Die Science: Handling production splitting problems, Part II

Possible Cause 5— Excessive Blank Holder Pressure

Excessive blank holder pressure restricts metal from flowing inward. Restricting metal flow causes the metal to stretch, which could result in a split. Be sure to monitor blank holder pressure carefully, because nitrogen/gas systems can leak over a period of time.

To achieve constant pressure, hook the system up to nitrogen tanks and adjust the pressure on the tanks. This allows the leaking gas to be replaced immediately while constant pressure is marinated. Excessive blank holder force also can be controlled through the use of equalizers.

Possible Cause 6— Insufficient Blank Holder Pressure

Insufficient blank holder pressure can allow the metal to wrinkle when it’ subjected to compression. Wrinkled metal causes a resistance to flow, especially when trapped in the side wall with enough clearance for one one metal thickness.

Remember that wrinkles can cause fractures, and fractures can cause wrinkling. Metal that fractures in tension may wrinkle when tension is lost (see Figure 1 ).

Possible Cause 7—Poor Pressure Distribution

During drawing it is very important that the pressure exerted on the blank holder be distributed as evenly as possible through the entire blank holder surface. This is especially important when equalizers are not used.

Excessive pressure in any given area can cause the blank holder to deflect and apply more holding pressure to the metal in that area. This prevents the metal from flowing inward, possibly resulting in a split.

Poorly maintained press cushions are notorious for causing poor pressure distribution. For a press cushion apply even force, it must be machined perfectly flat and be thick enough to resist deflection. The cushion pins, typically made of low-carbon steel, must all be machined to the same length.

If the pins are exactly the same length and the press cushion is flat, clean, and well-maintained, pressure distribution will be even throughout the entire blank holder surface. However, if the cushion pins are damaged, bent, or of unequal lengths, binder deflection can occur, resulting in a poor fit between the die face and the blank holder and possibly resulting in a loss of metal control. In addition, if the press cushion is damaged, deleted, or dirty, regardless of how accurately the cushion pins are machined, binder deflection might occur. It also is important to make sure that the bottom surface of the blank holder is flat and not dented or dirty.

Problems typically occur during die setup and removal, especially when the cushion pins are removed. Often damaged or squashed pins get placed into the cushion pin holes, resulting in inconsistent pressure distribution from one setup to the next (see Figure 2 ).

Possible Cause 8 — Improperly Adjusted Equalizes

Equalizers maintain a specified gap between the die face and the blank holder, regardless of excessive pressure that might be exerted on the blank holder. Equalizers also allow simple adjustments to be made from minor sheet thickness variations.

All too often equalizers get pounded into the die face or blank holder surface. This penetration into the surface causes the gap between the die face and the blank holder to be reduced, resulting in excessive pinching of the material and a loss of flow. If your equalizers are squashed or have penetrated into the blank holder or die face surface, recalibrate and shim them as necessary (see Figure 3 ).

Possible Cause 9— Poor Blank Holder Fit

If the die has been broken or has been in production for several years, there is a good chance that the blank holder doesn’t fit the die face well. Trying to control metal flow with a severely eroded blank holder or die face is extremely difficult.

Take the time to resurface both the blank holder and the die face, and cruelly spot the blank holder with respect to changes that occur in the metal’s thickness during drawing. Keep in mind that metal thickens during compression and thins in tension. This special spotting process is often called a running spot or draw spotting.

Possible Cause 10— Improper Lubrication

Lubrication seems like a fairly obvious controlling factor, but surprisingly it’s dismissed often.

Make sure that you are using the proper lubrication for the job and that it’s mixed correctly. Unless you’re doing just basic cutting and bending, it’s unlikely that you’ll be able to use a single lubricant for all drawing applications.

In addition, make sure that both sides of the material are lubricated.

Possible Cause 11— Excessive Press Speed

Excessive ram velocity can cause metal to split. Although increasing the strokes per minute may increase production, be sure not to exceed the process speed limits.

Possible Cause 12 — Bad Die Surface Finish

Erosive, corrosive, and adhesive galling can result in a severe increase in friction, as well as part scoring. Make sure that the die surface is finished and polished correctly. Look for rough surfaces and signs of galling.

In addition, make sure that all radii are free of hooks or imperfections. Minor imperfection s in a punch or die entry radius can result in splitting. If galling persists, it may be necessary to replace the die section with a higher-quality, more wear-resistant tool steel or to have the die section coated.

Permanent Corrective Action

The key to solving splits is to find the root cause and take permanent corrective action. Periodically check your tools for galling, server erosion, and signs of production wear. Whenever possible, make corrections before the die is placed in the press.

Remember the old saying: An ounce of prevention is worth a pound of cure

Until next time… Best of Luck!