The economics of CNC automatics.
A recent study, performed at the Institute of Manufacturing Technology (University of Hannover, West Germany), focused on the economics of CNC automatics vis-a-vis CNC universal lathes and manual turret-type chuckers, and on CNC automatics compared with automatic bar lathes (both cam operated and program controlled). If you're trying to retire those workhorse screw machines and turret lathes on your shop floor, the results of this study could make justifying their replacements easier. The research was sponsored by Traub GmbH, a West German machine-tool builder. The ground rules
The CNC automatics used during the study were equipped with a controller capable of partitioning programs so one part could be programmed at the machine tool while another program was being executed (sometimes called parallel programming). The tools for these machines were automatically adjusted by in-process and off-line gaging, and tool-wear compensation and automatic tool changing were used. The CNC automatics also could automatically load workpieces by either a bar feeding magazine or a flexible handling system, thereby accommodating both chucked workpieces and parts turned from bar stock.
For chucking, a cast pulley was used as the test part, requiring two clamping operations to finish it. A sleeve, which needed to be clamped only once, was selected for the bar-turning comparison. Similar cutting paths and clamping methods were used on all machines.
The lathes were operated two shifts per day and lot runs were repeated five times. Maintenance contracts were used as a basis for calculating machine-maintenance costs. The study also considered the economic ramifications of assigning multiple machines to an operator. Here's a sampling of what was found. Chucking costs
During this phase of the study, an initial lot run on all machines required the same time to set up. Subsequent lot runs of the part, however, reduced setup time on both the CNC automatics and the CNC universal lathes. This was because no additional effort was required for writing and optimizing the NC program. Compared to the CNC universal lathes, the CNC automatics needed 30 percent more time for setting up the workpiece-handling system for repeat runs of a particular bath.
Cycle times per part on both machines were close. The manual chuckers, on the other hand, took about 1.5 times more time per part.
According to the researchers, machine costs per hour for the chuckers were almost 10 percent higher than those of the CNC universal lathes ($29 versus $26.50). Compared to the CNC universal lathes, hourly costs f the CNC automatics were about 20 percent higher due to the initial cost of the workpiece-handling system. But, taking into account that automatic handling on the CNC automatics permits an operator to control up to four lathes reduced thse hourly costs from $32 to $24.
The manual chuckers had a higher cost per part and a higher cost for repeated production of a lot run, but lower cost for order preparation, than the two CNC machines. Setup of the workpiece-handling system for the CNC automatics produced at 65 percent higher cost for repeated production than on the CNC universal lahte.
When revaluation multiple-machine assignments on CNC automatics, it was determined a two-machine operation was more expensive than running one machine for lot sizes up to 16 pieces requiring 80 min per part to complete. For lots greater than 16, though, assigning an operator two machines yielded up to a 20 percent savings per part.
For chucked parts, the study concluded that costs per part for the manual chuckers were considerably higher than those associated with either type of CNC lathe. Lot sizes greater than 30 pieces were produced more profitably on the CNC automatics; for smaller lots the CNC universal lathe had the advantage. Barring costs
During the bar turning tests, 10 hours were spent on setup for the initial lot run on the cam-operated bar lathes. Initial, as well as repeated production, on the CNC automatics required approximately 90 percent less time. All machines required the same cycle time to finish a workpiece.
Considering multiple-machine operation, the hourly costs of CNC automatics were 30 percent higher than those of cam-operated bar lathes. This was viewed as acceptable because of increased efficiency and shorter setup time of the CNC automatics.
Operating two CNC automatics (compared to single-machine operation) produced a cost savings of up to 15 percent per part for batches larger than 50 parts.
Operating three machines was more economical when producing batches greater than 35 parts with a total processing time longer than 57 min. Compared to a two-machine operation, cost savings per part were 12 percent.
Operating four CNC automatics was best when producing lots greater than 80 parts with a total processing time longer than 130 min. This yielded an additional 6 percent savings.
The complete report contains 20 charts and graphs covering many more cost and time comparisons. It's available from Traub Automatics Inc, Hauppauge, NY. For a copy, circle E47.
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|Author:||Coleman, John R.|
|Publication:||Tooling & Production|
|Date:||Apr 1, 1984|
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