Die maintenance. Die repair.

What's the difference?

By Art Hedrick

November 8, 2005

Unnecessary die repair stems from several basic shortcomings, namely poor die design, setup procedures, tool design, and maintenance techniques.

Figure 1 When you grind tool steel sections such as D2, M4, and powdered metals, you actually are grinding small carbide pools. Photos courtesy of Bohler Uddeholm.

If you find your daily activities include tasks such as fixing broken pads, welding a damaged die section after a dowel falls out of the upper half, digging two parts out of a draw die, or replacing pilots that have pierce holes in a progressive die, that's die repair.

Unnecessary die repair stems from several basic shortcomings, namely, poor die design, setup procedures, tool design, and maintenance techniques.

It's discouraging how many die maintenance programs revolve around a poor die design. For example, let's say coil springs are breaking in the die every 2,000 strokes of the press. To ensure that they are replaced before breakage, you could schedule spring replacement every 1,500 strokes. However, the springs should actually last at least 10 times longer than that.

The root of the problem most likely is that the die in which the spring is used was designed improperly or that the spring is deflecting far beyond its most efficient rate. The long-term solution is to fix the root problem by using a longer spring or a gas cylinder.

If your daily activities consist of sharpening cutting sections because of normal wear, replacing springs before their expected life cycle, periodically cleaning the dies and inspecting them for loose dowels or sections, and resurfacing and lubricating components as necessary, that's die maintenance.

Unfortunately, most of the individuals given the job title "die maintenance" spend little time executing actual preventive maintenance. Most of the time they are running around the stamping shop like chickens with their heads cut off, trying to put out major production fires—addressing splitting problems in a forming die; trying to make adjustments for a 90-degree bend; and shimming this, that, and the other thing in an effort to make the parts fit the checking fixtures.

Figure 2 Grind shear angles so the cutting action is balanced.

True preventive maintenance procedures involve items that need to be addressed regularly, regardless of how well the die was engineered.
Sharpening Cutting Sections

Cutting sections and punch edges break down over a period of time, resulting in part defects such as burrs, feeding problems, and safety issues, to name just a few. For this reason, these die sections and punches must be sharpened periodically.

'Here are some guidelines to follow when sharpening cutting sections, pierce punches, and buttons:

• Use the right grinding wheel— Remember that when you grind tool steel sections such as D2, M4, and powdered metals, you actually are grinding small carbide pools (seeFigure 1). Carbide is a very hard metal to cut. Use a wheel that breaks down and does not load up during the grinding process. Wheels that load up can cause excessive heat to build up in the die's sections, resulting in softening, heat checking, or cracking of the die section or punch. Using flood coolant methods will help keep your die section as cool as possible.
  
  Think of your grinding wheel as millions of single-point cutting tools (sand) held together by a bonding agent. Once the small granules of sand get dull, they must fall off the wheel. If they do not fall off, they will no longer cut smoothly and will begin to load up and burn the die section.
  
  Consult with your grinding provider to determine the best grinding wheels for your application. Keep in mind that there is no single miracle wheel that can be used for all tool steel and low-carbon applications.
  
  
• Grind the proper cutting shear—Angularity ground on the cutting sections helps to reduce the necessary cutting force and helps quiet down the die. The correct cutting shear also reduces the shock loading of the die and press, prolonging press and die life.
  
  Grind shear angles so the cutting action is balanced. This will help to maintain punch-to-button alignment and prevent excessive shifting of the piece part (seeFigure 2).
  
  Grind your die section so that small, weak appendages enter the die last. This helps prevent impact shocking and possible breakage of a small, weak cutting feature (see Figure 3).

Figure 3 Grind your die section so that small, weak appendages enter the die last, which helps prevent impact shocking and possible breakage of a small, weak cutting feature.

Shimming Procedures

Shimming die sections may be necessary to maintain the timing of each die station. Following are some guidelines for shimming:

• Avoid using multiple thin shims. An excessive number of thin shims placed under a die section creates a spongy die section, which may have a greater tendency to breathe and move. Use fewer, thicker shims instead.
  
• Make sure that there is sufficient clearance for screws, dowels, and slug drop holes. Inspect all pierce holes to make sure that there is no shim interference in the slug drop hole.
  
• Avoid shimming die sections crookedly, and avoid shim overhang on die sections.
  
• Remove all burrs from your shims.
  
• Make sure all sections and shims are free from debris before mounting.

Cleaning and Inspection

Clean your dies regularly and inspect them for loose dowels, screws, and broken springs. Following are some guidelines for die inspection:

• Look for loose fasteners.
  Check all screws to make sure that they are tight.
  Look for missing dowel pins.
  
  
  
• Look for broken springs.
  Determine why the spring broke.
  If the springs are close to the end of their normal life cycle, replace them.
  
  
  
• Look for galled die sections.
  Inspect wear plates and cam surfaces.
  Regrind and fit as necessary.
  Lubricate all necessary mating die surfaces.
  Inspect the die to make sure all safety guards are in place.
  Clean all debris such as slugs, slivers, and lubricant buildup from the dies.
  Dry the die to ensure that it does not rust.
  Look for dulled, chipped, or cracked die sections and replace as needed.
  Make sure all cutting punches are secure in their retainers.

These are just a few of the basic maintenance items that need to be addressed. Create your own list based on your needs, but be sure you're setting a true die maintenance program and not a die repair program.

Until next time ... Best of luck!