April 6, 2004
In today's competitive global market, stampers are looking for ways to reduce tooling and stamping cost by any means possible. Pitch notches, often referred to as French notches, are used commonly to prevent overfeeding and mis-hits in progressive dies. More often than not, however, using a pitch notch to make a part increases material consumption. Does removing the pitch notch save you money or cost you money? This article focuses on several factors to consider when deciding whether to use a pitch notch. It also presents a good alternative to a conventional slug-producing pitch notch and a few tips on designing pitch notches.
Typically, a pitch notch is a small section of material that is cut away from one or both edges of the strip stock at the beginning stations of a progressive die. See Figure 1.
Pitch notches have several functions. Perhaps one of the most important is to provide a solid stop for the material to hit against, so that the operator or die setup person is not allowed to overfeed the tool. Overfeeding a progressive die can result in severe die damage and is a safety hazard. For a progressive die to operate efficiently, it must feed smoothly. Edge camber, resulting from the coil slitting process, can make feeding a progressive die very difficult. By making a straight line cut on the edge of the incoming material, pitch notches remove any severe edge camber that might be present in the coil. This allows the material to feed smoothly through the die. Figure 2shows severe edge camber in a coil.
Often pitch notch stops are designed also to provide a first-hit solid stop. First-hit solid stops provide a starting place for the leading edge of the material to hit against when it is first fed into the die. When properly placed, the pitch notch stop can reduce or eliminate loose scrap or half hits in a progressive die. For large progressive dies that use wide and/or thick coil material, it often is difficult to locate the parts properly using only the pilot pins. Using pitch notch stops with pilots helps locate and register each part in its proper station. When using pitch stops for part location or registration, two notches commonly are used, one on each side of the strip. See Figure 3.
Misconception 1: New coil feeders that feed within 0.0005 inch don't need a pitch notch.
The reality: If your incoming material doesn't have severe edge camber; your operators and setup personnel are highly skilled and trained on progressive die setup and production; misfeeds are rare; and your die protection is always working properly, congratulations! You definitely are in the minority. Keep up the good work. If you haven't met all of these criteria and still don't have misfeeds or die damage, you're just plain lucky.
Misconception 2: Pitch notches cost money; they're wasteful.
The reality: Pitch notches can cost you or save you money. Even though adding a pitch notch most likely will increase material costs, it may prevent severe die damage. Ask yourself how much will it really cost if the die is damaged? Consider not only the die repair cost, but also the downtime cost. The truecan be substantial. I have seen numerous tools damaged from overfeeding because a pitch notch wasn't used. One severe die crash can cost 100 times more than the cost of the additional material consumed by a pitch notch.
Misconception 3: Sophisticated electronic die protection installed in the die to prevent over- and underfeeding negates the need for a pitch or positive stop.
The reality: All of the die protection in the world won't improve the ability to feed smoothly. Die protection does not remove edge camber.
I've seen numerous progressive dies with full electronic protection smashed to bits. Why? The die protection was not set up properly or was overridden during die set. Most progressive die damage occurs in the first 10 hits during die setup. It's amazing just how many dies are equipped with die protection and never hooked up to the press.
Using a conventional pitch notch stop requires slug removal. Shedding a small strip of material as little as two times the metal's thickness can be a problem. Using a lance and flange pitch stop eliminates this problem. A lance and flange pitch stop is created by first lancing a small area in the strip and bending it down to create a straight flange. See Figure 4. The flange not only provides a solid stop, it also stiffens the carrier and aids feeding. This type of specialty pitch notch works well with one-sided carriers, because the flange can be trapped in a bar lifter, preventing side bending during feeding.
By using a pivoting pitch notch stop, proximity switches can be activated to signal the press that the strip has been properly pitched forward and is ready for cycling. This method combined with an exiting sensor offers good die protection. See Figure 5.
Please keep in mind that I am in no way suggesting that you must have a pitch notch. Education is the key to success. Rather than adding numerous unnecessary sensors—which may be bypassed during die setup—educate your die setup and production personnel about the pitch notch and its function.
If you're not using a pitch notch, educate your personnel on the importance of die protection and how it works. Make sure they fully understand pitch, progression, and all of the factors that contribute to smooth feeding. Remember the old saying, "An ounce of prevention is worth a pound of cure." Sometimes it pays to add a pitch notch and reduce the risk of die damage.
A word of advice to die builders—unless directed otherwise, design a pitch notch in the tool and leave the decision of removing it to the stampers. Only they know their operation's capabilities.