Material & Process Selection

In consideration of our target consumer base, the main goals are to minimize overall weight and cost of the desk. A student is likely to seek mobility in large furniture, which requires adaptability to user's changing needs. Strength of the material will also be considered but is not our primary focus. We assume that only books, papers, and supplies will be atop the desk, and so not much weight should be applied. In order to further minimize the market price of the desk, all the necessary components of the desk will be packaged into a kit with a set of instructions for the customer to assemble on his/her own.

Target market price: $500. Target materials cost: $200.

Flat Surfaces

Materials Considered: Particleboard, fiberboard, Acrylonitrile Butadiene Styrene (ABS) plastic

Wood is best suited for our flat tabletop surfaces instead of common glass and metals. For the purpose of this material selection, we are focused on lightweight material while maintaining durability. The common wood particleboard density is around 50 pounds/ cubic feet, while common glass is between 150-175 lbs/cubic ft and aluminum/steel is at large relative to these.

Particleboard is made from a variety of woods and scrap wood such as sawdust or sawmill shavings. The scrap wood is then compacted grinded into fibre similar to a thermomechanical process. The wood chips are preheated and then runs through a hot pressing process. This hot pressing process is the manufacturing step to heating and molding the wood scraps together, which creates a compact and sturdy material. This result retains the high yield and cost-effectiveness. Usually 3 layers are optimal for improved strength with a core in between two surfaces. The final properties of the particleboard is a result of the raw material itself as well as the manufacturing process. ABS is also considered alongside with particleboard because of its properties to be a great resistance to chemicals, heat and impact.

Because wood scraps are relatively easy to obtain and manipulate in machine processing, we are expecting low cost expenditure on the flat surfaces of our desk. A good example of cheap and commonly used particleboard material are in tables. IKEA has many low-cost desk and tables under $10 made from particleboard, fiberboard, ABS plastics.

References: Wood densities, ABS , particleboard manufacturing process

Angle-adjustable surface

Materials Considered: Glass, plexiglass, acrylic, plastics, cloudy plastic (polycarbonate, PVC, HDPE, UHMW)

Our goal is to achieve a strong, sturdy, clear adjustable angle surface. To optimize creativity, we determined a clear surface would be best, allowing for drafting artists to use light to trace through multiple layers of paper. To optimize productivity and ergonomics, we chose to offer users a versatile design with the freedom to continuously adapt to the user's changing needs. Expected price for a glass surface ranges from $20 to over $100, while expected median price for a plastic surface is around $50. Plastic material is also lighter than glass, which is one of the critical considerations for the angular surface since we want the user to be able to move the desk angle up or down easily. Different grades and finishes of various plastic materials were considered. When we examined the Ashby chart (see Figure below) for technical ceramics, we determined it was possible to achieve a relatively high (as compared to metal) strength to weight ratio with certain types of glass and plastic. The best material we found to achieve the clear finish and strength required for our desk (able to support 200 lbs) is Poly(methyl methacrylate) (PMMA), which lies relatively high on the strength axis and in the middle density range of the Ashby chart below, meaning it exhibits a favorable strength to weight ratio.

Figure 1. Strength vs Density Ashby Plot

LED lighting strip

Material Considered : Light Emitting Diode (LED), Incandescent Lighting

To help students or our targeting consumers draw easily through multiple layers of paper, we decide to install lighting under the transparent part of our desk. We considered LED and incandescent lighting for our lighting and we decided to use LED. These are the reasons.

1. Cost

Our first goal is the cost and we can find the bulb costs of Incandescent lighting and LED are not very different. However, the energy costs are different (Figure 1) and we can find the bulb life span of LED is much longer than incandescent one.

2. Efficiency/ Sustainability

As you can see in Figure 2, The LED is more efficient than incandescent in terms of energy. Also the smaller additional circular graphs show in the manufacturing process, the LED is also sustainable.

In addition to that, LED has no warm-up time. However, Incandescent bulb needs warm-up time for filaments heated up.

3. Safety/Property

Usually incandescent lighting has bigger size than LED. Also, the incandescent bulbs emit heat which could make fire on our desk.

A 10 Watt LED light is equivalent to a 600 lumen light, which is the typical value for a standard flashlight. Using this comparison, we can also safely say that using LED lights with similar specifications would be sufficient illumination to light up the entire acrylic surface. Moreover, the lights should be powerful enough for use in a dark room.

Reference : Light Comparison: LED Lighting vs Incandescent Lighting

Legs + Frame

Materials Considered: Mild Steel, High Carbon Steel, Aluminum, Bamboo, Polymers

In the interest of minimizing the number of materials used in the production of this desk to streamline sourcing and manufacturing processes used, we decided that it would be best to manufacture both the legs of the desk and frame of the adjustable surface using the same material.

We sought to choose a material that minimized both weight and cost, while still maintaining the capacity to carry 200 pounds. Using a material properties graphing software by DataVis Material Properties, graphs of cost vs. compression yield strength (Figure 1) and cost vs. density (Figure 2) were made for all the materials available. It appeared that polymers were the most affordable and lightweight, but these were undesirable because all polymers exhibited lower strength values than any of the other materials considered. Given that the legs and frame support the weight of the desk itself, we decided to set our minimum strength constraint to 200 MPa, which is the lowest strength of aluminum metal. So, the most competitive options from both graphs were High Carbon Steel, Mild Steels, Aluminum and Bamboo.

Figure 1. Price vs. Compression Strength Ashby Plot

Figure 2. Cost vs. Density Ashby Plot

In order to choose from these materials, another graph was created in Matlab that compared these materials in terms of cost vs density (Figure 3). The bamboo appeared to be the best but concerns with inconsistency regarding material properties, dimensions, and aesthetics made it a poor candidate. While natural bamboo tubes are cheap (100 4-ft tubes cost $14.29), they have inconsistent diameters, and bamboo plywood, which would presumably be used for the frame, is relatively expensive (4'x8'x1/2" bamboo plywood costs $165). Moving on, with this graph (Fig 3.) alone, none of the aluminum and steel metals appeared to be the best, so a merit index to compare the materials was derived.

Merit Index: A merit index is essentially a quantity that relates a preferred parameter to the primary parameters that affect it. In this case, the density and material cost both need to be minimized so that a light weight but affordable material is chosen. In the below equation, M represents the Merit Index which is a function of the density of the material and the cost of the material.

We then looked up each material to see if they were available in common construction forms, in this case round tubing. The most available, and rational materials were A513 steel (comparable to AISI 1020), 4130 alloy steel, Aluminum 6061-T6, 6063-T6, and 6061-T6.

6063-O: M = 26,676

6063-T6: M = 29,700

6061 - T6: M = 34,020

A513: M = 45,646

4130: M = 233,145

Aluminum 6063 is the best choice out of the available materials because it has the smallest index.

According to the merit index derived above, 6063 Aluminum proved to be the optimal material, with the lowest cost and weight given our constraints. For the legs, 1x0.055x72" tubes would be bought at $19.24 each, and for the frame, 0.5x2x96" rectangle bars would be bought at $54.65 each.

The legs would be cut to 30" lengths and the frame would be cut to 2' and 1.5' lengths using a band saw. The remaining tube lengths would be welded horizontally onto the legs on either side of the desk to add stability. The bar lengths for the frame would then be welded together to create a rectangular support around the perimeter of the angle-adjustable portion. Two additional 1.5' bar lengths would be attached to the inner sides of the flat surfaces using tie plates and screws, and protruding curved notches would be welded to their top surface. These notches would be milled using a desktop CNC, such as the Othermill or Shopbot, from the rectangle bar. A flat U-shaped component would also be created from welding three pieces of this bar (two 0.75' and one 2' lengths) together, and it will be fastened to the rectangular support frame at a halfway point. This would hook onto the notches when the surface is lifted for the angle adjustability.

Welding pieces together minimizes the number of components and increases structural integrity while allowing our team to construct these parts of the desk using only two shapes of metal. Milling using a desktop CNC allows for easy repeatability and accurately machined notches. This simplification in the sourcing and flexible use of material helps to reduce the cost of labor and amount of waste produced. Moreover, since only one material is used, any waste created can easily be collected and recycled.