Die Science: The basics of troubleshooting die problems
Gimme three steps
In my career as a consultant, I have experienced numerous die-related problems. Some were complex, and some were easy to solve. But most were solved by following basic die design and troubleshooting guidelines.
For a stamping die to function properly, you have to follow three basic die design guidelines, and follow them in order. Disregarding these rules or taking them out of sequence can cause problems such as inconsistency, die damage, misfeeding, and inaccurate dimensions, to name just a few.
Step 1: Locate the Part/Strip Accurately
To ensure part dimensions are completely correct, you first have to locate accurately within the boundaries of the die. To do this, place gauging within the perimeter of the part or blank, locate the part on a punch replicating the shape of the part, or use pilot pins to locate the part or blank.
Pilot pins are used commonly in progressive dies to locate the part. They typically are hardened and ground, cylinder-shaped tool steel pins that enter into prepierced holes and register the location of each part or the strip within the boundaries of the die.
The pilots are placed a consistent distance apart. This distance is called the pitch or the progression length. Pilots don't always have to be round, but they must enter into a prepierced hole that fits the pilot point and geometry precisely.
The clearance around the pilot pin is a judgment based on the thickness of the metal, the size of the part, and the tolerance requirements for the part. For medium to large progressive dies, the pilots typically are 0.002 to 0.003 inch smaller than the prepierced hole they enter. The pilots must be dimensioned so they locate the part accurately without sticking to the part material.
The pilots must extend beyond the surface of the pressure pad enough to locate the strip or parts accurately before the pressure pad holds the material down tight to the die. However, if the pilots extend too far from the surface of the pressure pad, you might have difficulty stripping the metal from the pilots.
The accepted rule is that the bearing surface of the pilot—the point at which the taper turns into a diameter, which is very close to the prepierced hole size—should extend beyond the pressure pad about two times the material thickness. Keep in mind this is just a guideline and doesn’t work for every application. If the material is very thin or very thick, the rule won’t work.
It is good practice to place low-pressure spring pins around each pilot used in the die. These small, low-pressure spring pins help to strip the material from the pilot points as the die opens to ensure that the strip or part remains on the bottom die and does not come up when the upper ram raises. If you are using perimeter gauges, make sure they fit accurately around the outside of the part while still allowing the part to be placed easily on the die and removed. If you are using the shape of the part as a means for locating it, make sure it fits very closely to the punch on which it sits and does not move or tip at all.
Step 2: Hold and Control
Pressure pads, which hold, strip, and control the metal flowing into the die, are critical. If the pressure pads are not holding the material properly or not holding the part tightly to the die, the part or strip may move or become distorted while being formed or cut. The pressure pads must have enough force to prevent the material from shifting during cutting or forming. The amount of force required under the pad is a judgment based on the type, thickness, and deformation taking place.
The pads must fit the shape of the part and die very accurately. A bent or distorted pressure pad might not control the metal, allowing parts to move out of position. If the pressure pad does not fit the geometry of the part accurately, it might cause the part to distort while the cutting punches are stripping from the metal.
In drawing operations, the draw pad must fit very closely to the die face to prevent the metal from wrinkling while it is being drawn into the cavity. Pads used in bending operations must hold the material tight enough to the die so that when bending occurs, the metal is not allowed to shift on the die or pad surface.
The pads must distribute the holding force as evenly as possible throughout the entire part. Use springs of the same force underneath the pad or nitrogen gas springs plumbed to a common control pressure box to ensure uniform pad pressure.
Step 3: Perform Work
Perform whatever duty or task the station was intended to perform. This could include cutting and various types of forming.
The Logic Behind the Sequence
Each one of these steps must happen in sequence. If you try to perform work before the part or strip is located properly, the work performed will likely be in the wrong location. Items such as bend lines and pierce holes will be mislocated and out of position.
If you attempt to draw metal before locating the blank properly, the form will be out of time with the edge of the blank, and splitting might occur. Too much metal outside of the punch on one side of the part may cause a resistance for the metal to flow, resulting in splitting. On the opposite side, too much metal might flow into the tool, causing wrinkling.
If you attempt to hold the parts tightly to the die before locating them, you will have difficulty locating them. As a result, the pilots or the gauge will have a lot of trouble moving the part or strip. If you try to locate the parts and then form or cut, the material will not be clamped down tightly to the die. This most likely will result in part distortion and nonconforming geometries, as well as the inability to feed the tool accurately.
Following these three steps in sequence can prevent a lot of die problems and headaches. Take the time to ensure your parts are precisely located. Take the time to ensure that your pressure pads fit properly and are exerting enough force to control and strip the material.
STAMPING Journal is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping.