Page 32 - MetalForming March 2015

CNC Punching
Fig. 2—Slitting operations are best per- formed with fully guided tooling. The punch point should enter the center of the die to deliver the best edge quality while reducing tool and machine wear.
For long slitting operations, use fully guided slitting tools, especially during partial hits. These securely and precise- ly position the punch points so they enter the centers of the dies (Fig. 2). This allows the press to operate at high speed with good part quality. Also pay attention to the tool’s punching-force calculation, ensuring it is correct for the material being punched. As tools become dull, punching force increases, which wears the tooling and the press. It is best to limit the punching force for a particular slitting tool to 80 percent of press capacity.
Relieved Blanking Tools Prevent Damage to Small Forms
Tabs, embosses and other small forms often are formed ahead of being punched or blanked from the sheet material. When blanking occurs, these
Fig. 3—Prepunched forms are preserved when follow-up blanking is performed with a custom blanking tool.
small forms often are damaged or destroyed, wasting material and press time.
A custom tool with reliefs allows prepunched forms to be included in the part, preventing damage to the forms (Fig. 3). The small forms may be placed directly in the sheetmetal prior to part blanking. This allows processing of small forms in the press rather than in a secondary operation. Including a relief in the tooling allows forms to enter the relieved area of the tool dur- ing blanking. Large areas can be relieved to accommodate multiple forms for a part or an area specific to a particular form.
Overall, good-quality blanks depend on part size, die clearance, height, length and width of part forms as well as location of the part. Punches should be flat using straight taper dies. As always, quality punching requires sharp punches and dies.
Get More Forming Capacity Using the A-Station
Due to a limited number of press stations, production challenges occur
Fig. 4—Punch forms, such as this embossed feature using both the A- and B-station, can expand the range of a punch press.
when stations fill. To expand the form- ing capacity of a CNC punch press, consider using the A-station as well as the B-station (Fig. 4). Fabricators most often use the B-station for forms such as louvers, knockouts and embosses. The following tools find possible use in the thick-turret A-station: centerpoint up-form and down-form, countersink up-form and down-form, emboss, extrusion down-form, shearbutton,
and V-line stencil for logos and sym- bols. Using these forming tools in the A- station expands the punching range of the press, reduces tooling setups and increases punching output.
Punch Special Shapes More Efficiently Using the Auto-Index Station
Most presses feature an auto-index station that can be used effectively to punch large contours in a part. Using
Fig. 5—An inside/outside radius tool is ideal for punching a large radius. Such tools punch higher-quality edges with fewer strokes than smaller nibbling tools.
special shape punches and dies, the auto-index station can be rotated from the machine program to any designat- ed angle, creating the desired contour. This proves far more efficient and pro- duces better edge quality than nib- bling large holes with a small punch. It also eliminates multiple strokes using small punches that create excess punching debris, tooling and machine wear (Fig. 5).
Three shaped tools in particular can provide more efficient contour punch- ing. Quad radius tools produce large holes with smooth edges, and inside/ outside radius tools retain punch slugs or parts in the sheet. Banana tools, similar to inside/outside radius tools, offer the added benefit of using the inner part of the tool to nest smaller parts. Also, unlike inside/outside radius tools with sharp points that can break, obround banana tools may be more effective in thicker material.
Form Knockouts that Exceed Station Range or Press Tonnage Capacity
To achieve knockouts larger than the presumed capability of the punch
30 MetalForming/March 2015
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