Design
Design decisions were made based on how the torsion test will be performed. The device is designed to hold rods with diameters ranging from 2mm to 5mm. The rod must have a 90 degree bend at both ends pointing in the same direction. The length of the bent parts can range from 2 cm to 5 cm. The part of the rod which undergoes torsion has a length of 20 cm. The device performs the torsion test by firmly holding the rod in place at one end while holding and twisting the other end using a pulley attached to a weight. The parts of the device are made entirely of acrylic in order to be cheap and easy to manufacture.
Base Design
The base of the device needed to be stable and allow the end of the rod to be firmly held in place while also allowing the length of the rod to rotate freely. The end of the rod is securely held in place by the top plate (orange), base plate (grey), and the two plates in between them (blue). In order for the clamping mechanism to be capable of holding rods of varying diameters, slots were made in the base plate and top plate so that the space in which the end of the rod fits is adjustable.
There are also two long pieces (blue) that run parallel to the length of the rod. These pieces act as rails to ensure that the rod only undergoes torsion and not deflection. These pieces do not firmly clamp the rod however, as this would introduce friction and not allow the rod to freely rotate.
The base plate is supported by four rectangular pieces that link to the base plate and with each other to form a solid platform. These support pieces have tabs that fit into the base plate. The tabs contain slots in them so that a small piece or pin can be inserted into them to prevent the tabs from coming out of the slots in the base.
Pulley Design
The pulley is made up of three acrylic discs that are bolted together. The acrylic disc in the middle (blue) has a smaller diameter than the outer discs (orange and purple), creating a space for string to be wound up. The middle disc has a slot that can fit rods with diameters as large as 5mm. The rear disc has a hole near the edge so that string can be tied to it. The first disc (orange) also has protractor markings etched into it. This allows students to determine the amount of angular deflection more easily.
One of the challenges for this design was to come up with a way for the pulley to be able to securely hold rods of varying diameters. The solution for this is an L-shaped piece (pink) with slots in it. The slots in the middle disc and front disc (orange and blue) have a width of 5mm. In order to firmly hold smaller specimens, the L-shaped piece has diagonally positioned slots which allow it to push the rod into the top corner of the slots in the front disc and middle disc. This allows smaller diameter rods to stay as close to the center of the discs as possible. Also, the corner of the slots in the front and middle discs is offset from the center of the discs by 2mm in the x and y directions. The reason for this is to ensure that the center of the disc is as close as possible to the center axis of the rod. This allows the pulley to spin as concentrically as possible.
Manufacturing
Lasercutting
Since this device is made of acrylic pieces, the manufacturing process of the device consisted mostly of lasercutting. All of the pieces were made from lasercutting 4.5 mm thickness sheets of acrylic. The lasercutter reads a dwg file of the piece and cuts along the lines of the drawing. When cutting parts that need to mate together, it is important to consider how lasercutting affects the measurements of the piece. The laser is able to cut pieces by melting the material along the specified lines, which causes a certain thickness of the material to be lost. This means that holes made by the lasercutter will be slightly larger than what was specified in the drawing, and extrusions will be slightly smaller. If a hole is dimensioned to have an exact fit with a screw (the diameter of the hole is equal to the OD of the screw), the fit will be noticeably loose.The protractor design was also etched into the front disc using the lasercutter. (more on lasercutting can be found here)
Preparing the Specimen
