Tooling by Design



Tool and Die Quality

By: Peter Ulintz

Sunday, March 1, 2015

When metal stampers deal with acceptance criteria for new stamping dies, they’re asked to comply with product-drawing specifications; fulfill fit, form and function requirements; achieve statistical capability; and reach run-at-rate speeds. While these metrics may provide useful 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 demonstrates that a new die will run at rate and also produce parts that meet print specifications, the following assessments also can be made—while the die remains in the press and before final acceptance of the die.

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 calculations to compress all of the springs in the die at its home position. If the actual and calculated forces differ significantly, look for interference problems 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.

Most importantly, use the tonnage monitor to ensure balanced loads at each of the four corners 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 quality die will not produce reverse tonnage 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 minimum, the incremental increase in stock thickness will result in an equivalent 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 interferences in the die assembly.

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 contacting 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 advancement 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 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 constructed? 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 setting and same feed/pilot release settings. Only the die setting has changed.

Toolroom Inspection

After completing die tryout, perform these additional checks with the die open in the toolroom:

• Have any of the punches or any of the die sections been shimmed? If so, why?

• Do the pins and bushings show wear or score lines?

• Make sure it’s easy to release the stripper shoulder bolts and remove the strippers.

• Bluing the punches should reveal any punch-to-die interference.

• Remove all of the springs and carefully check that the stripper can bottom without any interference or resistance.

• Spot-check punches and dies for proper Rockwell hardness.

• Check that every lifter and shredder spring exhibits even pressure. Are the springs long enough to prevent premature failure due to fatigue?

• How many slugs are retained in the die bushings and die sections? Typically there should be no more than three slugs, four maximum.

• Inspect all wear plates—surfaces should be smooth, not scored. Pay particular attention to wear patterns that appear on one side only, indicating misalignment or uneven pressure from incorrectly set heel blocks.

• Welding and shimming sometimes are unavoidable, but they should not be present in critical areas. Workmanship for any welding or shimming should be high quality.

• Observe how the die components are reassembled. Toolmakers never should have to tighten screws using a pipe over a wrench. Fasteners are designed to function at specific clamp loads. A torque wrench is always required to ensure use of proper clamping loads for each fastener.

If tool and die quality is important to you and your stamping operation, your die acceptance criteria must extend well beyond requiring your tool suppliers to produce parts to print or meet run-at-rate criteria. MF


Related Enterprise Zones: Tool & Die

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