.g. hexagonal, square with rounded corners, rhomboid, triangular star and key.
These special are used for specific purposes, while the common ones represent 80% of the perforated sheets production.
Sometimes the customer may require the perimeter of the hole not to be on the same surface of the metal sheet. For this reason our company provides countersunk of flared hole.
This kind of hole is obtained trough the deformation of the perimeter of the hole either upwards or downwards, maintaining the flatness of the sheet.
Graepel has developed the method of combining the two directions of a countersink in a single sheet.
The hole obtained by punching is divided in three areas. On top the deformation area, in the middle the cutting area and on the bottom the breaking point area.
The measure of the hole is defined by the measure of the cutting area.
During the standard punching process the section of the hole is slightly conic in the breaking point area. It is relevant that the ratio between the bridge “n” and the thickness of the metal sheet isn’t lower than 1, otherwise it is possible a fracture of the bridges between the holes.
PITCH "p": Distance between the centers of two neighboring holes.
BAR "n": The shortest full area between two neighboring holes.
Hole punching pattern:
The holes are arranged at 60° also called alternated arrangement. It is the most wide spread pattern because it combines high stiffness of metal sheet with the high open area. Since it is a standard perforation arrangement, unless otherwise specified the perforation will be automatically realized this way.
the holes are arranged at 45°. This punching pattern is rarely used
the holes are arranged in parallel rows. It is very requested for the furnishing of shops, display panels and decorative purposes.
Equal arrangement; the most common perforation pattern for square holes is in parallel rows. This kind of perforation is widely used especially for ornamental purposes.
Alternating arrangement; in this case we are not referring to a 60° arrangement because the centers of square holes are not disposed at the vertices of equilateral triangles. In case of square holes, the pitch is measured parallel with the side “a” of the hole.
Diagonal arrangement ; holes are arranged in parallel rows turned 45° respect to the sides of the plate
Oblong and rectangular holes
For these two types of holes there are three kinds of arrangements are available:
Alternating arrangement; most common in case of oblong hole
Equal arrangement; most common in case of rectangular hole
Arrangement in alternate rows; mostly used for grain sieves
Two perforation system
1) Sectional perforating press, also known as CAPS machine
Sectional perforating presses (CAPS) perforate the sheets section by section, processing individual metal sheets fixed on the working table.
In these case the mold is less wide than the metal sheet. After the perforation in the y-axe, the mold moves on x-axe to complete the punching process on the remaining part of the metal sheet.
Possibility to perforate very thick metal sheets reducing the number of passes and the amount of power required
Short time for setting up the machine
Long perforation time and therefor high incidence of labour
Impossibility to perform continuous perforation, because of the time needed to load/unload metal sheets
Irregular perforated areas
2) All-across perforating press, also known as perforation line are made up of
A decoiler with straightener
A punching press
A shear for cutting in length
In this kind of presses the mold covers the whole width of the coil, which is dragged by pair of feed rollers. To any downward movement corresponds a movement forward of the rollers, equal to the perforation pitch.
After the desired length of the sheets is reached, the punching system stops and the shear cuts the sheets. If the customer asks for a whole perforated coil, at the end of the perforation process the material is only rewinded and not cut by the shears.
Short perforation time and therefore low incidence of labour
Possibility to perforate coils instead of sheet and therefore lower cost of raw materials
Possibility to supply sheets with/without edge on the short side
High costs of the molds
Limited possibility to realize irregular perforated areas.
Low and medium thickness
Relatively long time for setting up the machine
Low and medium thickness
Whole punched coil
Edges, unperforated areas and their measurement
One of the main features of perforated metal sheets is the possibility to have edges and unperforated areas.
Usually, the two long sides of perforated metal sheets up to 3 mm thick have small unperforated edges, while the two short sides do not have any edges.
In case of metal sheets with thickness over 4 mm, the standard provides for full unperforated perimetrical edges.
Unperforated external edges can be sheared. It is also possible to have unperforated areas in different positions, and especially with sectional presses we can satisfy almost every request of shape and position of unperforated areas.
EDGES MUST BE MEASURED STARTING FROM THEIR LIMIT ON THE OUTER SIDE OF THE METAL SHEET, NOT STARTING FROM THE CENTRE OF THE HOLE.
A defining feature of the CAPS machine is the ability to perform irregular punching areas or unperforated areas.
Thanks to computer numerical controls many of these presses are able to program and monitor every single punch.
Perforated metal sheets can be supplied with mounting holes of whole types: round, oblong or other..
We can supply metal sheets with notched corners or with carving of any shape.
Last but not least we can provide metal disked/shaped metal sheet.
Start and end perforation
For technical reasons of construction of the molds, the distance between the punches is generally double or triple compared to the perforation pitch.
So only after the second / third stroke, the perforation pattern is accomplished.
This punching beginning is called "double" or "triple initial jump". With CNC modern it is almost always possible to eliminate these initial jumps.
By the way it has to be previously specified in the request! Nowadays, many molds work with double feed rate to limit times and costs of production.
Unfortunately, not all these molds allow to obtain a regular start and end of perforation.
Often the customer waives this aesthetic requirement to reduce the costs of the perforated plate. The double or triple jump not only affect the round holes but also all other punching holes. The perforated area is measured between the circumferences or the outer sides of the holes of the first and last row of holes.
Cutting and shearing burr
Just as in any other shearing system, perforation involves the formation of burr on the exit side of the punch. The amount of burr depends on various factors (type of material, tool grinding, clearance between punch and matrix, etc.).
It is worth considering that on the whole perimeter of the metal sheet there are cutting burrs caused by shearing and trimming.
Important: unless otherwise agreed, the entering side of punches is shown on technical drawings.
Relation between hole, bridge and sheet thickness
The more the ratio of the hole diameter and thickness of the sheet approaches the value 1, the more the sheet is subjected to tension and deformation. For the hardest metal sheets, such as stainless steels, this problem is emphasized and the ratio should never be below 1.5.
The ratio between empty and full spaces, which thanks to the after-written formulas is easy to determine, is expressed as the percentage of the total perforated surface. Therefore it does not consider edges and unperforated areas.
Flattening of perforated sheets
Punching causes tension and deformation. For this reason it is necessary to flatten and level the sheet surface with sheet straightening machine.
In case of large sheets, large unperforated surfaces, wide or unequal borders, very hard materials or with "difficult" punchings, the tension may compromise the flattening. Perforated coils are not subjected to any kind of flattening process. For this reason coil's flatness is not guaranteed.
Shearing and other processes
Often the customers request cut-to-length material.
We realize the cut using CNC guillotine shearing machine.
Generally it is possible tu cut sheets up to 4000 mm length.
Obviously, not all perforated plates are cut after the perforation: the sheets obtained from the standard width coils, (1000, 1250 and 1500 mm), as well as from 4 mm or thicker sheets, are supplied without trimming, therefore their measures are within the standard of steel industries tolerances.
It's clear how the utilizer of perforated sheets takes a double advantage: avoids the wastage of metal and has the material ready to be used.
There is a multitude of other possible working of metal sheets: from cutting to size to punching, (fixing holes, grooves, notches of the corners, etc ..), from bending to deep drawing, molding, from welding to surface treatments..
Graepel Italiana Spa, unlike the majority of the perforators, can offer all of the services, and thanks to its long experience and the well-trained staff, can ensure a perfect execution.
Extract by "Manual of the perforated sheet" by Riko Graepel