Page 43 - MetalForming December 2011
P. 43

 Regardless of their final use, all hole extrusions are produced through a combination of sheetmetal bending, stretching and compressing. This action generates a tremendous amount of pressure, heat and friction. Long production runs require use of premi- um tool steel, extreme-pressure lubri- cants and highly polished working sur- faces with wear-resistant coatings. To optimize punch life, metalformers must provide lubrication at the point of operation, especially during punch extraction.
The best tooling materials and coatings, however, cannot compen- sate for inadequate tool design or manufacturing quality. Fig. 2 illus- trates a proven tooling arrangement for forming hole extrusions. The tool must provide a pilot point (A) for loca- tion in the pre-punched hole. The extrusion punch nose radius (B) must blend perfectly with the straight sec- tion of the punch. Both areas must be highly polished, preferably along the working direction of the punch, as this portion of the extrusion punch is subjected to extreme heat and pres- sure. Under these conditions, a very small scratch, unperceivable to the naked eye, will quickly lead to galling in just a few press strokes.
To reduce wear during punch extraction, provide a back relief (C) behind the straight section of the punch. This allows lubrication to reach behind the punch to lubricate the tool surface when the punch extracts. (The relief area in the illustration is exag- gerated for clarity.) As little as 0.001 to 0.002 in. per side can provide great benefits. Note: It may be necessary to plumb an oil line in the die to apply lubrication at this point.
The die button (D) requires a tapered wall on the inside diameter to facilitate easy removal from the die matrix. Provide at least 1 deg. of taper— more if the tolerance band for the extrusion O.D. allows.
Finally, be sure to provide a knock-
out pin inside the die matrix (E) to help lift the extrusion from the bushing. This is especially important to prevent misfeeds in progressive dies.
Another type of extrusion is the pierce extrusion. The difference between this type of extrusion and a hole extrusion: a pierce extrusion does
not produce a slug. These extrusions always develop fractures (Fig. 3) and dimensional stability is very unpre- dictable. These extrusions are usually used as locating features or screw- attachment points for assembled structures with little or no loading conditions. MF
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