Workholding definitions

accuracy

The accuracy of a chuck/expanding mandrel measured in mm TIR is comprised of two components:

• The distance between the centerline of the machine spindle and the centerline of the workholding device.
• A random variation about the centerline of the workholding device caused by conditions built into the chuck such as internal friction and sliding fits.

actuator
A device that clamps the chuck or expanding mandrel through the aid of pneumatic or hydraulic cylinder.

chuck
A chuck is a specialized type workholding device usually used to hold a rotating workpiece (such as the bar or blank in the headstock spindle of a lathe)

clamping cylinder
See actuator.

collet
A collet is a holding device—specifically, a subtype of chuck—that forms a collar around the object to be held and exerts a strong clamping force on the object when it is tightened, usually by means of a tapered outer collar to grip the work piece. 

collet chuck
A device that uses a collet for workholding. 

compensating chuck
A chuck in which the jaws individually locate their gripping diameter without altering the position of the workpiece.

 

crank Shaft Machining Chuck

A special power chuck designed for crank shaft Eccentric Turning and milling operations. This chuck can be tailor made to suit machine. Crankshaft is held between centres of RH and LH chuck. Aligning jaw of the chuck facilitates job clamping. Chucks are actuated by hydraulic cylinders. 

dead length
The action of gripping a workpiece without changing its axial position and uses the face of the spindle as its locating surface for the inner collet. 

diaphragm chuck
A chuck that uses the deformation of a flexible material to grip a workpiece and is ideal for thin walled components. Diaphragm chucks are primarily used for light accurate turning, fine boring, facing and grinding operations.

 

expanding mandrel
A workholding device which is used for internal diameter clamping of component to machine in a single setup

 

face driver
Face Driver allows roughing and finishing operations completed in one clamping and you will able to machine the entire length of the workpiece in one operation.

 

first operation chuck
A workholding device designed to grip on unmachined castings, forgings or bar stock.

 

fixture
A work-holding or support device used to securely locate or support material, workpiece or a tool during machining operations ensuring that all parts produced using the fixture will maintain conformity and interchangeability.  Usually custom designed to hold a specific workpiece. 

indicating band
A diameter on the chuck body designated for measuring runout of a chuck.

 

mandrel
see expanding mandrel

 

master jaw
The moving part in a chuck to which the top jaw is mounted. 

pull-back chuck
This Chuck is ideal for the finishing operation, it simultaneously grips and pulls the piece part back against a workstop. The Radial pull back function of the chuck ensures a positive resting of the component after clamping 

repeatability
The repeated accuracy of chucked workpieces, especially the workpieces with a relatively higher length/diameter ratio used for production of duplicate parts.

retracting jaw chuck
A chuck which grips shafts for turning the complete length by retracting jaws into the chuck face to allow the area previously gripped to be machined. Either a face driver or expanding arbor provides the torque when machining the area previously gripped by chuck jaws.

 

rotational part
A piece part symmetrical about a central axis.

 

second operation chuck
A workholding device designed to grip on previously machined surfaces.

self-centering chucks
A self-centering chuck, also known as a scroll chuck in which all jaws move to or away from the workpiece and maintain on common center.

 

self-contained
A Chuck with an integral or built-in pneumatic or hydraulic power cylinder.

 

soft jaw
Top jaw blank that can be machined to specific dimensions Soft jaws are mounted to the master jaws.

 

top jaw
The component that clamps the workpiece and is mounted on the master jaw.

Accuracy, applications and features of GMT Planetary Lapping Machines

Accuracy:		Planetary Lapping Machine Lap plate with work carriers Optical flat
	One light band to two light band over 75Ø component. (i.e. 0.3µ to 0.6µ over 75mmØ).
	Surface finish : 0.2µ Ra.
	Parallelism : 0.002mm over 75mmØ.
Applications :
These machines can be very sucessfully used in:
	Electronic industries for lapping silicon wafers, germanium wafers, ferrite and piezo electric ceramics.
	Pump industries for lapping mechanical seals made of tungsten carbide and ceramics.
	Air compressor industries for lapping compressor valve plates.
	Automotive industries for lapping thrust washers and distance pieces used in power steering.
Features :
	The machine is constructed on a rigid welded hollow section steel frame, which houses a AC motor offered with variable frequency drive and a worm and worm wheel reduction gear unit.
	The central gear, internal ring gear and work carriers are driven a AC motor.
	The drive from the AC motor is taken through a worm and worm wheel reduction unit and then goes centrally through the main spindle to the central gear to the internal ring gear, through the work carriers.
	The drive from reduction gear unit is through a unidirectional clutch arrangement. This arrangement ensures a smooth motion when the machine is started. This feature also eliminates breakage of fragile components like silicon and germanium wafers while starting and stopping the machine.
	Both the lap plates are stationary. The differential motion given to the work carrier results in a random figure of '8' motion. This ensures that the component comes in contact over the entire area of the lap plate in so far as is feasible in line with the geometry of the component.
	A special camel back straight edge permits rapid and accurate assessment of the flatness of the lap plates.
	A set of special cast iron lapping gears are supplied with the machine for relapping the top and bottom lap plates.
	An independent slurry pump feeds the mixture of abrasive and oil to an abrasive feed assembly.It is then distributed through a spider to the working face by means of strategically located holes. A separate stirrer motor is attached with the pump to continuously stir the abrasive with oil.
	The main spindle bearings are sealed against ingress of the abrasive slurry. The used slurry is carried away by a suitable arrangement and is collected in a removable tank.

BEHAVIOUR OF GRANITE SURFACE PLATES

BEHAVIOUR OF GRANITE
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.

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.