Introduction: Outie Headphones: Parametric Bent Wood Lamination

About: I tend to have too much energy, so I try to do everything so I can sleep at night.
This is part of a two-Instructable project on outie (or reversible headphones). The main Instructable can be found here. If you have no interest in headphones but all the interest in parametric wood lamination, my hope is this one can stand alone.

Wood lamination is a fairly messy process. It is difficult to create pieces with critical dimensions, where the functionality is lost if those dimensions aren't met. The end result of the lamination is only as good as the mold/jig you create for it. And if you want to vary the final form even a little bit, you have to create an entirely new mold, usually by hand.

I wanted to explore automatic generation of wood lamination mold and profiles from a parametric 3D model of the finished product. For this I needed:
  • 3D model of the finished product based on critical dimensions (essentially head size for a headphone)
  • 3D model of the mold and wood lamination sheet, automatically generated from finished product

Once I have these, I can:
  • automatically generate cut files for ShopBot and laser cutter for a particular design
  • ensure laminated piece holds to the intended critical dimensions
  • change the critical dimensions in the model for a different head, and automagically generate new molds and profile cut files

The intent is that by going through this process for a simple part, I could gain insight on the process of "automated" wood lamination creation that would later enable me to apply generative art concepts to wood laminates.

Step 1: Headband CAD: Robust Modeling Discussion

In parametric CAD packages (Autodesk Inventor, Solidworks, ProE), making basic models can be a fairly straightforward process. However, making robust models can be much trickier, even for the same shape. If you've ever built something without thinking about it too much, then wanted to go back and make changes, you've probably experienced the pain of spending waaaaaay too much time making the simplest changes. Usually this is because you didn't design your project to be changeable: you made design decisions that were optimized for speed, cost, etc instead. You can do the same thing with modeling. I think of a robust parametric model as one that is easily changed without needing to constantly fix broken parts of the model.

There are many methodologies and best practices for creating robust parametric models. Most focus on the way you create references between your features, since references are the most common thing to break when changing models. The form of a headphone headband can be almost completely described by 2 perpendicular drawings (one from the front view and one from the side). The strategy we'll take is to create 2 base drawings that every feature references directly. This way, whenever we want to change the model, we only have to change the 2 base drawings to update every feature, rather than update every single feature manually.

Step 2: Setup Your Inputs and Equations

My goal is to create a robust set of models that include:
  1. the final form of the headband
  2. the mold I will create to laminate the wood
  3. the profile of the wood laminations for laser cutting
all linked to each other. These models will be used to output vector DXF files for the ShopBot CNC router and the laser cutter.

By parametrically linking the the mold and profile features to the base sketches (which dictate the final form), any updates I make to the final form will instantly propagate to the mold and profile without any additional work from me. It will take longer to build the references needed for this robust model, but it will enable me to iterate very quickly and generate files for headbands for different size noggins.

To achieve this you need a logical flow of references. First, let's define the key dimensions that will drive our headband model.

Given a particular size of head...
  1. Final headband radius (input) - pick this to match the width of the head
  2. Band leg length (input) - pick this so that combined with the headband radius the earbuds sit at the right height on the head
  3. Wood veneer thickness(input) - thickness of a single sheet of wood laminate
  4. Number of veneer sheets(input) - combined with the thickness, gives the...
  5. Total thickness (computed)

Before even creating drawings, let's create equations that define all these quantities. Some equations are used as inputs to the model, and some are used to compute values needed to create the model. The computed equations we'll need are:
  • total thickness = wood veneer thickness x number of veneer sheets

If we change any of our inputs, the total thickness.changes automagically. These equations will become much more useful for the later models.

Step 3: CAD the Final Form

Create 2 orthogonal base sketches that completely define the final form of the headband (snapshot coming once I fix my Windows partition...). Link the dimensions in the sketches to the equations you created. Dimension the headband curve to be the final headband radius, the extensions past the curve to be your leg length.

This model is used mostly for aesthetic iteration: we can change the base sketches to alter the final form until we like it.

Step 4: CAD the Mold

Next up, the wood lamination mold.

The wood lamination process creates internal stresses trapped in the wood and glue that cause the wood to try to springback to its natural state when removed from the mold. Due to this phenomena, molds need to be designed with profiles that are "more bent" than the final desired form.

The equation for springback radius from a book on one of my shelves:

Springback radius (computed) - reduction in radius of the mold (compared to headband) to take into account the springback of the laminate when released from the mold

I entered it into the model as:
Rform (in the model) = (number of layers - 1) / (number of layers) * (original radius)

You'll notice in the equations I also keep track of the curved length of the original sketch and the total length (including the headband legs). I use these to properly dimension a new sketch for the mold using the springback radius. The circular arc with the springback radius has the same length as the original form's curve, and the legs are the same length.

I build up the mold model around the new base sketch, and add the following features:
  1. empty space to hold the laminate equal to the thickness of the band
  2. break the outer mold into three parts so I can clamp it one section at a time
  3. add dowel pin holes into each mold piece for easy alignment (each section will be made from 3 stacked pieces of MDF)
  4. clamp holes on the inner mold, 2 for each outer section to ensure even clamping
  5. flats on the outer mold pieces that are parallel to each clamping surface on the inner mold (these clamping features are going to make clamping SO easy)

To use the mold on the ShopBot, just export the top profile of each piece to a DXF.

Step 5: CAD the Wood Laminate Profile

The wood profile will be laser cut and needs to be designed so when it's laminated it will match the original form. Create a new sketch and draw the headband as if it were rolled flat. To get it to match the original, we add dimensions like the curved length, leg length, and leg width. Then set these equal to the equations from earlier to get the profile to match the form.

The only additional feature here is extra length to the straight legs. When we laminate the parts together, the outermost laminations will naturally curve less than the inner and therefore have shorter legs. The extra length (I added 1/2") will ensure all sheets are long enough when laminated, then we can sand them to the proper length. As with the mold, simply export the face of the flat laminate to DXF for laser cutting.

Now we have 3 models, whose features are all linked together. Try changing some of the original dimensions and watch it automatically the form, mold, and laminate profile.

Step 6: Generate Files and Cut Parts for Mold

Time for building. I'll let the pictures do more of the talking.

  1. Transfer the mold model to a DXF
  2. Import into VCarvePro (program that translates your DXF to ShopBot language), layout the parts, setup the toolpaths
  3. Find 3/4" MDF and cut to size
  4. Affix to ShopBot and run your files (notice I left tabs that keep each section attached to the big piece of MDF, made my life easier for holding the parts to the ShopBot bed)
  5. Remove parts from the base
  6. Sand away the tabs, being very careful to preserve the surfaces that will contact the laminate

Step 7: Glue and Assemble Mold Sections

  1. Get wood glue, pins, hammer, and clamps
  2. Liberally spread wood glue on sides where pieces will touch
  3. Place parts together, hammer pins into holes to force pieces into proper alignment
  4. Clamp pieces together to achieve better bond, clean up excess wood glue around the edges with a wet rag
  5. Wait 24 hours for parts to dry

Step 8: Unclamp and Finish Mold

  1. Remove clamps
  2. Using a sanding block, sand smooth the profiles (the part that will touch the laminated wood) of the mold. Be careful not to change the shape of the parts and eliminate any creases or bumps

Step 9: Lasercut Headband Profiles

  1. Export DXFs of the headband profile
  2. Place wood veneer on laser
  3. Determine the proper settings for your machine and material, cut your profiles

Step 10: Mold Preparation

  1. MDF mold
  2. Wood veneer profiles
  3. Oodles of clamps
  4. Wax paper
  5. Scissors
  6. Wood glue
  7. Small foam paint roller
  8. Some kind of tape (I used painter's)

  1. Cut a strip of wax paper to fit around the outer perimeter of the inner mold. This will protect the surface of the outermost laminate during clamping, as well as prevent the laminate from adhering to the mold.
  2. Cut one large wax paper strip several inches longer than the wood laminates, and set aside. This will be used on the outer mold pieces to prevent the laminate from adhering to them.
  3. Cover the base MDF with wax paper and put the inner mold on it.

Step 11: Assemble Lamination Profiles

  1. Create your glue rolling station.
  2. Pour glue into a paint roller basin.
  3. Lay out a sheet of wax paper to prevent the laminates from bonding to the table.
  4. Piece by piece, roll a thin layer of glue on the topmost laminate, then lay the next laminate on top, covering the glue.
  5. Continue until all the laminates are in a wood - glue - wood - glue - etc stack.
  6. Before the glue becomes too tacky, align the edges of the laminate profiles by pressing in on the sides.

Step 12: Form Laminate Stack to Mold

  1. Place the centerline of the laminate stack (the line that would go front-to-back over the center of your head if you were wearing the headphones) against the top-of-head surface of the inner mold.
  2. Take the long wax paper sheet and lie it against the unprotected surface of the laminate.
  3. Position the top outer mold piece at in the picture, and tighten the clamp until the mold has just started bending the laminate.
  4. Add other clamp, move slowly, adjusting each clamp a little at a time, constantly readjusting to make sure the laminate remains centered and the stack of laminates don't get shifted out of alignment. Do this until the top piece is secure. Don't over-tighten to avoid crushing the laminate.
  5. Continue with the outer mold side pieces, adding clamps, alternate between clamps and close them little by little, ensuring the laminates remain in correct position.
  6. Once finished, the mold assembly should be sturdy and move as one piece. Leave to dry for 24 hours.

Step 13: Cleanup the Wood Lamination

  1. Carefully relieve the pressure from the clamps one by one, remove the outer mold pieces from the assembly.
  2. Once freed from the outer mold, carefully free the wood laminate from the inner mold.
  3. Time to prepare the laminate for finishing. Very, very carefully use belt/disc sanders to remove the excess length of laminate from the ear-side legs. Trim to the length appropriate from your CAD model.
  4. Most edges will have glue that squeezed out during clamping and needs to be removed. You can either use sandpaper with hand (I mostly used 220 grit) to clean up all the edges, or use a powered sander for larger chunks. If you use a powered sander be incredibly careful not to push too hard and sand away parts of the actual piece. I learned the hard way disc sanders disintegrate wood laminate faster than ice melts in a bonfire (need to work on better metaphors).
  5. If you were careful enough while laminating (or got lucky enough), you won't have to sand the the outermost surfaces of the laminate. Since it's a messy process though, chances are there's some glue dried on the outside. Use sandpaper to remove, being careful not to sand through a layer of laminate. If necessary, use finer grit sandpaper to leave the surface nice and purty.
  6. Finish up the sandpapering by using a 220-ish grit to uniformly round the edges of the laminate until you're happy.

Step 14: Wood Finishing

There are a billion combinations of finishes you can apply to wood, and an equal number of websites telling you the best way to do it. I've tried a variety of combos. If you want a glossy finish, and don't work with wood tremendously often, it's hard to beat the effort-to-quality ratio of a simple stain with a polyurethane coat. You can get really small containers for relatively cheap, nearly any hardware store has it, and doesn't need more than several minutes attention at a time and a careful hand holding a brush.

After satisfactory sanding, use a tack-free cloth to clean all dust and dirt from surface. Cover the surface you'll use to protect it.

The goal is to create a perfectly smooth thin coat with no bubbles over the entire surface. It will take some practice and technique depends on your brush and finish, so try a practice first.

  1. Dip foam brush into polyurethane, soaking just front 1/4" of brush (don't want to apply too heavy of a coat, or else it will run and we'll get an uneven surface)
  2. Be careful to avoid creating bubbles with the polyurethane, they might get transferred to the surface and create ugliness
  3. Slowly apply polyurethane over entire surface. Be careful not to brush over polyurethane thats set more than 30 seconds: it may have become tacky enough that you'll leave streaks in it.
  4. Once finished, set on surface as not to ruin the coat, and allow to dry for 2 hours. If done correctly, the surface will be blemish free, and all you need to do is additional coats in the same manner until you're happy with the thickness (for me this was 3 thin coats).
  5. If you need to fix imperfections in the coating, use > 400 grit wet-dry sandpaper with a little water to smooth them out.
  6. Dry, clean, and repeat the process.

Step 15: Back to Headphones / Future Work

Additional headphone work
If you want to get the inside scoop on the headphone electronics, 3D printing, etc, check out the main Instructable.

Future work
This project's main goal was to evaluate the usefulness of CAD to flexible wood lamination design and fabrication. The next thing I want to explore is linking generative design techniques (code that creates designs according to a set of rules rather than specifying every feature in the design manually) to these types of wood laminations CAD models through CAD systems APIs (APIs are functions you can use to access a program from another program). I'm imagining a large model of wood laminates with many sub-models linked together to form a huge structure, running generative scripts to create structures with repeating micro-forms and emergent macro-forms, and automatically generating all the necessary molds and cut files. I think it sounds better in my head :)