Forming on the modern punch press: sheet metal fabrication’s Swiss army knife3月 20, 2018
A punch press isn’t just a punch press anymore. A Swiss army knife of metal fabrication, the punch press now can mark, deburr, and form. And when it comes to forming, the opportunities abound.
Automated punch presses have been punching holes in sheet metal for more than 60 years. How many holes have they punched? Who knows? My guess would be somewhere in the quintillions (that’s with 18 zeros).
However, with their stroke control and the availability of adjustable-length punch holders, CNC punch presses are also being used as mini press brakes. Many types of forms are being produced in parts before they come off the turret or rail-type punch machine.
Countersinks and small embosses are among the basic forming tools being used in CNC punch press machines to reduce or eliminate secondary operations. Lance and forms (tabs, bridges, knockouts, cable ties) and extrusions are among the advanced class of forms being produced in punching machines today. More complex forming tools include those that produce louvers, continuous embosses, and hinges.
Forming tools add value by eliminating secondary operations such as forming, welding, and installing fasteners. The press brake isn’t going anywhere, of course. Space between the punch and die will always limit what can be formed on the punch press, at least for the foreseeable future. But if a part needs shallow forms like an emboss, a short flange, or even a hinge, chances are those forms can be made right on the punch press machine bed—no secondary ops required.
How Punch Presses Form
Infinite stroke control in many of today’s punch press machines allows the operator to adjust the ram stroke in 0.0005-in. increments, improving forming accuracy. Stroke control accuracy also can help with small adjustments needed because of variances in material thickness.
Most modern punch machines also have control of the ram’s upstroke. This feature allows the programmer or operator to set the upstroke for the forming tool high enough to clear any forms produced while also raising the tool to a height lower than top dead center, thus maximizing speed and efficiency.
Upforming—forming with a lower ram, forming cylinder, or sliding wedge mechanism—sometimes allows the punch press to make tall forms. Many forming tools are designed to form upward with spring-loaded strippers in the die assemblies, resulting in tall die heights. Still, the tall dies do take up room in a turret or punching head, and thus reduce the space available for some forms.
In most machines capable of upforming, the die sits lower in the die pocket or holder, providing extra space between the upper and lower tools to create tall forms such as tabs, louvers, large embosses, and hinges.
Adjustable-length punch holders not only allow for the fine-tuning of forms, they also allow some older-model punch presses to use a wider range of forming tools. These punch holders allow operators to adjust the upper tool length even if stroke control is not available. This improves accuracy and reduces setup time when adjusting for forming heights.
Countersink tools create countersinks in punched holes to accommodate flathead screws and other specialty hardware and fasteners. These tools eliminate the need to set up a drill press or other secondary operation to make the countersink.
The features of a formed countersink are the form angle, major diameter, minor diameter, and depth (see Figure 1). Unlike a machined countersink, a formed countersink does not remove but instead displaces material.
The point and the surface of the punch apply force and displace material as the sheet is pressed against a blank die. The prepierce size is ultimately responsible for the finished minor diameter and depth.
Countersink tools come in dedicated and universal varieties. A dedicated tool, good for deep countersinking in thick material, creates a specific countersink geometry. A universal tool can vary its depth and, in so doing, create countersinks for a range of screw sizes. Although specifics vary depending on the tool and workpiece material, a dedicated tool generally can countersink a greater percentage of the sheet’s overall thickness. Certain countersink tools (such as those with a special feature called a pilot nose) allow for countersinks up to 100 percent of the material thickness.
Using emboss tools, the punch machine can produce simple embosses such as a stamped company logo, a raised surface to allow clearance for specific hardware, and small dimples placed into a sheet to help strengthen the part or enhance its visual appeal.
Embossing on the punch press occurs either via forming or cold forging. Fabricators usually use cold-forge embossing to create text and designs on the sheet metal surface. A normal punch stroke shears metal and creates a slug. But the material also can be forged upward, often by as much as 50 percent of the material thickness, by stopping the punch stroke before the metal fractures and shears off as a slug.
The formed emboss (see Figure 2) is created by bending the material around the tool, compressing the inner surface and stretching the outer surface at the locations where the material changes direction. As the material flows over and around the tool, material is also pulled into the formed emboss from the surrounding sheet. The amount of material being pulled into the form and the ductility of the material (its ability to be stretched) can affect the flatness of the surrounding sheet.
Note that as the form takes place, material within the emboss is stretched and thinned, which creates stresses that can exceed the strength of the material, resulting in failure—usually at the transition points (the radii that form the emboss) where a change in direction occurs, as shown in the cross section in Figure 3. Also, thick material with high ductility will stretch more before failing than thin material with low ductility.
Reducing those stresses can help make embossing more consistent. Ways to reduce stress include increasing the forming angle and radii at those transition points, using more ductile material, and prepiercing a hole in the center of the emboss.
For instance, a combination of material properties and emboss proportions may cause the flat top of an emboss to arch into a domed shape after forming. If a domed emboss shape is unacceptable, prepiercing a hole in the top of the dome will remove much of the stress that causes the material to arch, and the form will remain much flatter across the top (Figure 4).
A continuous emboss tool creates forms that are longer than can be achieved with a single-hit tool (see Figure 5). It also can be designed to form large-diameter round embosses or extrusions. The height of these forms is limited based on the material type and thickness, as well as the machine type and configuration. For best results, the radius of the lower tool must be at least two times the material thickness.
Stencil Tools: Snapping and Bending
V-line stencil tools place stamped logos, identifiers, and instructions into the face of the sheet metal. The V-line concept is also used with tools designed for tabbing thin-material parts into a nest for easy removal after punching, as well as creating snap lines.
As the tools penetrate the sheet, they create a line of weakness—that is, a snap line—in the upper and lower surfaces of the sheet material. This allows operators to snap a burr-free part loose from the rest of the sheet. Snap-type tools also can help break down sheet skeletons, minimizing the space required for scrap material.
V-line stencil tools also can create bend lines, which allow operators to bend workpieces by hand to a designated angle (see Figure 6). Bend lines can work in material up to a certain thickness and length (for example, up to 0.078-in.-thick mild steel, with bends up to 12 in. long, depending on the tool and application).
Fabricators need to consider a part’s design requirements as well. But done in the right way for the right application, forming bend lines on the punch press can eliminate all or part of a press brake operation. In these cases, the V-line stencil tool creates a V-shaped groove in the sheet—not so deep that the sheets snap apart, but deep enough so that the operators can bend them. The angle of the V stencil point determines the angle that can be formed.
Lance and Form
A variety of lance and form tools can produce a multitude of different features, including shear tabs, clip attachments, card guides, wire harnesses or cable clamps, and features for airflow or airflow deflection. Using these tools, the punch press lances (cuts) the material and creates a form. Lance-and-form assemblies can be any size and shape, and forming them generally requires less press power than punching a hole of the same size.
Using good lubricant on the sheet can help make the forming process consistent. Replacing the tool’s cutting components when they dull is always good practice, as is being sure the tool is designed to handle the application’s material type and thickness. If problems arise, decreasing the form height, increasing the form radii, and using more ductile material can help make the operation more consistent.
Electrical knockouts give electrical enclosures access points for wires. A punch press usually produces them by punching a slug but leaving small uncut tabs, which hold the slug in a slightly offset position, either slightly above (formed up) or below (formed down) the parent sheet, depending on the part’s design requirements. The retained slug can be knocked out (hence the term) at a later time (see Figure 7).
Typically, knockout punching uses a single tool to create the slug and tabs. But a punch press also can perform multiple, or segmented, knockouts for larger geometries. This reduces required tonnage, making it easier on the punching machine. Note also that when the knockout is formed down, a tool can work over a large footprint (surface area) without increased tonnage requirements. This is because upforming a knockout uses a spring stripper that requires additional force.
Various locking tab-type forming tools can produce spring-loaded tabs that lock into place when paired and assembled to other components (see Figure 8). This capability can eliminate the need for spot welding, riveting, or fastening with threaded hardware. An assembler need only slide an adjoining sheet or other component into the tab, and the lanced form locks the material together.
Such forms can eliminate expensive secondary operations and allow parts to be attached by hand in assembly. They also allow for the joining of dissimilar material thicknesses and material types, such as stainless steel to aluminum.
Extruded holes are formed up or down to create a formed collar on one side of the material (Figure 9). Commonly tapped as well (also on the punch press), extruded holes are used with thread cutting screws or other fasteners, or they are used as locators for other press-fit applications.
Extrusions can be formed in clusters, with cluster tools piercing and forming multiple extruded holes at once. Some tools can even prepierce the hole, emboss, then form an extruded hole, all in a single hit.
Note that extruded holes in some material, especially stainless steel, can distort during forming. In these cases, a fabricator can apply a good forming lubricant to the material before making the extrusion. Not only will the material release from the die better, it will slide over the die surface smoothly during forming. This gives the material a better opportunity to distribute the forces of bending and stretching, preventing distortion in the formed wall and tearing at the root of the extrusion.
Ribs and Offsets
Rib and offset tools add strength and stability to sheet metal components. In many situations, adding stiffening ribs may allow the use of thinner materials, resulting in material cost savings.
These tools come in two varieties: single-hit and continuous. Single-hit tools stamp a rib or offset, be it a straight line, webbed shape, or anything else, in a single hit. Continuous tools form a rib or offset along a continuous path.
Roller tools fall into this category (see Figure 10). In a rib configuration, the tool uses roller inserts of equal height. The offset configuration consists of one standard insert in the upper tool and one shortened insert. This creates an offset, or multilevel form. With some creative programming, the offset configuration can create a flared window opening or flared part edge.
Louvers on enclosures allow air to flow while protecting the components within from the elements such as sun, wind, rain, and sand. Louver types abound, but when it comes to sheet metal fabrication, closed- and open-end louvers are probably the most popular (see Figure 11). Louvers can vary in width and length and may have an angled or radius-form profile.
Closed-end louvers are cut on one side, and the material adjacent to the cut edge is raised to create an opening in the sheet. Closed-end louvers often have a curved form profile. The punch press also can create a straight-back closed-end louver with a progressive-hit tool that nibbles a louver opening with multiple hits, creating a continuous louver. (The concept is similar to a continuous emboss, shown in Figure 5.) Continuous-louver tools can create closed-end louvers with a curved form profile as well.
Note that a single-hit louver usually will look better aesthetically. That’s because the continuous-louver tools require the material to advance in short increments. During these incremental moves, the material is stretched and subsequently compressed. This operation may work-harden the material and will leave noticeable overlap marks.
Continuous louvers come in various forms, including those with level tops—that is, the top of the louver is not at an angle but is instead level with the parent sheet. For best results, an operator should start a level-top continuous louver in the center of the louver shape, then form one side and then the other, moving in small increments such as just 0.030 in. The tool can rehit the center of the louver to ensure ultimate flatness (see Figure 12).
Open-end louvers are “open” on three sides—front, left, and right—with the back edge bent to create the form. These louvers offer increased airflow, but their exposed ends are best contained inside panels and enclosures. The cuts on each side of the louver are made with what’s known as a draft angle (usually about 10 degrees), which creates a shorter front edge of the form. During forming, the shorter front edge rises and the longer back edge bends to allow the sides to strip easily.
Say a press brake operator needs to form a multitude of shallow flanges. He stands all day by the brake, bending flange after flange. Is there a better way?
Punch presses now have tools to create these and other kinds of bends (see Figure 13). To create a flange, an upper forming tool descends and presses the sheet against a lower die with a rotating V opening. Pressure forces the die to rotate and fold the flange around the upper tool. The flange needs to be of a certain height (about 0.24 in., depending on the tool) to allow the rotating die to initiate the forming process. These tools can produce form heights between 0.300 and 0.700 in., depending on the height of the upper tool.
One of the most challenging sets of forming tools for CNC programmers and operators are the hinge tools. Hinges formed in a punch press are created by standing up a tab and then curling it over using three forming steps.
Forming hinges in a turret or rail-style punch press is a challenging application, but a highly rewarding one when the fabricator understands the relationship among the required hits, locations, and stroke settings. Besides using standard tools to create the tabs that will be formed into a hinge, the process typically involves two forming tools—the initial form tool and the knuckle tool—and three forming strokes.
After the tabs have been punched in the sheet to be formed, the initial forming tool makes the first two strokes. The first stroke produces the leading edge of the tab that will slide around the interior of the knuckle tool during the final forming stroke.
The second stroke then bends the tab up to a specific angle. At this point, the spring-loaded lower assembly, or die, pushes the sheet off the lower insert before the sheet advances to the next forming location.
The knuckle tool performs the final stroke, descending toward the raised tab (see Figure 14). That raised tab enters the upper tool and makes contact near the front edge of the tool. As the tool continues to descend, the tab is forced to slide around the perimeter of the upper tool, curling around to form a hinge.
Knockouts, extrusions, shaped embosses, flanges, radiused or curved tabs, ribs, multiple bend tabs, oversized louvers—myriad forms can be produced in a CNC punch press machine.
And all this just scratches the surface of the forming and marking options possible on the modern punch press. If your machine has the ability and space needed to create a specific form, a tool probably can be designed to produce it.
John Ripka is application technician
for Mate Precision Technologies, 63-421-0230, www.mate.com. Images courtesy of Mate Precision Technologies.
Publication: The Fabricator