Page 36 - MetalForming December 2012

Tooling Technology
Matching the Grinding Wheel to the Tool Steel
Tool Steel
Grinding Wheel
A2 and S7 tool steels
Grinding-wheel hardness: G-J Grit: 46-60
M2, PM tool steels (inc.M4PM)
Grinding-wheel hardness: D-G Grit: 46-60
mum finished-part quality.
Turret alignment—It is important to
regularly check the press turret for mis- alignment, which can damage and wear tools prematurely. Follow the press and tooling manufacturers’ rec- ommendations for turret maintenance.
Tool material—Punch and die toughness and hardness vary by the type of tool steel used in their manu- facture. Tool components also vary by product grade (good, better, best) and tool coating. And, tool quality and tool life vary among suppliers—your engi- neers should know the differences.
Punching speed—High punching speeds, under certain conditions, gen- erate enough frictional heat to soften a punch. A softer punch will wear more quickly.
Lubrication—A lubricant will sig- nificantly increase tool life.
Material thickness—Thicker mate- rial will cause punches to wear more quickly.
Workpiece properties—Physical and mechanical properties of the work- piece 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 fac- tors described above.
When to Sharpen
Indicators that tools may be dull include:
• The piece part has begun to exhib- it excess rollover;
• The press is making more noise than it should; or
• The press is working harder than it used to.
At this point, a small amount of sharpening is in order, to “touch up” the cutting edge. Frequent touch up works better than waiting for the punch to become overly dull. Tools will last longer and cut more cleanly with less punching force, ensuring consistency in workpiece quality. The maximum amount of sharpening depends on workpiece-material thickness, punch length and width, and the punch sta-
Fig. 3
tion. Generally, following specific tool- sharpening instructions and sharpen- ing frequently can double tool life.
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, nib- bling or shearing), lateral forces can deflect the punch tip and tighten clear- ance 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 direc- tion of the punching motion. This will polish the surface that contacts the material, and minimize the likelihood of future galling. Do not use harsh abra- sive methods such as sandblasting or belt sanding, as these techniques can create a coarse surface finish that pro- motes 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, with- out impacting grind life.
Grinding Considerations
When grinding punches, take care to match the hardness of the grinding wheel with the requirements of the
tool steel. Generally, harder tool mate- rials require softer sharpening wheels, and softer materials require harder wheels (Fig. 3). Balancing hardness and coarseness results in a wheel that opti- mizes the sharpening action. Follow the tool manufacturer’s recommenda- tions 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 con- suming, 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, organ- ize and load tools into their presses— nonproductive time—and take care to properly organize and store their tool- ing 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
34 MetalForming/December 2012
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