Page 44 - MetalForming June 2013

The Science of Forming By Stuart Keeler
Tooling Technology
Stamping is No Good–Who Gets the Blame?
Happens all the time—rejected stampings start the finger point- ing, and the press-shop owner’s cry for an explanation is answered by a material supplier refusing to accept responsibility. “Thousands of other stampings are made daily by other press shops using the identical mate- rial,” says the material supplier. “There- fore, the press/die combination must be faulty.” The press-shop owner coun- ters that the material must be at fault “because similar material from other suppliers results in perfect stampings in my press/die combination.”
The blame game has begun, and when it ends (hypothetically), both the workpiece material and the press/die are off the blame list. Instead, the rejec- tion occurs because the material prop- erties do not match the requirements of the press and die. The supplier and the press shop share the blame because nei- ther has acquired and exchanged the data needed to ensure a successful match resulting in an acceptable stamping.
Metalformers and their material suppliers have struggled to match material properties to the needs of the press/die for several reasons:
1) Metalforming as a system is com- plex and difficult to understand.
Stuart Keeler (Keeler Technologies LLC) is known worldwide for his discovery of forming limit diagrams, development of circle-grid analysis and implementation of other press-shop analysis
tools. Keeler’s metalform- ing experience includes 24 years at National
Steel Corporation and 12 years at The Budd Com- pany Technical Center, enabling him to bring a very diverse background to this column and to the sem- inars he teaches for PMA.
Keeler Technologies LLC
P.O. Box 283, Grosse Ile, MI 48138 Fax: 734/671-2271 keeltech@comcast.net
Decades ago, the properties of low- strength mild steel and high-strength low-alloy (HSLA) steel were simple, easily measured and well-controlled. Yield and tensile strength, total elon- gation and hardness readings were suf- ficient to predict material formability. A material’s hardness predicts its resist- ance to surface wear, and has little cor- relation to formability. Instead, the more important forming parameters of sheetmetal forming to track include work-hardening exponent (n-value), strain-rate hardening (m-value), direc- tionality of properties (r-value), form-
each forming mode requires the amount of deformation required to reach the final shape. The interaction between forming modes initially is unknown and available only after the stamping is completed.
4) We are great at post-mortems but poor at predictions. We can use SPC to chart historical trends and identify pro- duction blips in great detail. We rarely, however, can predict the exact behav- ior of a future press run, even if we know the performance of the previous 10 or 20 runs. Compare this to doctors proficient at identifying the cause of death but unable to accurately predict
ing-limit curves (FLC) microstructure during forming.
and change of
2) Unfortunately,
much of this charac-
terization and under-
standing has been con-
fined to the research
laboratory and has
been slow to be inte-
grated into day-to-day press-shop operations. A tremendous disparity exists in the level of metalforming knowledge between the OEM/Tier 1 stampers at one end of the supply chain and lower-tier stampers at the other end. Similar disparities exist between major mills and smaller serv- ice centers and warehouse suppliers.
we achieve an acceptable breakage rate (ideally zero) at die buyoff. The concept of continuing process modifications until attaining a safety margin or a robust process often is viewed as a waste of time and money.
6) Demands are made for the con- sistency of one or two system inputs, while ignoring the roles played by the other 50 or more system inputs. Some believe that constantly forcing improve- ments in the consistency of sheetmet- al will create a more consistent output of the forming system. This ignores the fact that other inputs may be more inconsistent or affect the system much more strongly than material properties.
7) The massive job of monitoring so many system inputs starts with monitoring system output. Stampers need accurate and sensitive output data to detect the onset of input devi- ations. They lose such information
Stampers need accurate and sensitive output data to detect the onset of input deviations.
how long a suggested treatment will extend a patient’s life.
5) One main press- shop rule appears to be, “If it ain’t broke, don’t tinker with it.” At die tryout, we modify the forming process until
42 MetalForming/June 2013
www.metalformingmagazine.com
3) In all analysis projects, disas- sembly is easy while shops struggle with synthesis. A laboratory can dissect a complex stamping into a number of unique forming components, each capable of being studied in isolation. Engineers can quantify relationships between each forming operation and the controlling material properties, as well as other process variables. The reverse, though, proves difficult. Dif- ferent forming operations are con- trolled by different material proper- ties; assigning levels of difficulty to