Avoid Galling Problems on Punches

A TiCN coating on a cluster punch increases hardness. Photo courtesy of Trumpf.

To coat or not to coat? That should be the question when choosing a punch. The answer, as may be expected, depends on the application. In some cases, a coating can be well worth the added cost.

Ensuring that quality tool steel is used for a punch should always be the first criteria. Paired with lubrication, a punch may not need the assistance of a coating. But for difficult jobs, like those that generate a great amount of friction and, therefore, heat, or have downstream requirements that preclude using oil, spending money on a coating can improve the bottom line.

Coatings offer lubricity, increased hardness, and, depending on the specific composition of the coating and the manufacturer making the claim, extended punch life by up to 10 times that of untreated tools.

The added measure of slipperiness, the main goal of any coating, reduces friction and resists combining with particles resulting from the process, which translates into less heat buildup. That means less galling. Less galling keeps the punch perimeter clean, smooth, and capable of creating a higher number of quality features.

A hard tool coating, when applied to a lesser grade of steel, reduces the chance of the tool combining with particles being sheared off from the tool edge, potentially increasing the number of hits between sharpening or replacing the punch.

Choose by Application

Carl Peterhansel, tooling sales and service manager at TRUMPF Inc., Farmington, Conn., said that punches made from high-grade tool steel are good for many jobs, but using a coated punch should be recognized as an option for aggressive applications like nibbling, when the tool diameter is close to material thickness, and when punching highly abrasive materials.

"There's really no substitute for strong, wear-resistant tool steel," he said. "But if a customer is working with a plated or prepainted material that quickly wears down the punch surface, a coating can extend punch life.

"Or if the addition of a lubricant creates a downstream issue, a coated punch can reduce the coefficient of friction without introducing oil to the process."

More Hits

Scott Tacheny, special applications engineer for the Punching Division of Wilson Tool International Inc., White Bear Lake, Minn., said, "In one comparison we found that when an M2 steel was used, 100,000 hits were made in 0.075-in. 304 stainless with a 0.312 round before the tool had to be sharpened. When a coating as minimal as the TiN [titanium nitride]—one of the oldest coatings around—was added, we got 225,000 hits before resharpening. With the newer TiCN [titanium carbonitride] coating, the number of hits before sharpening was north of a million."

The lubricity a coating adds can all but eliminate the irksome challenge of galling, Tacheny said, and help avoid breakage and other tooling issues when the machine tool is operating at maximum tonnage.

"There is really no application where you shouldn't consider using a coated punch, but you would not want to use the wrong coating. There's no one-size-fits-all. For example, a TiCN is fine for steel but can cause galling when used with an aluminum or galvanized—it'll cause the tool to stick," Tacheny said. "You have to have the right coating for what you are doing. Ask your supplier about the best choice for each application."

Variety of Options

Multiple layers of coatings may be added to a punch to achieve the desired properties, and different coating processes affect the results. For example, OptimaX, a proprietary coating from Wilson Tool, uses physical vapor deposition (PVD) to layer a molybdenum disulfide (MoS2) metal composite coating for lubricity over a TiCN layer for hardness.

Regardless of the coating process used, the three most common being PVD, chemical vapor deposition (CVD), and thermoreactive diffusion (TRD), protecting the tool surface against abrasive, adhesive, and corrosive wear while maintaining dimensions and tolerances is the ultimate goal. Thickness of the coating varies depending on the material being coated and the process used, but it usually measures between 1 and 5 microns. Tacheny said, "We are physically adding something, so we have to be concerned with dimensions, and sometimes we need to compensate for those thicknesses."

Resharpen Coated Punches? Yes.

Punches, coated or not, wear even under the best of circumstances, particularly on the cutting edge. Regrinding isn't a problem.

"The two advantages of coatings, increasing hardness and lubricity, are needed on the perimeter of the punch. Sharpening removes only some of the worn or damaged portion of the punch, leaving a fresh perimeter with all the characteristics of a new one," said Tacheny.

According to TRUMPF, its TiCN-coated punches reduce the frequency of resharpening by up to 50 percent but maintain hardness and wear resistance after regrinding.

Coatings and tool steels are continually advancing to keep up with changes in materials and the need to punch more, faster, and with less downtime for tooling maintenance.

"Historically, coatings have been a quick and easy solution to tooling problems, but in the grander scheme of things, work is being done to make better tool steels. That takes away from some of the need for coatings," Tacheny said. "But everything evolves, including the materials being punched. Materials are getting rougher, harder, and more abrasive and will wear down a punch, so we need to combat that with harder coatings along with harder steels."

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