Tooling Tips to Promote Profitable Punching
To help ensure quality in your punching projects while simultaneously driving out waste, fabricators should focus on three key areas of their CNC punch-press operations:
• Die clearance and press-station alignment;
• The condition and appearance of the slugs and punched sheets; and
• Basic preventive tool maintenance.
Die Clearance and Turret Alignment
Proper punching-tool use starts with careful handling and accurate placement in the turret.
A good lean work practice: Develop a standard die-clearance chart and train press operators to understand why it’s important to use the correct clearance (Fig. 1). This will help them understand any problems that occur due to incorrect clearance.
Turret-press uptime also will increase when operators maintain accurate station alignment. This ensures the punch enters the center of the die, resulting in even die clearance around the cutting edge and optimum tool life. Extremely accurate tooling products are available to help maintain this important relationship.
Operators can use a specially designed alignment device to ensure proper tool alignment, which will help minimize tool wear and improve part quality.
Slugs Tell a Tale
The condition of the slugs and the punched sheet say much about the condition of the tooling and the quality of the machine setup. The slug is a mirror image of the hole, while the punched sheet represents the final result of the punching sequence. Examine both to verify correct punch-to-die clearance and tool angularity, and for clues that the tooling may have dulled.
Ideal slugs result when the fracture planes coming from the top and bottom of the material have the same angle, and form in alignment with each other (Fig. 2). This minimizes punching force and creates a clean hole with minimal burrs. Excessive clearance will create slugs with a rough fracture plane and a small burnish zone. The larger the clearance, the greater the angle between the fracture plane and the burnish zone. Excess clearance makes a hole with large rollover and fracture, so that the profile is somewhat pointed with a thin burr. Excess clearance also reduces tool life.
If clearance is too small, slugs will exhibit a fracture plane with little angle and a large burnish zone. Inadequate clearance makes a hole with small rollover and steep fracture, so that the profile is more or less perpendicular to the material surface. Insufficient clearance also will reduce tool life.
Punch and Die Maintenance Tips
| Fig. 1
Hole size—Small punches wear more quickly than larger punches.
Hole configuration—Sharp corners will wear more quickly than straight or curved edges, particularly on punches. Narrow sections will wear more quickly than heavier sections.
Punch face—The portion of the punch that strikes first does most of the work, and will therefore wear faster.
Punching stability—Reducing hitting shock while holding the sheet perfectly flat allows the punch to cut cleanly, prolonging punch life.
Stripping cleanly—Even stripping of the workpiece from the material contributes to longer tool life and optimum finished-part quality.
Tool material—Punch and die toughness and hardness vary by the type of tool steel used in their manufacture. Tool components also vary by product grade (good, better, best) and tool coating. And, tool quality and tool life vary among suppliers—your engineers should know the differences.
Punching speed—High punching speeds, under certain conditions, generate enough frictional heat to soften a punch. A softer punch will wear more quickly.
Lubrication—A lubricant will significantly increase tool life.
Material thickness—Thicker material will cause punches to wear more quickly.
Workpiece properties—Physical and mechanical properties of the workpiece material will greatly impact tool life.
Punch and die wear—Punches will generally wear more quickly than dies, since dies are less affected by the factors described above.
When to Sharpen
Indicators that tools may be dull include:
• The piece part has begun to exhibit excess rollover;
• The press is making more noise than it should; or
• The press is working harder than it used to.
When Punches Dull Prematurely
…clearance may be too tight. Total clearance (not per side) should be 20 to 25 percent of material thickness. During partial hitting (notching, nibbling or shearing), lateral forces can deflect the punch tip and tighten clearance on one side. Sometimes the punch tip may move far enough to shave the side of the die, which results in rapid deterioration of the punch and the die.
Punch-tip galling—adhesion of the workpiece material to the punch tip—results from excess pressure and heat. Operators can remove galling from the punch tip by rubbing with a fine stone, taking care to rub parallel to the direction of the punching motion. This will polish the surface that contacts the material, and minimize the likelihood of future galling. Do not use harsh abrasive methods such as sandblasting or belt sanding, as these techniques can create a coarse surface finish that promotes galling.
Where galling persists, fabricators can opt to use punch-tip lubrication pads. Another solution is to specify punches with a 2-deg. total back taper. This small size difference facilitates stripping and minimizes galling, without impacting grind life.
When grinding punches, take care to match the hardness of the grinding wheel with the requirements of the tool steel. Generally, harder tool materials require softer sharpening wheels, and softer materials require harder wheels (Fig. 3). Balancing hardness and coarseness results in a wheel that optimizes the sharpening action. Follow the tool manufacturer’s recommendations for sharpening.
Lightly dress freshly sharpened punch edges with an oil stone. This removes the corner of the tool—most vulnerable to chipping. Although this dressing operation creates only a tiny radius, it will strengthen the tool edge by distributing stresses, and minimize the likelihood of tool flaking.
Streamline and Organize
Certain punching operations wear out tooling more quickly than others. Shops that routinely evaluate their operations and look for more efficient operations will minimize tool wear and avoid damage to the tooling and the press.
For example, only perform nibbling with a turret press when it offers the most efficient process for making the part. Nibbling not only is time consuming, but it also can prematurely wear the tool’s cutting edges, cause galling and lead to guide and turret wear.Lastly, lean fabricators will track the time their operators take to find, organize and load tools into their presses— nonproductive time—and take care to properly organize and store their tooling inventory. Likewise, only sharpen and lubricate tools when it’s convenient for the operator, not at the time the tools are needed for a press run or are in desperate need of maintenance. MF
See also: Mate Precision Tooling
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