International Journal of Industrial Ergonomics, 1 (1987) 231-240 231 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

AN ERGONOMIC APPROACH TO DESIGNING A MANUFACTURING WORK SYSTEM

Biman Das

Department of Industrial Engineering, Technical University of Nova Scotia, Halifax, Nova Scotia, B3J 2X4 (Canada)

(Received July 28, 1986; accepted November 10, 1986)

ABSTRACT

This paper highlights the methodology that was applied systematically to incorporate ergo- nomics principles and data to design a manu- facturing work system. The manufacturing task comprised of drilling four holes on a prepared steel plate. The components of the manufactur- ing work system included: manufacturing task, power-feed drill press, jig, fixture and other

equipment, workplace layout, operator training and (hard) production standard and feedback. The ergonomically designed manufacturing work system proved to be effective and efficient in terms of manufacturing processing time, safety, training time, and worker productivity, satisfaction, and job attitudes.

INTRODUCTION

Ergonomics deals with the engineering of machines for human use and with the en- gineering of human tasks for operating ma- chines. It is concerned with the ways of desig- ning equipment or machines, facilities and work environments, so that they match hu- man capabilities and limitations. The objec- tives of ergonomics are to increase the ef- ficiency and effectiveness with which work is performed and to maintain and promote worker health, safety and satisfaction (Mc- Cormick and Sanders, 1982). Ergonomics principles and data should be applied ad- vantageously for optimum design of product, job, workplace, training method and system

safety. Human performance can be improved considerably from such an application. In the context of task execution, human perfor- mance is usually evaluated in terms of how efficiently a designer deals with manual processing time, errors, training time, safety and user satisfaction.

In designing a manufacturing work system, the designer should not only attempt to maxi- mize worker productivity, but also try to im- prove worker satisfaction and job attitudes and minimize safety hazards. It is possible to achieve such a desirable goal through proper application of ergonomics principles and data. Despite their importance, poorly designed manufacturing work systems are common- place in industry (Konz, 1983).

0169-8141/87/$03.50 © 1987 Elsevier Science Publishers B.V.

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The main objectives of this investigation are to apply systematically appropriate ergo- nomics principles and data to design a manufacturing work system (drill press oper- ation) and to evaluate the system in terms of manufacturing processing time, safety, train- ing time, and worker productivity, satisfac- tion and job attitudes. This paper will em- phasize the methodology that was especially used to achieve the objective of this research.

DEVELOPMENT OF A MANUFACTURING WORK SYSTEM

The manufacturing task involved drilling four holes on a prepared steel plate. In the past, the drill press operation was performed in a standing position. The jig plate was located parallel to the table edge causing un- necessary hand motion in loading the steel plate to the drill press. The fixture employed a sliding clamp with special nut which had to be screwed in and out to secure and remove the plate. The outgoing material bin was placed on the right side of the drill press table which caused unnecessary long reach motion to pick up the next plate.

To alleviate the problems stated above, a new manufacturing work system was devel- oped by applying ergonomics principles and data in a systematic way. The components of the manufacturing work system included: manufacturing task, power-feed drill press, jig, fixture, incoming and outgoing material bins, compressed air hose, workplace layout, operator training and (hard) production standard and feedback. To develop the opti- mum method of operation, short (15 min) and long (one hour) trial runs were per- formed. An MTM (Method-Time Measure- ment) analysis was conducted to eliminate the unnecessary motions and improve the neces- sary motions.

Manufacturing task, machine, cutting tool and lubrication

The manufacturing task involved drilling 1,,

four z diameter holes into steel connector ,~3,, 1" ]" plate, ,. 4 long × wide × 3 thick (con-

version factor to S.I. unit: 1 " = 2.54 cm). To perform the task a power-feed drill press was used. For 0.2 carbon mild steel material and 1 " z diameter high-speed steel drill bit, the re- commended machine or drilling speed is about 80 fpm or 1 222 rpm. In the present situation, the machine speed was set at 600 rpm in consideration of the excessive scale present on the steel plate, manual intermittent as opposed to automatic continuous lubrication of drill bit, and the cutting tool life. For the drill size and the material drilled, the sug- gested feed is about 0.004" per revolution; however, in the present situation the feed was set at 0.0048" per revolution. By decreasing the speed and increasing the feed, a com- promise was achieved between the actual and the recommended machine set-up. The selec- tion of the speed and feed was made to minimize excessive heat generation in the steel plate during continuous production work and chip removal problem. Duct tapes were used around finger tips to protect the operator's fingers from the heat generated. A sulphur- base lubricant was used during the drilling operation.

To regrind the drill bit, a precision tool grinder was used. A special cylindrical jig was constructed to set up the drill-bit beyond the drill press chuck, so that the drill-bit point would go 1 / 8 " or half the hole diameter past the connector plate to ensure that the hole was clear through.

Jig

An interchangeable 13 /16" internal diam- eter bushing with necessary locking arrange- ment guided the drill-bit point to the steel or connector plate. The jig was designed in such

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a way that the drill-bit point would never be exposed to the operator or always stayed in- side the sleeve to ensure operator safety.

The jig plate was offset by 20 ° to the left or away from the horizontal centre line of the press (Fig. 1) to avoid unnecessary hand mo- tion in loading the steel plate to the drill press. Thus, due consideration was given to the human forearm-hand configuration in locating the jig. Also, the offsetting of the jig facilitated the operation of the fixture clamp by offsetting the hand motion from the power-feed attachment.

Fixture

From the ergonomic consideration the original handle of the De-Sta-Co fixture clamp was considered inadequate (rectangular shape, 2 ! " 4 long x 5 / 8 " wide x 3 / 8 " thick). Conse- quently, for the fixture clamp a (white) plastic

1 " handle 4" long × ~ diameter was especially designed and built by giving consideration to anthropometric specifications and intended use (standard for hand breadth at metacarpal for the 99th percentiles men -- 3.9" (Van Cott and Kinkade, 1972), Figs. 1 and 2). A U- shaped clamp or lock was provided to restrict the fixture clamp opening to a maximum of

]P! about ~ to avoid unnecessary motion and standardize the method of operation.

Incoming and outgoing material bins

In designing the incoming material bin, the following ergonomic factors were given con- sideration to: (1) ease the sliding of the plates toward the operator by providing the neces- sary 35° slope to the bin base, (2) facilitate the pick up or grasp of the plates by allowing

"7_3" a 4" d e e p x . 4 wide bin base with an in- clination of 15 ° from the horizontal surface, and a front opening height of 4", and (3) ease the loading of the plates to the jig by furnish- ing the height of the bin base similar to the jig base height or approximately 5" from the table surface (Fig. 2).

For the design of the outgoing material bin, consideration was given to the following ,ergonomic factors to: (1) ease the dropping of

Fig. 1. Ergonomically designed jig and fixture for drill press operation.

Fig. 2. Drill press operation (ergonomically designed)-~--,Front view.

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the finished connector plate into the bin by providing adequate horizontal bin area, (2) furnish baffles to facilitate the sliding of the plates into the bin, and (3) ease the inter- change of the full outgoing material bin by providing the guide rails and the necessary stop at the end of the rails for operator safety.

Compressed air hose

For disposing chips especially from the jig area around the drill bit, compressed air with a pressure of about 19 psi was used by means

- x

2s" \ L I

of a Lincoln,50-204.8 air hose nozzle with coupling. Necessary chip guarding was pro- vided around the drill press bed (Figs. 1 and 2). The operator always wore a face shield or safety goggles and the air-hose nozzle was directed away from the operator at all times. Due consideration was given to the OSHA (Occupational Safety and Health Adminis- tration) standard 1910.242 for the use of hand and portable power tools and equipment with particular reference to the compressed air used for cleaning (under (b)). The standard states that compressed air shall not be used for

m

Le_~nd: I . Incoming bin; 2. Fixture clamp handle; 3. Dr i l l feed handle; 4. Power feed handle; 5. Outgoing bin; 6. Electr ic to ta l iz ing counter; 7. Production standard stand; 8. Quality feedback stand; 9, Chair; and lO. Foot rest. Note: Normal and maximum working areas in the vertical plane,

based on women. Conversion factor to S.I. unit: I " = 2,54 cm.

Fig. 3. Workplace layout of drill press operation--side view.

cleaning purposes except where reduced to less than 30 psi and then only with effective chip guarding and personal protective equipment.

Workplace layout

Due consideration to ergonomic principles and data was given in the development and design of the workplace configuration. In par- ticular, the following operator-related dimen- sional factors that influence workplace design were considered: (1) postural control and dis- tribution of body weight, (2) reach envelope of hands, and (3) eye position with regard to display area. To determine workplace dimen- sions, advantage of the available anthropo- metric data was taken (Farley, 1955; Murrell, 1965; Van Cott and Kinkade, 1972; Mc- Cormick and Sanders, 1982). It was desired to design a well-organized, efficient and safe workplace. The components of the workplace

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included: (1) operator seat and footrest, (2) incoming and outgoing material bins, (3) jig and fixture, (4) hydraulic pump oiler, chip brush, steel plate surface, compressed air hose, chip remover gadget and hand rug, (5) electric totalizing counter and production standard and quality feedback stands, and (6) wall clock and operator instruction sheet stand. The important and critical anthropometric di- mensions that were used in developing the workplace are shown in Figs. 3 and 4.

In consideration to the nature of the pro- duction task, it was decided to perform the task in a seated position to: (1) provide high level of body stability, (2) give comfort to the operator, and (3) minimize operator energy expenditure and fatigue. The critical dimen- sion of the seat height was dependent on the work table height.

The workplace was designed to accommo- date both the male and female operators. Consequently, the limits of reach and

Legend

l . Incoming bin (Produc- tion standard stand)

2. Connector plate 3. J i g 4. Fixture 5. Hydraulic pump o i l e r 6. Chip brush 7; Steel surface 8. Air hose 9. Outgoing bin

I0. Chip remover gadget ]1. Hand rug ]2. Instruction stand 13. Electric to ta l iz ing

counter 14. Quality feedback

stand ]5. Chip guard 16. Magnet

and maximum workinN z o n t a l p l ane based on r s i o n f a c t o r to S . l . 54 cm.

Fig. 4. Workplace layout of drill press operation--plan.

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clearance requirements were based on the di- mensions of the smaller (female) and larger (male) operators, respectively. The concept developed by Farley (1955) for the women was used to develop the normal and maxi- mum working areas in the horizontal and vertical planes. He determined the normal working area as being equal to the volume circumscribed by the horizontal arm pivoting about a relaxed vertical arm. The maximum working area was represented by the volume circumscribed during the movement of the fully extended arm pivoting about the shoulder pivot point. An endeavour was made wherever possible to place or locate the vari- ous equipment within the normal working area. The outgoing material bin was placed near the incoming material bin to economize operator motion and to locate within the nor- mal horizontal working area.

The installation of the electric totalizing counter was constrained by the existing drill press spindle housing design, nevertheless, the counter was placed 13 ½" away from the mid- dle of the operator's eyes. The recommended minimum display distance is 13", although the preferred distance is 20" (Van Cott and Kinkade, 1972). The distance of the produc- tion quality feedback stand was 26" from the operator and the centre line of the card was placed at the opt imum operator eye level.

Operator training

For the drill press operation, a comprehen- sive operator training method was developed through the use of MTM analysis, operator instruction sheet, demonstration, practice ses- sion, feedback and guidance (Das, 1986). The operator instruction sheet contained informa- tion with regard to: (1) part, operation and machine names, (2) machine speed and feed and cutting tool and other equipment used, (3) connector plate critical dimensions and tolerances, (4) hole sequence, (5) workplace layout, and (6) operating procedure in terms

of left- and right-hand motions. It was em- phasized during the training session that the heat transfer to the fingers would be less, if the consecutive holes were drilled in a se- quence, diagonally opposite to each other. Also, it was pointed out that the product quality or dimensional tolerances depended especially on placing the plate correctly against the jig pins and engaging the fixture clamp handle properly. To facilitate the view- ing of the instruction sheet, a special stand was built which had a 30 ° inclination from the vertical plane.

Production standard

After standardizing the method of oper- ation in terms of the machine tools, equipment, workplace layout, working condi- tions and training, the product ion / t ime standard was determined through MTM and subsequently checked by means of an overall stop-watch time study. The production stan- dard for the operation was 60 holes/15 min or 240 holes /h (100% normal). The per- centage of cycle time that was machine con- trolled was 52%. The hard production stan- dard was established on the basis that the external work elements would be performed at a pace of 130% of normal standard to achieve an overall hard performance standard of 112% normal or 268 holes/h.

The production standard was presented on v!" 5" a white -4 long x high card, written by

black quill pen with approximate letter size, all capitals, 7 /32" wide x ~" high x 1 /32" thick.

Production feedback

An electric totalizing counter with knob reset arrangement was selected to provide quantity feedback. For better legibility, the counter selection was made with white figures or numerals on black background as opposed to the reverse combination to take advantage

of the irradiation phenomenon (McCormick and Sanders, 1982). The phenomenon pos- tulates that white lines on black tend to ap- pear wider than they are and black lines on white tend to appear narrower. The size of the numerals were 0.18" high and 0.156" wide, i.e. the width-height ratio was 83%. This was more than the satisfactory level, which calls for a width-height ratio of above 70% (MacCormick and Sanders, 1982).

The production quality feedback was pro- vided in terms of the percentage of good holes produced in a unit time. The production quality output was determined by a go/no-go gauge. The production quality feedback was provided in the same manner as the produc- tion standard.

EVALUATION OF THE ERGONOMICALLY DEVELOPED MANUFACTURING WORK SYSTEM

The ergonomically developed manufactur- ing work system was evaluated in terms of manufacturing processing time, safety, train- ing time, and worker productivity, satisfac- tion and job attitudes.

Manufacturing processing time

A preliminary MTM analysis helped in identifying manufacturing method problems. Through subsequent use of ergonomic princi- ples and data, the unnecessary motions were eliminated and the required motions were im- proved. Specifically, motion economy was achieved by (1) offsetting the jig plate straight edge by 20 ° to the left or away from the horizontal centre line of the press (Fig. 4), (2) redesigning the fixture clamp handle and pro- viding restriction to the fixture clamp open- ing, (3) locating the outgoing material bin to the left side of the operator, adjacent to the incoming material bin, and (4) designing incoming and outgoing bins so as to facilitate

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the pick up or grasp of the steel plate and the dropping of the finish connector plate. Thus, as a result of the newly developed manufac- turing method, it was possible to drill 240 holes/h (100% normal) on the steel or con- nector plate. In the past or with the old method, only 208 holes/h (estimated 100% normal) could be drilled. Stated otherwise, an improvement of about 15% in manufacturing processing time was realized as a consequence of the successful incorporation of ergonomics principles and data in the design of jig, fix- ture, and incoming and outgoing material bins and workstation.

It should be pointed out that when the operators in the new manufacturing work sys- tem were provided with an assigned hard production standard (268 holes/h) and pro- duction feedback, they were able to reach a production (quantity) output level of 264 holes/h. In other words, a total improvement of about 27% in manufacturing processing time was realized due to the application of ergonomics principles or concepts and data in the design of the drill press operation. This matter is more fully discussed subsequently under worker productivity, satisfaction and job attitudes.

Safety

In performing the manufacturing task, safety features were incorporated especially in the (1) selection of machining parameters, (2) design of the jig, (3) operation of the com- pressed air hose, (4) provision of protective equipment, and (5) sequence of drilling holes. No safety problems were encountered during the performance of the drilling task.

Operator training time

In this study, the subjects were 56 male and female college students who were paid $3.50 per hour. The task was performed in the Machine Tools Laboratory, Park Shops,

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T A B L E 1

Average product ion quant i ty and quality output data

Product ion output

Training q u a r t e r # (t ime in min)

1 (0 -15)2(15-30)3(30-45)4(45-60)

Quant i ty (no. of holes) 44.89 51.68 53.88 56.32

Quali ty (no. of good holes)38.89 46.80 50.61 53.43

North Carolina State University. Each sub- ject was trained individually for one hour and twelve minutes (demonstration twelve minutes followed by one hour practice) in the perfor- mance of the task by means of the especially developed comprehensive operator training method (Das, 1986).

Table 1 shows the average values of the production quantity and quality output data for the four production quarters. The average quantity and quality output increased from the first to the fourth quarter by 26% and 37%, respectively. Thus the improvement in quantity and quality output was considerable. In terms of quantity and quality output, the subjects at the end of the fourth quarter re- ached the measured standard by 94% and 89%, respectively. All the subjects were able

T A B L E 3

Percentage increase or decrease in product ion quant i ty and quality output among groups

Compar i son between groups (experimental condit ions)

% Increase or decrease in product ion output

Quant i ty Quality

2(PS: 100% normal) vs. 1 (Control , no P S / P F ) 4.17 3.71

3(PS: 112% normal) vs. 1 2.39 < 1 4(PF: Quant i ty&qual i ty) vs. 1 - < 1 1.96 5(PS: 112% normal + PF:

Quant i ty&qual i ty) vs. 1 12.84 14.46

to perform the assigned task by employing the prescribed motions and came close to reaching the measured production standard (normal, 60 holes/15 min) after one hour and twelve minutes training and practice session.

Worker productivity, satisfaction and job attitudes

Worker productivity was determined in terms of production quantity and quality out- put. Worker satisfaction and job attitudes were measured through questionnaires or sub- jective scales, at the end of task performance.

TABLE 2

Compara t ive analysis of worker productivity, satisfaction and job at t i tudes among groups: Student ' s t test

Compar i son between groups (experimental condit ions)

Calculated Student ' s t Value

Worker productivi ty Worker satisfaction

Quant i ty Quali ty Modified Trunca ted JDS JDI

Worker job at t i tudes

2(PS: 100% normal) vs. 1 (Control, no P S / P F ) 1.49 3(PS: 112% normal) vs. 1 0.82 4(PF: Quant i ty&qual i ty) - 0 . 0 7 vs. 1 5(PS: 112% normal + PF: 4.42 * * Quant i ty&qual i ty) vs. 1

1.08 2.39 * 3.71 * * 1.97 * 0.04 3.27 * * - 1.37 - 0.22 0.60 3.44 * * 5.76 * * 4.52 * *

3.97 * * 6.09 * * 8.63 * * 8.04 * *

Note: The tabulated Student ' s t values for 5% = 1.68 (significant *) and 1% = 2.42 (highly significant **) ; Negative sign = decrease in group means.

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Worker satisfaction scores were determined by employing two measures: (1) modified JDS (Job Diagnostic Survey) scales (Hackman and Lawler, 1971; Hackman and Oldham, 1975), and (2) truncated JDI (Job Descriptive Index) scales (Smith et al., 1969). The second mea- sure was used to compare or confirm the results obtained by the first measure. The original JDS scales were modified to suit the requirements of the present study. The mod- ified JDS scales included the following job or work dimensions: (1) skill variety, (2) task identity, (3) task significance, (4) autonomy, (5) production feedback, (6) production standard, (7) working condition, and (8) pay. Each subject was asked to answer the questionnaire, which consisted of 18 questions on seven-point Likert-type scales regarding his or her perception of the various job attri- butes that were actually present. The JDI scales measure worker satisfaction in terms of five aspects of the job: (1) work, (2) pay, (3) supervision, (4) promotions, and (5) co- workers. The truncated JDI scales employed only the first two scales (work and pay) since they were relevant to the present study. The work and pay scales consisted of 18 and 9 adjectives or phrases, respectively, with re- gard to each particular facet of the job. Worker job attitudes were measured by using JDS scales (Hackman and Lawler, 1971; Hackman and Oldham, 1975). The JDS scales measure worker job attitudes in terms of four job attitude factors: (1) experienced work motivation, (2) job involvement, (3) general job satisfaction, and (4) specific job satisfac- tion. Each subject was asked 17 questions on seven-point Likert-type response scales for determination of worker job attitudes.

Experiments were conducted especially to determine whether production standard (PS) and production feedback (PF) could be pro- vided to operators singly as well as jointly to improve worker productivity, satisfaction and job attitudes in the repetitive manufacturing (drilling) task performed under a specially

designed manufacturing work system (Das, 1982a, b). Each experimental group consisting of eight subjects performed the task for one hour under a randomly decided specific ex- perimental condition. The results showed that only the combination of an assigned hard production standard (112% normal) in the presence of quantity and quality feedback had a significant positive effect on worker productivity, satisfaction and job attitudes, all at the same time (Table 2). The increase in quantity and quality output were 13% and 15%, respectively, compared to the control group, with no provision of production stan- dard and feedback (Table 3). The improve- ment in worker satisfaction and job attitudes was maximum for this experimental condition (Table 2). Consequently, to design an effec- tive manufacturing work system the operator should be provided an assigned hard produc- tion standard in conjunction with quantity and quality feedback.

CONCLUDING REMARKS

This case study has demonstrated the need for applying ergonomics principles and data systematically to facilitate the design of a manufacturing work system. The system proved effective and efficient in terms of manufacturing processing time, safety, train- ing time, production quantity and quality output, and worker satisfaction and job atti- tudes.

ACKNOWLEDGEMENTS

The author expresses his appreciation to Drs. M.A. Ayoub, W.A. Smith, Jr., L.H. Royster, and R.J. Hader. This research was partially funded by the Economic Develop- ment Administration, U.S. Department of Commerce.

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