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Die Science: Bridge strippers versus spring strippers
- By Art Hedrick
- March 6, 2017
- Article
- Bending and Forming
A recent consultation job reminded me of the importance of selecting the best method for stripping cutting punches. It’s not that I haven’t seen similar problems in the past; rather it’s the simple fact that using the wrong type of stripper can result in catastrophic tool failure, as well as multitude of other problems in the die.
During metal cutting there is a natural tendency for the metal to collapse around the body or shank of the cutting punches; this is especially true when piercing. That metal must be stripped from the punches.
Bridge Strippers
Bridge strippers, also known as positive or plate strippers, are plates that are solid-mounted to the lower die. They strip or pull the pierced metal from the punch. Bridge strippers:
- Are simple to manufacture.
- Are inexpensive compared to other stripping methods.
- Do not require the use of coil or gas springs.
- Require very low maintenance.
Now, for the not so good news: Bridge strippers present several fundamental problems.
First, they don’t hold the metal down to the lower die sections during the metal cutting and piercing process. This means essentially that the cutting punches will be punching the metal down tight to the lower die. If the metal has curvature caused by coil crossbow or coil set and the cutting punches are at different heights, the extra metal could be bound up between the punches and cause the cutting punches to deflect (see Figure 1). Severe deflection may cause the punches to chip or break.
In addition, punch deflection and binding probably will increase the force needed to strip the metal. If the force is too great, it may bend or break the bridge stripper. If your punches literally are being ripped out of the retainer, there is a good chance that the bridge stripper is the culprit.
Second, the metal is free to move back and forth during other operations, such as flanging or drawing. Eventually the cutting punches will enter the material, and the trapped punches will hold the metal. Even if the strip is designed with a stretch carrier, the entered punches will trap the material, likely causing the punches to deflect and chip.
Third, because bridge strippers don’t hold the metal down tight to the lower die, the metal is free to move from side to side during the cutting process. Unbalanced shear angle ground on the piercing punches can force this sideways movement. Excessive horizontal movement can cause the punches to break. In addition, during withdrawal, the metal slams or “hammers” against the bottom of the plate stripper (see Figure 2), sending a great shock into the pierce punch and possibly resulting in punch breakage during stripping.
Spring Strippers
Spring strippers are spring-loaded plates, either flat or contoured, that hold the material flat and tight to the lower die during cutting (see Figure 3). The spring stripper pad, mounted to the upper die shoe, surrounds the cutting punches. It is secured to the upper die using a standard pad spool, keeper block, shoulder screws, or special commercially available guided spools.
Unlike a bridge stripper, a spring stripper flattens the metal before punch contact. This prevents the material from binding between the cutting punches during entry and withdrawal. In addition, as the punch withdraws from the metal and the lower die, the stripper (which is still spring-loaded) holds the metal down flush with the lower die section, allowing the cutting punches to completely withdraw from the sheet metal or piece part before the spring stripper releases.
Stripper pads often are inserted with a small block of steel called a pad window. This pad window usually is small and light and can be removed easily to allow the die maintenance technician to free the ball lock-style pierce punch from the retainer without removing the entire stripper pad.
Because the spring-loaded stripper holds the metal flat during cutting and withdrawal, the problems typically associated with bridge strippers are nonexistent. However, spring strippers do have some disadvantages:
- They are more expensive to manufacture than bridge strippers.
- They require coil or gas springs to achieve the necessary stripping force.
- They require periodic spring replacement.
The Bottom Line
I’m a no-nonsense kind of guy, especially when it comes to die design and stamping, and in my opinion, although they can and do work, bridge strippers should rarely be used. My experience is that the manufacturing cost savings do not outweigh the trouble and risk that may result. Buy some springs, drill some spring pockets, and invest a few dollars. Lose the bridge stripper. If you are on a real tight budget, use a urethane stripper. I’m confident that you won’t regret your decision.
About the Author
Art Hedrick
10855 Simpson Drive West Private
Greenville, MI 48838
616-894-6855
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The Fabricator is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The Fabricator has served the industry since 1970.
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