Consistent part quality no longer is achieved by first-piece and periodic inspections. First of all, the process doesn’t work. Secondly, your customers won’t accept it. In fact, some customers may no longer accept “parts to print;” instead, they expect the same quality part, consistently, from container to container, lot to lot, and shipment to shipment.
Manufacturers who also weld or assemble metal stampings produced inhouse also demand that their press shop supply consistent part quality. Why? Because accepting “parts to print,” permitted to vary through the entire tolerance range from coil to coil or run to run, causes time-consuming adjustments to assembly fixtures, welding jigs, positioners and locators. In high-volume applications, this may require adjustments to multiple fixtures or process lines. Then, additional time must be spent fine-tuning so that the same-quality part comes off each fixture or process.
Part-to-part inconsistencies may result from lack of precision in the die when processing one part to the next; a sloppy press or ram slide that fails to repeat exactly with each stroke, in distance and parallelism; an inconsistent feed that causes the pilots to correct strip location in both directions; or camber in material that shifts the strip side to side within short distances. These are only a few of the more than 50 process input variables that can affect part quality and dimensional stability. These variables can be organized into groups.
Product design: Depth of draw, consistency of draw depth, features formed in opposite direction, stretch-formed features, size of radii, material thickness specified, material type specified, etc.
Press and press-line equipment: Flatness and residual-stress state after straightening, press speed, working tonnage, available press energy, load balance, press rigidity and press guidance.
Die variables: Die-set construction, die materials, surface roughness, surface treatments, spotting accuracy (draw dies), die rigidity, pressure-pad force and balance, thrust, die heeling, guiding system, alignment, die temperature, wear, dirt, draw beads (change with wear), shims on set blocks (load distribution), shims under tool components (die timing), press operator, etc.
Blank/stock strip: Rolling direction, blank shape (optimized), flatness, cleanliness, burr direction, workpiece temperature, etc.
Lubrication: Lube type, viscosity, amount applied, location applied and the operating temperature range all impact part quality. Lubricants are formulated to work within specific processing-temperature ranges (see figure).
The challenge, then, is to identify the process variables that contribute most to current part-to-part variation, and to control those variables. Let’s start with the obvious—the better the press, stock straightener and feeder, the better the results. The same goes for the material to be processed—better quality and consistency improves stamped-part quality.
Properly designed and built tooling that does not permit deflections and fluctuations helps as well. Designing dies to process critical dimensions simultaneously will result in stamped parts that are as accurate as the tooling.
Sometimes, small changes in press speed-due to insufficient flywheel energy—will contribute to variations in product quality.
Sleuthing the Prog-Die Strip
Progressive-die processes can be challenging, especially when trying to solve quality-related problems. But a comprehensive inspection of the die strip can tell much about what has occurred during the stamping process. The strip is the equivalent of an eyewitness of a car accident. Like the witness, the strip was physically present during the entire process, and experienced everything that occurred. It knows, for example, if:
• The material fed properly.
• The pilots accurately positioned the strip.
• Pitch length (the distance between pilot holes) grew or shrunk beyond spec.
• A stretch flange split due to a mismatched cut.
• Extrusions split due to a burr.
• The die is hitting too hard or not hard enough.
• Deep-drawn features split due to restricted material flow or an incorrect radius.
• The carrier design is appropriate.
• The die is hitting level in all stations.
• Die timing is correct with the die fully loaded.
• Cutting clearances change with the die fully loaded, due to tipping moments.
Press shops also must monitor tool wear and fatigue—breakage not caused by a die crash—and implement preventive-maintenance programs to avoid wear-related instability.
Finally, don’t overlook how changes in process temperature affect linear expansion of press members and die components. A press structure will change size as it warms to its normal operating temperature on a cold morning, or when it cools during a long repair or lunch break. In-die process heat, due to friction, can change the characteristics of punches and die sections by altering their size and clearances. When the press structure and die components change shape, the parts that they produce surely will be affected.Achieving consistent part quality relates directly to how well the stamping process inputs are understood and, most importantly, how well they are controlled. MF
Related Enterprise Zones: Tool & Die
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