Page 40 - MetalForming July 2016

Tooling by Design
By Peter Ulintz
Progressive-Die Carriers
With all material running through progressive stamp- ing dies we find the carrier— the material that connects, holds and transports the stampings from station to station as they move sequentially through the die. Also known as webs, strips, ties and attachments, the types of carriers and their shapes vary based on product geometry, material thick- ness and strength, material flow during deformation, part stability in the die- tip position, lifting distance, and press speed. The carrier-strip profile may be straight, zig-zagged or looped, depend- ing on the optimum location of the attachment points on the part, and the clearance required in the die.
Tool and die design engineers select among several types of carriers:
• Center carriers, created by cutting a periphery around the part that leaves a strip of material near the center of the part. A narrow strip allows the die to perform forming and other work all around the part, while a wide strip allows work to be performed along the outside of the part.
• Inboard carriers, attached to the part somewhere between the center and the outside of the part. This type of carrier stays within the stock width,
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His back- ground includes tool and die making, tool engi- neering, process design, engineering manage- ment and advanced product development. As an educator and technical
presenter, Peter speaks at PMA national seminars, regional roundtables, international conferences, and college and university programs. He also pro- vides onsite training and consultations to the met- alforming industry.
Peter Ulintz
Technical Director, PMA pulintz@pma.org
avoiding the need for extra material to create the carrier.
• Lance carriers, created by lancing the coil stock rather than trimming material away. Lancing eliminates scrap material between parts but tends to leave shavings in the die—this prob- lem can outweigh the benefit of reduced scrap.
• Outside carriers, generally attached to the sides of a stamped part to allow work to be performed on or around the center of the strip. Outside carriers make it easier to carry the strip on out- side lifter rails, and also provide good balance to push-feed
the strip through the
die. Outside carries are
created by trimming,
lancing, or by punch-
ing dog-bone shapes
for drawing.
be 1⁄8 to 1⁄4 in. for medium-sized dies requiring carriers that must flex, while larger dies may require wider carriers in order to push the coil stock through the die.
When using two carriers, keep their lengths as consistent as possible to balance strip movement. Consistent carrier length will help prevent carri- er-strip distortion and twisting of the stamped parts. Use a “natural” attach- ment point on the parts that allows for easy cutoff at the end of the die. But remember: The direction of the cutoff burr can be a significant factor for the
• One-sided carri-
ers, which attach to
the stamped part on
one side only. This allows work to be performed on three sides of the part. However, one-sided carriers may allow the strip to veer off to one side of the die, creating problems with part loca- tion in the die stations.
• Solid carriers, which find use when there is no material movement expect- ed during the stamping process. The coil stock remains solid until the cutoff or blanking station generates the final periphery.
Design Considerations
Try to design carriers within the stock width and pitch required for the blank. Otherwise, additional material must be allocated for the coil-stock width or the progression to accommo- date the carrier. Strive for a minimum carrier width not less that two times the material thickness to help ensure good cutting. Typical carrier width may
Try to design carriers within the stock width and pitch required for the blank.
part edge.
If the action of the
die requires the carriers to stretch or flex, be sure to design the car- rier with loops long enough to stretch or flex without breaking. The resulting carrier must be strong enough to
feed all of the parts through their full progression. In some cases, three flex carriers may be required.
Make the radii in flex loops as large as practical. Avoid mismatch steps or nicks in the cut edge of the carrier. Small radii and other carrier-edge defects concentrate stresses in these areas, causing the carrier to break when flexed.
When upper punch blocks must extend below the coil stock, or when lower die blocks extend above the coil stock when the die closes, clearance will be required relative to the parts and their carriers. Here, a loop carrier can be stretch-formed vertically (up in the die) to provide the necessary clearance.
Large parts produced from thin materials often require carriers with stiffening beads to add strength and stability. Another stiffening option is
38 MetalForming/July 2016
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