Page 48 - MetalForming January 2015
P. 48

  Tooling by Design
By Peter Ulintz
Metal Stamping Inconsistencies
Failure to recognize that the pro- duction of metal stampings is a highly interactive process con- sisting of many input variables, all of which must be controlled, contributes mightily to our process and dimen- sional inconsistencies.
The stamping process comprises more than 50 processes inputs—some studies have identified more than 100 inputs—that can affect part quality and dimensional stability. When problems arise, the stamping die, certainly one of many process input variables, often receives disproportionate and unnec- essary attention. One example: Making adjustments inside the die to improve part flatness, when the real problem lies with improperly straightened coil material that releases trapped stress after the coil has been notched or punched.
Part-to-part inconsistency also may be caused by a sloppy press or ram slide that fails to repeat in distance and parallelism with each stroke; an inconsistent feed system that forces the pilots to correct strip location in both directions; or camber in the mate- rial that shifts the strip from side to side within short distances.
Other sources of part-to-part incon- sistency:
Peter Ulintz has worked in the metal stamping and tool and die indus- tries since 1978. He has been employed with the Anchor Manufacturing Group in Cleveland, OH, since 1989. His back- ground includes tool and die making, tool engi- neering, process engi-
neering, engineering management and product development. Peter speaks regularly at PMA semi- nars and conferences. He is also vice president of the North American Deep Drawing Research Group. Peter Ulintz
Incoming Material
Stampers often blame the quality of the incoming material, sometimes with justification. However, the real problem may be how the purchasing manager orders the material, or his reluctance to pay a premium for the higher-quality material that the process requires.
Purchasing material with little tech- nical information—ordering 1010 AKDQ steel, for example—allows for the supplier to provide a wide range of material properties. A low-carbon vac- uum-degassed steel would fall into this specification, as well as a 1006 EDDQ material or a 1008 DDQ. All have less than 0.13-percent carbon content and all have been aluminum-killed. And, all will perform different-
material is simply flattened. On the other hand, a precision straightener can work the incoming material hard enough so that it becomes flat, with minimum or no crossbow, and present the incoming material to the die con- sistently in this condition.
Stampers also can purchase stock feeders that practically eliminate the need for pilots in a die. And, some feed- ers will force pilots to work beyond their original intent. Not presenting the work accurately to the next event surely will result in inconsistencies of product as well as other problems, such as breakage and premature tool wear.
Stamping-press motion is simple and basic. The ram (or
 ly in the die.
It also may be nec-
essary to purchase material with closer- than-standard thick- ness tolerances, a con- trolled yield-strength range or a minimum yield-to-tensile ratio to help process tight-tol- erance stampings.
“Don’t overlook how changes in process temperature can affect linear expansion of press members and die components.”
slide) reciprocates down and up, causing the stamping tool to process the work mate- rial. What we often take for granted is that press repeatability in three- dimensional space (left- right, fore-aft, up-down) produces part consis- tency. Worn or sloppy
Straighteners and Feeders
Straighteners that inadequately straighten the incoming material may require frequent adjustments and com- plex tool compensation.
Stampers select from three basic types of coil-correction equipment: flatteners, precision straighteners and levelers. A five-roll flattener may not adequately remove coil crossbow, but a 13-roll precision straightener will. When crossbow causes quality prob- lems in a particular stamping, con- stant die adjustments (from coil to coil and within a coil) will be required to maintain part quality if the incoming
gibbing, crankshaft bearings or pitman connections will produce inconsisten- cy of the press and, consequently, of the parts being produced.
Don’t overlook how changes in process temperature can affect linear expansion of press members and die components. A press structure will change size as it warms to its normal operating temperature. And, in-die process heat (due to friction) can change the characteristics of punches and die sections by altering their orig- inal design size or clearances. When the press structure and die compo- nents change shape, the parts that they produce surely will be affected.
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