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Technology Focus

Who's Making the Rules?

by Mark Batson Baril

May-June, 1999
In today's aggressive converting market, the sea of information on choosing steel cutting rules is vast. Our intention with this article is to compile information important to rule buyers and rule users within a very specific market. The concentration for this information is for 2 point - (.028"), 71mm thick Center Bevel cutting rules used within the converting industry. Applications include folding cartons, gaskets, labels, nameplates, membrane switches, medical applications, plastics, etc. According to rule manufacturers, this 2 point standard cutting rule is by far the largest volume item produced. The following areas are the most important factors in knowing what makes a steel cutting rule work for you.

Edge Construction

There are two methods used within the industry to put the cutting edge on a flat strip of steel rule-grinding (-round edge) and shaving (shaved edge). Grinding uses a series of grinding wheels to grind the edge on the rule as it passes through the production line, The wheels typically grind in a perpendicular direction to the length of the blade. They can be turned in some cases to create an angled grind direction. Shaving uses a series of hardened solid tools to peel back or shave away the steel in order to create the cutting edge.

Grinding gives a sharper edge than shaving. The grinding process leaves a very sharp edge that has thousands of microscopic points per inch, almost like a serrated edge rule. Because of this inherent sharpness, the rule cuts well in situations where aggressive materials like plastics are being converted. When cutting gaskets, labels, membrane switches, nameplates, laminated folding cartons, etc., there is no question that you will produce better parts more quickly with ground edge rule.

One disadvantage of grinding is that it leaves microscopic grooves in the edge that can cause stress fractures when trying to create a very tight radius bend. Grinding at an angle instead of perpendicular to the length of the blade can help to eliminate this effect.

Shaving the edge is typically faster than grinding and, therefore, is usually less expensive. Also, shaved edge rules can usually achieve slightly tighter radius bends without cracking. Traditionally, shaving has delivered a smoother edge finish, resulting in less dusting, especially when cutting recycled paperboards. New production methods in grinding are now delivering smoother finishes on most ground edge rules.

Bevel Angles

The concentration for reducing dusting and poor edge finishes in the final product has been re-focused within the industry from the construction method to the cutting edge bevel angle. The basics are these: the steeper the angle, the less pressure needed to cut, and the more shearing effect you get. Greater shearing effect will allow for smoother, less fragmented cuts, which eliminate or reduce dusting and chipping. There are three bevel angles commonly in use around the world today:

60 Degree - The American Standard Steep

52 Degree - The European Standard Steeper

42 Degree - New for Dust Reduction Steepest

In general, the steeper the angle, the weaker the rule. Although there is less cutting pressure needed to penetrate the material, there is less strength in the rule (especially the cutting edge) to absorb the punishing effects of the impression. The advantage to staying with a standard is that the tools used to cut and miter the rule are typically set up to work with only one angle. The best joints and ruling job will be done with tooling that matches the rule angle exactly. Although you can get away with slight differences in angle of rule to tooling matches, it is best to stay at least close to the angle the tool is meant to cut. Changing the machines can be expensive and time consuming-one of the big deterrents to change within the diemaking industry.

Body Hardness

Because most rule manufacturers are now hardening the cutting edges of at least some of their rules, you must be careful in making the distinction between the body hardness and the cutting edge hardness. Both are very important!

All manufacturers are using the Rockwell C scale to measure hardness. Although each has its own method of testing. the measurement can generally be used as a good comparison. Keep in mind also that the body hardness is a measurement of the inner part of the body and does not include the softer decarburization layers that most rules have.

Body hardness affects a number of things. The softer the rule, the easier it is to bend consistently, especially multiple bends in automatic benders. The softer the rule, the tighter radius you can bend with it as well. The harder the body, the better it will stand up to the punishment of the cut. Be careful when choosing rules with a drastic difference in cutting edge vs. body hardness. In some cases the body can collapse or shrink before the cutting edge does. This is especially a concern with cylinder presses where the rule not only has a downward pressure exerted on it, but has a sideways push. When diecutting with a cylinder press, the hardest possible body should be used.

Cutting Edge Hardness

The harder the rule, the longer it will last. That's the basic rule (no pun intended). Of course, the harder the rule, the harder it is to work with too. This means there must be compromise when choosing the right rule to use for each project. It used to be that all rules had the same hardness in the cutting edge that they had in the body. Many rule producers, however, are now offering the service of edge hardening their rules. Laser, plasma, high frequency, and induction are some of the methods used to concentrate energy on the cutting edge, resulting in the affected area becoming harder than the rest of the rule. To this date, there has not been a great deal of marketing or research to say that one method is better than the other.

The reasoning behind hardening just the edge is to try and keep some of the benefits of the softer body while having the long lasting edge. Be careful again, as the harder that edge is made, the more cracking you will get on the tighter radii. If a manufacturer shows that no hardening has taken place, it means that the entire rule has been through-hardened and the body and the cutting edge are the same Rockwell.

Body Coatings

Most manufacturers are coating their rule to protect it from the elements. Often times, this is merely to prevent rust from taking over. Most are using some type of petroleum‑based product that is put on as the last step in the process, while others use no coatings at all. One of the major factors in the coatings area is how they react to automatic processing equipment. Not only is the grabbing power of the machinery affected, the residue left within the machine can help in some cases and hurt in others. Be sensitive to the maintenance requirements and internal workings of your particular machine when choosing a rule. Other specialty coatings like Teflon are sometimes used, but are uncommon as stock items.

Decarburization Thickness

Decarburization is the decrease of the carbon content at the surface of a steel due to interactions with the environment at elevated temperatures. Carbon has a huge influence on the mechanical properties of steel. Decreasing carbon content causes a degradation of these properties. The decarburized layer in steel rule is the weakened outer layer on both flat surfaces of the rule. The only significance this has to the selection of the right rule for your application is related to bending, and, in particular, to automatic rule processing machines. This "decarb" layer is what tends to be scraped off during processing. In many cases it is what the machine or bending tool is able to grab onto, and its thickness and consistency can affect your final results. Build-up of this debris, along with the coatings mentioned earlier, should be carefully watched by operators and maintenance people as well as those involved with calibrating the equipment. The thickness of this "decarb" layer also has a direct impact on the bendability within most rules. Since this outer skin is softer and weaker than the inner body, the thicker the layer is, the easier it is to bend.

Minimum Bend Radius and Angle

This measurement is for the smallest bend a rule can take without suffering a stress crack at that corner. Most of today's rules are capable of much more than what most applications need for a minimum radius. With the most bendable of today's rules, a bend can be made that will actually allow the rule to be completely folded over on itself at 180'. Even when this bend is made with the sharpest of tools, like a Standard Helmold X3, the minimum radius that can actually be achieved, past 90' in depth, is somewhere between .015 (.381mm) and .028 (.711mm). In order to get a smaller radius on these deeper angle bends, steel must be removed at the bend location. Removal of this steel is called "broaching." Broaching the rule means that a section of the face of the rule is taken away in order to relieve the corner where the bend will be made. This will usually allow for a very tight, flat bend to be made at the cost of slightly weakening the rule. Some manufacturers' rules will bend to a tighter radius than the typical minimum. In these cases it must be noted that broaching may be necessary to get this extreme from the rule.

Standard Heights and Availability

The industry standard height is .937" (23.8mm). Other very common heights are .918" (23.3mm) and the conventional cylinder press height of .923" (23.44mm). Depending on the application and the type of press, rule heights can and will vary greatly. Most rule producers will offer at least the three heights mentioned here as stock items in at least a couple of their rule types. Expect to be able to find the .937 (23.8mm) height available in just about all categories of rule and quantity needed.

Purchasing rule in strips has been common for many years, These strips vary in length from the American standard of 30" to the 1 meter standard used around the world. Coils of varying lengths are now becoming more common with the use of rule processing machines. Less waste and speed of processing are both keys to the coils' new-found popularity. Most all rule manufacturers are now making their rule available in both strips and Coils.

Height Tolerances/General Specifications

Specifications set forth by the IADD (International Association of Die Cutters and Diemakers) control height, thickness, temper, cut edge fidelity, cut angle fidelity, camber, dish, twist, and coil set. Holding the best possible height tolerance is becoming more and more important as today's presses and makeready systems become more sophisticated. If extremely tight tolerances are needed, talk to your manufacturer about its tolerances within a specific single batch of rule. It is usually the case that, within a batch, the rule will be extremely uniform in height.

Pricing

This is an interesting area to tackle. For the large production user of steel rule, pricing can and should be one of the large factors in deciding on a rule. Quality should not be sacrificed for a good price, yet in many cases the lower priced rules may be sufficient to get the job done well. Often times a lower price does not mean the rule is of lower quality or feature, it may simply mean that the rule is produced in such high volume that the price is able to be reduced. As your volume of rule used goes up, it becomes more and more important to know all the factors involved, including price! In the real world steel rule diemakers like to choose one or two rules that work for most of their basic workload within a given market. It makes sense from all standpoints, including the customers', to do this. Substantial savings are made by buying in volume, reducing set-up times for all machinery involved, and controlling quality and trackability.

We have compiled this information as an aid to businesses trying to make decisions on what rules may fit their needs. All of the tangible factors presented here must be mixed with intangible effects, such as relationships and loyalty, customer service, and commitment to improvements. It is through this "mix" that a good decision can be made.

Cut Smart Manufacturing is a full service design and manufacturing company that specializes in helping businesses cut products or purchase cut products. They can be contacted at 800-465-4141.