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Technology

A Closer Look at CNC-Engraved Dies

by David Bohne, h+m USA Dies

August-September, 2014

CNC engraving is no longer the “new kid on the block” when it comes to engraving dies for the print finishing industry. The technology is recognized as providing a higher definition in the stamping image due to optimization of the engraving relief angle, and there is very little tolerance variation between multiple-up dies. The use of CNC-engraved dies also considerably reduces the set-up time on press, as all dies are absolutely identical regarding frame size and centered position of the image. All factors considered, this streamlines the production process and increases productivity. Still, there are questions about correct artwork preparation and cases in which CNC engraving may not be the best choice.

How does artwork need to be prepared when ordering CNC-engraved dies?

Since a milling tool must follow a path, it is crucial to provide vector art to the CNC diemaker (i.e., .AI, .EPS or .PDF). Most stampers capably provide these vector formats; however, there remain several sectors in the industry still using artwork more suited for output to film used for producing etched and hand-engraved dies. All artwork must be inspected by the diemaker's graphics department and prepared for the CNC production process. The graphics department will isolate the elements that make up the die's design and make certain that the vector shapes will correctly translate into the tool paths that the equipment will follow.

It's important that those vectors are just the shape and do not have a stroke applied. If a stroke has been applied, (most often) the center of the stroke is where the actual path is and, if given to the CNC equipment without adjustment, that's where the tool would cut. This would make the stamped area look as if it had 'shrunk' by exactly half of the size of the stroke applied to that path.

In addition, the points on the shape to be engraved influence the result. Think of the artwork required for CNC machines from the milling tool’s perspective: Along a curve, a tool is programmed to travel from point A to point B to point C. If there are only a few points along the curvature, the milling result will look choppy. The more points installed, the smoother the curve. The engraver will determine the optimum number of points needed to create a smooth curve. Clean artwork helps reduce time and costs, while also increasing the quality of the die.

How does the cost of CNC-engraved dies compare to other engravings, such as hand engravings and etched dies?

When evaluated next to hand engravings, the cost for a CNC-engraved die can, in some cases, be comparable. And, there can be a clear advantage to CNC engravings when considering multiple-ups and, particularly, for combination dies. For a single die, the diemaker must consider the time it takes to create the engraving program and the subsequent time on the CNC machine vs. that made by hand engraving, particularly regarding detailed feathers or animal hair, for example. If argument that hand-engraving is a dying profession is considered, the pressure to produce comparable results via CNC (in terms of quality and cost) remains the challenge. Supporting that argument is the notion that the creation of sculptured motives is (in today’s world) made on the computer rather than modeling it.

Compared to chemically etched dies, the CNC die usually is more expensive, although it can be argued that the technical advantages of CNC engravings easily compensate for the price difference.

What are some of the advantages in utilizing a CNC-engraved die vs. an etched die? When does an etched die makes more sense?

Advantages of CNC engravings include increased precision and exact replication; crisp, sharp beveled edges, particularly on fine print; the ability to choose the desired depth and bevel; and a longer life span of brass. In addition, the motive can be optimized for hot stamping and precision precasted counterdies can be made (rather than using the etched die as a mold for the counters).

Combination dies typically are CNC engraved in brass. In this case, the additional cost can be offset since the job is running in one pass vs. two passes. For short runs on very complex images, a two-pass solution using etched dies may prove to be optimal.

Etching can be advantageous for motives that have extraordinary fine lines, thereby requiring a long engraving time. Microembossing large areas also takes extraordinary time via CNC. Therefore, a double-etched copper die may prove to be the more cost-effective approach, which also provides greatly improved lead times, particularly for multiple-ups.

Have there been recent developments in CNC-engraving technology?

Today’s CNC equipment offers an improved ability to run at higher feed rates, particularly for the smaller tools. This has prompted improved tooling quality, as higher feed rates can heat the tool excessively, causing it to break. Today’s CNC equipment also features much-improved user-friendly controls and programming software.

Optimum quality in any CNC-engraved result depends on the capabilities and choices of the CNC programmer. Often, the CNC programmer must weigh the benefits of running a larger tool, even knowing that some quality may be sacrificed. In the best cases, the sacrifice is negligible while still enabling the programmer to produce the engraving – or multiple engravings – in a timeframe that is most cost-effective.

What methods are available for reproducing highly complex engravings?

It generally is more cost-effective to hand engrave (or, in some cases, CNC engrave) a master engraving, from which composite duplicates can be made. In order to achieve optimal cost-effectiveness when producing the master die, the engraver must assess the time to program and CNC engrave the master vs. the time needed by a hand engraver.

Additionally, when no art file is available, there are occasions when someone requires one or more duplications of a new or worn hand engraving via CNC engraving. In this case, check with a diemaker who has a digitizing machine. This is a type of equipment that scans the engraving contours into a digital file, which perhaps can be optimized before CNC duplications are produced.

InsideFinishing would like to thank David Bohne, general manager, h+m USA Dies, a provider of CNC-engraved brass dies and a member of the Kurz Foil Group, now with six locations worldwide. For additional information, contact h+m USA at 704.599.9325 or email david.bohne@hmusadies.com.