Die Forensics--Find the Root Cause of Piercing and Blanking Failures
1) Excessive wear and galling, leading to short punch life, poor-quality holes and generally unacceptable performance.
2) Slug-control failures, leading to rejected parts at best and damaged die components at worst.
3) Punch breakage or deformation, leading to downtime, additional die damage and breakage and delays.
4) Excessive burrs, leading to short runs, defective parts and additional downtime.
A general practice in designing and building a stamping tool is to apply a common punch-to-matrix clearance to all of the perforated holes, regardless of size. Unfortunately there comes a point when hole size becomes too small in relation to the part material thickness for that clearance to be effective. This results in high punch loading, longer burnish in the hole and excessive burr. This phenomenon begins to occur when the hole size drops below 1.5 times the material thickness. At that point it becomes more difficult to bend and cleanly break the slug free.
Hole characteristics vary with clearances. Regular clearance typically results in a high percentage of shear or burnish with minimal rollover and break. The hole tends to be smaller than the punch point. Engineered (increased) clearance achieves a low percentage of shear or burnish, with greater rollover and break. The hole size with engineered clearance will be larger than the point of the punch.
An engineered punch-to-matrix clearance of 9 to 20 percent per side places the part material under a much lower tensile load, minimizing compressive load and associated problems. When the part material exceeds its tensile limits, the slug suddenly separates from the part. This sudden unloading of pressure on the punch generates a reverse shock that often leads to punch-head breakage.
Slug pulling has many causes, including sticky lubricants, punch over-entry and loose punch-to-matrix clearance. When a hole is perforated, the slug bows a from the center of the punch, creating a vacuum pocket that can cause the slug to stick to the end of the punch, resulting in slug pulling. Lubricants create a seal around the vacuum pocket, further increasing the chance of slug pulling. Punch over-entry generates a similar problem— the further a punch is over-entered, the greater the vacuum it creates at withdrawal.
Regular punches with no means of slug control must maintain a relatively tight punch-to-matrix clearance. The resulting tight fit of the part material around the point of the punch can cause galling and heat damage to that area of the punch. Rings around the point indicate that the part material has sprung back on the point at snapthrough, grabbing the end of the punch. The tight fit on the point generates heat, discoloring the area just behind the tip and potentially damaging its heattreat and reducing tool life.
Punch withdrawal from the part material can generate as much as two-thirds of the punch wear. Because regular clearance can produce a hole that is as much as 0.002 in. smaller than the point of the punch, it creates a press-fit condition on the point of the punch with every hit. With regular clearance, the hole in the part contracts and grabs the end of the punch. The slug expands and becomes jammed in the matrix. Abrasive wear on the punch and matrix will be excessive.
Slug jamming, most commonly occurring when perforating thin and or soft material, can lead to punch breakage or matrix splitting. Along with a tight punch-to-matrix clearance, excessive land length in the matrix can cause slug jams. Reducing the land length in the matrix will lead to fewer slugs held in the land, requiring less force to drive them out. Land length should not exceed four times material thickness.
Increasing the clearance will reduce the size of the slugs and allow them to fall freely through the matrix; it also will reduce wear on the punch and matrix, improving tool life.
Tool Steels to Battle Compression
Compressive strength is a little known and often overlooked characteristic of tool steels. It is a measurement of the maximum load an item can withstand before deforming or resulting in a catastrophic failure. A common failure location due to excessive compressive loading is in the radius-blend area. However, not all radius-blend failures result from compressive load. Grinding burn and lateral deflection of the punch point also may result in failures with similar symptoms.
Punch deformation is another form of compressive failure. Deformation can occur at the point or behind the guide in the stripper. In most cases, properly tempered tool steels will bend or deform before breaking. Punch-point bending often is considered a sign of insufficient hardness. Proper hardness-testing methods require special procedures when checking round or bent items. Accuracy of hardness testing is greatly enhanced by grinding opposing flats.
Reducing the diameter or chamfering the mounting-surface portion of the head minimizes or eliminates compressive loading and flex of the unsupported outer diameter at impact. It also is important to chamfer the bottom of the counterbore in the retainer to avoid interference with the fillet radius under the head of the punch.
The three most effective methods to prevent head breakage: Increase the punch body diameter; apply a shear configuration to the punch point; and hold the backing-plate hardness between Rc 45 and 48.
As a general rule, spring strippers should be used whenever possible and punches should not be allowed to pump in the retainer.
Punch-Face Shear Angles
A double-flat shear has two flats on the face and often is referred to as a roof-top shear. This design works best on oblong and rectangle punch-point shapes. Concave roof-top shear leads to punch-point chipping and splitting and should be avoided.
A bevel shear will reduce punch load and minimize punch-point chipping; however, this design tends to induce wear. Conical shear is the best configuration when perforating with a round punch. The load reduction is greater than the bevel shear, wear is evenly distributed around the point, and the slug is deformed enough to minimize slug pulling.
Although shear angles can reduce initial impact and total load, the angles themselves can induce other types of failures. Flank chipping often results from a lateral concentration of load generated by the shear angles on the punch face.
be accomplished with relative ease; however, current manufacturing capabilities still make it difficult to wire-EDM or grind inside corners on die sections and matrixes to a sharp corner. Because EDM wire is round, the corners in the die section or matrix will have a radius of approximately 0.006 in. Grinding these corners will produce similar results due to corner breakdown of the grinding wheel. The radius actually provides a benefit: If a sharp corner existed, it would create a stress riser that could lead to premature failure.
If you leave sharp corners on a punch, they likely will become chipped and worn through regular use. This generates a high amount of burnish and an excessive burr in the associated areas of the hole. To eliminate the burr and minimize punch wear, apply a generous radius of at least 0.020 in. to the profile corners of the punch. In fact, to achieve optimum tool life, a small radius should be applied to all sharp external corners when possible.
Care should be taken when sharpening punches and matrixes.Heavy passes with a grinding wheel, use of the wrong grinding wheel, an undressed grinding wheel or lack of coolant are just some of the things that can lead to grinding damage. Types of grinding damage include discoloration, surface cracks and flaking. Internal stress and changes in hardness also affect tool life.
The punch face can flake off due to rapid surface heating—the surface expands at a much faster rate than the rest of the punch, causing it to separate.
Improper sharpening can lead to a catastrophic failure, as the punch can split due to load and surface cracks caused by grinding heat. Cracks occur when heat builds up in the punch. Unlike flaking, cracks result from a much deeper heating.
One might also find grinding burn and a heat-checking pattern on punches. Grinding damage occurs when the temperature of the part comes within 50 deg. of the part’s tempering temperature. A color change from brown to blue and then black indicates burn severity. MF
See also: DAYTON Lamina Corporation
Kambiz, Do not fret, I found this reference very helpful too. I googled and found it at daytonlamina.com. or Google "dayton_tech-steels.pdf" bottom of p.11 is figs 44/45 "Grinding Burn". There's your reference!
could you please put your reference for that three last line above dealing with tempering and color change? tnx