Introduction: Pulse • a CNC-cut Social Bench

Within our social toolset, conversation is imperative to communicate and connect with those around us. Each new encounter affords us the opportunity to interpret the world through fresh perspectives, simultaneously extending a deeper understanding of our own self. Unfortunately, modern lifestyles and daily routines often generate distance from others and missed opportunities to connect and grow. Pulse is a response to this lack of communication between strangers within our society. By creating a casual, inviting atmosphere, conversation is inevitable.

A sense of warmth radiates from each new bond and inspires other interactions to spawn thereafter. This sense of connection creates a magnetic field and Pulse stands at its core, drawing users to learn and discover their yearning desire to be recognized and connected within their community.

Step 1: Model Form

Top Surface:

I analyzed the ideal curve for a Chaise Lounge, based on human dimensions and roughly following the curvature of Eames chairs and other successful furniture. I drew out this curve in Rhino 3D and continued the curve to include a bar stool seat in the back as well. I then mirrored this curve and pulled it out 40 inches (in the X direction) from the first curve. This allows ample space for both seating “strips”. I slightly raised one curve to create a more ambiguous topography of seating spaces. I put a few more curves in between these two to further frame the transition from one curve to the other and finally lofted these curves to create a smooth surface which I finessed quite a bit using the Autodesk T-Splines plug-in for Rhino.

Bottom Surface:

The bottom surfaces were a result of designing the two side elevations to exemplify the curves and attempt to create the most efficient form utilizing the least material whilst retaining structural integrity. I developed two curves on each side, and again placed a few intermediary curves for a smooth transition from on face to the other. I then lofted and finessed these surfaces in T-Splines as well.

Step 2: Preparing Model to Be Built

Slicing the Piece into Three Components:

I divided the Chair into three pieces so that it can more easily be moved around by a minimum of two people. This involved creating indents where the slices are made and ensuring these slices happen in sync with the 2.25” increments described in the next step (“Preparing Cut Files for CNC router).

Joiner Pieces and Dowels: I modeled two joiner pieces on the front and back end and two more on the bottom. These get bolted in each time the piece is moved to a new location and keep the three units together. Each piece does stand alone so these don’t structurally hold the piece up, but they are strong enough to take the load and retain stability as people get on and off the piece. Four small dowels were also modeled in each corner of the piece (some are discontinuous) to help align the slices once they are cut and ready to be glued down. These should be drilled out by the CNC cutter because it is nearly impossible to guarantee accuracy by hand-drilling these holes, and alignment is everything when glueing the pieces together. The discontinuity is ok, because it is less important that the dowel goes all the way through, what is important is that each two pieces being glued together will need at minimum two points for dowels to go in once glue is applied to hold the pieces stable and perfectly aligned when clamping down these pieces.

Final Steps of Model-making: The surrounding surfaces are simply flat surfaces that can be lofted as planar surfaces from the created curves along the perimeter. This is to give the illusion of one solid mass that ties all the users together and allows for smooth comfort and efficiency only where humans are intended to interact with the piece.

Step 3: Preparing CNC-routed "Ribs"

Selecting Material:

I chose to use 3/4 inch ultra light medium-density fiberboard (uldf) because it is soft, easy to cut and relatively light-weight. In the future I will choose to use exterior grade mdf so I don’t have to worry when leaving it outside. With this in mind, and previous experience which proved that laminating 4 4’x8’ sheets is too heavy to lift onto the CNC bed, I decided to laminate the sheets in 3-ply sets. So I sliced the model in 2.25” (3 x 3/4”) increments. I decided the two elevation pieces (because they are solid rather than hollow) should only be made of 1-ply material so for these two edges I sliced the model by 3/4”.

Alignment and Preparation of "Ribs":

Once the model is sliced it is time to lay out all the pieces. It is imperative that from the midpoint on the pieces are all rotated so the sloped surfaces are all facing the same direction. This is important because the CNC can only cut slopes in one direction, it cannot undercut. It is also very important that these slices are rotated to get the slope on the right side, rather than mirroring the pieces which will result in backwards, unusable pieces.

With a fully sealed perimeter for each “rib” member I offset this curve inward to create a 1.5” thickness. This allows the interior to be hollow and will significantly cut down the weight. In the first iteration I used a 3” thickness and the chair wound up far too heavy. Because it is fully solid, and gains stability from this solidity and overall mass, 1.5” is a good thickness, not too thin. It still was somewhat heavy so 1” may suffice, but may not guarantee sufficient gluing surface, which is the main structural source for this piece.

Lastly I divided each rib member into about 5 pieces using finger joints to re-connect the pieces once cut. This allows for maximization of space on each sheet of MDF as the pieces can be more easily nested together than if you were to cut each rib out whole. When making these finger joints make sure to account for the curvature of the CNC blade. Instead of making rectilinear finger joints I made them with rounded corners using the radius of the CNC bit I planned to use for the profile cuts. This allows each piece to fit together perfectly with no additional cutting necessary.

Step 4: Preparing for CAM Software

Numbering and Sheet layout:

In a project with so many pieces it is absolutely crucial to number each member in Rhino, in the cut files, and also once they are cut in pencil on the piece. This is the only way to keep track of each piece and smoothly manage putting all the pieces together. Next draw out rectangles for the 4’x8’ sheets and start placing the rib pieces on the sheet. I was able to nest most pieces and only used 5 3-ply sets (15 sheets total) for the ribs, (one of which was flip-milled). On top of this I used three sheets to create the end pieces and joiner pieces, so a total of 18 sheets. While placing the pieces on the sheets it is important to account for the drill bit thickness in between pieces to prevent cutting into any pieces. The closest each piece can go is the diameter of the drill bit. A little safety room is nice to ensure you don’t lose any of your piece. It is especially important on the top surface of the pieces. As this will create the surface of the final piece and must remain fully-intact for optimal results. One of the sheets will need to be flip-milled. This is the piece containing the middle transitional members because they are doubly sloped. Make sure to put all these doubly sloped pieces on one sheet.

CAM software:

What was required for me, using RhinoCAM, was a perimeter curve of each for the profile cut toolpath, a perimeter of the sloped surface for the roughing toolpath, (separate the two sloped perimeters on the flip-milled piece because they will need to be two separate files), and the sloped surface. It is best to put each of these paths on a different layer so it is easy to select all the curves when setting each toolpath in RhinoCAM. Also make sure to include the point for the drilling of the dowel alignment holes, and the number of each piece (not for RhinoCAM but simply for your own reference). A final important check is to make sure the slopes are all facing the right direction.

Step 5: Preparing Wood for CNC Cutting

This step requires two people. Lay out the Wood and create a perimeter of scratch paper in case any glue drips out. Laminate 3 pieces of 4’ x 8’ 3/4” uldf using a spreader, which can be purchased or simply made of scrap wood and wood glue. I used Titebond II from Home Depot. It is imperative to be quick and evenly spread a full, even surface of wood glue on each piece before setting the next one on top. Quickly check each corner for alignment then press along the full surface (I gently walked on the piece without shoes) to ensure no air-pockets are trapped inside - this could damage the CNC router or simply create imperfections in your piece. Once you have 3 pieces together, you can create another layer of perimeter protection (I used trace paper and masking tape). This is especially important in between sets so they don’t get stuck together. Repeat this process for all 5 pieces and set some weights on the final top piece to hold it down and prevent expansion. If doing each set separately you can clamp the pieces down with horizontal and vertical 2”x4”s to evenly compress the pieces together. Doing them altogether allows their own weight to compress the pieces together but weights will be needed for the top piece. The wood takes 24 hours to dry, so make sure to account for this before scheduling time for CNC-routing.

Step 6: CNC-routing

I used Rhino CAM to create the CNC cut files which is nice because it can use the rhino file and is a familiar platform for me. It took me 2-3 full days of CNC-routing. The laminated pieces are still quite heavy and may require 2-3 helpers to lift onto the CNC bed. The flip-mill piece is also challenging

Step 7: Laminating the Pieces

Once all the pieces are cut you can begin organizing them in numerical order. It is helpful to lay out the wood on bricks or blocks so that you can clamp each set of layers while they are drying from the glue. Wood glue tends to expand quite a bit, so using as many clamps as possible and clamping right away is ideal. The wood glue takes 24 hours to dry. This process took me about a week.

Step 8: Applying Bondo

I didn’t sand the pieces from the roughing path grooves because these grooves allow the Bondo to stick to the MDF more easily. Apply Bondo to all sloped surfaces and sand until smooth. It took me a total of 7 tubs of Bondo.

Step 9: Applying Joinery

Drill holes through piece and joinery. Make sure there are at least two bolt holes on each piece (and four on the middle piece). I used 1/2 inch nuts and bolts. If planning to leave the piece outside choose the galvanized nuts and bolts. You will need 32 nuts and bolts.

Step 10: Applying Paint

To ensure a smooth even surface it is recommended to use a spray gun. First apply a layer of primer. If planning to leave the pice outside choose an outdoor protecting layer. Next you can choose a color for the whole piece. Or select two colors, one for the sloped surfaces and one for the planar surfaces.

Step 11: To Conclude

Public exposure to others allows individuals to develop a criticality to their life by comparing it to other views and lifestyles. This social connective intervention creates spontaneous encounters and fosters a richer understanding of a city’s inhabitants as well as of one’s self.

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Bio: I graduated from Cal Poly San Luis Obispo with a B. of Architecture in 2015. I have worked at a few architecture firms in the ... More »
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