DIVIDERS, CALIPERS & MICROMETERS


These tools are used for measuring or laying out work. Two general types of calipers are in common usage: two hinged leg with various tips and sliding bar with measuring readout. Hinged type with two sharp points are usually referred to as dividers; while those with a pencil or pen tip on one leg are often called compasses. Micrometers are usually more precise alternatives for critical measurements.

Leg calipers come in a variety of types. The less expensive rely on hinge pivot friction to hold settings made by forcing legs into position. The more expensive usually are sprung with a screw, thumb wheel adjustment. Many have extension devices or bars and some have bendable legs.

With their sharp points, dividers are most frequently used to transfer measurements from a scale rule to a drawing or vice versa. They probably derived their name from the drafting process of marking off equally spaced points on lines or curves to divide them into equal parts. When properly pressed, they produce almost imperceptible prick marks in the drawing paper or other materials. Often in sheetmetal layout, they are used as compasses to scribe arcs or circles into the surface. Often a removable dark coating, such as Dykem, is applied to aid visibility.


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DRAFTING DIVIDERS & BENDABLE COMPASS w/ PENCIL TIP AT TOP.

For eraseability, compasses with a a fine hard pencil tip are usually used on original drawings, for layout and arcs. Pen tips are used to ink in the final tracing. Today much of this work is done with computer aided design (CAD) programs.


The SELF MEASURING DIVIDER is a useful device for direct measurement having scales marked 1/10, 1/64, 1/32, 1/16 and 1/8 in. plus mm.


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SELF MEASURING DIVIDER.


For transferring measurements from one scale to another, proportional dividers provide a quick caculationless method of transfer. By adjusting the pivot point, a fixed ratio between opposite end divider points is achieved.


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PROPORTIONAL DIVIDER.


This more expensive machinist's type from Tacro Inc. has guideline graduations for scaling linear distance from 1/2 to 10. The shorter tips are used for the smaller scale. A second graduation set is marked for dividing circles in equal parts from 6 through 20. The locking nut is on the shown, front side, while a rack and pinion adjusting knob is on the back. This provides very quick and precise settings.

Back to CLOSE COUPLING.

For extended use in copying drawings, a device known a as PANTOGRAPH was commonly used. Precision types are frequently used by jewelers and machinists for scaled engraving. Three dimensional types are often used to copy scale solids. Lately CAD or CAD-CAM (computer aided machining) techniques have replaced these.

BACK TO SCALE RULES & TAPES

With more blunt, and possibly curved ends; the heavier, leg calipers are usually used to measure solid objects in tighter spaces. Outside types have ends curve inward to ease outside measurement of balls, cylinders and odd shapes, while inside have them curved outward to use inside hollow spaces or slots. Commonly called a hermaphrodite ; a third type, with one leg curved inward and one outward or straight, is useful in measuring stepped items.


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CALIPERS
Divider
Inside
Turnable leg hermaphrodite.
Rings are springs.



For longer distances, some types have movable legs mounted on bars. Those with drawing legs are mostly referred to as beam compasses; while for cutting, measuring and other uses, they are called trammels. Carpenters and builders use movable points on yard sticks for measuring and layout. A machinist's version of this is the ADJUSTABLE CENTER DRILL FLY-CUTTER.



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SMALLER XACTO TRAMMEL OR BEAM COMPASS w/ cutting blade.



One of the more common, flexible and precision measuring devices is the bar type indicator caliper. These usually permit outside, inside, step and depth measurements. Commonly measuring to about 6", very long sizes are available. Older types had a linear scale with a vernier to provide more accurate readings. Later geared dial types appeared, greatly easing accurate readings. Digital readouts have furthered the ease. On metal versions, tips are frequently sharpened for scribing arcs during layout.

CAUTION: Plastic versions are subject to wear, when used on sharp metallic edges. They can also be sprung by higher forces. While on metal types, sharp edges can scrape paint and finishes or even flick away fine detail, like rivets, on plastics .


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PFM MITUTOYO HO VERNIER CALIPER.

Available several years ago from Pacific Fast Mail (PFM), the top vernier reads to 1/4 HO scale inch while the lower reads to 1/1000". The top left blades are for inside measurements, while the lower are for outside. At the right end, the small extension is for depth. The top screw locks position.

For quick and dirty measurements, while shopping or in the field, a small, shirt pocket , light 3" verier comes in handy. Included in a set with a slightly better 6", one was found in a cheapy bin in a hardware store at about $ 5. Surprisingly the accuracy is very good, within .001 when compare against a digital caliper.


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SMALL PLASTIC VERNIER


The upper scale reads to 1/128" = .0078" = .68" HO = .375" O = 1.25" N. While the lower states it reads to .05 mm, actually it reads to .1 mm = .003937" = .343" HO = .189" O = .63" N. This makes it a little awkward, requiring a calculator or conversion chart..


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GENERAL PLASTIC HO DIAL CALIPER.


Less expensive and accurate, main scale reads in feet, dial shows 3' and reads 1/5" per division. Upper scale reads directly to 1/4" in O scale.

NOTE: In the General caliper shown, both inside and outside tips had noticeable gaps when fully closed at zero, producing an error of about 1/2". The dial should be calibrated at some other point, by using accurately measured stock or feeler gauges and setting dial at this value.


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DIAL CALIPER.

Reads to .001".


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DIGITAL READOUT CALIPER.


The LCD is driven from minute magnets along the bar, using the Hall effect. The two buttons at bottom, switch between readouts in .0005" or .01 mm. and reset zero to find differences between points. There is a port at the top right to feed data to a computer, if the cable, driver and software can be found. For those with poor eyesight, a larger readout, 6" version is available at about $30 US and a 12" at about $50 US.


Less flexible, but considered more accurate is the micrometer. Resembling a question mark the most common type is a 1" capacity, reading to .001". Base on a precision 40 pitch screw, each full turn of the spindle equals .025". The frame is graduated in .025", while the spindle has 25 graduations of .001". Work is placed between the anvil and the spindle tip, which is tightened against the work. The measurement is the sum of the frame and spindle readings. Often with novices, repeated readings may vary by a couple of division, due to a lack of feel in tightening. More expensive versions have a pressure adjust to secure repeatability. Many have vernier scales on the spindle to obtain readings down to .0001". Precision ground blocks are often used to check accuracy.



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BASIC MICROMETER.


Usually in one inch increments, larger sizes are available, up to many feet. Most have only one inch range of adjustment. Some read to finer graduations. Today very expensive digital readout versions are available.

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