Punch press
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A punch press is a type of machine press used for forming and cutting material.
The punch press can be small and manually operated and hold one simple Die set, or be very large, CNC operated, and hold a much larger and complex die set.
A Die set consists of a set of (male) punches and (female) dies which, when pressed together, may form a hole in a workpiece or may deform the workpiece in some desired manner. The punches and dies are removable with the punch being temporarily attached to the end of a ram during the punching process. The ram moves up and down in a vertically linear motion.
Today's machines are commonly CNC controlled and are mostly powered with an hydraulic ram to enable the punching process. Commonly machines are large metal framed equipment having two types of machine frames. A C type frame or a 'portal' type frame. the C type commonly has the hydraulic ram at the top foremost part to enable the punching process to be carried out, whereas the portal frame is much akin to a complete circle with the ram being centred within the frame to stop frame deflection or distortion.
All punch press machines have a table or bed with brushes or rollers mounted in the tables to allow the sheet metal workpiece to traverse with low friction. Brushes are commonly used in production environments where minimal scratching to the workpiece is required, such as brushed aluminium or high polished materials. The main bed of most machines is called the 'Y' Axis with the 'X' Axis being at right angles to that and allowed to traverse under CNC control. Dependent on the size of the machine, the beds and the sheet metal workpiece weight, then the motors required to move these axis tables can vary in size and power. Older styles of machines used DC motors to move, however with advances in technology, today's machine mostly use AC brush less motors for drives.
The process of operation begins with the CNC controller commanding the drives to move a particular axis to a desired position. Once in position, the control initiates the punching sequence and pushes the ram to Bottom Dead Centre and returns it to Top Dead Centre. the Origins of BDC and TDC go back to older machines where this was a pitman type press with a Pneumatic or Hyrdraulic operated clutch system. On today's machines BDC/TDC does not actually exist but is commonly used as a term to derive the top and bottom of a stroke of the ram. The Punch enters the Sheetmetal, and pushes it through the die, obtaining the required shape of the punch and die set. This will form a slug of metal that is collected underneath the die and ejected to a scrap container. The whole punching process on modern machines is extremely fast compared to older pitman style machines and thus gives rise to increased production volumes. The sequence takes approximately 0.5 milli seconds to complete ( variant from machine to machine and manufacturer)and signals to the control the next movement command allowed after the ram has reached the top of its stroke.
As a metal forming process, the punch press is used for the highest volume production. Cycle times are often measured in sheet yield as a percentage of waste to parts required ratios per sheet processed. As most programming is done by skilled CAD/CAM operators parts within the sheet workpiece are commonly nested.Machine setters are mostly used to set up tooling and programming but thereafter once the machine is running an operator of low skill can oversee its continued operation. Often one operator will monitor several punch presses simultaneously making this one of the lowest cost metal manufacturing processes.
Punch presses are usually referred to by their tonnage. In a production environment a 20 ton press is mostly the prevalent machine used today. The tonnage needed to cut and form the material is well known so sizing tooling for a specific job is a fairly straightforward task.
Most punch presses today are hydraulically powered, however there remains a legacy of older machines which are mechanically driven rams, meaning the power to the ram is provided by a heavy, constantly-rotating flywheel. The flywheel drives the ram using a Pitman arm. In the 19th century, the flywheels were powered by leather drive belts attached to line shafting, which in turn ran to a steam plant. In the modern workplace, the flywheel is powered by a large electric motor.
Mechanical punch presses fall into two distinct types, depending on the type of clutch or braking system with which they are equipped. Generally older presses are "full revolution" presses that require a full revolution of the flywheel for them to come to a stop. This is because the braking mechanism depends on a set of raised keys or "dogs" to fall into matching slots to stop the flywheel. A full revolution clutch can only bring the flywheel to a stop at the same location- top dead center. Newer presses are often "part revolution" presses equipped with braking systems identical to the brakes on commercial trucks. When air is applied, a band-type brake expands and allows the flywheel to revolve. When the stopping mechanism is applied the air is bled, causing the clutch or braking system to close, stopping the flywheel in any part of its rotation.
Hydraulic punch presses, which power the ram with a hydraulic cylinder rather than a flywheel, and are either valve controlled or valve and feedback controlled. Valve controlled machines usually allow a one stroke operation allowing the ram to stroke up and down when commanded. Controlled feedback systems allow the ram to be proportionally controlled to within fixed points as commanded. This allows greater control over the stroke of the ram, and increases punching rates as the ram no longer has to complete the traditional full stroke up and down but can operate within a very short window of stroke.
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