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Weighing the options

Comparing punching technologies to make the best choice for the application

by Pierre Comhaire, punch press product manager, and Dan Caprio, punch press product sales manager, LVD Strippit

 

 

When it comes to batch runs and producing 3-D or formed parts, it’s tough to beat the cost-per-part efficiency and productivity of the modern CNC punch press. Unlike other sheet metal fabricating technologies that have advanced significantly in recent years (think fiber laser), punching is a relatively mature technology. But it can offer a number of advantages over other fabrication methods depending on the application. 

 

Knowing the application needs is key but so, too, is selecting the right type of punch press. With today’s single-head and turret machine design offerings, fabricators must choose wisely to make the best investment for their business.

 

 

 

Single-head or turret

The choice of punch press comes down to two machine styles that are fundamentally different in design: single-head (tool changer) or turret. Both styles are offered in hydraulic and servo-electric designs. The electric-drive punch press has gained popularity recently for its energy-saving design.

 

The turret punch press holds tools in a “turret” that rotates to bring the required tool to the punching position under the machine’s ram. Some turret stations are fixed, others are indexable. If the turret station has an indexing mechanism, the tool itself can be indexed (rotated). The number of turret and index stations varies by machine make and model. Machine tonnage can range from 20 to 50 metric tons. 

 

The single-head punch press features a rotary or linear “magazine” that actively loads the tooling required into the punching head. The punching head features index motors, so all tools are indexable a full 360 degrees to any angle in 0.001-in. increments. 

 

As a result, this style of punch press requires fewer tools per job than the (non-indexed) turret punch press. Most single-head punch presses have about 20 standard tool stations. This capacity can be enhanced using indexable multi-tools (5 or 10 station) or an extended tool magazine housing additional tools (up to 40 tools in the LVD design) that can be automatically placed and removed while the machine is operating.

 

A small manipulator positioned between the machine and the extended tool magazine is used to exchange the tooling from the extended tool magazine to the magazine and vice versa. The punching force of a single-head punch press is typically 20 to 30 metric tons.

 

In both the turret and single-head designs, multi-tools can be used to increase the machine’s tooling capacity. With the use of a multi-tool, a turret punch press can have more than 160 tools. In a single-head system that is also outfitted with an extended tool magazine, the capacity could be as high as 400 tools.

 

One design is not inherently better than the other. Deciding between the two is a matter of weighing the pros and cons of each design against current needs and growth plans.

 

 

 

Upfront outlay 

Application dictates the technology level required and the size of the investment. Generally speaking, a single-head punch press is a larger investment than a turret punch press. Shops producing one-off parts or prototypes may be better suited to an entry-level turret punch press; those handling production work can look to more feature-rich machines. 

 

As the manufacturing environment has shifted from large batches to small, shops in search of more economical fabrication methods are moving toward entry-level turret punch presses to gain punching capacity at a low price point. Shops looking to add extra value into each part gravitate to machines with more tooling capacity, forming capability and speed, which can mean a single-head punch press or a mid-range to high-end turret punch press. 

 

There is no standard formula, though. A forward-thinking small shop looking to expand its capacity could have good reason to justify the higher price tag of a single-head punch press.

 

 

 

 

Machine setup

A turret punch press is set up in a two-step process: assemble the punch and then load the tool into the turret. To keep tooling costs down, most shops don’t have dedicated punches and guides for all their tools, so it’s common to assemble and disassemble units. To minimize downtime due to setup, it’s recommended that the operator assemble the tooling for the next job while the machine is running. 

 

A single-head punch press with a rotary magazine uses a pre-setup cartridge. Once the cartridge is assembled, the operator must take the extra step of measuring the tool heights and inputting this data into the control. However, when this is completed, loading the cartridge into the magazine/carousel is a fast process.

 

Of the two technologies, a turret punch press offers the fastest station-to-station positioning time. Bi-directional turret rotation selects the shortest possible route to the next punching station, so a turret punch press can travel from station 1 to station 3 in much less time than it takes to change out a tool on a single-head punch press. In fact, station-to-station positioning time is measured in seconds.

 

With the turret punch press, individual turret stations can accept only one size of tool, from 0.5 in. to a maximum of 4.5 in. The feed clearance between the upper and lower turret is typically limited to approximately 0.984 in., which restricts the possible forming height. The feed clearance also limits the use of whisper/shear tooling, which is used to reduce noise and tonnage and, in some applications, minimize sheet deformation.

 

In a turret system, the punch moves in the guide and the punch must be aligned in each station. As a result, there could be a slightly increased amount of tool wear when compared to the single-head system, but this is minimal.

 

In a single-head design, the punch is clamped and guided with the punching head, so alignment is simple. The longer guiding on the single-head system (approximately 14 in.) helps alleviate alignment issues and thus results in less tool wear.

 

In a single-head punch press, every station can accept any tool size (0, 1 or 2) as well as multi-tools. Only punch and die holders are required; no stripping springs or A/B/C/D holders are needed. The punching head can index all tools, which reduces the number of tools required for a given job.

 

Multi-tools are easily dropped in and out of the magazine. This style of punch press also has more capacity for forming and can form profiles from 1 in. up to 3 in. in height. The tradeoff is that the tool change process is slower than a turret.

 

For punching material thicker than 1/8 in., a turret is a good choice because tooling options abound to handle heavy-duty punching, including high-performance tools. While the single-head design can also handle thicker materials, it’s not ideally suited to heavy punching applications.

 

 

 

Additional factors 

These days, more fabricators are doing forming in the punch press, whether it’s to add value to a part or to eliminate secondary processing. Forming operations range from countersinks, tabs and knockouts to louvers and even continuous embosses. 

 

Space between the punch and die (feed clearance) limits what can be formed on the punch press. The maximum forming limit on a turret machine is 0.984 in.; on a single-head system it’s 3 in., giving the latter the advantage when it comes to flexibility in forming. When producing parts in polished stainless steel or for applications that call for a highly cosmetic part, such as a medical device, a single-head punch press has the upper hand because of its retractable die. The die can be lowered out of the way while moving the workpiece into position for its next hit, then raised again once the workpiece is in position. 

 

 

This virtually eliminates any scratching on the workpiece. Scratching of the underside of the workpiece is common when punching using a turret punch press because the workpiece moves on top of the dies in the turret. Using dies with brushes helps minimize scratches on the workpiece.

 

If keeping noise levels to a minimum is a priority, a single-head design can employ whisper tools and special urethane strippers. These features reduce the decibel level by as much as 50 percent. 

 

Keep in mind that today’s turret punch presses are also quieter than past designs, so, they, too, afford a level of noise control. A certain level of noise is to be expected when punching, but there are other solutions, such as acoustical curtains and enclosures that offer a way to lower the decibel level.

 

The servo-electric punch press offers the advantage of energy efficiency with a low consumption of approximately 20 kVA. This is especially beneficial for companies with high electricity usage needs (operating many machines) or for those with a maximum kVA limit. 

 

However, certain models of hydraulic punch presses are also energy-efficient thanks to a low-pressure/high-pressure system that helps lower energy consumption by better regulating the hydraulic pump and motor during machine operation. Using two pressure circuits, the system applies low pressure, high flow for lower tonnage, high hit rate tools, and high pressure, low flow for those applications requiring higher tonnage tools.

 

Automating the offloading of punched parts is easier on a single-head system because the larger feed clearance makes it possible to pick a broader range of part sizes. The automation device has more space between the tool magazine and the table to access these parts. 

 

 

 

Because a turret-style punch press has less feed clearance, the turret can interfere with this parts picking operation as there’s more chance of the automated device colliding with the turret. As a result, parts picking on a turret is restricted to larger sized parts. This is important to keep in mind if adding automation.

 

When it comes to maintenance, a hydraulic single-head punch press and a hydraulic turret punch press involve the same amount of upkeep on the hydraulic unit. However, the electric-drive punch press, because it has no oil and fewer components than a hydraulically driven punching machine, may require less maintenance than either of these designs.

 

Overall, the choice of punch press is as individual as the application. Now with more machine designs on the market and greater capabilities across a range of punching machines, it’s important to take a close look at what each punching technology offers and consider the system that can deliver the best all-around solution for the requirements.

 

LVD Strippit

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