To ensure a seamless transition between design and manufacturing, production drawings were made for the most critical components of the project such as the cam shaft, the bearing holders and the knife carriage.
Limits of 3D printing in the scope of this project
The majority of components of this project needed to be machined because the final demonstration would involve cutting a vegetable causing significant loads to be applied to many components. Furthermore, other components such as the rails and barrel cam indexer, though not highly load bearing, would be sliding against other parts requiring a smooth surface finish to negate the effects of friction as much as possible. The resolution of 3d printed parts would have just not been sufficient to achieve such a finish.
Other parts were machined because a very tight tolerance fit was necessary. One example of this was in the bearing holders for the indexer cam. These bearing holders had a H8 +0.0012 / -0.000 fit on the inside bore of these parts. 3D printing would not have been able to achieve this tolerance.
Some parts, however, were 3D printed. These included the spacers under the cutting board as well as the pulleys for the timing belt. The spacers were printed to save time, since they were not a critical components and 3D printed parts have a relatively high yield strength in compression normal to the print layers. The pulleys were printed to save money and were based on a previous design with proven success.
The Rails
Slider rails were used to constrain motion in both the slicing and indexing directions. The rails were machined out of aluminum and the sliders out of Delrin. Delrin was chosen because it is an inexpensive well lubricated resin of good toughness, low wear, and good machinability.
The same T-slot rail profile used for the Slider-crank project earlier in the semester was used for the rails in this project. The T-slot in the Delrin sliders were cut with a custom ground cutter made from a 2-flute endmill. Because the surfaces cut into the rail are sliding surfaces, high RPMs, low feed rates and stable work holding were used to ensure a smooth surface finish. Below is a photo of the profile between the white Delrin slider and the aluminum rails.
The Making of the Barrel Cam Indexer
By far the most difficult part to make in this project was the barrel cam indexer. The challenges with this part arose from the complicated channel geometry as well as the tight tolerances on the sides with the bearings.
The machining of this part was done in two stages. First it was machined on the lathe and then after it is taken to the 4th Axis Rotary CNC and milled there. Turning between centers was done to ensure concentricity until the shaft ends were the correct dimensions. Constant measurements were taken with micrometers to ensure that the barrel was within tolerance. A final measurement was taken on a cold shaft. Because metal expands when it is heated, and the process of machining is a high friction process, it is often necessary to let a part cool down before making a final measurement especially at such tight tolerances. This is because the shaft will actually be smaller when it is in operation than just after machining.
Once lathe turning was finished, the part was taken to the 4th axis CNC machine. This was used to cut the indexing groove into the cam. Wrapped tool paths were created using Fusion360 CAM (computer aided manufacturing) and these parts were posted as G code for the machine to follow. The part was trued into the rotary and indicated in using a dial indicator. This ensures alignment with the axis of the rotary as well as alignment with the X axis of the machine. A cutting tool was inserted and the CNC began cutting. The blacklash and slack in the rotational axis caused a build up of error during the run and cross milling began. The CNC machine had to be stopped and a single cut in one direction was made to create the channel. This proved successful.