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When to Sharpen Tools

If a piece-part is starting to show too much roll over, if the punch press is making more noise than you think it should, or if it’s working harder than it used to – perhaps a tool is dull.

Mate recommends sharpening tools when the edges are worn to 0.01(0.25) radius. At this point, just a small amount of sharpening will “touch up” the cutting edge. Frequent touch up works better than waiting for the punch to become very dull. Tools last longer and cut cleaner with less punching force, and you get improved consistency in work quality. The maximum amount of sharpening depends on thickness of material being punched, size of punch (length and width), and punch press station.

How to Sharpen Tools

1. To sharpen, clamp the punch squarely in a V-Block on the magnetic chuck of a surface grinder. Only 0.001 to 0.002(0.03 to 0.05) should be removed in one “pass”. Repeat until tool is sharp, normally  0.005-0.010(0.13-0.25)total.
2. Use a standard vitrified bond, aluminum oxide wheel: hardness range “D” to “J”; grain size 46 to 60. A “ROSE” wheel made especially for grinding high speed steel is a good choice but not mandatory.
3. Dress the wheel using a rigid single or multi-point diamond:
   • downfeed 0.0002-0.0008 (0.005-0.020)
   • crossfeed quickly 20-30 in/min (508-762 mm/min)
4. Apply coolant with as much force and as close to the tool and wheel as is practical. Use a good general purpose grinding coolant used to the manufacturer’s specifications.
5. Feeds and feed
   • Downfeed (wheelhead), 0.001-0.003 (0.03-0.08)
   • Crossfeed (infeed), 0.005-0.010 (0.13-0.25); for nitride punches, 0.002-0.007(0.05-0.18)
   • Traverse (sideways), 100-150 in/min (2,540-3,810 mm/min)
6. After the sharpening, lightly stone the sharp cutting edges to remove any grinding burrs and to leave a 0.001-0.002 (0.03-0.05) radius. This reduces risk of chipping. Do not use a file of any kind.
7. Demagnetize the punch and spray on a light oil to prevent corrosion.

 

FEED RATES PER PASS

Downfeed:
0.001-0.003(0.03-0.08)

Crossfeed:
0.010(0.25)

Traverse:
100-150 in/min.
(2.50-3.80 m/min.)

Fixing Sharpening Problems

Problem	Cause	Cure
Discoloration** and/or surface cracks and/or “fish scale”	Insufficient coolant Improper Wheel Improper Dress	Increase or redirect flow Use coarser grain, softer grade grinding wheel Drop wheelhead 0.0002-0.0004 (0.005-0.010) and redress Move crossfeed approx. 50 in/min. (1.25 m/min.)
Harsh cutting sound and/or poor surface finish	Excessive stock removal Improper wheel Improper dress or glazed wheel	Less downfeed; lower crossfeed rate Use coarser grain, softer grade grinding wheel Redress wheel, break glaze on wheel surface

**Dark discoloration indicates damage not necessarily limited to the tool surface. Removal of burned surface will not rectify damage. Recommend replacement of the tool.

When Punches Dull Too Fast

Clearance may be too tight. It should be 20-25% of material thickness TOTAL clearance (not per side). In partial hitting (notching, nibbling, shearing), lateral forces may 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. This results in rapid deterioration of both punch and die.

Sharpened Tool Edges Stay Sharp Longer if Edges are Dressed

The microscopic irregularities in workpieces and tools attack each other with each impact. Protrusions and sharp corners get flattened and knocked off. With very sharp tool edges (A), flakes in various microscopic sizes are knocked off as the tool wears. Each flake leaves a rough surface vulnerable to additional flaking. For this reason, we recommend lightly dressing the edge of freshly sharpened punches (B) with an oil stone (India Oil Stone STO29807). This removes the corner, which is most vulnerable to breaking off. Although the radius is tiny, it strengthens the tool edge by distributing stresses which cause flaking.
With a radius of just 0.001-0.002(0.03-0.05), the tool can still be considered very sharp, and it stays that way longer. This small radius is applied with only one light pass of the stone per edge.

If Punches Overheat

1. Use a lubricant. This will decrease friction
2. Use more than one punch of the same size in the sequence if lubricant is unacceptable or if slug pulling occurs. By rotating the punches, there will be a longer time for each punch to cool down before it is used again
3. Give the tool a rest. Plan the program so that the tool that is overheating alternates with different punches. Or stop the press for awhile.

Die Maintenance

As with punches, keep dies clean and watch for wear. Use the same sharpening procedures –– hold die on surface grinder’s magnetic chuck; use same wheel and feed rates. Check die thickness after each sharpening and add shims as necessary.