Page 16 - MetalForming November 2011

Die Timing
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To complete the work, the flywheel or hydraulic pump sup- plies energy. As this occurs, the press members deflect.
An analysis of the quantity of energy involved will show why a gradual reduction in cutting pressure prior to snapthrough is critical. A general rule for snapthrough (or reverse load) that a press can withstand without sustaining damage is 10 percent of rated press force or tonnage. Reverse loads significantly higher than 10 percent of total capacity may damage the machine. Particularly critical is the slide con- nection—the attachment of the pitman to the slide. Should this connection fail, the slide may fall unexpectedly.
Some presses are designed to withstand higher reverse loads. For example, manufacturers can supply presses designed to withstand repeated reverse loads of 50 percent of rated capacity or more.
Fig. 4 illustrates the waveform signature of a combined punching, cutoff and joggle-bending operation. Here, AISI-SAE 1039 steel 0.500-in. thick by 2.0 in. wide has two holes punched, and a 0.562-in. joggle formed. The part, an engineering- class chain sidebar, also is cut off in this combined operation.
The die is correctly timed, and snapthrough energy release is well below10 percent capacity of the 300-ton straightside press used for the operation.
Technique for Die Timing
In addition to providing angular shear on the punch and die, the entry of individual punches may be timed to reduce cutting forces. In most cases, the punches penetrate one-third of stock thickness, when rapid plastic yielding (fracture) occurs. Therefore, the entry of the punches usually is stepped in increments of approximately one-third stock thickness.
Tighter-than-needed die clearances increase cutting forces and snapthrough energy. Good tool-engineering prac- tices allow the process to determine the clearances used rather than following arbitrary rules. Some mild-steel jobs work best at 18 percent side clearance, while others (such as hard brass) require very little clearance to avoid a shaving opera- tion. This analytical tool is valuable to optimize processes.
Optimizing punch and die shear, together with stepping punch entry, can reduce peak cutting dramatically. It is important to note that the total flywheel energy required per stroke is not reduced.
The process of optimizing cutting forces can be aided by the use of force monitoring and waveform signature analy- sis. These methods are valuable process-control tools.
Applying This Technology
Metalformers should keep records of die timing, and note the optimum die timing for each job. This information will avoid trial-and-error work when a die is resharpened. Proven timing data also proves invaluable for adjustment of new dies.
Nearly all tonnage monitors start with a DC signal con- verted for a digital signal for display and remote communi- cations. A chart recorder uses the DC signal obtained from a connector on the monitor. MF
14 MetalForming/November 2011
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