Figure 69 (Maynard) is a sketch showing how the normal and maximum working areas for the hands in the horizontal plane are usually determined. In drawing the sketch, it is assumed that the worker is comfortably seated at or standing by his bench or table of proper height. His arms hang naturally from the shoulders. Placing his right hand on the near edge of the table approximately opposite his left side, he can sweep his right hand through the arc AMB. The area included between this arc and the edge of the table is generally said to represent the normal or most comfortable working area for the right hand.
The points along the arc AMB can be reached with a motion of the third class. To reach all other points within the area bounded by the arc, a fourth-class motion must be employed. It requires more time to make a fourth-class motion than it does to make a third-class motion of the same length. Hence, the arc AMB should receive preference when making layouts.
Even when third-class motions can be employed, motions of equal length cannot be made in the same length of time at all points, along the arc AMB. Motions are made most quickly near point A most slowly at point B. When motions must be made much beyond point M in the direction of point B y fatigue increases materially. The closer the hand approaches B } the more unnatural is the position that the arm must assume. In fact, if the elbow rests on the table, the point B cannot be reached at all.
The arc which bounds the maximum working area is traced
by the fingers when the arm, fully extended, is. pivoted about the
shoulder. For the right hand, this is arc CKD in Fig. 69. The
limitations discussed above do not apply to the maximum area.
All points can be reached by fourth-class motions, and motions
can be made as quickly in one section as in another. In posi-
tioning material within "this area, the chief concern should be to
keep the length of the movements at a minimum. If possible,
the section near BD should not be used. Besides involving
maximum travel, it requires a rather awkward and fatiguing
wrist motion to reach material located in bins anywhere except
at point D, or in other words, when the arm is not fully extended.
The above discussion applies equally to the areas used by the
left hand and arm.
In order to confine all motions to the third class, material
should be placed along the paths that the hands normally follow,
or along the arcs FLE and AMB of Fig. 69. The only point at
which the hands can work together without Involving the use of
shoulder motions to -change the position of the arms is the point J.
In reality, this is not a point but a small area, is determined
by the ^rist and finger motions that can be used without moving
the arms. , ,
In the vertical plane, the arc described by the fingers when a third-class movement Is made is the arc AB of Fig. 70, and the arc CD is the maximum arc made employing a fourth-class move- ment. These arcs determine
the efficient placement of
materials in the vertical plane.
When positioning tools that
are suspended above the work
area, care should be taken to
locate them within the sphere
which would be generated if
the arc CD, Fig. 70, were to be
rotated about the body of the
operator as an axis. If no
other equipment or material
interferes, the tools should be
located On the Sphere which
WQU ^ b e generated by similarly
rotating the arc AB ; but in any
case, they should be located so that they can be reached without
the necessity of employing body movements.
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