Gauge Blocks and calibration Calibrate: Klibrert / v.t.

1. to mark (a gauge) with a scale of readings. To correlate the readings of (an instrument etc.) with a standard

2. To find the calibre of.- Calibration / Calibrator ( The Oxford reference Dictionary)

To determine the correct reading of (an arbitrary or inaccurate scale or instrument) by comparison

with a standard. (The Penguin pocket English Dictionary)

Brief Historical Note of Measurements:

Measurement standards are based on light waves using a Interferometer. The first experiments

concerned with the interferometric determination of length, was carried out by A.A Michelson using an interferometer of his own design.

However it is interesting to note that as long ago as 1827, the Frenchman, Mr. J.Babinet put forward the idea of using wavelength of monochromatic light as natural and

invariable units of length.

During 1892-1893 Mr. Michelson in association with Mr. J.R Benoit, measured the metre in wavelength of cadmium red light. Later in 1905-1906 the metre was again

measured by Mr. Benoit, Mr. Fabry and Mr. A. Perot (France) using a Fabry-Perot interferometer.

These experiments provided a basis for the adoption, by spectroscopists, of the wavelength of

the red line of cadmium as a reference standard for wavelength measurements.

General Conference of Weights and Measures

11th conference in Paris (France) 1960, The metre was being redefined as being equal to 1 650 76373 vacuum wavelengths of the orange line emitted by

the krypton atom of mass 86.

17th conference 1983 The metre is defined as the distance travelled by light in vacuum during a time of 1/299 792 458 of a second .

Brief History of Gauge Blocks

The inventor of the Gauge block set (also known as "Slip gauges") was a Gentleman called Mr. Carl Edward Johansson (Sweden) later known as the "Master of Measurement"

Mr. Johansson founded his own company (C.E Johansson AB) in 1911, primarily to manufacture his own invention of 1895.

For many years Johansson's were the only gauges of their kind available

It also interesting to know that after inventing and manufacturing the first "Universal

Combination Gauge Block Set" The master standard necessary for the implementation

of manufacturing tolerances and part interchangeability enabled Henry Ford to create and mass produce the famous "Model T" (15 Million)

Although block gauges were available early in the century it was not until the advent of "The Great War" (1914-1918) when industry was forced

to mass produce engineering components on interchangeable basis, that they were seriously considered for workshop use.

A gauge block length (the distance between the working faces) is made very accurately using a fine tolerance. For the successful use of block gauges as working standards

it is essential that each pair of working surfaces should be truly flat and parallel. For Steel, this requires an extensive manufacturing process. Rough machining to size,

hardening, rough grinding, low-temperature heat treatment to ensure the finished blocks

are in a stable condition, demagnetised, and a lapping operation would finalise the manufacturing process until exact size has been achieved.

Which Material Do You Need?

Steel Steel gauge blocks have proven their

reliability of more than a hundred years. Steel gauge blocks provide high

resistance to wear with a good property to adhere to other gauge blocks.

However,steel ,must be protected against corrosion.

- Highly alloyed steel -Hardness guaranteed to 800 HIV

-Artificially aged for optimum form and dimensional stability.

-Coefficient of thermal expansion (11,5 1,0) x 10-6 K-1

Tungsten Carbide

Tungsten carbide is 10 times as much resistant than steel gauges. They are intended for frequent use,also where

superior wringing quality is required.

-Hardness guaranteed to 1500 HIV -Coefficient of thermal expansion

(4,23 0,1) x 10-6 K-1

Ceramic

Extremely resistant to wear and scratches. Up to 10 times more abrasion resistant

than steel. Virtually no warping of material, great material stability. Corrosion resistant (alkalis, acids,oil, sweaty hands,grinding fluid and other aggressive media). Non-magnetizable,antistaticand antimagnetic

as well as being non-conductive. Low weight and no need for lubrication. -Coefficient of thermal expansion

(9,7 0,8) x 10-6 K-1

Which Accuracy Grade Do You Need?

Grade 2 Are commonly used as "Working

Standards" may be used for preliminary setting up or comparatively rough

checking, where product tolerances are relatively wide. These blocks are

somewhat coarser than those normally associated with precision

measuring equipment.

Grade 1 Most commonly used in inspection rooms within the production to set and calibrate

measuring instruments and other equipment. They are also used for the production of components, tools and

gauges as well as to inspect fixtures and machines

Grade 0 These gauge blocks are designated for

use as " Company Standards" in calibration

laboratories or environmentally controlled inspection room to set and calibrate plug gauges as well as measuring equipment.

Grade K Intended for use as "Reference Standards"

in metrology oriented laboratories of National Institutes, precision measuring

rooms and other laboratories of National Calibration Services. They should be used as

"masters" to calibrate gauge blocks, length standards of same accuracy and measuring instruments as well.

(Note) The new standard ISO 3650 does no

longer take accuracy Grade 00 into consideration

The use of Glass gauge blocks was developed and made popular during the Second World War to save the use of steel. Despite the fact that hardly no one used them any longer, glass gauges possessed some advantages over steel gauges. No corrosive

effects due to perspiration of the hands, low coefficient of expansion, and showing abrasive-resistant qualities at least equal to steel.

Tungsten Carbide gauge Blocks offers excellent "wringing" capabilities.

Wringing: The phenomenon of "wringing" of flat surfaces is of fundamental importance to the use of gauge blocks. Block gauges are wrung together by hand through a

combined sliding and twisting motion, when they are found to adhere very strongly.

The phenomenon of "wringing" was noted first in the 1870's by Sir Joseph Whitworth. It was found that two surface plates would adhere together, after having been cleaned

by a dry cloth and the two working surfaces were brought together with a sliding motion, air being expelled in the process.

Research has indicated that the phenomenon is due to molecular adhesion between a liquid film and the mating surfaces of the plates concerned. It was also noticed that

the longer two optically flat plates remained wrung together,the more difficult it became to pull them apart. This has been proved to be the effect of the gradual diminution of the

liquid film between the surfaces.

Optical flats are used to determine irregularities in a hardened, lapped and polished

plane surface, such as aworking surface of a block gauge. However, today they are more popular for testing Micrometers and other flat measuring faces.

The optical flat is held against the lapped surface, and the interference fringes form an accurate picture (similar to a contour map) of the surface in comparison with the ideal

surface. Optical flats may be made of glass or of fused quartz. The later material, although more expensive, possesses

greater wearing properties.

Since Micrometers is one of the most common and most popular forms of measuring, it is therefore one of the most calibrated instrument into the Engineering Industry.

Optical flats are often used for checking the flatness of the micrometer measuring faces but also the lack of parallelism at some,or all parts of the scale.

For example a Micrometer ranging from 0 - 25mm would require 4 optical flats for the following length:

12,00mm / 12,125mm / 12,25mm / 12,375mm The difference in length of the 4 optical flats used would correspond to a quarter or

a third of the micrometer spindle pitch of 0,5mm

Interference fringes: As per above photo, the colourful fringes spacing are measured at between

0.25 to 0.15 For example the fringes that can be seen on the optical flat fitted between the micrometer

measuring faces clearly demonstrate an acceptable parallelism and flatness finish.

On the photograph bottom right. The left fringes demonstrates a clear parallelism but an evident lack

of flatness. The center fringes shows a faulty parallelism and the right fringes shows a clear parallelism but also a relatively acceptable flatness. The number of

interference fringes or rings acceptable during calibration procedures for

Micrometers can be found in the DIN 863 (German Industrial Norms) standards.

The Micrometer and its possible errors: a) Lack of flatness of the measuring faces

b) Lack of parallelism of the measuring faces c) Inaccurate setting of the zero reading

d) Inaccurate readings following the zero position e) Inaccurate readings shown by the fractional divisions on the thimble

Inaccurate readings from zero onwards are generally found to be caused by pitch errors in the screw. This is a problem I have seen many times with low and high quality

micrometers. Periodics errors are commonly found when the instrument has been

improperly used. Forceful contact against measured parts instead of using the ratchet for example, thus resulting in damaged spindle threads

Pitch errors are being termed as "the progressive error" and "the periodic error"

The progressive error: exists when the pitch of a screw is uniform, but either exceeds or is less than the nominal figure. The periodic error: is that which recurs at regular

intervals. It varies in magnitude when measured from thread to thread along the screw.

Both progressive and periodic errors can be determined by using block gauges

Yes,even Gauge Blocks get calibrated. The instrument first developed for calibrating gauge block was an improved external micrometer, which was manufactured early in

the 19th century by Brown & Sharpe for the inventor of the gauge block set composition, Mr.C.E Johansson.

21st Century technology! TESA UPD Gauge Block Comparator. Needless to say that I cannot

use this equipment at my place today, have woken up this morning of June 2002 with an ambient temperature of 7 degrees!