Design Description
The current process used to assemble the adhesive tabs to the bioabsorbable webbing is a tedious assembly process for the operators. The fine motor skills required to assemble the device create ergonomic issues for the operators. The current alignment method of the adhesive tabs and bioabsorbable webbing is done visually as well as the final inspection of the device.
Our design has the following elements involved. The final design can be seen on the Final Machine Design page.
1. Design Elements
1.1. Fourbar
The process requires the application of adhesive tabs to bioabsorbable webbing repetitively. A fourbar linkage is “the simplest pin-jointed mechanism for single degree of freedom controlled motion” (Norton 47). Thus, a fourbar mechanism is the best choice for performing the repetitive motion required by our process. This is a CAD model of our fourbar.
Figure A. Fourbar assembly.
1.2. Webbing Fixture and Mask
The webbing fixture is the rotating part of the machine that holds the bioabsorbable webbing. The webbing fixture has been customized to fit the bioabsorbable webbing. As shown in Figure 1, the webbing has a unique shape. The small radii on the four tabs are used in order to fully define the webbing as it is placed into the fixture. The radii allow for a 1/16th inch dowel pin to fit into the notches. With placing dowel pins into every notch, the webbing is fully defined in the fixture and can easily be placed in the same spot on the fixture repeatedly.
Figure 1: Webbing
The adhesive tabs are almost identical to the four tabs of the bioabsorbable webbing. They are the same shape, width and have the same radii patterns as the webbing. The tab layout can be seen in Figure 2. These similarities allowed for the dowel pins to not only serve as alignment pins for the webbing, but also to align the adhesive tabs correctly with the bioabsorbable webbing.
Figure 2: Tab
Figure 3 shows the webbing fixture with a piece of bioabsorbable webbing correctly placed within the alignment pins. Because of the ergonomic issues of trying to remove the webbing from the fixture an internal lifting support cylinder was designed into the fixture. The lifting mechanism lifts the device up off the flat surface of the fixture and away from the alignment pins. This allows the operator to easily remove the device without damaging it. This can be seen in Figure 6. In order for all four adhesive tabs to be correctly placed on the bioabsorbable webbing the fixture was designed to rotate around its center to allow for placement of all four tabs. Two small detent pins are located on the underside of the fixture 90 degrees apart. This allows for the fixture to lock into place every 90 degrees ensuring that the four adhesive tabs will be correctly placed on the bioabsorbable webbing. The grooves for the detent pins to lock into can be seen in Figure 3.
Figure 3: Webbing Fixture
Due to the fact that the webbing and tabs are thin and flexible, it is possible to apply a tab that would overhang the edge of the webbing. This would result in a rejected device. Currently the method to detect an overhanging tab is by a visual inspection. In order to achieve a 90% yield and to improve the time to assemble the device a go – no go gauge was designed to aid the operator. A mask was designed that could be placed on the top of the webbing fixture; the mask can be seen in Figure 4.
Figure 4: Mask Fixture
The mask was designed so that it would fit around the alignment pins while still allowing for the bioabsorbable webbing to be placed down onto the fixture. The webbing tabs are a ¼ inch wide, so the mask was designed to be cut with a ¼ inch end mill. The ends and sides were cut appropriately to allow clearance for the 1/16th inch dowel pins. The way the fixture and the mask align can be seen in Figure 5. The mask just clears the dowel pins and acts like a shield. If a tab was going to be placed off of the webbing, it would stick to the mask and easily be seen by the operator as an incorrect placement.
Figure 5: Webbing Fixture with Mask Fixture, top view
Once all four tabs are placed onto the bioabsorbable webbing, the operator would then lift the lever removing the webbing from the fixture in order to see if one of the tabs had stuck to the mask. Figure 6 below shows how this process would work. Since the webbing lifter is only the size of the center portion of the bioabsorbable webbing, the tabs fan out allowing for the device to be easily removed.
Figure 6: Webbing Holder with Webbing Lifted
1.3. Tab Holder and Grippers
Proper operation of the fourbar mechanism requires the next available tab to be located in the same spot every time to allow the grippers to grasp it. The clear backing of the tabs allows the adhesive tabs to be removed consistently. The clear backing is bent and placed into the groves of the tab holder. The bending of the clear backing allows for the non-adhesive end of the tabs to extend off of the clear backing and allow for the tabs to be gripped and removed, as can be seen in Figure 7. The friction that is created due to the material wanting to return to its original shape is sufficient to hold the strip in place while in the tab holder. Figure 7 below shows a strip of tabs being held by the tab holder.
Figure 7: Tab Holder
SMC Corporation of America is the world’s largest manufacturer of pneumatic automation products. One of SMC’s pneumatically actuated grippers with customized tweezer ends was attached to the fourbar mechanism. The tweezers on the end of the fourbar are capable of grabbing a tab, putting it on the webbing and then releasing it without causing damage to the tabs or webbing. A pneumatic gripper allows for control over the amount of force that was being applied to the tab. At the base of the machine are a pressure regulator and a pressure gauge. This allows for the operator to adjust the air pressure being applied to the gripper until the proper grip is achieved. The grippers with the customized tweezers are shown in Figure 8. The tweezers were designed to be the same width as the tab, This allows the operator to visually determine if the grippers have correctly gripped the adhesive tabs.