Wednesday, July 31, 2013

GMT Expanding Mandrel

Expanding mandrels are the ideal solution to various internal clamping problems. Expanding mandrels provide the benefits of versatility and accuracy when compared to other workholding methods. The power operated, flange type expanding mandrels can be used on any machine with drawbar facility. The natural pull back action of the sleeve ensures the component is resting against the locating face. There is high clamping force due to the clamping sleeve being in contact along its entire length through double angle.

Expanding mandrels provide the following workholding features:
  • Quick and secure clamping
  • Positive location against backstops
  • Controlled datums
  • Uniform expansion
  • Positioning accuracy
  • Sleeve interchangeability - As sleeves in one type are interchangeable, one single expanding mandrel covers a very large clamping range.
  • No pressure marks in the clamped bore. Easy loading.
Expanding mandrels may be dedicated to one component or a range of component types and sizes, locating in and gripping component bores.

GMT Collet Chucks

Designed to eliminate part pullback, GMT Collet Chucks have tapered sleeve that pushes forward over a collet to compress it, resulting in precise Z-axis positioning. Units are balanced for high-speed operation. GMT Collet chucks are offered gripping capacities from 10mm to 63mm diameter.

GMT Dead Length Collet chucks are actuated by the draw tube moving forward away from the machine spindle. GMT collet chucks are actuated as draw tube moves towards machine spindle and slides the bar against pusher, resulting in consistent Z axis positioning of bar.

Features :
  • High gripping
  • Actuated by rotary cylinder
  • All components are hardened and precision ground
  • Permissible run out accuracy as per DIN 6386
  • Compact design
  • High concentric accuracy
  • DL for all second length operation applications
  • Interchangeable with GMT power chucks
  • Used in CNC lathes
GMT collet chucks for increased production from bar feeding. Uses both multibore and spring collets and is made in three sizes: 26mm, 42mm and 60mm.

 You can hold certain range of diameters within the  h9 tolerance range according to the design range of the collet chuck, using different collets as shown in the picture above.

GMT Power operated pneumatic chuck

Power Operated Pneumatic Front end Chucks are designed to utilise in full, the spindle bore for bar/pipe work. This design avoids rear mounting of the actuating cylinder and necessary draw bar connections. The chuck can be mounted on the lathe spindle with an adaptor flange as needed. The actuating cylinder is built into the chuck. The stationary air distribution system with the ports to supply air to the respective sides of the piston is mounted separately on the chuck.

These chucks are available in various sizes ranging from size 160mm Ø to 1000 mm Ø. The chucks are offered with standard jaw stroke of 4.2mm to 12mm and extended jaw stroke of 19mm to 25.4mm. The extended jaw stroke to hold pipes, casings and tool joints.

For holding long oil pipes, one chuck is mounted on the front end of the spindle, and one chuck at the near end of the spindle. Power Operated Pneumatic Front end Chucks have twin non-return valves, which prevent the clamping air from returning through the port unless air is let in on the other side of the piston. This ensures retention of clamping force even when the air supply is switched off. The maximum operating pressure of the chuck is at 10 bar pressure.

Principle of Operation
Power Operated Pneumatic Front end Chuck has two special rubber seal rings on the distributor assembly. The special seal ring is compressed when the chuck is pneumatically actuated to facilitate opening and closing of chuck jaws. Once the job is clamped due to venting of the air supply, the special seal rings lift to ensure rotation of the chuck. This design eliminates wear on the special seal rings.

The chuck body is made of forged medium carbon steel. The guideways are flame hardened and ground. The wedge and base jaws are made of nickel chrome steel; case hardened and ground on all working surfaces. Serrations are provided on top face of the ground base jaws. The various radii on the hard jaws in conjunction with serrations are designed to grip a wide range of job diameters.

Electro Pneumatic Control Unit
Each GMT Power Operated Pneumatic Front end Chuck is supplied with an Electro Pneumatic Control Unit. This control unit consists of Lubro Control Unit, Electrical Contactors, and Fuses and in addition, solenoids for interlocking with the main spindle.
Push button or foot operated switch is supplied with the Electro pneumatic Control Unit for actuating the chuck.

Features :
  • Power operated pneumatic front end chucks are designed to utilise in full, the spindle bore for bar/pipe work.
  • This design avoids rear mounting of the actuating cylinder and necessary draw bar connections.
  • The actuating cylinder is built into the chuck.
  • The stationary air distribution system is mounted separately on the chuck.
  • Power operated pneumatic front end chucks have twin non-return valves.

GMT Twist Finger Chuck

FINGER CHUCK : GMT has developed specialized chucks for second operations where radial clamping is not possible. A suitable locator is used to locate the component center. The fingers located in a rod, which has a helical groove ensure that the component is flat against the chuck. This enables loading yet ensures accurate location. GMT-make hydraulic rotating cylinder is used to move the chuck fingers swings in and clamps the component in a single motion. Actuator pin is located on helical groove for actuating the finger. Fingers during de-clamping lift and swings out for component unloading and loading. The chuck firmly clamps the component without any distortion or surface damage.
Features :
  • Chuck has a locator for locating the component.
  • Twist finger chucks cannot perform without locating the component.
  • Fingers are located in a rod, which has a helical groove.
  • Actuator pin located on helical groove for actuating the finger. Fingers during clamping and de-clamping lift, swings in and out, for component loading and unloading.
Application :
Automotive, Wheel turning, Aerospace, Aircraft, Commercial vehicles, Energy, Heavy duty machines, Defence Production, Oil, Wind generators

GMT Hand Operated Chuck

Hand Operated Chuck (HOC) : The production capacity of a lathe is entirely a function of the chuck used, the gripping power and the true running of the chuck.

A lathe with the excellently engineered GMT chuck enhances the productive capacity of the lathe which provides the answer to faster and better guaranteed production.

The conventional bevel gear and the scroll have been replaced by a worm and wormwheel with three circular cams, to operate the jaws. The chrome case hardened wormwheel-cum-camring ensures maximum resistance to wear. The entire reaction of the clamping forces of the three jaws are confined in and taken up by the worm wheel-cum- camring and not by the chuck body or component parts.
The chuck body and the wormwheel-cum- camring are made from steel forgings, which provide the desired grain flow and thus minimize distortion under the clamping forces. All the parts are amply proportioned, made from high tensile alloy steels, hardened and ground wherever necessary, to close limits of accuracy. The hard jaws are provided with gripping serrations to grip the work piece firmly.
The normal dimensional variation in the gripping diameter of jobs is taken care of by the adequate stroke of the base jaws, thus avoiding frequent resetting of the top jaws.
On each circular cam, slides a chrome case- hardened button with a groove of the same radius as the cam. This provides a large pressure bearing area to the load transmitting surfaces. The large bearing surfaces reduces the stress per unit area to the minimum and completely inhibits any denting of the circular cam.

Because of the comparatively small number of well designed parts used in the chuck, frictional losses are reduced to minimum. The worm, wormwheel and the circular cam give the chuck an amazingly high mechanical advantage resulting in a grip many many times more powerful than that of a scroll chuck, for the same effort applied at the chuck key. Appropriate cam rise, combined with worm drive, ensures self locking action of the chuck over the entire clamping range and this guarantees the safety of the operator.
The top jaws are positively located on the base jaw by serrations ground after the jaws are case hardened. The top jaws can be quickly reset for different sizes of work pieces by merely loosening two socket head screws, shifting the jaw to the appropriate serration and re-tightening.
Three Jaw chuck of sizes from 125 mm dia. to 315 mm dia. are offered with serrations of 1.5 x 60o on the base jaws.
Sizes 400 mm dia. to 800 mm dia. are provided with 'V' Serrations of 3/32" x 90o.

Soft Jaws are made from low carbon steel and used for second operation. They are used for gripping finished diameters and are supplied as blanks. They are reversible, so that both ends can be used.
The gripping diameters are to be machined in position on the chuck. This will ensure true running of the job on soft jaws, within close limits.

GMT Standard Power Operated Three Jaw Chuck

Power Operated Three Jaw Chuck with Through Bore(PHNC) : GMT Power Operated Hollow High Speed Chucks are designed to rotate at high speeds on CNC lathes. The compact construction of the PHNC Chucks offer further advantage of less weight and low inertia which have positive influence on the dynamic effect of CNC machine spindle. These chucks have a large through bore and are therefore suitable for bar work. The base jaws and top jaws weight are reduced in these chucks. This not only reduces the mass but also lowers the centre of gravity, which make the centrifugal losses low. 

Chucks on CNC machines need to deliver high initial gripping force. GMT takes care of this in the design by making the chuck operate with a large drawbar pull. Consequently, the wedge is designed to have large contact area with the base jaws. The weight of the body, base jaws and hard jaws are less, without the other essential criteria being sacrificed. The chuck body is recessed to remove a large extent of material to reduce the weight. (from sizes 250 mm to 500 mm). The chuck body is forged steel. The guideways are hardened and ground. 

The wedge, made of nickel chrome steel, is case hardened and ground on all the working surfaces. The base jaws also made of nickel chrome steel, are case hardened and ground to match both the wedge and the body guideways. The base jaws are guided in the deep, wide, hardened slots in the body, which provide the ample bearing area necessary to withstand the forces resulting from high gripping action. Provision has been made for manual lubrication of sliding Serrations are ground on the top face of the base. The serrations on the bottom to match the base jaws. The various radii on the hard jaws in conjunction with the serrations are designed to grip a wide range of diameters. hardened chrome case hardened and ground base jaws and the wedge ensure high load carrying capacity over a very long period. 

Chuck performance detail
2 Jaw and 4 Jaw chucks with through bore can be offered on request
Chuck Size dia
Clamping Range
Max Drawbar pull (Kgf)
Max gripping force (Kgf)
RPM max
Weight (without top jaws) Kgs.
Flywheel effect GD2 (KPm2)
Max top jaws weight (per set) Kgs.

GMT universal ball lock chucks

GMT universal ball lock chucks (UBLC) are designed to clamp components and ensure face butting. Ball Lock Chucks have certain distinctive advantages over wedge and lever type chucks. Jaw movement is achieved by sliding and swivel of the base jaws, which are amply proportioned.
This results in less wear, less bearing pressure and less stress concentration in the jaws when compared to same sizes of wedge or lever type chucks. 

Apart from the radial clamping forces, these ball lock chucks provide an axial force to pull the component against the butting face. This is achieved as the jaw movement is along an arc. Wedge, base jaw, spherical aligner, spherical bushes are made of Nickel Chrome Steel, case hardened and precision ground on all working surfaces.

Eccentric compensating type of ball lock chuck is essentially designed where the component is usually located between two centres, one on the chuck body and one on the tailstock of the machine. Since the line of the centres is defined, concentricity is guaranteed and the ball lock chuck design allows adequate freedom for the jaws to adjust and accommodate the irregularities of the job before full clamping force is applied.

Features :
  • Positive pull back action against jaw face after clamping due to spherical movement.
  • Positive sealing to prevent entry of chips.
GMT UBL chucks are offered in 2 versions:
  • Self centering
  • Eccentric compensating


In geological terms more commonly used, rocks for surface plates are Granite, Diabase / Dolerite and Gabbro. Granite consists of a fairly coarse grained aggregate of quartz and feldspar with small quantity of black or white mica. More often granite is identified by shades like black, grey, pink, etc,. The other and rather more commonly used rock for surface plate is the Diabase or Dolerite.

The properties, which help in selecting a rock for manufacture of surface plates, are hardness, density and the grain size. Uniformity of grains and structure is also a good feature. In general fine-grained Dolerite has better scratch resisting property than harder grey granites,and this has been confirmed by scratch test using a sharp diamond.

Another important property of rocks is the density. The compressive strength of a rock increases with an increase in the density. This higher density rocks have generally smaller grains and lesser porosity. Dolerite stays ahead in this category with very fine and smaller grains when compared with other igneous rocks. Diabase / Dolerite is a variety of a stone type known as Gabbro.

While it is closely related to granite, it is not the same. Dolerite lacks several of the minerals (such as quartz and mica) which give granite its distinctive grain structure. The lack of quartz means that Dolerite is slightly less resistant to wear than the hardest granite. However, this is offset by the lack of mica, which is very soft, and tends to flake out of granite plates, leaving pits. Dolerite is denser than granite, weighing about 190 lbs. per cu.ft. While most granite weigh between 160-170 lbs. per cu.ft.

Dolerite is less porous and more stable, which means that a smoother and more uniform surface finish can be achieved. It can used for multi-dimensional, very high-accuracy applications that require a very smooth, stable surface like angle plates, parallels, master squares, straight edges and so on.


A medium to coarse-grained intrusive igneous rock composed of potassium and sodium rich feldspar, quartz, minor plagioclase, and small amounts of ferromagnesian minerals such as biotite or hornblende. It is the intrusive equivalent of rhyolite.
When we inspect granite specimen it shows orange to pink feldspar, white plagioclase, grey quartz with minor dark mafic minerals. Another form of coarse-grained granite consisting of white feldspar, grey quartz and black biotite it also contains an inclusion of dark metamorphic rock.

(From Handbook of Physical constant)

SiO2 70.18
TiO2 0.39
Al2O3 14.97
Fe2O3 1.57
FeO 1.78
MnO 0.12
MgO 0.88
CaO 1.99
Na2O 3.48
K2O 4.11
H2O+ 0.84
H2O- 0.03
P2O5 0.19
Quartz 38.8%
Orthoclase 16.7%
Albite 28.3%
Anorthite 9.2%
Corundum 0.3%
Hyperthene 4.3%
Ilmenite 0.6%

CNC Lathe Chucks - Do’s and Don’ts


Always clamp the component in the middle stroke of the Base jaw / Master jaw.

Perform proper boring of Soft Jaws


Ensure a gap of 0.5 mm between the wedge and the chuck body inner face, when jaws are at radially outer most position.


Use proper Top jaws
Height of top jaws should be less than its length.
Ensure at least two screws are used to attach the top jaws with master jaws.

Do periodic maintenance
  • Remove the chuck from machine and clean it properly with kerosene and lint free cloth. (frequency every 25000 cycles.)
  • Replace the damaged or worn out parts with new one.
  • Ensure the smooth working of sliding parts.

Ensure that the Jaw Clamping Screw does not touch the base jaw


Jaw clamping screw should not touch the base jaws

  • Don’t attempt to redesign the chuck
  • Do not attempt to braze or weld any internal parts of the chuck.
  • Do not use welded top jaws as this will cause imbalance on the Chuck.
  • Do not attempt to modify the design of the chuck.
  • Don’t use the chuck after removing the front cover. 
Don’t tighten the screws beyond specified limit.

Don’t use excessive leverage (pipe) to tighten the chuck mounting screws, and top jaw clamping screws.  

Don’t use long screws for chuck clamping and top jaw clamping.

Don’t exceed recommended parameters of chuck.
Don’t exceed the recommended draw bar pull of chuck.   
Don’t exceed the recommended R.P.M. of the chuck.

Don’t use steel hammer on any parts of the chuck
Instead, use Nylon Hammer or Aluminum shaft


When we investigate the changes in granite surface plates over  time and with usage, we find that the surface topography is not retained with changes in atmospheric conditions like temperature. So the behavior of the plate has  to be studied under varying conditions like,
•  Temperature
•  Change in humidity
to determine their effect on the accuracy of the surface plates. 

First the surface plate is divided into squares of 100X100mm. An electronic level with a sensitivity of 0.0005/100mm placed on a bridge is used to measure the flatness of the surface. Readings are taken along and across the surface and the surface topography is computed to attain the contour. This bi-directional approach to every point gives accurate measurements of the surface.

Now the behavior of the granite is observed at various temperatures. We can see a significant change in the contour of the surface due to temperature rise. But the magnitude of the change cannot be explained by the coefficient of thermal expansion of Dolerite rock, which is 2x10 -6 to 4X10 -6 per degree centigrade. At 26°C, we can notice the surface contour seems to be concave with overall flatness accuracy of 5.5 microns. When there is a rise in temperature to 40°C the surface contour changes to convex with overall flatness accuracy of 8.6 microns. This is the behavior of granite due to change in temperature.

Changes in contour due to a change in atmospheric humidity is very insignificant. The change in contour of the surface with change in humidity of 20, 40 and 60% has been shown to be negligible.When the surface was soaked with water and measurement taken within a short period the variation in surface contour is negligible. However a change was observed when the surface was allowed to be covered and soaked with water for 12 hours. For a study, we took a granite plate of size 400X400X100mm. Before soaking in water the flatness was checked as 3.48 microns, when the top surface was soaked in water briefly, the flatness was recorded as 3.24 microns. Now the top surface was soaked in water for 12 hours, the reading of flatness was recorded as 2.34 microns and when all sides soaked in water for 3 hours, we got the flatness reading to be 1.91 micron. This was the effect of water on the accuracy of the surface plate.

However it is observed that for both the water soaked condition and as well as for temperature change the surface returns to the original surface topography, once the surface plate is brought to its original atmospheric condition. Studies on igneous rocks such as Granite and Dolerite showed that,
•  The igneous rock has pores and capillaries
•  The material consists of distinctly different crystals of different materials.

As stated above granite has pores and these can have with them entrapped moisture. The moisture content is maximum at the surface and decreases in the deeper layers. When the stone is soaked in water and the surface is dried and after some time it is broken into two halves, the newly exposed upper layers have distinctly higher content of water whereas it is less wet as the depth increases. We know that the igneous rock like Granite, Dolerite, etc., consists of various substances such as Quartz, Hornblende, Calcite, Orthoclase, etc., Due to this various ingredients of stone have uneven volumetric or linear expansion within them. For example quartz expands four times more than the feldspar and twice as much as hornblende. Due to rise in temperature the quartz exerts a pressure against its surroundings and causes an expansion of the surface, which even if small, affects the surface accuracy. The variation of the surface topography thus is an inherent phenomenon due to the properties of a stone. Its value changes from stone to stone. It is more in stones with higher porosity and quartz content.

Further investigation reveal that, granite expands only about 0.036% at 25ºC whereas water expands more than 0.32%, at the same temperature if it is not compressed. If the walls of the pores prevents water from expanding it can exert about 70 atmospheric pressure against the pore walls. Thus the entrapped moisture or water in the pores comes to an equilibrium pressure due to capillary action and there will be a pressure on the walls of the pores, which act as small pressure vessels causing the surrounding portion of the rock to expand due to this internal pressure. However this pressure will be maximum at top layer of the surface and reduces in the deeper portion. Thus the upper layers of the stone expand more than the lower causing the surface to swell.


High degree of hardness (High quartz- high hardness)
Low heat conductance
Low temp sensitivity
Electrically Non-conductive
Rust and acid resistance
Matt surface, non-reflective surface
Low water absorption
High modulus of elasticity- Torsionally rigid, sag free, good slid's prop
Dolerite - a better scratch resistance
Modulus of elasticity - 26-86 GPa
Compressive strength - 107 Mpa / 15600 PSI
Water absorption - 0.07 - 0.31
Density - 3g/cc
Hardness - 6-7 Mohs scale
Tensile strength 5 Mpa
Co-efficient of thermal expansion - 6.1X10 -6/°C
It is  observed that fine-grained rocks have relatively higher density and hardness. They have higher resistance to scratch and are less porous. However stability of granite under varying temperature and humidity conditions is a major limitation even though the coefficient of thermal expansion is low compared to other engineering materials like cast iron and steel. If there is any small impact or scratch made over the granite surface plate, it results in crushing of localized zone and a broken piece or powder comes out without harming the surface accuracy. However in the case of cast iron or steel surfaces, under similar circumstances a bulge would result and the surface accuracy is affected and often these remain unnoticed by the naked eye.

The granite color alone is not an indication of the physical qualities of the stone. In general, the presence or absence of minerals determines the color of the granite and this may have no bearing on the qualities that make good surface plate material. There are pink, gray and black granites that are excellent for surface plates. However the consistency of several made from the same vein in a quarry can be unpredictable.this inconsistency is much less in pure black granite.

The aging qualities of these rocks are also excellent. This is conformed by a measurement carried out on a 1.6m X 1m surface plate kept in a metrology room immediately after its lapping and sparingly used over a period of four years. The variation in shape and the accuracy of this surface is negligible. Thus granite has been found to be a useful material for providing an accurate surface.


The basic requirements for an ideal Metrological Laboratory are:

The Room:
  1. Should be below the ground level to certain meters (say 6m), to take advantage of constancy of underground temperature and to isolate it from the vibrations, acoustics and other external influences, which affect the lab environment.
  2. Should be insulated with expanded polystyrene insulation on all six sides.
  3. Should have double walls with air gap on all the four sides to be made.
  4. Should have special RCC floor blocks, cooled with circulation of chilled water is required.
  5. Should have swirl type air diffusers to be fixed for thorough air mixing.
  6. Should have an system so return air flows all along the wall surface.
  7. Should have a high 30 air changes per hour.
  8. Should have an air shower and air lock at entry, to keep the environment controlled under standard requirements.
  9. Should have a vibration damping foundation with massive 1.5m monolithic RCC blocks. 
  10. Should have Anti-static PVC flooring.
  1. For effective handling of instruments and stainless checking of the readings, high efficient fluorescent lamps to be used as per NABL recommendation.
  2. Importance to be given in temperature and humidity control all over the lab premises by strategically locating sensors and direct digital control.
  3. Stainless steel pipes and fittings should be provided for compressed air supply system.
  4. Separate pre-cleaning room with ultrasonic cleaning machine to be maintained.
  5. Separate material stock room for storage of the calibrated instruments and gauges is a must.
  6. Built-in emergency lighting and fire alarm system to be installed as a preventive maintenance inside the calibration laboratory.
Parameters to be considered before designing for the underground metrological laboratory.
20 ± 0.2° C
50 ± 5% RH
Clean room Class
10,000 (5VDI 2083)
Air velocity
< 0.2 m/sec
Differential Air Pressure
> 10 pa (1mm Wg)
Ground vibration (@<10 Hz)
< 0.2 µm
400 lux
Noise level
< 45 dB (A)
Floor covering
Anti static PVC
200 – 250 Sq.m.


A product or service should be 'Fit for purpose', conforming to specified requirements with respect to National standard / International standard / User's specification. This requires the services of a calibration laboratory, which should be accredited. 'Accreditation is a process of formal recognition of technical competence in accordance with the prevailing international standard' by a recognized accreditation institution.

The National Accreditation Board for testing and calibration Laboratories (NABL) is a full member of Asia Pacific Laboratory Accreditation Co-operation (APLAC) and International Laboratory Accreditation Co-operation (ILAC). Because of a comprehensive requirement of combining inspection and testing in many areas, NABL  provides accreditation  for inspection bodies in accordance with ISO/IEC 17020 International standard, and ISO/IEC 17025:1999. 

A  Calibration laboratory shall be accredited according to their 'Best measurement capability'. It means the least uncertainity of measurement (±) at a confidence probability level of 95%. The calibration laboratory is not permitted to report a smaller uncertainity of measurement than the best measurement capability on its endorsed documents. Obviously, the actual uncertainty of measurement can never be smaller than the best measurement capability. 

The calibration laboratory shall be organized in such a way as to ensure the integrity and training of its staff and operations for ensuring unbiased calibration. There should be an authorized signatory for the calibration certificates/reports issued by the laboratory. It should have a quality manual, which should be maintained up-to-date and available for scrutiny, in compliance with ISO/IEC 17025 and NABL requirements. The quality manual should contain a quality policy statement, objectives, commitments by the top management, names, qualifications, experience of persons responsible for the managerial, scientific/technical activities, measurement capability, Traceability of calibration of all measuring instruments to standard, calibration procedures adopted, National/International standards referred/used. 

The laboratories should have authorized signatories for approving and issuing calibration certificates for each calibration parameter as mentioned in the scope of accreditation. The laboratory should have an adequate number of qualified and trained staff with at least a degree in Physics or Diploma in Engineering. The team should be headed by a P.G. in Physics or Graduate in Mechanical/Instrumentation Engineering. 

The calibration area shall be adequately free from vibrations generated by air-conditioning plant, vehicular traffic or any other such sources. Acoustic noise level shall be maintained for proper performance of calibration. A threshold noise level of 60 dBA is to be maintained as recommended. Adequate level of illumination has to be maintained, fluorescent lighting is preferred to avoid localized heating and temperature drift. The recommended level of illumination is 450-700 lux on the working table with glare index of 19 for the laboratory. 

The environmental conditions for the activity of the laboratory shall be such as not to adversely affect the required accuracy of measurements. As possible, only the staff engaged in calibration activity should be permitted entry inside the calibration area. Laboratory should have equipment of required accuracy in respect of each parameter covered. Stability of the standards, accuracy of the values realized through them and repeatability, should be regularly monitored.

Calibration certificates, performance history sheets, working standard details should be held safely by the laboratory. Each equipment should have a record of name, manufacturers name and address, type, range, identification and serial no., date of procurement and commissioning, details of calibration, details of maintenance, performance history with dates. The service manual has to be available at all times.

All the standard equipment of the laboratory should be calibrated periodically against calibration standards of a laboratory accreditated by NABL/ equivalent MRA partners having superior measurement capability or NPL/ other international NMIs. To give further assurance to the accuracy or uncertainty of measurements, a laboratory should be required to participate from time to time, in Proficiency Testing Programmes. There the abnormalities of equipment are detected in terms of En number, through inter-comparisons and the appropriate corrective actions should be taken. The standard equipment shall be replaced/ repaired and re-calibrated with a higher accuracy standard. Reports on such inter-comparisons should be documented with reference. This Proficiency Testing practice should be included in the Quality Manual.

Why use a 17025 Calibration?

ISO 17025
calibration is our guarantee of confidence in our calibration results through internationally recognized technical auditing of a laboratory's competence. ISO17025 is virtually now the only Internationally Recognized Calibration System. Traceable Calibrations from Non ISO17025 Accredited Calibration Sources are now generally technically questioned. To quote from the ISO Standard, Certification against ISO9001 and ISO9002 does not of itself demonstrate the competence of the laboratory to produce technical valid data and results."

In the United Kingdom , UKAS Calibration is the only Accreditation Body who assesses Test & Calibration Laboratories to ISO 17025. National & International Technical Trade Barriers for your company's products and services become transparent when all your calibrations to the ISO17025 standard.

A Background to ISO 17025
ISO 17025 addresses every element of laboratory management. It is not exclusive to the laboratory manager, assistant laboratory manager, or quality manager. The standard involves all laboratory staff whose functions relate to the quality of laboratory data generated. It replaces NAMAS M10 and laboratories, which are currently accredited to M10, have until 30th June 2002 to meet the requirements of ISO 17025 if they are to retain their certification.
The ISO management system will provide a defined, ordered process for operating all facets of a laboratory. The laboratory community throughout the world has expressed, through the International Organization for Standardization, the essential elements for a laboratory management system in the text of ISO 17025. That document provides the nucleus for most laboratory accreditation programs today. ISO 17025 provides the framework for a Quality Management System for a testing or calibration laboratory.

The Opportunities
The Transition Management Tool gives the following benefits:

  1. Clear identification of the new requirements in ISO 17025
  2. Simple workbook format 
  3. Addresses every clause and sub-clause of the standard
  4. Cuts through the jargon of the standard
  5. Breaks the standard down into manageable chunks
  6. Identifies the gaps in the existing system 
  7. Fully costs the transition project 
  8. Assigns responsibilities for all the key project tasks
What is driving accreditation?
  1. The automotive standards QS 9000 and 16949 require testing and calibration laboratories to be accredited.
  2. Laboratories may need a quality system that is recognized internationally.
  3. The marketplace - laboratories must keep up with competitors who are becoming accredited.
  4. Internal motives - laboratories can improve their internal systems. 

ISO 9001 certified organisations : Calibration of measuring instruments

ISO 9001
ISO 9001 certified organisations have to make decisions regarding where to send their measuring instruments for calibration. Nowadays, many calibration laboratories have ISO 17025 accreditation. But some are not accredited, but still doing calibration work for other organisations. Also, many accredited laboratories are not accredited for all the services they offer. The use of non-accredited calibration labs, or non accredited services of partially accredited labs, may reduce operating costs in the short term, but could turn out to be costly in the long term. Examination of ISO 9001 (2000) and ISO 17025 suggests that ISO 9001 certified organisations should select their calibration labs carefully and make sure that the labs they use are properly accredited for the services they provide.

Organisations certified to ISO 9001 are required to calibrate all their measuring equipment used to verify or control quality, and all such calibrations are required to be traceable to national or international standards (ISO 9001 1994 section 4.11, ISO 9001 2000 section 7.6). All the records of calibrations are required to be maintained properly and corrective action to be taken when measurement equipment is found to be out of specification. Some of the implications of calibration and traceability requirements for ISO 9001 certified organisations and for calibration and test laboratories begins with the investigation, regarding the meaning and components of the term `traceable'.

Many calibration laboratories claim accreditation to ISO 17025. Here we go for NABL for accreditation and in Australia NATA is the accrediting body. Accredited labs are entitled to use the NABL logo on their documents and web pages. ISO 17025 is an international standard that specifies quality and technical competence requirements for testing and calibration laboratories. ISO 17025 replaced ISO Guide 25 in 1999. 

Hiring and keeping competent technical staffs, internal audits, maintenance of in-house quality checks and participation in proficiency testing programs will improves the likelihood of an error-free service but it alone can never guarantee complete absence of calibration or other errors. However, customers of reputable ISO 17025 accredited labs can expect to be informed promptly and fully of errors when they are discovered, and of the particular consequences related to the calibration of their equipment (as per sections 4.9, 4.10). 

Calibration, uncertainty and traceability
The ISO 9001 requirement for traceable calibration of test and measurement equipment raises questions concerning the term `traceable'. When we examine definitions and components of traceability extracted from ISO 9001, ISO 17025 and other documents, we get the answer for it in clear way and along with that discussion regarding the implications were also made in following lines. 

Tractability is defined as, ‘the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties....' The unbroken chain of comparisons is called a `traceability chain'. An unbroken chain of comparisons is a logical and easily understood component of traceability. The manager of a non-accredited lab might claim that his calibrations are traceable because he is able to trace only the calibration pedigree of the references and standards, which he uses.

This aspect can be analyzed by an example. Assume we keep a set of weights which we use to check balances in a chemical laboratory. The balance should have a resolution and repeatability within the uncertainty required in the final result, when compared with a set of calibrated weights. It must be properly serviced and maintained, mounted on an appropriately rigid and vibration-free bench in a temperature controlled environment. Air movement around the balance may need to be restricted. If the weights to be compared are of different density, compensation for buoyancy might be necessary. Buoyancy compensation might require measurements of air temperature, humidity and barometric pressure. If the lab provides other calibration services then the presence of other equipment nearby may alter the environment in the vicinity of the balance, e.g. a temperature calibration oven might alter the mean radiant temperature in the vicinity of the balance. 

If we appreciate the potential complexity of the calibration process then we should require that the lab calibrating our weights employ a technician with sufficient competence and training to appreciate all the potential sources of error in the calibration. He should be capable of setting up the equipment properly and deciding which errors are significant and which can be ignored for a particular calibration. Competence as a component of traceability is addressed in ISO 17025 section 5.6. The Section states that traceability of measurement shall be assured by the use of calibration services from laboratories that can demonstrate competence, measurement capability and traceability. The use of the word 'shall' in a standard usually means that there is no other way to achieve compliance. ISO 17025 further states that, any calibration laboratories fulfilling the requirements of this International Standard are considered to be competent . A calibration certificate from a calibration laboratory accredited to this International Standard for the calibration concerned is sufficient evidence of traceability. 

Uncertainty as an essential component of traceability
No measurement is ever true. There is always a difference between the true value of a measurand and the output of an instrument. Measurement uncertainty is a quantitative statistical estimate of the limits of that difference. The measurement uncertainty is ' a parameter associated with the results of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand'

Uncertainty estimates document the rationality and consistency of the comparisons. A traceability chain is a documented set of comparisons between consecutive pairs of instruments or measurement systems: A-B, B-C, C-D, etc. Usually instrument A is compared with instrument or standard B for the purposes of calibrating A, and the uncertainty estimated is that associated with that calibration process. The contribution of instrument or standard B to the overall calibration uncertainty is typically 4-10 times smaller than the contribution of A. Similar process to be handled between C & D also. Properly calculated and documented uncertainty estimates in a calibration chain indicate the `direction' of traceability. 

Everyone should view uncertainty estimates as confirmation that his instrument is calibrated against a reference of adequate performance and that all-potential sources of error are under control during the calibration process. The essential component for traceable calibration is stated as below, Traceable calibration involves comparisons with traceable standards or reference materials. Only laboratories, which demonstrate their competence can perform traceable calibrations, by accreditation to ISO 17025. A traceable calibration certificate must contain an estimate of the uncertainty associated with the calibration.  Organizations using non-accredited calibration labs do not conform to ISO 9001 and  should not claim conformance. At the same time, some calibration laboratories offer a wide range of calibration services but are accredited for only a subset of those services. 

In some cases labs claim `ISO 17025 accreditation' but are vague about exactly which services are accredited and which are not. ISO 9001 organizations should be careful to select calibration labs that are explicitly accredited for the services they are using. NABL keeps an up-to-date ,publicly available list of accredited labs with details of the calibration services for which they are accredited and their least uncertainties of measurement. In a manufacturing environment it is often the case that more than one measurement system is used to monitor or control the quality of the product, and inevitably some measurements contribute more than others to uncertainty in product quality. ISO 9001 does not require all measurement systems to be rigorously calibrated - only those that contribute significantly to the control or verification of the quality of the product. One approach to this problem might be to perform uncertainty analyses on quality-related measurements using techniques similar to those outlined in the ISO to determine which measurement systems require calibration and the maximum associated uncertainties. 

Thus, an ISO 9001 certified organization should analyze the measurement systems it uses to verify or control quality, make informed decisions on which instruments require calibration, and have these instruments calibrated by selected ISO 17025 accredited labs.



Errors in Measurements
There is a true fact that, no measurement is exact. All measurements are subject to some error. It is therefore necessary to state not only the measured dimension, but also the accuracy of determination to which the measurement is made. As for as possible the errors inherent in the method of measurement used should be kept to a minimum, and having minimized the error, its probable magnitude, or accuracy of determination should be stated.
Along with the actual gauge block size details, there should be details regarding the measured error in the block and the accuracy of determination with it enclosed. The accuracy of determination can be improved by repeating the measurement a number of times and stating the mean value.
Types of errors:
There are two types of errors,
•  Those which should not occur and can be eliminated by careful work and attention.
•  Those which are inherent in the measuring process. Misreading an instrument, arithmetic errors, alignment errors, parallax error, errors due to temperature were some of the errors that we can eliminate on proper procedural handling of the system.
When we can truly believe a measurement?  
We can never have 100% confidence in a measurement. No measurement is ever correct. There is always an unknown, finite, non-zero difference between a measured value and the corresponding true value. Most instruments have specified or implied tolerance limits within which the true value of the measurement should lie if the instrument is functioning correctly. One can never be 100% sure that an instrument is operating within its specified tolerance limits.
There are steps we can take to minimize the probability of a measurement falling outside specified tolerance or uncertainty bands. Regular traceable calibration is a method for gaining quantifiable confidence in a measurement system.

For example, if we consider about the pressure transducer, there are a number of modes in which electronic circuitry and the digital display can fail or malfunction. Most of the faults and malfunctions would not be visible to an operator therefore it is impossible to verify the absence of faults and electronic drift by simple inspection. We cannot tell by inspection if the instrument has recently been dropped, subjected to an over-range pressure or otherwise mistreated. When we make a measurement in the field we are forced to trust the instrument. The only way we can gain confidence in the electronic manometer is by regularly comparing its response with another similar or preferably superior instrument in which we have a high level of confidence. A quantitative comparison or verification of the performance of an instrument is called a calibration.