In certain operations, the operator's time is not fully occupied during the operation cycle.
For example, when the machine makes a cut under power feed, the operator stands by with nothing to do until the end of the cut is reached. Whenever an operation is temporarily under the complete control of the machine or whenever the operator must wait for a part to heat or cool, a chemical reaction to take place idle time exists. This idle time can be utilized for other activities.
The first step in undertaking the elimination of idle time from an operation cycle is to determine exactly when the idle period occurs, how long it lasts, and what its relation is to the part of the cycle during which the operator is occupied. This may be shown clearly by constructing what is commonly known as a "man and machine activity chart."
As an illustration of the idle time of machine operator, the elements of the milling-machine operation and the allowed time for each are given below.
Element Description Allowed Time in Hr.
Pick up small part from table . 0007
Place in vise 0.0009
Tighten vise 0.0020
Start machine .0003
Run table forward 4 in . 0.0012
Engage feed 0.0003
Mill slot 0.0080
Stop machine 0.0015
Return table 6 in 0.0017
Release vise 0.0013
Lay part aside in tote pan .0009
Brush vise 0.0009
The man and machine chart was constructed from these data. When the man is working, the machine is idle and when the machine is working, the man is standing watching it. The desirable condition is, of course, to have both working without idle time. The analysis shows that the machine works 0.0080 hour and the man 0.0117 hour. If two machines were provided, the man could work continuously. It would then require 0.0117 hour per piece plus 0.0008 hour to turn from one machine to the other, or a total of 0.0125 hour to produce each piece. This is an improvement over the first method with its time of 0.0197 hour per piece, but each machine would be idle 0.0125 - 0.0080 = 0.0045 hour per piece. But the elements of work can be improved to reduce their time.
Accordingly, the elements were analyzed to simplify them or improve them. Several possible changes are identified and the best ones were implemented. A vise with a quick-acting cam-actuated clamp is provided, thereby reducing the time to tighten and release vise to a total of 0.0006 hour. The machine is allowed to run until the clamp has been returned past the cutter and is then stopped while the table is being run still farther back so that the vise is out of the way of the cutter. A trial shows that this does not affect the finish of the slot enough to spoil it for the purpose for which it is to be used, and the " stop-machine; element is thus eliminated, reducing the operator's time by 0.0015 hour. An ejector is provided which kicks the part out of the vise as it opens, and the part slides down a chute to the tote pan. Thus, the "lay aside part in tote pan" element is eliminated, and 0.0009 hour is saved.
These changes reduce the operator's time by 0.0051 hour, or to 0.0125 - 0.0051 = 0.0074 hour. Thus, under the new setup, the machines operate continuously, and the operator has 0.0006 hour idle per piece or can work at a slower pace with less fatigue. The "brush-vise element could be eliminated by attaching an air hose to the machine and arranging a foot control on the floor. The operator would step on the control as he opened the vise, and the chips would be blown out while he was picking up the next piece. Since the machines are working continuously, however, this change would only increase the idle time of the operator, and as his fatigue is not great on this operation, the change need not be made.
Typical Problem.- The operation just described is simple, and an experienced analyst could undoubtedly solve it mentally without the man and machine chart. Many problems are more complicated, however,
and the charts are essential if the most efficient arrangement is to be made.
For example, for a certain bracket machined on a Potter and Johnson automatic turret lathe, time study shows that it requires 0.0231 hour to load and unload the machine and 0.1990 hour to make the cuts. After the machine is loaded, the operation is entirely under the control of the machine, and the operator has nothing to do. The man and machine chart for this operation was prepared. The problem is to make a setup that will permit the operator to utilize the 0.1990 hour of idle time effectively.
Many different arrangements will probably be possible, but for a given set of conditions, there is usually one that is better than the others. For the bracket job, there are three Potter and Johnson lathes available. If these are all set up for bracket machining, the operator can run all three at one time. Even with this arrangement, however, an intermediate man and machine chart, shows him to be idle 0.1528 hour for each three brackets machined.
The next operation performed on the brackets is a drill-press operation on which an allowance of 0.0535 hour is established. There is no power-feed drilling on this operation, and so the operator cannot do anything else while drilling.
The lathe operator may be given the drill press to run. If lie drills two brackets to every three machined
on the lathes, he will be idle 0.1528 (2 X 0.0535) = 0.0458 hour per three brackets. If he drills three brackets to every three machined on the lathes, each lathe will theoretically be idle 0.0026 hour per bracket. The drill-press time value is the allowed time, however. By exerting better than average effort and by developing better than average skill, the operator will shorten this time and hence will be able to eliminate all idle time from the cycle.
The most efficient arrangement in this case, therefore, will be for one man to run three lathes and one drill press, machining three brackets per cycle. The man and machine process chart, shows the sequence in which the operations must be performed to work most effectively.
Multiple Machine Operation on Miscellaneous Work. The operations thus far described have been quantity operations. A sufficient number of pieces is produced to justify assigning special machines to these jobs only. When the setup is worked out and put into effect, it will remain in operation for weeks and even months.
When work is of a miscellaneous nature, however, with jobs changing daily, machine coupling is more difficult although not impossible. Because of the rapidity with which operations change, it is impractical for the supervision to attempt to plan the work so that one operator can run more than one machine.
If, however, surplus machine equipment is available, two, three, or even four machines may be placed facing one another in such a position that one operator may run all of them. Whether or not the operator runs all machines or only one of them is left entirely to him. He is given a number of jobs to do and has several machines to do them on. If he is an operator who can plan effectively, he may be able, by working jobs with long cuts with jobs requiring short cuts, to keep all machines in operation most of the time. If he is unable to think ahead and to see possibilities of making combinations, he will not be able to turn out so much work.
Principles of Multiple Machine Operation. Where machine coupling has never before been practiced, a certain amount of preparatory work must be done to overcome prejudices that are quite likely to exist. Where for years it has been the custom to have one man at each machine, operators may feel that they are being asked to exert themselves unreasonably if they are asked to work in a period when they would otherwise be idle.
There is little logic in this attitude. Other operators such as assemblers, welders, molders, fitters, and so on, work steadily all day as a matter of course. Outside of occasional necessary breathing spells if the work is heavy, they do not expect to be idle. There is no reason other than habit why machine operators should expect different rules to apply to them. As a matter of fact, on automatic screw machine work and certain special-purpose machines, one operator has tended several machines for many years. It is only when the principles of machine coupling are extended to lathes, boring mills, milling machines machines for which the idea of machine coupling is new that questions are raised. If this is likely to occur, however, the matter should be fully discussed with all concerned before the installation is made.
It should be made clear that machine coupling is not a speeding-up process. The operator is not asked to work more quickly and to rush about from machine to machine, but rather only to occupy himself during a period when he otherwise would have nothing to do. In no case is he asked to exert an effort that is in any way detrimental to his own safety or physical well-being. When operators become accustomed to the idea of handling more than one machine, they usually have no objection to doing so. Since no intelligent individual finds sitting or standing by a machine in enforced idleness a particularly interesting task. Rather, it is more interesting to try to keep several operations going simultaneously, getting the best performance from each.
Quality is not impaired by this arrangement, since nothing that the man does after the cut is started in any way affects quality. The only precaution that should be taken is to make sure that the machine will not do any damage if the operator is forced to be away from it longer than the time required to make the cut. Most modern machines can be set to stop automatically at any desired point.
Operators who run more than one machine customarily receive a somewhat higher rate of pay than operators who run only one, the thought being that they have greater responsibility and hence are entitled to higher compensation. " On miscellaneous work where the running of two or more jobs at one time is left to the operator, he is usually paid the full amount for each job under the piece work or bonus plan used. Since the combining of jobs is left to the initiative of the individual, it is felt that he is entitled to the extra earnings, the employer being satisfied with increased production at the same overhead cost.
A typical production setup making use of the principles of machine coupling: Two different types of electric-motor frames are handled simultaneously in this setup, and one man runs all machines. The frames are delivered by the double-deck roller conveyer on the right to the milling machine in the foreground where the feet are milled. From the milling machine, one type goes to the automatic turret lathe on the right and the other to the automatic turret lathe on the left. The frames are bored and faced complete and are then placed on the double-width gravity conveyer above the turret lathe on the right for delivery to the double-spindle multihead drill press. Then I-bolt and setscrew holes are drilled and tapped on another drill press, and the foundation bolt holes are drilled on the multiple-spindle drill press shown in the left background. This completes the machining cycles, and the frames are placed on a gravity conveyer in the left background not visible in the picture, for delivery to a press for pressing the core into position.
Full Knol Book - Method Study: Methods Efficiency Engineering - Knol Book
Ud 13.1.2024, 28.11.2021
Pub. 3.11.2013
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