GMT METROLOGY DIVISION - SYSTEMS OF MEASUREMENT

SYSTEMS OF MEASUREMENT

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.

GMT GRANITE MASTER SQUARE

GRANITE MASTER SQUARE 

 

Made of fine black, Metrology Grade Granite, GMT 6-Face Master Squares may  be used for  checking of squareness or parallelism. Master squares have 6 finished faces . One large square face is finished flat and perpendicular to the four sides, which are parallel and perpendicular to one another.

The working surfaces are ground and lapped parallel to each other, remaining sides are polished. They are provided with through-holes for weight reduction and ease of handling.
GMT 6-Face Granite Master Squares are ideal for accurately checking the X-Y-Z axes on CNC machine tools and coordinate measuring machines (CMM).

With the square in the horizontal position, the X and Y axes can be checked for 90° squareness. With the square in the vertical position, tracing along the vertical edge of the square can check the perpendicularity of the Z axis. By tracing along the top edge of the square while in the vertical position, it will check parallelism to the table in the X and Y axes.

Custom sizes are also available on request.
Accessories: Wooden case.

GMT Granite Surface Plates-Application and Accuracy

Application
Surface plates are used as reference planes for dimensional inspection and calibration of mechanical artifacts. This plate acts as a datum line or surface. So, the flatness of the plate is very important. Dolerite is the perfect material for metrology, because of its inherent characteristics mentioned in an earlier blog. The definition of flatness is, ‘all points on the surface being contained within two parallel planes, the base plane and the roof plane’. This flatness measurement commonly carries a tolerance and may include a grade designation. Flatness tolerance of the surface plate is provided in the standards with different grades based on flatness.

GMT Metrology Granite Surface Plates meet the DIN and IS Standards specified in the table above. Granite Surface Plates are manufactured to precision standards using the highest quality granite.

Accuracy
Rigorous quality control inspection is carried out at all stages. Final inspection is carried out with a High Precision Computerised Electronic Level calibrated by an NABL accredited lab at regular intervals..

Accuracy under Load
Granite Surface Plates are capable of supporting a total normal load of 25 kgs. per 0.1 sq.mtr.

Certificate
A certificate of accuracy is supplied with each plate as per standards specified.

Technical Data
Granite Surface Plates are manufactured in five grades. The typical application of these five grades of plates is as under:

• GRADE 00 as per DIN : 876 : 84 is of laboratory grade. It is specified for precision measurement in gauge rooms and metrology laboratories for calibration masters.
• GRADE 0 is of inspection grade. It is specified for general work in quality control.
• GRADE 1 is of tool room grade. It is specified for work shop use.
• GRADE 2 & 3 are intended for use in shops for general inspection. The choice of Grade 2 & 3 will depend on the application needs of the customer.

Material
Granite Surface Plates are made from select granite with uniform distribution of light and dark constituents. In such granite the minerals are evenly distributed resulting in a homogenous appearance.
The following are the representative properties of our granites :
Texture                           Fine
Specific Gravity             3.08 g/cm3
Hardness                        Scleroscope over 90
Modules of Elasticity     9.0 to 12.0 x 105kg/cm2
Moisture absorption       Nil

Flatness Tolerance
The flatness deviation of a local area of 250 mm x 250 mm of the working surface shall not exceed :

1. 3.0 µm for plates of Grade 00
2. 3.5 µm for plates of Grade 0
3. 7.0 µm for plates of Grade 1
4. 15.0 µm for plates of Grade 2
5. 30.0 µm for plates of Grade 3

Granite Surface Plate Stands
These are fabricated of heavy angle iron and reinforced to ensure rigidity. The stands are designed to withstand the weight of the granite surface plates and are supplied with 3 leveling screws for leveling the granite surface plates.

Optional Accessories
Plate will be supplied with a suitable Rexine Cover to protect both the working surface and side faces of the plate at extra cost.

GMT PRECISION GRANITE SURFACE PLATES

Why GMT Granite Products?
GMT's metrology division is located at Hosur, Tamil Nadu. This region is home to large deposits of world famous Indian black diabase granite which is is best suited for metrology application as it has  a very fine-grained structure. The properties, which help in selecting a rock for metrology application are density, water absorption & porosity, flexural and compressive strength.

Advantages of Indian Black Granite:

• Low coefficient of expansion and thermal stability
• Water absorption and porosity is low and hence long term dimensional stability is good
• It is fine-grained and the density & hardness is suited for dimensional metrology
• Flexural strength, Compressive strength, Modules of rupture and Tensile stress
• Totally corrosion free, granite does not rust nor corrode or react chemically with other substances
• Non Magnetic and non-conductive
• Non-reflective matt finish
• High flatness accuracy
• Ideal for guideways for measuring instuments and air bearing surfaces
• High bending strength
• Insensitive to mechanical overloading
• All calibration equipment is to national and international laboratories standards
• Hand lapped for maximum accuracy

SPECIFICATIONS

FLATNESS TOLERANCE IN µm FOR ACCURACY GRADE 

SIZE OF PLATE             AS PER DIN 876:84                      AS PER IS 7327-2003                                        WEIGHT

   (in mm)            |  GRADE "00" | GRADE "0" | GRADE "1" | GRADE "2" | GRADE "3" | (Kgs) (Approx.)

250 x 250        |            3          |       3.5        |         7          |      15          |      30          |      10
300 x 300        |            3          |       3.5        |         7          |      15          |      30          |      22
400 x 250        |            3          |       4.0        |         8          |      16          |      32          |      20
400 x 400        |            3          |       4.5        |         9          |      17          |      34          |      40
500 x 300        |            3          |       4.5        |         9          |      17          |      34          |      35
630x 400         |            4          |       5.0        |        10         |      20          |      39          |      65
630 x 630        |            4          |       5.0        |        10         |      21          |      42          |     100
800 x 500        |            4          |       5.0        |        10         |      21          |      42          |     120
1000 x 630      |            4          |       6.0        |        12         |      24          |      49          |     320
1000 x 1000    |            4          |       7.0        |        14         |      28          |      56          |     500
1200 x 800      |            4          |       7.0        |        14         |      28          |      56          |     475
1600 x 1000    |            5          |       8.0        |        16         |      33          |      66          |    1100
2000 x 1000    |            6          |       9.5        |        19         |      38          |      75          |    1650

TO MEET SPECIAL REQUIREMENTS WE OFFER :

1. Sizes larger than 2000 X 1000 mm. The thickness of the plate increases with size.
2. Granite Surface Plates with inserts with drilled and tapped inserts
3. Granite Straight Edges
4. Granite Master Squares
5. Granite Cubes
6. Master Cylinder made of hardened and ground steel

CNC Lathe Chucks: Troubleshooting common problems

Possible causes and solutions for common chuck problems such as:

1. Component slippage 
2. Excessive Vibration
3. Master Jaw not moving
4. Poor Accuracy
5. Base Jaw Breakage

Troubleshooting: Component slippage

1. Lack of lubrication on master jaws & moving parts -Lubricate master jaws through the grease nipple
2. Component Clamping at end of the stroke -Clamp the component at the middle of the Jaw stroke to avoid slippage.
3. Speed is excessively high -Reduce the speed level to obtain necessary chucking
4. Cutting forces too high -Calculate cutting force & check against chuck specifications
5. Insufficient chucking force -Check hydraulic pressure setting

Troubleshooting: Excessive Vibration

1. Run out on chuck body beyond permissible value-True the chuck body within permissible value as specified
2. Top jaws holding the job not symmetric with the rotation axis and therefore causes imbalance -Chuck with the special jaws and component to be dynamically balanced
3. Draw tube / Draw bar may be out of centre or out of alignment -Check and correct the alignment

Troubleshooting: Master Jaw not moving

1. Chuck with dirt from burrs & chips - Dismantle & do periodic maintenance.
2. Internal parts of Chuck broken/ jammed -Replace broken parts & do periodic maintenance.
3. Check cylinder for malfunction.

Troubleshooting: Poor Accuracy

1. Excessive height of top jaws -Reduce the height of top jaws as recommended by GMT
2. Guide ways of the internal parts of the chuck could be worn out -Recondition the chuck /Replace the chuck with new.
3. Too much chucking force - Reduce the chucking force to optimum level

Troubleshooting: Base Jaw Breakage

1. Component  Clamping at end of the stroke -Clamp the component at the middle of the Jaw stroke.
2. Excess Draw Bar pull (causing breakage of Master Jaw & Wedge) - Select suitable cylinder from ‘Selection Chart’ specified in GMT Instruction Manual and maintain the Draw Bar pull as specified.

Recommended torque for jaw screws

Recommended torque for jaw screws should be applied while tightening the top jaws with master jaws.

Use screws of reputed make only. (Eg. TVS, Unbrako). Length of the screw should be as per GMT standards. Write to esales@gmt.co.in for a free electronic copy of GMT PHNC Instruction Manual.

RECOMMENDED TORQUE

                          SCREW SIZE                                                      TIGHTENING TORQUE

                                        M8                                                                         39 Nm

                                       M10                                                                        77 Nm

                                       M12                                                                       110 Nm

                                       M16                                                                       220 Nm

                                      M 20                                                                       400 Nm


Important Instructions:
If you use screws that are  longer than recommended size, the top jaw will not clamp with master jaw, as the clamping screw will rest at the bottom face of master jaw T-Slot. This will result in master jaw breakage.If the soft-jaws are purchased from outside source, ensure that the jaws meet GMT standards.

Click here for further information and instructions on chuck maintenance. or visit our website www.gmt.co.in

Chuck Lubrication: What can go wrong if ignored?

What if you don’t lubricate as per schedule and don’t use the right lubricant?

 Improper lubrication results in;

• Loss of Gripping Force
• Component Slippage
• Clogging, restricting lubricant flow and Jaw Stroke
• Corrosion  

If the chuck is not lubricated regularly, the chuck body, wedge and the sliding parts will get corroded, resulting in high friction and loss of clamping force. The curve of the clamping force of a new lathe chuck, depending on the RPM, shows as a continuous curve. An improperly lubricated lathe chuck, shows a considerably reduced clamping force and shows as a dashed curve. This result in unsafe clamping of the work piece and the chances of falling out during machining is high. To avoid this, follow the lubrication and maintenance instruction provided in the chuck instruction manual. 

GMT recommends you to slide the master jaw through full stroke (clamping and de-clamping) to ensure uniform distribution of grease on sliding surfaces several times, without the component, after 500 jaw cycles. This helps to convey the lubricant to the sliding faces and maintain the clamping force longer.

For lubricating the lathe chucks, GMT recommends one of the following grease:

·         OKS  265            

·         ALTEMP  QNB50

·         MOLYKOTE  TP42

GMT chucks are equipped with several lubrication nipples for even application of grease. To ensure  even distribution of the grease inside the chuck, slide the master jaw through full stroke (clamping and de-clamping).

Remember, in extremely contaminated environment such as machining C.I work pieces a chuck can lose clamping force. In such cases, the complete chuck has to be disassembled, cleaned and lubricated. 

Chuck  with improper or no lubrication causes corrosion as shown in the photos below

                                                 CHUCK BODY                       BODY SLOT

                                                  WEDGE                                    BASE JAW

Write to us at esales@gmt.co.in for a free electronic copy of GMT Instruction Manual or visit our website www.gmt.co.in for more information