Page 44 - MetalForming March 2015

Tooling by Design
By Peter Ulintz
Tool and Die Quality
When metal stampers deal with acceptance criteria for new stamping dies, they’re asked to comply with product-drawing spec- ifications; fulfill fit, form and function requirements; achieve statistical capa- bility; and reach run-at-rate speeds. While these metrics may provide use- ful information regarding the quality of metal stampings, they reveal nothing about the quality of the die itself. It is possible, after all, for poorly designed dies to produce parts that meet these criteria, at least for a short awhile.
An industrial definition for “quality die” does not exist because quality interpretation can vary greatly between industries and manufacturers. None- theless, I’ll present some suggestions to allow metal stampers to better assess the quality of their new tools, beyond meeting part-print and run-at-rate requirements.
In-Press Evaluation
After the tool-build shop demon- strates that a new die will run at rate and also produce parts that meet print specifications, the following assess- ments also can be made—while the die remains in the press and before final acceptance of the die.
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
pete.ulintz@toolingbydesign.com www.toolingbydesign.com
Set the empty die in the press (no material loaded) and bring the press slide down to its home position. The load reading on the tonnage monitor should be very close to the force cal- culations to compress all of the springs in the die at its home position. If the actual and calculated forces differ sig- nificantly, look for interference prob- lems in the die.
After checking the empty die, load each station; for progressive dies, feed the coil material through the die to load all of the stations. Place lead solder in the machined grooves of each set block, then cycle the press once (one stroke) and compare the calculated cutting, forming and pressure loads to the actual loads.
Look and Listen Carefully
Extraneous sounds from the press often can be attributed to the die or its components.
For example:
• Do the pressure pads and stripper plates make slapping sounds when con- tacting the strip on the downstroke, or make snapping sounds on the upstroke?
• Does the strip or part pop back up erratically, or does it rise smoothly? In progressive dies, is stock advance- ment smooth and free of vibration?
• How does the finished part or strip eject from each cavity or pad surface? Are the parts or strip being pulled up with the stripper on the upstroke?
• Do you hear squeaking sounds when the die closes? This
Most importantly, use
the tonnage monitor to
ensure balanced loads
at each of the four cor-
ners of the press slide.
Any off-center loading
must not exceed the
maximum rating for the press being used. Also verify the reverse-tonnage loads, ensuring that they do not exceed the press manufacturer’s limit. A qual- ity die will not produce reverse ton- nage or off-center loads that exceed the maximum ratings of the press.
Now measure the solder thickness at each set block—it should be 0.005 in. thicker than the grooves machined in the set block when the stock thickness is at the low limit of its tolerance. For stock thicknesses greater than the min- imum, the incremental increase in stock thickness will result in an equiv- alent increase in solder thickness. Ensure equal thickness readings for each of the set blocks. If they differ from one side of the die to the other, ensure that the press ram is level at the home position. If it is, look for inter- ferences in the die assembly.
may indicate a lack of lubricant or binding of the pressure pads or guidance systems, due to off-center loading.
• How well are the scrap chutes construct- ed? Poorly constructed scrap chutes can indicate that the die builder does not understand the need for the scrap
and part to freely fall away.
Unload all of the die stations; for
progressive dies, cut the strip. Leave the remaining material in the feed equipment, and completely remove the die assembly from the press. Then reinstall the tool and load all of the die stations. Apply any lubrication as before and begin stamping parts. If good- quality parts cannot be reproduced without simple adjustments—usually to compensate for temperature changes in the tool and press —there may be a problem with the tool. Remember, nothing else in the process has changed —same press, same lubricant, same coil material, same straightener set- ting and same feed/pilot release set- tings. Only the die setting has changed.
Do you hear squeaking sounds when the die closes?
42 MetalForming/March 2015
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