CNC Punching sheet metal work

CNC Punching sheet metal parts using a CNC Punch Press

Computer numerically control is known simply as (CNC) piercing and punching is a production / manufacturing process that is carried out by using hydraulic, mechanical or electrically powered CNC punch presses. The action of the press is to force a punch (the top tool) into a die (the bottom tool) in a controlled manner to pierce a hole in a sheet of metal. All CNC punch presses have at their heart this process to produce holes. There is also a plate that sits between the top punch and bottom die and is known as the stripper plate. The punch passes through the stripper plate and then pierces the sheet and pushed the slug of metal through the bottom die. When the punch is removed from the die and back through the sheet the stripper plate helps pull the material off the tip of the punch freeing it ready to punch the next hole. On a machine like the Trumpf this stripper plate is programmable and hovers just above the top surface of the metal sheet. The whole cycle happens in a fraction of a second leading to very high levels of productivity.

These CNC machines can come in vast range of sizes and production speeds but are mainly either a single head design with a tool rail (Trumpf) layout or a multi-tool turret design. They may be single stand alone machines where each full sized sheet of metal or smaller panel is placed into the machines clamps and then punched or they may have fully automation. The automation can take the form of automatic loading of sheet and removal of finished components and scrap which is sometimes known as "lights out" production as the process is fully automated and nobody needs to be involved once the material and tooling is loaded.

Whichever machine format is used to punch the holes it is simply programmed to move a sheet into position via x and y axis movements under the CNC machine’s punching ram ready to punch / pierce a hole. A series of holes is created by continuing to punch while moving either the x-axis or y-axis at the same time. The processing thickness range for most CNC punch presses is from 0.5mm to 6.0mm thick in a range of sheet materials including mild steel, galvanised steel, zintec coated mild steel, stainless steel and aluminium alloys. Over this thickness the processing of sheet becomes plate and is better suited to laser cutting. The choice of hole punched / pierced can be as plain as a circle or rectangle through to special 2D shapes to suit a specific cut out design e.g. an electrical connector cut out. By using combinations of single hits and overlapping hits more complex sheet metal work shapes may be produced using standard CNC tooling. The CNC punch press can also punch 3D forms such as stand-off dimples, plunges for taptite® screws, and electrical knockouts on either the top side or bottom side of the metal sheet, which are often employed in sheet metal enclosure housing designs. Some more modern CNC machines may have the ability to form/tap threads, fold small flaps, punch continual sheared edges without any tool nibble marks making the machine very productive and reducing second operation handling. The instruction to control the CNC machine to create the correct sheet metal component geometry is commonly known as the CNC program.

An industry standard CNC programming software package such as Radan can be used to communicate with the machine. We use this software to create 3D models of the sheet metal components and full fabricated assemblies. The software also allows us to generate the flattened developed blanks which we can add tooling to and find the best layout to suit a given sheet of metal, this is called the CNC nest. Once this CNC next has been created it can be set to the CNC punch press and is its instruction to produce the actual sheet metal parts on the CNC punch press.

In today’s fast moving production climate it is vital to be able to communicate design data quickly and accurately between customers and their suppliers. V and F Sheetmetal has addressed this requirement over the last 12 years with the inclusion of the complete Radan® software suite of CAD/CAM tools for 2D drafting, 3D solid modelling, sheet metal tools and part nesting.

3 dimensional software generated models of sheet metal fabrications and fully working assemblies can be created to eliminate design errors before any sheet metal manufacturing is started. Using 3D models we can check the correct alignment of parts by ensuring inserts will be in line with screw holes, pop rivets will fit through their holes in fixing panels and angles, covers have sufficient clearance over boxes after powder coating, front panels align correctly with electronics chassis on rack mounted equipment to name just a few applications.

3D and 2D images can then be used to help you visualize your designs for production suitability or to relay your design ideas to your own customers. Where you only have a sheet metal sample we can still create fully detailed dimensioned 2D drawings to work to in our manufacturing factories in Fareham, Hampshire, UK and for your future inspection records. These detailed dimensioned drawings can be output in AutoCAD® compatible file formats DWG and DXF. We can also handle imported 3D data from AutoDesk® Inventor and Solidworks® software (SAT and STEP file formats) to speed up CNC punching program creation and reduce the chances of translation errors. Exact 2D sheet metal flat pattern developments can be automatically and accurately produced by unfolding the 3D sheet metal model ready for the selection of the best CNC punch press tooling and the most efficient program nesting for a given sheet size. The sheets used are normally 2000mm x 1000mm or 2500mm x 1250mm in size. Other special sheet sizes can be sourced directly to reduce material wastage and of course smaller components of sheets are used for smaller batch sizes or smaller components making the process of CNC programming very flexible for batch production.

The use of Radan’s automatic component sheet nesting capabilities enables V and F Sheetmetal to quickly try out different CNC punching nests options to evaluate the best material utilization. We can experiment with the position and alignment of components within the sheets which can be a very powerful tool when manufacturing a series of parts for a family of parts. We may be able to punch small components from within the inner windows of other parts, between the punch press clamps or in the dead areas between larger parts. With raw material prices rising faster than general inflation, the material costs are becoming a larger percentage of the overall component costs. Radan CAD / CAM sheet metal manufacturing software helps us to keep this cost to a minimum which in turn is passed onto our customers in more competitively quoted components and assemblies. As an aside, all the sheet metal that is punched out down through the CNC dies and left in the CNC frame skeletons are separated into material types and 100% recycled so nothing is really wasted.

When I look at holes punched in sheet metal by either CNC punching or fly press piercing they are not the same size on the top to the bottom of the sheet?

When we punch out a hole in sheet metal material either with CNC punch press machines, fly presses with simple tooling or power presses with custom made power press tooling die sets the action of the tooling in the sheet material is the same. For example, the diameter of a pierced hole on the top (punch) side of the sheet metal work is produced by the punch and will therefore be the same diameter as the punch tool diameter. The hole diameter on the underneath (die) side of the sheet metal is created by the die which has a “die clearance” and is slightly larger than the piercing punch which then produces a hole with a slightly tapered cross section, this looks like a cone.

The “die clearance” should really increase with the increase in the sheet metal thickness being pierced but in practice some dies can be used cross a range of types of material and material thicknesses (gauges). The “conical” cross section of the pierced hole is not an even taper throughout the material thickness. In fact approximately the first third of the sheet metal thickness is sheared by the action of the punch and die and should have parallel sides. The last two thirds then tares (known as the break) and tapers out to the edge of the cutting die with it’s clearance, producing a two stage cross section to the hole. Let’s think about a round hole with a diameter of 10mm. We will need a punch which will be 10mm in diameter and a die with 0.2mm die clearance if we are punching 1mm thick mild steel. With this tooling set up we will produce a hole in the sheet metal that is diameter 10mm on the top (punch) side of the sheet and a diameter of 10.2mm on the underneath (die) side of the sheet.

What might the smallest hole diameter be that can be punched in a certain thickness of metal?

With most common commercially available sheet metal materials such as mild steel, aluminium alloys and stainless steel alloys we apply a simple rule of thumb that a diameter of hole can be punnched no smaller than the material gauge is thick.

How near to the edge of a sheet of metal can I design for a hole to be punched out?

The same rule as with the hole being pierced no smaller than the thickness can be applied for the amount of material left between the edge of the sheet and the edge of the hole. If you have a 5mm hole being CNC punched in 2mm mild steel we would say that 5.5mm to the middle of the hole from the sheet edge is the nearest you can punch the hole before the material will start to bulge out past the sheet metal edge profile.