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Question and Answer

Guide to Selecting Steel Rule

by Mark Batson Baril

November-December, 2002
In today’s aggressive converting market, the sea of information available for selecting steel rule is vast and the nets used to gather and compile that information are often full of holes. Our concentration for this article is for 2 point - .028² (.71mm) thick Center Bevel cutting rules used within the converting industry for a vast number of applications like folding cartons, pocket folders, display cards, labels, nameplates, membrane switches, medical applications, plastics and gaskets, etc. According to all the rule manufacturers, this 2 point standard cutting rule is by far the largest volume item produced and is by far where the largest amount of marketing and research dollars go. At the time of this article, we have found that there are eleven rule manufacturers that concentrate on this market, worldwide. Most of the considerations talked about here can be applied to other less common rules as well.

What edge construction method should I use?
There are two methods used within the industry to put the cutting edge on a flat strip of steel rule:

  • Grinding (Ground Edge) This method 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 (Shaved Edge) This method uses a series of hardened solid tools to peel back or shave away the steel in order to create the cutting edge.

Grinding gives you 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 very well in situations where aggressive materials like plastics are being converted. If you are cutting gaskets, labels, membrane switches, nameplates, coated or laminated paperboard, etc., there is no question that you will produce better parts more quickly with ground edge rule.

There are two disadvantages of grinding. Grinding the rule leaves microscopic grooves in the cut 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. Shaved edge rules can usually achieve slightly tighter radius bends without cracking. Grinding will typically be less accurate than a shaved rule as the cut edge will tend to wander from the center of the blade more than with a shaved rule.

Shaving the edge is typically faster than grinding and therefore the rule is usually less expensive. Up until recently, shaving has delivered a smoother edge finish resulting in less dusting especially when cutting recycled paperboards. New production methods in grinding are delivering as smooth finishes on most ground edge rules by all manufacturers. The concentration for reducing dusting and poor edge finishes in the final product has very quickly been re-focused within the industry from the construction method to the cutting edge bevel angle.

What bevel angles should I use?
The basics are as follows: the steeper the angle the less pressure needed to cut and the more shearing effect you get. More shearing effect will allow for smoother less fragmented cuts eliminating or reducing dusting and chipping. 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. Many die manufacturers are now using more than one bevel angle within the same tool. This allows additional pressure control within the tight/busy areas of the tool. 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 - For Dust reduction : Steepest

Most manufacturers produce at or around these angles. 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 and this has always been one of the big resistances to change within the diemaking industry around the world.

What is the difference between body hardness and cutting edge 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 their 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 it does not include the softer decarburization layers that most rules have.

The body hardness effects a number of things. The softer the rule is the easier it is to bend consistently, especially multiple bends in auto benders. The softer the rule is the tighter the radius you can bend with it. The harder the body is 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. In cylinder presses especially, where the rule not only has a downward pressure exerted on it but also has a sideways push, the hardest possible body should be used.

The harder the rule the longer it will last. That’s the basic rule. 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. This is still the case in many rules that work well for today’s market. 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 that result in the effected area becoming harder than the rest of the rule. 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.

What are the standard heights and availability for rule?
The industry standard height is .937² (23.8mm). Other very common heights are the old type high .918² (23.3mm) and the conventional cylinder press height of .923² (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 .937² (23.8mm) height available in just about all categories of rule in just about any quantity needed.

Purchasing rule in strips has been the most common way for many years. These strips vary in length from the American standard of 30² to the 1 meter standard used around the world. Coils of varying length are now becoming more common with the more common use of rule processing machines. Less waste and speed of processing are both keys to the coils new found popularity. All rule manufacturers are now making their rule available in both strips and coils. For this reason we have not made this part of our technical charts. The one area to be careful of is to make sure your manufacturer can easily coil in the proper direction for your operation. Clockwise feed means that with the cutting edge facing up the rule feeds the same way the hands of a clock would move. Counterclockwise means the opposite. Most manufacturers can coil both ways.

The information presented here was compiled 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 the intangible effects that present themselves like relationships and loyalty, technical expertise of production people, customer service and commitment to improvements. It is through this “mix” that a good decision can be made on what’s best for your business.

Mark Batson Baril, is President of Cut Smart Manufacturing, Inc., a full service design and manufacturing company that specializes in extreme circumstance cutting projects. This article is a portion of a more complete technical article and guide. For more information, contact Cut Smart Manufacturing at (800) 465-4141 or www.Cutsmart.com.