Cutting Plates for Flatbed Diecutting

Written By Mark Batson Baril

Much attention has been placed on cutting rule height tolerances, degree of bevels, type of edge (ground or shaved) and the abilities of the make-ready artist to achieve the best cut. What should one expect from a steel cutting plate? Is the thickness tolerance of the cutting plate equivalent to today’s steel rule height tolerances? How smooth should the surface be? Under normal conditions, how often should parallelism be checked?

Wait a minute - did someone say artist? In today’s’ world of computer generated this and computer aided that, can there possibly be room for hand/eye/brain type skills anymore?
Certainly!

As we have talked about several times in other articles, the diecutting and the diemaking functions are completely interrelated in everything from information gathering to the materials and skills used to connect them. The cutting rule and the cutting plate are the very last parts to make the connection in the process. If all goes well this last connection is a smooth one and the perfect cut or "burst through" is achieved. In the perfect world, with many materials, the rule and the cutting plate actually never make contact with one another, as the final particles of material are actually pushed apart or burst through. In reality the connection can sometimes be a very aggressive strike.

Over the years, as the industry as a whole has strived to improve upon itself; new methods of manufacture, new materials, improvements in die design, etc.... have been applied to the areas that relate to the question posed. The idea being that if we can improve upon each of the individual, yet related areas, the final process and product will get better.

The areas that apply to this final connection are;

The Cutting Press and the quality of its’ parallelism, whether it be across the entire flat surfaces that the die and the cutting plate are applied or around the cylinder that the jacket is applied to. Wear or poor adjustment will effect die and plate life as well as help to produce a poor product. Often times this is where the artist (press person) is asked to turn a school kids’ finger-painting into a Picasso.

The Steel Rule Cutting Die and the quality of its components and design. Today’s best ground edge-hardened rule will achieve a consistent height tolerance of ± .0005* (.0127mm). This is unbelievably close to being perfect! The bevel may drift much more than this from side to side yet this will not effect the height. Designed in balancing knives or blocks that distribute the weight of the presses’ stroke across the entire cutting surface (press, die and plate) are tremendously important to maintaining control over height and pressure. Without these balancing knives the press is forced to rock across the cut, defeating any efforts to take advantage of close tolerance presses, rules or plates. Help, call in the artist!

The Cutting Plate - Finally getting around to the meat of the question. Given the perfect situation (press, die, press operator, etc..) you should expect the world from a good cutting plate. Billions of impressions? What are the limits? Anyone out there feel like bragging? (resurfacing doesn’t count). The thickness tolerance of today’s best quality cutting plate will be better than or equal to ± .002" (.0508mm). So to answer the second question; NO, the thickness tolerances are not as close in plates as they are in rule height. In fact, in talking with several companies that build the plates, the feeling is that they may be as close as they are going to get to being perfect.

Many of todays best cutting plates are built per the following;

1. The initial blank sheet is ground to a rough state that is close to its’ final thickness. 2. The plate is hardened to the desired Rockwell (usually 47-52 RC). 3. The plate is then further finished by hand to its’ close to perfect state (talk about artists!). 4. The plate then goes through its final machine grinding process to create the desired smoothness and final tolerance requirements. The smoothness of the surface is produced in a number of different ways and each manufacturer will claim that their method is best or most cost effective. Everyone’s cutting situation is different and therefore it is hard to say what is smooth enough.

There are companies that produce the entire plate with no hand work at all. Who’s are better and why? may be a great future subject.

Parallelism - What is this anyway? Parallelism is the relationship that the top surface of the plate has with the bottom surface of the plate. The best situation is to have a plate who’s’ surfaces have no deviation from parallel (a parallel plate). An un-parallel plate means that your plates’ overall shape is that of a wedge, or perhaps several wedges. Cutting plate parallelism with a deviation of .0015" (.0381mm) is common. Although we have not been able to develope a hard fast answer for how often parallelism should be checked, the basic principals of process control would dictate that each companys’ maintenance routines will vary and will also be constantly changing as newer and better technologies are applied.

In many cases a soft "under the cutting plate" material help adjust differences in die/cutting plate heights and deviations and take a step towards eliminating the need for press foot printing at all. Expensive on a short term basis, yet one of the cheapest products on the market if looked at from a long term cost standpoint.

All in all, a great subject that goes to the very core of the diecutting process!

Calculating Die Cutting Tonnage Continued...

Let’s Get Really Technical:
 
A couple of us have actually talked about developing an on-line tonnage calculating website. It would be comprised of a database that held values and asked questions like; Strength values (Tensile) for most common materials (A), Shear strength values for several processes/rule types/ejection, etc…(B), Number of inches being cut (C), Thickness of the material (D). A X B X C X D = Tons 2,000

In fact, this is exactly how many software stress analysis programs work. They take a set of very obvious variables and make a simple calculation based on these (and other) numbers. It gives you a very consistent way of looking at every project you take on. Right or wrong, the answer is a base number to start with, and that is what we have gathered is the trick to determining proper starting tonnage numbers. Once you have this standard formula in place and trust that it will give you that base number, you can then depend on it and translate it to work in different machinery on your shop floor. Perhaps you have a string of ten punch presses and they all cut a little different. One is hydraulic, one is pneumatic, one is mechanical off a simple small cam while another throws off a giant flywheel that was welded back together by Uncle Joe a few years back. They all cut differently but they all have a factor you can use as a multiplier against that base number we just calculated out. It’s beautifully simple really, it just takes some time to develop and work out in your own shop, on your own equipment. Once you have that number, everyone can plan around the equipment you have vs. the projects you have with more confidence.

So then the formula may look like this;
(A X B X C X D) F = Tons (Where F is a press factor based on experience and/or a manufacturers guidelines.) 2,000 Putting this into a real life situation may look something like this; I have a ten up steel rule die cutting and creasing .018” paperboard. There are 1,000 total inches of cutting, creasing, stripping, support knives etc… I am using modern ejection materials. I am cutting on a platen style press.
17000(A) X 1(B) X 1,000(C) X .018(D) X 1(F) = 153 Tons. 2,000
Simple Formulas from above; C/6.5 = Tons (1,000 / 6.5 = 153.8 Tons) or (C X 400) / 2,000 = Tons (1,000 X 400) / 2,000 = 200 Tons

Both formulas work and give us a range that is safe and a good starting point.

Now Let’s Get Really Simple:
 
What seems to happen with all this fancy calculating in real world situations is that the base theory gets boiled down to simple formulas that work for similar situations. Most of us deal in very similar tooling and materials everyday and having a very fast and simple way of coming up with a safe base number is natural. If you are always working in paperboard in about the same caliper, taking the total periphery and dividing by a single proven number is a fantastic way to approach tonnage calculating. The same goes for plastic, steel, leather, or anything else you cut on a regular basis.

So, this article is not going to give a catch-all formula for determining tonnage for all materials on all press types, with all tools, because there are too many factors involved and nobody would ever use it in real life. What we can do is offer a base calculation where you plug in your own numbers based on experience. Your own situation will provide the best formula for you.

That base calculation would look like this;
Total Periphery to Convert X Material Factor / 2,000 = Tons Needed

Developing a living chart of MATERIAL FACTORS will then be the key to making this work in your business. We’ve been using paperboard a good deal in our discussions and it seems that a starting point for a folding carton manufacturer on a Flatbed style press would be a MATERIAL FACTOR of 300. The heavier gauge the material is the bigger the Material Factor. (1,000 Inches X 300) / 2,000 = 150 Tons Keep in mind that if your cutting process changes, maybe it’s as simple as going to a harder rubber or steeper bevel rule, you will have to use a multiplier to compensate for this change.

There is no trick of the trade in calculating the tonnage you need for a project but as you develop a more and more sophisticated list of materials and how they process on your equipment, you will have an estimating and production tool that will help you predict with greater accuracy how well a job will run, where it should run, how many up it can run, and whether or not it will run at all. You will have a leg up on the competition that is still shooting from the hip and this will really put the pressure on them…..

We’d like to thank all of the operators out there that are trying to improve their production techniques and came to The TECHTEAM with their questions!