Kerf Cutting Timber is the process by which kerf cut lines are used to program bending, stretching, and warping in hardwood timber. This instructable walks through an exploration of material and process to make solid wood take on new physical properties. The research was done during an artist residency at Autodesk's Pier 9, where I had access to an amazing wood an metal shop and facility including an Omax Water Jet and room for a steambag set up. Translating from small study models to large physical prototypes, this instructable can act as a springboard for future applications and inspiration for objects and furniture.
Step 1: Precedents
Kerf cutting sheets of wood has been around for a long time. In fact the manipulation of wood for construction purposes is one of the oldest technologies known to man. Tracing the properties of wood from the tree in the forest through logging, to milling results in a huge variety wood cuts, grain types and properties. A wonderful article on the current state of wood processes can be found at: alternative11.liverjournal.com as well as Arron Porterfield's instructable at https://www.instructables.com/id/Curved-laser-bent-wood/ which explores the properties of lattice hinges on thin plywood using a lazer cutter. This project in contrast looks specifically at scaling Arron's process up replacing the lazer cutter with a waterjet cutter and plywood with dimensional lumber.
Step 2: Wood Selection
The species and cut of the wood is extremely important to understand before trying to manipulate it in any way. The difference between hardwood and softwood will have significant effects on how it bents, splits, and splinters. In tests I used cheap douglass fir 2x4 and 2x6 for quick studies and then scaled up to hard woods. I went sequentially from Poplar to Maple to Ash to European Beech. Each have particular strengths but the quality that is best is the consistency of the grain. It is essential to not have any knots or checks. For this you must inspect every piece of wood closely and plan your geometries accordingly. Personally I like Ash as it offers great bending potential and is very consistent however explorations with Mahogany or Spruce may be good for future tests.
Step 3: Patterning
The patterning of the kerf cuts will result in various outcomes. For this study, I used the most minimal patterns to produce the greatest effect producing an alternating hinge repeated at various frequencies. I varied the length of the cut, the depth of the hinge, and the width of the spacing. A longer thinly spaced cut with small hinges produces the most expansion while reducing the strength. A shorter wider spaced cut with large hinges produces the least expansion but also the most strength.
An important detail is terminating each kerf cut at a hole. This hole distributes the force of spreading and prevents splitting. Play around with these patterns and make variations staggering lines or varying the width across a single piece. Future studies will explore more intricate patterns.
Step 4: Tooling: Omax Waterjet
There are many factors that need to be considered when cutting hardwood on a waterjet.
1. Securing the wood is important. Leaving the stock long allows the ends to be clamped without interfering with the path of the nozzle.
2. Wood is not consistent and so the water stream passing through the material can vary. The tool first starts with a pierce shooting water all the way through the wood. This can take a up to a couple seconds during which the water is impregnating the grain. This can cause serious checking when the wood dries. Predriling and indexing these holes can prevent this but is challenging. Using the jet to cut a hole at the end of each line works well but is is important to sequence the tool path so that the hole occurs right before or after the line.
3. The water from the waterjet will saturate the wood. Wood can expand when saturated so depending on the length of the job, the wood may be totally soaked. Keeping the water level low can help.
4. Aligning the edge of the wood to the xy direction of the wood may not be perfect. I offset the outside cuts so that they were inside of the dimension of the wood and would be parallel to all the other kerf lines.
Step 5: Preparing the Wood
Once the wood has been cut and dried its time to prep the wood for steaming and bending.
1. Cut the clamp ends off freeing the kerf cuts.
2. Inspect the wood for checks or cracks.
3. Wood glue or epoxy cracks and wait to dry.
4. Sand smooth.
5. Spread the wood slightly using dowels and a pry bar. This opens the wood up exposing more surface area for steaming.
Step 6: Steaming Wood With a Steam Bag
After the wood is cut it can be bent dry or it can be steamed for increased flexibility. For steaming the wood I used 2 Earlex Steam Generators available from Amazon and a bag enclosure for specific wood sizes. These are the steps for construction.
1. Measure witch of 4 or 6 mill drop cloth to be able to wrap around the wood with minimal slack.
2. Lay a piece of 1/16" silicon rubber down, cut a piece of parchment paper and plug in an iron.
3. Iron a seem down the length of the plastic drop cloth enclosing the dry wood.
4. Use an ABS or PVC cap piece and drill a hole and screw in the coupling nozzle.
5. Wrap the end of the bag around the cap, wrap that with a rubber strip and secure it with a hose clamp tight.
6. Screw in the steam generators and turn on.
7. Cut a small hole in the bag at its lowest point to drain the condensation.
8. Leave to steam for 1 hour per inch thickness of wood.
9. Enjoy the smell.
10. The Earlex Steam Generator has a reservoir that will last about 2.5 hours. This will work for up to 2 inch stock. But, make sure to be ready to take the wood out before the steam runs out. It is best to act fast while the wood is hottest and most soaked.
Step 7: Expanding the Wood
When the wood has steamed for up to 2.5 hours now it is time to take it out and expand it. Spreading the wood can take two methods. 1 is using wedges and hammering them in the gaps at the end. The other is to use a pry bar and round inserts. And the third is using a jack system with threaded rod. I like the control of the pry bar and round insert as it is fast and gets the wood into shape while its in its most flexible state. The threaded rod on the other hand is slower but more precise. The downside is that the threaded rod, once in place is hard to interchange or remove entirely once the wood dries and holds form. For future tests, I believe there can be a template that the wood is spread onto the works with the holes in the pattern.
Step 8: Furniture or Sculpture
The crux of the project was battling pragmatic decision making for artistic expression. The sequential step by step study acts more as a materials test rather than a full piece of furniture. But here, we can see the applications.