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Screw gauge or micrometer screw gauge is a measuring instrument used for precision measurement. A measuring device used extensively in the mechanical engineering field, micrometer screw gauge is used for measuring extremely small dimensions. A screw gauge can even measure dimensions smaller than those measured by a Vernier Caliper. Belonging to the family of calipers, micrometer screw gauges also consists of two different scales.
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Micrometer Screw gauge works on the simple principle of converting small distances into larger ones by measuring the rotation of the screw. This “screw" principle facilitates reading of smaller distances on a scale after amplifying them.
To simply it further, let’s take a normal screw with threads. While inserting a screw, the thread rotates for a particular number of times. Each rotation of this screw can be correlated to an equivalent axial movement, which is known as the lead or pitch of the screw.
If all the threads of the screw are made with a constant pitch, which is already known, the amount of axial movement generated can be easily traced on a scale. The micrometer screw gauge uses this principle to measure dimensions.
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Construction and Parts
A screw gauge consists of a “U" shaped metallic structure, which is attached to a hollow cylindrical tube on one end. The hollow tube has a uniformly threaded nut inside it. A long stud with a plane face is fitted into this nut. Exactly on the opposite side of this nut and on the other end of the U shaped frame, a smaller stud with a plane face is also attached. Faces of both the studs are exactly parallel to each other.
This U shaped metallic structure is known as the frame of the micrometer screw gauge. The smaller stud is known as the anvil and the longer one is known as the spindle. The anvil is the fixed part of the device, whereas the spindle moves as and when the head is moved. The frame carries both the anvil and barrel, and is also heavier than the rest of the parts. The object to be measured is held between the anvil and the spindle.
The Barrel or sleeve is connects the frame to the cylindrical tube. It is a non-movable part of the screw gauge and has a scale inscribed over it which is the main scale of the device. Moreover, it also carries the most important part of the micrometer- the screw.
The screw is the heart of the micrometer and is located inside the barrel. The screw converts small dimensions into measurable distance using a scale. The thimble or head is the end of the cylindrical tube and is turned to move and adjust the spindle. The thimble carries the vernier or secondary scale. There is one more part called the ratchet which is provided at the end of the tube. The ratchet is kind of limiting device which applies a pressure by slipping at a predetermined torque and thus prevents the spindle from moving further. Some screw gauges also consist of locking devices which holds the scales at a particular position for prevent any kind of error while taking readings.
Thus the main parts of a micrometer screw gauge are:
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A micrometer screw gauge also uses two scales – main and secondary scales. The secondary scale is provided on the thimble and is the measurement of the pitch of the screw. This means that the reading on the secondary scale measures the distance moved by the thimble per rotation. The scale on thimble is divided into 50 equal parts and measures hundredths of a millimeter. The thimble scale rotates over the spindle or the main scale. The main scale is a millimeter scale subdivided into equal parts with half a millimeter distance. When the object is to be measured, it is placed in between the anvil and the spindle. Readings from both the scales are taken into account for arriving at the final measurement.
Micrometer screw gauge is a delicate device and thus special care should be taken while handling it. Moreover, it is also important that the micrometer is well calibrated to prevent any kind of error in the final reading.