Applications for noncosmetic stamped parts
December 13, 2001
This article explores the possibility that servo feeds can be used with pull-through straighteners as opposed to a conventional feed line that uses a powered straightener. Using the servo-PTS (pull-through straightener) can save money on equipment and material. The only limitation may be marking the material so noncosmetic applications are recommended.
Servo feeds have been in use for many years now. People have become comfortable with this technology and it is becoming commonplace. What is not commonplace is the knowledge that servo feeds can be used with pull-through straighteners as opposed to a conventional feed line that uses a powered straightener.
In a conventional feed line, either a reel or a cradle holds the coiled material, which then passes through a powered straightener. The powered straightener has driven rolls that pull the material off the coil, drive it through a series of offset rolls that flex it to remove coil set, and fill a slack loop between the powered straightener and the feed.
Coil set is the curvature or shape material retains after it is uncoiled. This "memory" leaves the material curved in the direction the coil was wrapped, and in this condition it cannot pass freely through the die. A bad part results if the coil set is not removed.
A slack loop is the excess material between the powered straightener and the feed. In a conventional line it has two significant attributes: depth and length.
Depth creates free material that the feed can pull from. The straightener on most systems is not synchronized with the feed. It needs time to refill the slack loop before the next cycle of the feed. The loop stores enough material for several feed lengths.
Length affects the radii of the curves in the loop, which must be gradual enough so it doesn't put coil set back into the straightened material. Lengthening the loop makes the radii more gradual.
In a system using a pull-through straightener, the feed pulls material through a nonpowered straightener. The straightener is located within inches of the feed rolls. It still has offset rolls that flex the material to remove coil set, but none of these rolls are driven. The straightener rolls turn freely on bearings as the material passes through when the feed advances.
The slack loop in a system with a pull-through straightener is different from the slack loop in a conventional line. Consideration should be given only to providing free material for the feed to draw from. No consideration is given to preventing additional coil set in the material because it is straightened after the loop, just before it goes into the feed.
The coil payout system, either a reel or a cradle, is powered when used with a pull-through straightener. As the material is pulled from the slack loop, the payout system refills it to maintain free material for the feed to draw from.
Another difference in the slack loop on a system that has a pull-through straightener is its shape. The shape of a slack loop in a conventional system is a gradual drop to the floor and rise back up to the feed. The slack loop in a pull-through straightener system can take several possible shapes that are determined mostly by the thickness of the material.
Air feeds with pull-through straighteners have been used in factories for decades. Thousands are still in use and earning income for their owners. Pull-through straighteners on air feeds always have offered stampers a low-cost feed line, floor space savings, and ease of use. A servo feed can offer these advantages too.
|Photos courtesy of Dallas Industries, Troy, Mich.|
A servo feed with a pull-through straightener (servo-PTS) offers several advantages over a conventional line with a powered straightener; however, it cannot be used for all applications. Understanding the benefits it can offer, as well as its limitations, is critical to applying the servo-PTS properly.
A primary benefit of a servo-PTS is cost savings. If a stamping operation chooses to replace an air feed-PTS with a servo-PTS, it needs to replace only the feed. The existing powered payout system that supplied material to the old air feed-PTS will work just as well with the new servo-PTS.
Savings still can be achieved if an entirely new feed line is purchased. A servo-PTS does not require a powered straightener. The investment for a servo-PTS and payout system typically is 20 percent less than that for a conventional line with the same capacity.
A conventional line (servo feed, powered straightener, and pull-off reel) has capacity for 0.250-in.-thick cold-rolled steel that is 12 in. wide and generally about 35 ft. long from the edge of the press to the back of the reel. A servo-PTS feed line with the same capacity typically is about 22 ft. long.
Servo-PTS feed lines also can help save material. The amount of coil set increases as the coil is run out because the inner wraps are much smaller than the outer wraps. As the coil set increases, the original straightener settings no longer remove enough coil set, resulting in bad parts and sometimes even causing die jams.
With a servo-PTS, the straightener is right at the feed and therefore typically within 5 ft. of the die. Results of an adjustment to the straightener are seen in the die quickly. If the adjustment does not work, another adjustment can be made with little wasted material.
In a conventional line, the straightener may be 24 ft. or more from the die. That 24 ft. of material has to be run out just to see if the adjustment worked. If more than one adjustment is needed, a considerable amount of waste can be generated, reducing the profitability of the job. Besides this material savings, the results of straightener adjustments on the servo-PTS also are quicker, which results in increased uptime for the feed line and the press cell it is part of.
A powered straightener has its own drive and a system of sprockets, chain, and gearing to drive the straightener rolls. A pull-through straightener has none of these, and its simplicity means that its maintenance costs typically are lower.
A servo-PTS feed has one limitation that must be considered. Pull-through straighteners sometimes mark material, because the material moves first and the rolls sometimes leave a burnish mark across its width. Anyone who has an air feed-PTS probably has seen this condition.
This means that these feeds are not suitable for applications in which the part requires a highly cosmetic finish. Parts such as outer surfaces of appliances, cosmetic trim, and covers should not be run on these systems. Internal brackets and supports, hitch components, internal car parts, and other parts that do not require a highly cosmetic finish are appropriate for servo-PTS systems.
Servo-PTS feeds can run materials ranging from 0.020 in. up to coil stock more than 0.500 in. thick. They are used in many shops to feed material into progressive dies with pilot punches. The feed rolls are released to allow the material to be moved the few thousandths of an inch required for piloting. Because the straightening rolls are not driven, the material can move in response to the pilots as they enter the material.
In the right application, a servo feed with a pull-through straightener can be effective. The advantages of the system, such as floor space savings and fast adjustment, combined with its relatively low investment, can reduce operating costs and provide the end user with a quick return on the investment.