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Surface Coatings for Draw Steels--A Time and a Place

By: Peter Ulintz

Monday, August 01, 2011
 

The tool and die and metalforming industries spend a significant amount of money on specialized surface coatings for die components. The primary purpose of these coatings is to protect the tool surface against abrasive, adhesive, and corrosive wear. The surface coating’s high hardness and low friction coefficient often can help increase tool life during production.

However, there are applications when surface coatings are unnecessarily applied. This occurs when metalformers fail to detect other process errors causing premature tool wear, such as faulty programming, machining or assembly practices. In these instances, the coating appears to improve tool life and productivity, but in actuality the coating only adds to the cost of die construction and maintenance.

The corner of a box or irregular draw corner forms like a cup.

Applying a surface coating to a die section often is the first response when galling occurs in the corner of a rectangular box or other irregularly shaped draws. If the coating works, the situation worsens, as someone suggests that the company alter its die-design standards to require a surface coating on all dies of similar design or producing similar parts. This adds unnecessary cost to the company’s dies, because the future dies will be designed and built to a standard that accommodates poor die-construction techniques—a worst-case scenario.

If you prefer to do things more methodically and wish to keep your tooling and production costs under control, then read on.

The advent of CAD models and CNC machining has substantially reduced the amount of time required to design, machine and assemble stamping dies. One practice for machining draw and form dies is to machine clearance between the punch and die cavity by offsetting the CNC cutter path by maximum material thickness. This works well, until a drawn corner is encountered.

The corners in box-shaped geometries form similarly to cup drawing. If the four straight walls of the box are removed so that the corners join together, they form a cylindrical cup. These corners are compressive on the workpiece material moving toward the die radius, and tensile on the material drawn over the radius (see the figure). As a result, thickening occurs in the vertical wall (A) and the flange remaining on the draw pad (B) near the draw corners.

This thickening phenomenon sometimes is overlooked during CNC programming and machining. Worse yet, the condition may not be identified.

A table may be used to determine the die clearances for drawn corners.
To reduce the likelihood of galling in drawn corners, additional punch-to-die clearance must be machined in the vertical wall area. This may sound easy, but it generally requires analytical tools such as computer simulation. A properly executed metalforming simulation can provide accurate thickness plots in drawn parts, which can be used to assess material thickening locations and determine the correct clearance needed for a particular process. In the absence of this data, stampers can use tables found in die-design handbooks.

After machining the proper clearances in the drawn corners, polish the corner die sections as though they were going to have a surface coating applied and install the uncoated components in the die. You may find that a thorough polishing will yield the performance improvements previously attributed to surface coatings.

Another phenomenon to consider: During metalforming, plastic deformation and friction generate heat, which causes die components to expand. This can decrease the punch-die clearances machined into the tool. If this clearance is sufficiently reduced, excessive stretching can occur and the stamping may eventually tear or break. When this occurs, the metalformer often decides to reduce press speed to minimize heat generation.

A better solution: Increase punch-to-die clearance to allow for predicted tool expansion at the expected operating temperature. This might require scrapping a few stampings or preheating the die until reaching its operating temperature, but it allows the die to run at higher speeds during normal operating temperature.

A Note About Lube

Modern lubricants are formulated to provide tremendous surface adhesion and tenacity. Extreme-pressure (EP) additives provide excellent anti-galling tendencies under extreme heat and pressure conditions. Selecting the proper lubricant for the temperature range in which the forming process operates is crucial when galling becomes a problem. While adding greater quantities of an insufficient lubricant can aid the forming process, the practice wastes money and creates a mess in the workplace that requires cleanup. A better solution: Work with your lubricant supplier to choose the appropriate lubricant for your process.

Applying more lubricant, reducing press speed and adding surface coatings to die steels are common solutions when galling occurs, but are not always the proper solutions. Machining proper die clearances and using forming lubricants designed for the specific operating temperature of your process may be cheaper and more reliable. MF

 

Related Enterprise Zones: Tool & Die

 


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