Tuesday, November 23, 2010

DIe Cutting Wood

Written By Mark Batson Baril

The question/problem came to Cut Smart basically in this form:

Designs in Wood, Inc.(alias name for case study) manufactures over 400 different sizes and shapes of small wooden parts in Eastern White pine ranging in thickness from 1/8" (3mm) to 1/4" (6mm) with a surface area under 6 square inches (152mm). Generally the surface areas are 3 to 4 square inches (75 - 100mm).

Our current process involves bandsawing 8/4 stock and then slicing and sanding each part. This is time consuming and we are looking for a way to lower our manufacturing costs. We have looked at laser cutting but have ruled it out because our secondary process requires a finished, unburned edge.

I am not completely familiar with steel rule die cutting, and wonder if it is something that we might be able to use. I would be interested in the following:
  • Can this type of wood be cut with a steel rule die (tolerances of .010" to .020" (.25 to .50mm are OK)?
  • What kind of equipment (press tonnage/manufacturer) would be required?
  • The cost of a typical steel rule die?
  • The life of such tooling in terms of number of impressions?
  • The finished edge appearance?

We answered in this way;

Because we don't know all the shapes you are cutting it is hard to say what your final results will be. The more flowing and rounded your shapes are the better the results will be. Sharp corners and thin areas of image will be tough to cut. There are several types of dies that could work including steel rule dies, clicker dies, EDM cut specialty punch dies and matched metal tooling. All of these are possibilities depending on the shapes you are cutting and your overall volume. Tolerancing like you mentioned will be tough to hold on any but the machined tools and punches.

Eastern white pine is a fairly soft wood that can be cut on a steel rule die. The 1/8" (3mm) thickness will be a great deal easier and will give much better edge results than the 1/4" (6mm) material. We have worked with several companies that build models from wood. They use steel rule dies as well as other cutting tools that cut in one hit. They have had excellent results with all of the types of cutting dies mentioned above. Tools other than the steel rule die will work well, but the steel rule die may be the place to start because of its relatively low cost.

The type of press and the tonnage needed would largely be a factor of how many you plan to cut at the same time on a sheet. One at a time like you describe would require very little tonnage 1 - 5 tons and a very common hydraulic type press would work well. Costs may range from $5,000 used to $20,000 (USD) new depending on the size and style.

A simple one up steel rule die would cost in the range of $100 to $300 (USD) depending on the shape and who you buy it from. The more images you add to the tool the cheaper each image becomes. Specialty punches and machined tools would cost substantially more.

Although we have seen manufacturers with millions of impressions on their tools, the material you are cutting is tough. I would estimate no better than 10,000 hits from a tool before it needs a reknife.

Generally you will find that an extremely hard, thin rule with a very long bevel will work well. Support the rule as high as you can with your base material for best results. There is a rule called "Razor Rule" that works excellent for cutting wood. If diecutting is still something that sounds like it would fit your needs, I suggest connecting up with a local qualified diemaker or diecutter that would be willing to cut a few samples for you. This will show you the type of product you can get and how economical this process may be for you.

Depending again on the shape of the cut, your edge results will probably have a slight roundness to the top and a square, sharp bottom. Grain, moisture content, sharpness of the tool, cutting surface wear, will all effect the results. Your with grain cut will most likely be of better quality than the cross grain cut. Knots will be a problem!

High speed CNC routering is another method we have seen used that performs the same way the laser does without the burned edges. Although slow compared to cutting with a die, the method may make sense if laser cutting came close to making sense for you.

Tuesday, November 2, 2010

Die Cutting with Rule Joiners

Written By Mark Batson Baril

Cut Smart recently dealt with this question:

Does anybody out there know how to create a perfect joint where radii come into one another on a steel rule die? We have more than one customer that insists that their radius cornered gaskets be run with a common cut in both directions to save material. On the other hand we have a diemaker that insists that he must have a double knife in order to put in the radius corners. There must be a way but we’re diecutters not diemakers and have no idea how. Should we find a new diemaker or is there some information out there they could use? Thanks!

    For the common application where a steel rule die will be used in some type of flatbed cutting operation, the best answer we can give is to use Rule Connectors - A.K.A. - Rule Joiners - These are a love/hate product. Some people swear by them; others swear at them! Rule Connectors are a solid steel machined punch which replaces the regular steel rule at tough to make joints. Rule Connectors typically replace normal rule where rules meet at a radius corner.

The Plus Side is this -
  • At the point where most diemakers have an major problem making a joint that works and is accurate, especially in tough materials, the rule is replaced by a virtually indestructible piece of machined steel that is perfect. The joints are moved to an easier and more desirable location usually on a straight-away and the problem is solved.
  • They are readily available, in a variety of different radii.
  • The Custom possibilities are endless.

There are two main drawbacks -
  • One is how the rule and punch is installed. Rule Connectors typically have "V" notched ends that join rule to the punch. If you do not cut the rule to the right size or the bevel on your rule is off-centered, you will pull your hair out trying to get the tool to work properly. However, if it is installed correctly, you will have virtually no spaces or natural nicks in the rule pattern. When you put it together right, it works great, especially on materials that love to separate rule.
  • The other drawback is the cost of the Rule Connectors. They cost roughly $20 to $30 (USD) each. Most of the time the cost can be justified by eliminating downtime, rule repairs and material waste. If you have a small run, the cost may be prohibitive.

To answer the question more pointedly –

The diemaker may be right! Even though there are rule joiners on the market, you will leave yourself open for more actual natural nicks than if you had allowed for space between your cavities and had made only one or two natural nicks in each cavity. Depending on your customer’s final product, this may be a big factor when selling the job in the first place.

Other answers in a case like this one are numerous - Could the product be more easily cut in a rotary application where a solid machined or chemically etched tool may be of use? No joints here! Also it may be worth thinking about a fully machined punch or die that actually outlines and cuts the entire image without any joints. More expensive, but in some cases that doesn’t matter at all. Have you thought about, for short runs, laser cutting, waterjet cutting or CNC Knife cutting machine production? All of these are methods that are being used and are working well in the right situations.

Rule joiners are not new to the market. As is the case in many situations, the usual for one manufacturer is the unusual for the next. Good luck in all your cutting adventures.