1. Micrometer and Types of Micrometers
Introduction to micrometer and types of micrometers:
The micrometer is an improvement over the measurement of the vernier calliper scale discussed in the past article.
The accuracy of vernier calliper remains to be 0.02 mm, but most of the engineering precision work demands greater accuracy with sensitivity for which an instrument having both these should be used.
The most familiar precision measuring instrument in the workshop is micrometre.
Micrometer works on the principle of screw and nut. The longitudinal movement of the spindle during one rotation is equal to the pitch of the screw i.e., the distance moved by the nut along the screw is proportional to the number of revolutions made by the nut.
Therefore by controlling the number of revolution and fractions of a revolution made by the nut, the distance it moves along the screw can be accurately predicted.
To apply the above principle to a measuring device, it requires:
- Precision screw.
- A means of cunting the whole revolution of the screw.
- A means of measuring the extent of the partial revolutions.
Any micrometer will show all these principles included. The screw thread is rotated by the thimble which indicates the partial revolution, the whole revolutions being counted on the barrel of the instrument. The setup is shown as in Figure below.
2. Types Of Micrometers
The following are the four common types of micrometers and 3 special purpose types of micrometers.
- Outside Micrometer
- Inside Micrometer
- Micrometer Depth Gauge
- Bench Micrometer
- Special Purpose Micrometer
- Screw Thread Micrometer
- Vee-Anvil Micrometer
- Thickness Micrometer
2.1 External or Outside Micrometer:
In types of micrometers Outside micrometer:
Micrometer or side micrometer is used to measure the dimension of small components for greater accuracy. It provides direct reading and is made in various patterns to suit particular applications.
Below shown the general arrangement of outside micrometer and it’s various parts. Regardless of the type or size of an outside micrometer, they contain the basic parts like:
- Anvil and spindle
- Ratchet driver
- Thimble and barrel
- Adjusting Nut.
- The micrometer’s frame is U-shaped to permit measurements of the cylinder.
- If diameter equal to the measuring range of micrometer and the shiftiness of the should be such that a test load of 1 kg weight does not alter the distance them by more than 1.5 μm for range of 0 to 25 mm, 2μm for range 25 to 50 mm.etc.
2.1.2 Anvil and spindle:
- To allow the attachment of measuring wire support, certain anvil of micrometer screw gauge should be removed at least 3 mm from the frame.
- Measuring faces are hard depended to about 800 HV (62 HRC) and aged. It should be accurately ground and lapped with its measuring face flat and parallel to the measuring face of the spindle.
- The diameter of the anvil should be equal to the diameter of the spindle within 0.04 mm and the axis of both in exact alignment.
- A spindle is the movable measuring face.
- The spindle and screw are initially lubricated with a thin, light non-corrosive oil. in that condition, There should not be any backlash between the spindle screw and nut.the spindle should run freely and smoothly throughout the length of its travel.
- There is a diamond knurled spindle locknut provided so that the spindle retains its perfect alignment.
2.1.3 Ratchet Drive:
- The ratchet at the end of the thimble is used to assure an accurate measurement and to prevent pressure being applied to the micrometer.
- It ensures a constant measuring pressure.
- The torsional moment of this drive should be so regulated that the force exerted between the measuring faces is between 0.5 to 1 kg.
2.1.4 Thimble and Barrel:
- The thimble has 50 equal division around its circumference. Each division having a value of 0.01 mm.
- The sleeve is accurately division and clearly marked in 0.5 mm divisions. It is pearl chrome plated and adjustable for zero setting.
- All graduations lines on the barrel (sleeve) should clearly engrave and for easy reading, the surfaces of thimble and barrel should have a dull finish and the graduation lines should be blackened.
2.1.5 Adjusting Nut:
- Micrometers are provided with a adjusting nut to compensate for wear between the screw portion of the spindle and nut.
- These adjustments are carried out by suitable spanners and keys which are provided with micrometer for this purposes.
As we know that the screw thread is rotated by the thimble which indicates the one-sided revolution. and the whole revolutions being counted on the barrel of the instrument.
The screw has a lead of 0.5 mm and the thimble and barrel are graduated as shown in the figure below.
As the pitch of a single start screw of a standard metric micrometer is 0.5 mm and the barrel division are 0.5 mm apart, one revolution of thimble will move a distance of one barrel division.
As thimble has 50 division and one revolution of thimble equal 0.5 mm, then a movement of one thimble division is 0./50= 0.01 mm.
The micrometer reading equals:
- The largest visible ‘whole’ millimetre +
- The largest visible ‘half’ millimetre +
- The thimble division coincident with the datum line.
For the given figure, the reading is:
9 whole millimetre = 9.00
a half millimeter = 0.50
48 hundredths of mm (48 Χ 0.01) = 0.48 = 9.98 mm
Testing The Accuracy Of A Micrometer
Periodically the accuracy of a mircometer should be tested to assure that the work produced is the size required. Before micrometer is used to measure the size of the component it is necessary to zero the instrument.
To do this,
- Initially clean the measuring faces and then turn the thimble until two anvils are touching and the ratchet slips.
- At this point, a reading is taken and this should read zero i.e., the thimble should coincide with the zero reading of the barrel. and zero reading on the barrel and zero marking on the thimble coincide with the barrel datum marking.
- If the reading is not zero, the adjusting spanner can be used to allow the micrometer to be set to zero.
External or Outside Micrometer Measurement
External Micrometer Measurement
- Clean the contact surfaces of the part and of the micrometer.
- Open slightly larger than a part feature.
- Seat anvil squarely against a reference surface of the part.
- Using the ratchet, slowly close the micrometer until the ratchet clicks once.
- Record reading.
- Repeat entire procedure several times and average readings.
- Clean the contact surface of the part and of the micrometer.
- Open slightly larger than a part feature.
- Seat anvil squarely against the reference surface of the part.
- Rock back and forth across the diameter, closing micrometer by small steps.
- When the first contact is felt, rock sideways to find the position over the centre.
- Repeat steps 4 and 5 until the perpendicular position is found and spindle just contacts the measured point as it passes over the centre.
Feel in Micrometer:
- Size of the part. If the micrometer is very large it will be awkward and maybe heavy. The person using it must be more intent upon support than on feel.
- Closely related to the first is the positions of the measuring. Even a 25-millimetre micrometer may provide inadequate feel if use at arm’s length through a recess in a large machine.
- The shape of the part. It the feel is checked against gage blocks and then duplicated on a cylindrical part of the same size, the reading will very 0.02 mm or more.
- Surface finish affects the feel. The coarse finish will produce a more pronounced feel than fine finish.
Micrometer Dos and Dont’s:
- Do use it as a memory device to preserve a reading until repeated.
- Don’t use it to make the micrometer into the snap gage.
Douse it for every measurement between flat surfaces.
Don’t expect it to guarantee reliable measurement if:
- The micrometer is dirty.
- Micrometer is poorly lubricated.
- A micrometer poorly adjusted.
- The micrometer is closed too rapidly.
2.2 Inside Micrometer Calipers
The figure below shown an inside micrometer. This types of micrometers, they does not have U-shape frame and spindle. The measuring tips are constituted by the jaws with contact surfaces which are hardened and ground to a radius.
One jaw is held stationary at the end the second one moves by the movement of the thimble. A locknut is provided to check the movement of the movable jaw. These are used for inspecting of small internal dimensions. Its range is from 5 to 50 mm. It is not so widely used.
Parts of Inside Micrometer
Inside micrometer are used for the measurement of larger internal dimensions. It consists of four parts:
- measuring head or micrometer unit
- Extension Rods.
- Spacing collars.
The figure below shows the four components.
Good quality steel is used for the body of the measuring head and is its spindle, but terminal measuring faces are made of high-grade tool steel. The measuring face of the anvil is hardened to about 800 HV or 63.5 HRC.
The micrometer screw has a pitch of 0.5 mm and the threads of the screw and nut are truncated so as to continue contact to the flanks of the thread. The screw should travel smoothly throughout the length of travel. An adjusting nut is provided to arrange for the thimble to be sufficiently tight for the micrometer to retain its reading after being set.
There should not be any backlash between the spindle screw and nut. General means are provided for compensation of wear between the screw and nut.
The measuring head with its associated extension rods and collars should conform to the following until of error at 20° C, ± 0.005 for 25-150 range, ± 0.010 for 105-300 range, ± 0.015 for 300 – 45 range and ± 0.20 for 450 – 600 range. Irrespective of the progressive or periodic type of errors in the traverse of the micrometer screw should exceed 0.003 mm.
These are provided to enable any measurement to be made throughout the range specified for the set and spacing collars are being used whenever necessary. These are suitably hardened to 800 HV and measuring faces are tipped with tungsten carbide or any hard material and they are finished by lapping operation,
These extension rods may either incorporated their own measuring anvil or they may take the form of different pieces to which a separate common anvil piece may be fitted.
These are used for smaller or fine adjustments in the range of measurement. The ends of spacing collars are finished by lapping and are flat, mutually parallel and square to the axis. Non-sharp edges and at least one face of the spacing collar is provided with dirt clearance groove.
The micrometer set for the measuring range 25 to 50 mm is generally supplied with a suitable detachable handle so that micrometer head can be easily lowered into deep holds.
Test for Accuracy of an inside Micrometer
An accuracy of internal micrometer reading is verified in two-phase i.e., checking the accuracy of the traverse of the measuring head and checking of the accuracy of overall lengths when the measuring headset to zero is associated with the various extension rods in turn.
The accuracy of the traverse of the measuring head is determined by clamping it in the Vee block with its axis in line with a sensitive indicator and the feeler of the indicator touching the rounded contact face of measuring head.
The feeler is set initially such that it reads zero when the micrometer head is also reading zero and a slip gauge of convenient dimension will be introduced between two faces.
A micrometer head is then set at same reading and correspondingly the slip gauge size is reduced.
Any error in the reading is revealed by a corresponding departure of the indicator pointer from its initial position.
The accuracy of the overall length is determined by using a vertical comparator. The indicator of the comparator is set at a height corresponding to that of the overall length to be measured, from a flat base.
After this getting the internal micrometer is placed under it in the maximum position and the error is noted down.
2.3 Micrometer Depth Gauge
This types of micrometers are used for mesuring depth of holes.
Micrometer depth gauge is used for measuring the depth of holes, slots and recessed areas.
It has got one shoulder which acts as the reference surface and is held firmly and perpendicular to the centre line of the hole.
For the large range of measurements, extension rods can be used. The screw of the micrometer depth gauge has a range of 20mm or 25mm.
The length of the micrometer depth gauge caries from 0 to 225mm. The rod is inserted through the top of the micrometer. The rod is marked after every 10mm so that it could be clamped at any position.
2.4 Bench Micrometer
In this types of micrometers, the bench micrometer principle makes use of a magnifying technique i.e., a gap of 0.01mm between the anvils is equivalent to a division width of the thimble of about 1mm.
Thus the actual distance is magnified approximately by 100 times. The greater the diameter of the thimble the greater modification is possible.
Following procedure is adapted to measure the size of the component,
- A suitable standard (M) is chosen which will be near to the nominal size of the component.
- A reading (R1) is taken over the standard followed by reading (R2) over the component. The difference in the reading R1 and R2 will be the size difference between the standard and the component.
- The actual size (x) of the component will be x = M + (R2 – R1) which assumes that the standard is smaller than the component. If larger then the expression becomes x = M – (R1 – R2).
Advantages and Disadvantages of Bench Micrometer
- Large diameter thimble permits the greater number of divisions around the large circumference, thus promoting better accuracy.
- The fixed anvil is replaced by a fiducial indicator to ensure constant measuring pressure. This divide is more reliable than the rachet.
- Micrometer screw errors will have minimal effect since the screw is used over a very small large during measurement.
- Only one disadvantage is that it can only be used as a compare to and is extremely sensitive and embodies scientific principles to enable measured errors to be greatly magnified.
2.5 Special Purpose Micrometers
The basic principle of micrometer remains the same even for special purpose micrometers, but based on the application they are classified into,
- Screw Thread Micrometer.
- Vee-Anvil Micrometer.
- Thickness Micrometer.
2.5.1 Screw Thread Micrometer
In this types of micrometers It is similar to the ordinary micrometer with the difference that it is equipped with a special anvil and spindle.
The anvil has internal Vee which fits over the thread and is free rotate. Thus the Vee can accommodate itself to any rake range of thread.
The spindle has a ground conical shape. When the conical spindle is brought into contact with the Vee of the anvil, micrometer reads zero, depending upon the threads to be measured difference sets of anvils are provided.
Thread micrometer is used for the measurement of the pitch diameter and the accuracy very much dependent on the helix angle of the thread.
Screw thread micrometer as shown in fig and designed to measure the pitch diameter of screw threads to thousands accuracy for varying diameters of work and each normally covers a range of threads per mm.
2.5.2 Vee – Anvil Micrometer
As their implies, have a tapered spindle and a V-shaped carbide tipped anvil. They are designed for measuring odd-fluted taps, milling cutters and reamers, as well as checking out of roundness to tenths of thousands accuracy.
In this types of micrometers, the angle of Vee equals 60 degrees and the apex of the Vee coincides with an axis of the spindle.
The zero reading of micrometer starts from a point where the two sides of Vee meet.
It is interesting to see that as the angle of Eve is 60° the micrometer will measure a distance of 1.5d for a round piece of diameter’ It is simple to obtain from reading.
2.5.3 Thickness Micrometer
It is not convenient to use the ordinary micrometer for measuring the thickness of the tube (cylinder) or sleeve because of the concavity of the internal surface.
In this types of micrometers for measuring the thickness of cylinder walls meant for this purpose the anvil is provided with a spherical measuring surface of the frame is cut away on the outside to permit the anvil being introduced into tubes of diameter as small as 5.00mm in an alternative design shown in fig.
The anvil is made of cylindrical form, its axis being perpendicular to the axis of the spindle. Micrometer for measuring the thickness of tubing no less than 12 mm inside diameter.
3. Precautions In Using A Micrometer
The following precautions must be adopted to obtain the accurate readings.
- Micrometer should be cleaned of any dust and spindle should move freely.
- The part of the dimension to the checked is held in left and micrometer in right hand. The way for holding micrometer is to place the small finger and adjoining finger in the U shaped frame. A forefinger and thumb are placed near the thimble to rotate it and the middle finger supports the micrometer holding it firmly.
- Then the micrometer dimension is set larger than the size to the measured and the part is slid over the contact surfaces of micrometer gently. Then the thimble is turned till the measuring tip touches the part and the final movement is provided by ratchet so that uniform measuring pressure is applied.
- The micrometer is available in variations sizes and ranges and the corresponding micrometer should be chosen depending upon the dimension.
4. Care of the Micrometer
The following care should be taken when micrometer is being used.
- Never drop a micrometer, always place it down gently in a clean place.
- Never place tools or other materials on a micrometer.
- Don’t lay a micrometer in steel ship or grinding dust or handle with oily hands.
- Never attempt to use it on a moving work.
- Keep the micrometer clean and accurately adjusted.
5. Errors In Micrometers
Some possible sources of errors which result in the incorrect functioning of the instrument are listed below:
- Lack of flatness of the anvils.
- lack of parallelism of the anvil at particular or all parts of the scale.
- The inaccurate setting of zero reading.
- Inaccurate reading following the zero position.
- Inaccurate reading is shown by the fractional divisions on the thimble.
6. Advantages and Disadvantages of Micrometer
- More accurate than rules.
- Greater readability than rules or vernier.
- No parallax error.
- Small, portable and easy to handle.
- Relatively inexpensive.
- Retains accurately better than verniers.
- Has to wear adjusting facility.
- End measurement.
- Short measuring range
- Single purpose instrument
- Limited wear area of the anvil and spindle tip.
- End measurements only.
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