The arc-on time is a measure of efficiency in an arc welding operation. As indicated in our discussion of arc welding in Section 8.5.2, typical arc-on times in manual welding range between 20% and 30%. Suppose that a certain welding operation is currently performed using a welder and a fitter. Production requirements are steady at 500 units per week. The fitter's job is to load the component parts into the fixture and clamp them in position for the welder. The welder then welds the components in two passes, stopping to reload the welding rod between the two passes. Some time is also lost each cycle for repositioning the welding rod on the work. The fitter's and welder's activities are done sequentially, with times for the various elements as follows:

Because of fatigue, the welder must take a 20 minute rest at midmorning and mid-afternoon, and a 40 minute lunch break around noon. The fitter joins the welder in these rest breaks. The nominal time of the work shift is eight hours, but the last 20 minutes of the shift is non productive time for cleanup at each workstation. A proposal has been made to install a robot welding cell to perform the operation. The cell would be set up with two fixtures, so that the robot could be welding one job (the set of parts to be welded) while the fitter is unloading the previous job and loading the next job. In this way, the welding robot and the human fitter could be working simultaneously rather than sequentially. Also, a continuous wire feed would be used rather than individual welding rods. It has been estimated that the continuous wire feed must be changed only once every 40 parts and the lost time will be 20 minutes to make the wire change. The times for the various activities in the regular work cycle are as follows:

A 10 minute break would be taken by the fitter in the morning and another in the afternoon, and 40 minutes would be taken for lunch. Cleanup time at the end of the shift is 20 minutes. In your calculations, assume that the proportion uptime of the robot will be 98%. Determine the following:
(a) Arc-on times (expressed as a percent, using the eight hour shift as the base) for the manual welding operation and the robot welding station, and
(b) Hourly production rate on average throughout the eight-hour shift for the manual welding operation and the robot welding station.

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Seq. Worker and activity Time 4.2 min. 2.5 min. 1.8 min. 2.4 min. 2.0 min. 1 Fitter: load and clamp parts 2 Welder weld first pass 3 Welder reload weld rod 4Welder: weld secondpass 5 Welder: repositioning time 6 Delay time between work cycles min Seq. Fitter androbot activities Times 4min. 4.0 min. 1.0 min 0.3 min. 1 Fitter: Load and clamp parts 2 Robot: Weld complete 3 Repositioning time 4 Delay time between work cycles