Die punching is a manufacturing process best suited for forging, contouring, and cutting jobs.
It is performed by a group of machine tools called die punches.
Die punches are machine tools that apply pressure to form or cut sheet workpieces like metals, plastic, fiber, paper, fabric, etc. The dies used in these machines have a specific shape, allowing them to accurately make repeatable parts much cheaper and faster than popular manufacturing techniques like machining.
This article discusses die punches in detail by looking at various machine configurations, workflow, operations, and advantages.
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What are Die Punches?
Die punches are a two-part machine tool that bends and cuts workpieces by applying force.
It consists of a press called "punch' and a secure clamp called "die." When the punch is pressed onto the clamp, the material in between is pushed into a pocket, causing it to break.
In manufacturing, die punches are largely employed to work on sheet metals.
They can cut and shape metal workpieces based on the pre-set pocket shape.
Types of Dies and Punches
Die punches use various tools with different features and functions to perform various operations.
The die punch is pushed into the sheet metal clamped to the die with the help of a hydraulic press.
Depending on the application and operations performed, it can be of many different shapes, sizes, and hardness.
Another important part of a die punch is the forming die which is used for forming the workpiece.
The forming depends on the workpiece's ductility, malleability, die design, lubrication, and feeding mechanisms.
Die punches can easily form a highly malleable metal. Low malleable metals can easily break from the pressure applied by the press.
Forging dies forges metals using highly localized forces and temperature.
It produces parts stronger than machined parts and has a lower production cost.
Another type of tool used in die punching is a core pin. It is primarily used to make plastic molds.
It is fixated, and the material is poured around. When the material dries, it takes the shape of the core pin.
When using a die-punching machine, you must consider the punch-to-die clearance to get a uniform and clean cut.
Die punches produce fractures in the material cross-section on both the upper and lower surfaces. For a clean cut, these upper and lower fractures have to connect.
When the clearance is too tight, these fractures don't fall in the same line and can't adjoin, resulting in the development of secondary cracks.
Moreover, a tight clearance causes the material to grab onto the punch, which increases the punching force and causes higher wear and tear to the machine.
On the other hand, if you keep the clearance too high, then it causes the metal to get squeezed between the edges of the punch and the die and causes extreme burr on the metal edges.
It is important to have an optimum clearance between the punch and the die. Usually, a clearance range of 4-8% is adopted for metal-forming jobs.
Die Punching: The Process
The process of die punching has three phases
Deformation is when the punch comes in contact with the material and creates a slight deformation in the contour of the material.
Then the punch goes deeper into the material, which is called penetration. The material is still intact in this phase.
The punch must go deeper and induce a fracture to cut the part out of the workpiece.
Machining Operations Performed on Die Punches
Due to the different shapes and sizes of punches and dies available, you can perform various machining operations on die-punching machines.
In blanking, the punch cuts parts out of sheet material. The cut-out shapes are the expected part, and the remaining material is scarp.
Blanking is most efficient when the maximum amount of material is removed from a workpiece as it minimizes material waste.
Opposite to blanking, piercing is where the cutout parts are scrap, and the rest of the sheet metal is the needed part.
Piercing has different types: lancing, nibbling, shaving, and cut-off.
Lancing is when the geometry of the material is modified by shearing and bedding, no material is removed in this process.
Here the cuts are not closed, meaning a part of the cut is always left attached to the workpiece. It can be bent or curved in any desired manner.
In the nibbling process, a series of slits or notches are made at a high frequency of around 300-900 times per minute.
It creates contours on the metal surface and usually requires some degree of machining to make the cut smooth.
Shaving operation smoothens the edges of a blanked part and achieves an overall dimensional accuracy of up to 0.001”.
Lastly, the cut-off removes peripheral edges from a workpiece or shapes a stock to the desired dimension for punching operations.
In punching, you can resize the sheet metal and cut slots.
Unlike blanking, punching is most efficient when the least amount of material is punched from the workpiece, as this reduces material wastage.
Bending is a simple operation in which the punch bends sheet metal from a single axis to the desired angle.
There are three main types of bending - bottoming, partial bending, and coining.
Bottoming is the most popular bending technique, where the workpiece and the die make contact after the bend.
The die's opening width or V-width is usually kept between 6-12 times the material thickness.
In partial bending, as opposed to bottoming, the workpiece and the die don’t touch, and bending is done without reaching the die bottom. The die V-width is kept around 12 times the material thickness.
Coining is the most penetrative of the three types of bending. It is also considered a subset of blanking.
Here the punch cuts circular pieces from a sheet metal clamped to the die, part reaching deep. As a result, it minimizes spring back.
A V-width, five times the material thickness, is best suited for the coining operation.
In this process, the material is squeezed by the punch to create controlled deformations on its surface.
You use the broaching operation when a material is too thick to be cut. It uses a punch having a series of cutting teeth.
Its teeth increase in size from top to bottom, whereas the last tooth is the largest, representing the maximum amount of material the broach can remove.
Each tooth makes a small cut on the surface and chips away the workpiece. This technique mostly finishes holes, splines, and flat surfaces.
In curling, a special die called curling dies rolls sheet metals into curved shapes. This has applications in the manufacturing of door hinges, springs, etc.
When a piece of sheet metal needs to be shaped into something else, extrusion dies are used.
Here, the material only takes the shape of the die, not the shape of the punch and the die combined.
Extrusion dies are primarily used in food manufacturing and clay modeling industries.
Final Thoughts: Advantages of Using Die Punches
Die punching is an affordable manufacturing process. Die punches can perform manufacturing processes like blanking, punching, piercing, coining, etc., with minimal setup time.
It also accelerates manufacturing output and saves time and running costs.
Punching is one of the fastest ways to manufacture many identical parts like perforated sheets, provided the part is simple.
They also produce the lowest amount of debris and require the least accessories compared to other machining jobs.
Frequently Asked Questions
How is punching different from drilling?
Punching uses a press, usually a hydraulic press to penetrate the punch into the material and cause a fracture. It causes the punch area to cut out the material. Drilling, on the other hand, uses a high-speed rotating cutting tool that removes material from a workpiece to create a hole or a cavity. Drilling produces large amounts of debris, which needs to be cleaned regularly to maintain good drilling efficiency. Punching doesn’t create a lot of debris.
How is swaging different from bulging in the die-punching process?
In bulging, a fluid, like oil or water, is used for expanding a part, while a bulging rubber die forms the workpiece. You can use this process for expanding cylindrical parts like pipes. Swaging, on the other hand, uses a pressurized die to apply pressure and compress a workpiece rather than expand it. It can be performed in a hot or cold press and is used to make tapering features.
What are the materials used for making Die Punches?
The most common materials used for making die punches are tungsten carbide and High-Speed Steel (HSS). These materials are heat resistant and have high compressive strength.