Methods & Construction

Other than the initial vision of this project, the analysis and design was carried out at the CWU campus. The device is a collaboration of pre manufactured components and parts machined on the CWU campus. Pre-manufactured parts were purchased from various vendors and were typically of complex geometries or assemblies. A list of the parts can be found in Appendix C1.1 of the report. Parts made on the CWU campus were made from raw materials or modifications of previously existing components.

 

Assembly

The main parts purchased from suppliers are the two helical gears, thrust bearing assemblies, open ball bearing assemblies, keys, and external retaining rings. Components that are made on campus are the two gear shafts, top and bottom bearing carrier plates, cog holding plate assembly, and modifications to a previously existing cassette locking tool. Manufacturing will be completed in a specific order. First, the bearing carrier plates will be machined and the open ball bearings will be press fit into their respective holes on the plate. Second, the gear shafts will be turned and milled to achieve the appropriate features. Third, the thrust bearings, helical gears, bearing carrier, and gear shafts will be assembled together and secured by external retaining rings. A visual tree diagram of the main assemblies can be seen in Appendix C1.2 of the report.

 

Engineering Disciplines/Relevant Equations

Engineering discipline areas of interest for the project include statics, strength of materials, and machine design in general. Equations used multiple times for the design and analysis are Direct Shear , Torque , and Diameter of Shaft for Given Torque . These and other related equations were used to help optimize design parameters of several components of the device including bolt sizes, minimum shaft diameters, and appropriate gear ratios.

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Manufacturing

Manual machining on a lathe and mill along with CNC machining were used to produce essentially all of the parts that were not purchased. Below are images and videos of some of these parts during and after manufacturing.

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Manufacturing Issues/Modifications

During manufacturing, multiple issues were discovered and presented their own challenges. The first issue to arise was holding of the different work pieces in the lathe. All of the cylindrical parts in the device are rather small and do not leave much room for secure and accurate clamping of the work piece as well as room for the machine tool. Improved process and procedures help alleviate this issue by leaving the material stock full size and machining the part completely then cutting it off the stock material. This provided lots of stock material for work holding and would leave plenty of material accessible for machining. Image 3.1 displays the improved work holding setup.

Another issue was the change in material hardness of the modified cassette tool after it had been welded to the gear shaft. The material became very hard and the turning tools had much difficulty when machining light cuts. The tool tip would simply break or chatter against the material and create uneven rough looking cuts. Unfortunately there was no way to fix the hardness problem in a timely fashion so the rough finish would remain but it has no effect on the overall function of the device. During the welding process, there was a slight overlap of weld splatter that effected the key seat machined into the shaft. Once again, this is something that should have been done last instead of earlier in the manufacturing process

Tolerances for the press fit application of the bearings in the top and bottom bearing carriers was certainly a learning experience. The relative light load demands and almost non-existent speeds during the use of the device made it clear that even with matching diameters, the bearings and bores for the bearings would have been a plenty tight fit. Instead, the bearing carriers were machined to be 0.001” undersize of the bearing outer diameter. This made installation of the bearings more difficult than originally thought. Even with heating of the bearing carrier and cooling the bearings, the fit was still rather tight and would make any future removal of the bearings difficult.

 

When final assembly of the device was done it was apparent that the spacing between the bearing carriers was slightly smaller than what was needed to fit the shaft assemblies set of thrust washers, bearings, and the gear all in-between the two bearing carriers. Luckily the thrust washer assemblies each use two washers that are only 0.032” thick each. Removing one washer from one thrust assembly per shaft remedied the clearance issue.

 

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