PROBLEMS
4.5 GRADUATED SCALES
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Glass Surface Plates
Glass is an alternative material for surface plates. It was used during World War II when material and manufacturing capacity were in short supply. Glass can be ground suitably and has the benefit that it chips rather than raising a burr, which is a problem in cast iron surface plates.
the divisions in an architect’s scale, illustrated in Fig. 4.7, represent feet and inches, while the plumber’s scale would have divisions in terms of 1/32th or 1/64th of an inch. The divisions of a rule, on the other hand, are the unit of length, its divisions, and its multiples. Typically, the rules with which we are familiar have graduations (in centimetres, millimetres, or inches) and their decimal divisions throughout the length.
Steel rule Architect’s scale
1 inch = 1 foot 1 2 3 4 1 2 3
Millimetres Feet
Fig. 4.7 Illustration of the difference between a rule and a scale
Steel rules are most popular in metrology applications since they are more accurate and durable compared to rules made from other materials such as wood or plastic. While rigid rules can be used for laying out lines on a job, flexible steel rules can also be used to measure surfaces with circular profiles. Steel rules are either stamped or cut from a roll of spring steel.
The graduations are photo-engraved and tempered with satin chrome finish for good readability.
The ruler can be 150, 300, 500, or 1000 mm long; 19 or 25 mm wide; and 1. mm thick. The finer sub-divisions may be marked either throughout the length of the scale or in only a part of its length.
The use of steel rule requires consideration of the relationship between the reference point and the measured point. Figure 4.8 illustrates the preferred way of choosing the reference point for making a measurement. A graduated line on the rule, rather than an edge of the
rule, is selected as the reference point. This method improves the accuracy of measurement considerably, even though a little effort is required to align carefully, the reference and measured points. It is recommended not to use the edge of the rule as the reference point, as the edge is subjected to wear and tear and worn-out corners may contribute to error in measurements.
Sometimes an attachment such as a hook or a knee is used to facilitate measurement, as shown in Fig. 4.9.
Reference point Measured point
Fig. 4.8 Illustration of reference and measured points
Reference point
Knee Hook
Fig 4.9 Correct ways of using a rule with attachments
4.5.1 Errors in Measurements
Steel rules are often used on the manufacturing shop floors for making measurements. Even though the least count of rules is limited to 1 mm or at the most to 0.5 mm, the user should
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never be lax in the usage of any measuring device, however simple or trivial it may be.
Therefore, one should be aware of the two common errors that can creep up in measurements involving rules. The first one is an inherent error because of poor quality of the rule. This can be attributed to either poor quality of the material of the rule or poor workmanship in the manufacture of the rule. One can avoid this by purchasing a good-quality rule from a standard vendor.
The second error may creep up because of wrong usage of a rule and the observational error.
The rule should be properly aligned with the job being measured, ensuring that the reference and measured points are set accurately. The rule should not be twisted or bent in the process of measurement. The major observational error occurs because of parallax error. This is illustrated in Fig. 4.10. Parallax is the apparent shift in the position of an object caused by the change of position of the observer. If an observer views the scale along the direction B or C, the line of sight would be such that there is an apparent shift in the recorded reading by a division or two, as apparent from Fig. 4.10. The more the shift of the eye, from a vertical position right above the measured point, the more pronounced the error. It is needless to say that parallax error can be avoided if the observer recognizes this typical error and takes care to align his/her eyesight in the direction A, shown in Fig. 4.10.
Steel rules come in various sizes and shapes, depending upon the requirements of the component being measured. Accordingly, there are narrow rules, flexible fillet rules, short rules with holders, angle rules, measuring tapes, pocket tapes, and so on (Fig. 4.11). Narrow rules, fillet rules, and angle rules are used for measuring the inside of small holes, narrow slots, and grooves. Short rules with holders are convenient for measuring within recesses.
Short rule is an extension of steel rule and obeys the same principle For very precise measurements, temperature expansion and contraction must be considered.
Scale surface B A C
Fig. 4.10 Parallax error that can be minimized by direct eye measurements
Fig. 4.11 Types of steel rules (a) Narrow tempered steel rule (b) Flexible fillet rule (c) Short rule with holder (d) Angle rule (e) Steel measuring tape
(a) (b)
(c) (d) (e)