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.


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