Errors in Micrometer Question and Answer – Engineering Metrology

1. Which of the following is the possible source of error in the micrometer?

  1. Lack of parallelism
  2. Lack of squareness
  3. Inaccurate zero position
  4. All of the above

Answer.4. All of the above

Explanation

Sources of errors in micrometers Some possible sources of errors that may result in incorrect functioning of the instrument are:

(i) The anvils may not be truly flat.

(ii) Lack of parallelism and squareness of anvils at some, or all, parts of the scale.

(iii) The setting of zero reading may be inaccurate.

(iv) Inaccurate readings following the zero position.

(v) Inaccurate readings showed by fractional divisions on the thimble.

The parallelism is checked by measuring the diameter of a standard accurate ball across at least three different points on the anvil faces.

The squareness of the anvils to the measuring axis is checked by using two standard balls whose diameters differ by an odd multiple of half a pitch which calls for turning the movable anvil at 180° with respect to a fixed one.

The flatness of the anvils is tested by the interference method using optical flats. The face must not show more than one complete interference band, ie, must be within 0.25 µm.

 

2. The degree of accuracy in the micrometer is governed by the

  1. Simplicity
  2. Linearity
  3. Processing error
  4. Compatibility

Answer.2. Linearity

Explanation:-

The accuracy of the micrometer will be governed primarily by the following two factors:

  1. Degree of Calibration
  2. Linearity

The degree of calibration of the spindle movement, which will be affected by the lead errors of the Screw; the effect is usually cumulative and increases with the length of the spindle travel. (Note: The aggregate effect of inaccuracies originating from screw lead errors can be reduced by “balanced calibration,” that is, by adjusting the thimble to produce an error-free reading in the middle of the total, or of the most frequently used section of the spindle traverse.)

The linearity of the spindle movement, requiring that any fractional rotation of the screw should result in a proportional advance of the measuring spindle; “drunken” thread, or stick-and-slip condition of the screw in the nut, will have an adverse effect. Deficient linearity will become particularly harmful when superimposed on major calibration errors.

 

3. Which of the following statement is correct about micrometer error

Repetitive precision of measurements with a screw micrometer does not depend on process error

Precision measurement with micrometer depends on Inherent accuracy

  1. Both statements are correct
  2. Both statements are incorrect
  3. Only Statement 1 is correct
  4. Only statement 2 is correct

Answer.4. Only statement 2  is correct

Explanation

Considering the wide use of screw micrometers, it is desirable to know the degree of confidence that the results of micrometer measurements deserve. The repetitive precision of measurements with a screw micrometer depends on two sets of factors: the inherent accuracy of the measuring instrument, and the combined effect of process errors.

The process errors of micrometer measurement can be caused by heat transfer while holding the instrument, reading errors, inadequate alignment or stability in the mutual positioning of object and measuring tool, wear, and many other circumstances. Various design improvements serve to reduce the incidence of these errors in micrometer measurements. Particular design features directed at increasing the dependability of the measuring process by reducing the effect of some of these potential errors.

The accuracy of the micrometer will be governed primarily by the following two factors:

  1. Degree of Calibration
  2. Linearity

The degree of calibration of the spindle movement, which will be affected by the lead errors of the Screw; the effect is usually cumulative and increases with the length of the spindle travel. (Note: The aggregate effect of inaccuracies originating from screw lead errors can be reduced by “balanced calibration,” that is, by adjusting the thimble to produce an error-free reading in the middle of the total, or of the most frequently used section of the spindle traverse.)

The linearity of the spindle movement, requiring that any fractional rotation of the screw should result in a proportional advance of the measuring spindle; “drunken” thread, or stick-and-slip condition of the screw in the nut, will have an adverse effect. Deficient linearity will become particularly harmful when superimposed on major calibration errors.

 

4. Which of the following option is true for the given statements?

Statement 1: Main sources of error in micrometer include scale misreading, thermal expansion

Statement 2:  Minor error sources in micrometer is graduation accuracy, reference edge straightness

  1. Both statements -1 and statement – 2 are true and statement – 2 is the correct explanation of statement – 1
  2. Both statements are true but a statement – 2 is not the correct explanation of statement – 1
  3. Statement -1 is true and statement – 2 is false
  4. Statement -1 is false and statement – 2 is true

Answer.2. Both statements are true but a statement – 2 is not the correct explanation of statement – 1

Explanation:-

Source of error in the micrometer

Main sources of error in micrometer include scale misreading (parallax effect), the excessive measuring force causing jaw tilt, thermal expansion caused by a temperature difference between the caliper and workpiece, and small-hole diameter error caused by inside jaw offset.

There are other minor error sources such as graduation accuracy, reference edge straightness, main scale flatness, and squareness of the jaws. These sources are allowed for within the specified accuracy of a new caliper and only cause significant error in case of wear or damage.

Care must be taken to ensure that measurement is performed with an appropriate and constant measuring force since a caliper has no constant-force device and that the user must be aware of the increased possibility of error due to measuring a workpiece using the tips of the jaws.

 

5. The reading error in the micrometer can be removed by

  1. Use of Satin chrome finish
  2. Use of large diameter thimble
  3. Distinct graduation line
  4. All of the above

Answer.4. All of the above

Explanation

Reading errors are substantially reduced by design features as follows:

(i) By using Satin chrome finish to eliminate glare

(ii) Distinct graduation lines applied on a beveled thimble surface to facilitate reading with a mini-mum of parallax error. Some models of precision micrometers have the graduated surface of the thimble and sleeve mutually flush.

(iii) A particular type of micrometer has window openings on the thimble, where the hundredths and thousandths values of the measured dimension appear in digits, and only the tenths and the ten-thousandths of inch values must be determined by reading the graduation lines.

(iv) Large diameter thimbles for direct reading of the ten-thousandths by graduation lines coinciding with a single reference mark, thus eliminating the vernier as a potential source of reading error.

 

6. If the measurement on micrometer is taken using the stand that rests on the bench to clamp then we are increasing micrometer_______

  1. Heat transfer stability
  2. Thermal expansion stability
  3. Alignment and holding Stability
  4. Reading stability

Answer.3. Alignment and Holding stability

Explanation:-

Alignment and holding stability can be improved for measurements of small parts by using a stand that rests on the bench to clamp the hand micrometer instead of holding it in the freehand. For the repetitive measurement of light parts, the use of bench micrometers can provide definitive advantages.

 

7. To reduce the wear and tear in a micrometer the anvil is made up of

  1. Carbide
  2. Carbon
  3. Steel
  4. Aluminum

Answer.1. Carbide

Explanation:-

Wear will most commonly occur on the measuring faces because of their direct contact with the workpiece.

Carbide-faced anvils and spindle tips greatly reduce the wear on these surfaces, thus maintaining over a prolonged period of use the original setting of the micrometer, as well as the parallelism of the contact surfaces.

After prolonged use of the micrometer, wear will occur in the threaded members that can affect the original setting and measuring accuracy of the micrometer. Resetting the thimble position to the original calibration and readjusting the clearance of the spindle movement by tightening the nut will usually improve the functioning of the instrument to a level equal or comparable to its original accuracy.

 

8. Heat transfer in the micrometer can be reduced by

  1. Using Iron frame
  2. Using copper frame
  3. Using plastic frame
  4. Any of the above

Answer.3. Using plastic frame

Explanation:-

Heat transfer can be reduced by employing plastic insulating grips on the frame. With few exceptions, the micrometer frames are made of steel forgings. a material with practically the same coefficient of thermal expansion as most of the parts to be measured. Because of the differential in the rate of thermal expansion, aluminum frames, although light in weight, are seldom used for micrometers.

 

9. Due to the wear and tear of the micrometer the tip of the screw does not move, this type of error in the micrometer is called as_____

  1. Zero error
  2. Backlash error
  3. Positive error
  4. Negative error

Answer.2. Backlash error

Explanation:-

Backlash Error:- Due to the wear and tear of the grooves of the screw, it is found that when the direction of rotation of the circular scale is reversed, the tip of the screw does not start moving in the opposite direction immediately and it remains stationary for a part of the rotation. This introduces an error known as a backlash error. To avoid this error the screw must be rotated in the same direction duing the measurement.

 

10. The type of systematical or fixed error in a micrometer is/are

  1. Calibration Error
  2. Human Error
  3. Technique error
  4. All of the above

Answer.4. All of the above

Explanation:-

Systematic error in micrometer occurs due to recurring malfunction of one or more elements.

The type of Systematical or fixed error in a micrometer is/are

  1. Calibration error
  2. Human error
  3. Technique error
  4. Experimental error

Calibration error:- It is caused by False elements, design, and construction errors.

Human error:- Human error is due to Bias, physical peculiarities, behavioral traits.

Technique error:- It occurs when we use the known method but in a situation for which it is not satisfactory.

Experimental error:- It is caused when Using an unknown method in a situation for which it is not satisfactory.

 

11. Zero error in micrometer means

  1. There is a negligible gap between the spindle and the anvil
  2. Zero on thimble and datum line on sleeve do not coincide when measuring faces are in contact
  3. The micrometer range is negative
  4. Zero marks on the thimble are not visible

Answer.2. Zero on thimble and datum line on sleeve do not coincide when measuring faces are in contact

Explanation:-

When the anvil and spindle of the micrometer screw gauge are made to touch each other, then the zero on the main scale should match with a zero on the circular scale. However due to wear and tear or manufacturing defect the two zeros usually do not coincide with each other, then the vernier is said to have zero error. There are two types of zero errors.

Zero error in micrometer occurs when zero on the thimble and datum line on sleeve do not coincide when measuring faces are in contact.

 

12. Which of the following is incorrect about precautions in using a micrometer?

  1. The final movement is given by ratchet
  2. Thimble is turned till the Measuring tip just touches the part to be measured
  3. Part to be measured is held in right hand and micrometer in left hand for good results
  4. Error in reading is maybe due to lack of flatness of anvil

Answer.3. Part to be measured is held in right hand and micrometer in left hand for good results

Explanation:-

The part to be measured must be in left hand and the micrometer in the right hand for better results. To hold the micrometer, hold the heat-resistant plate on the frame with the thumb and index finger on your left hand, and pinch the thimble between the thumb and index finger on your right hand.

 

13. What is the value of total error for grade 1 micrometer when the micrometer is tested at 20°C?

  1. 4+(L/100)µm
  2. 5+(L/100)µm
  3. 6+(L/100)µm
  4. 7+(L/100)µm

Answer.1. 4+(L/100)µm

Explanation:-

Whenever tested at 20°C, the total error should not exceed the following values

For ‘grade 1’ micrometer =  4+(L/100)µm 

For ‘grade 2’ micrometer = 10+(L/50)µm

where L = upper limit of the measuring range in mm.

The micrometer must be so adjusted that the cumulative error at the lower and upper limits of the measuring range does not exceed half the total error.

 

14. The type of zero error in a micrometer is/are

  1. Negative error
  2. Positive error
  3. Both 1 and 2
  4. None of the above

Answer.3. Both 1 and 2

Explanation:-

Depending on the position of the zero of the circular scale, the zero error can be of two types namely

  1. Positive zero error
  2. Negative zero error

Positive error:- When the zero of the circular scale is below the reference line of the main scale. This micrometer has a positive zero error.

To find out the positive zero error, note the division of the circular scale that coincides with the reference line. The zero error is obtained by multiplying the coinciding circular scale division by the least count of the micrometer.

Negative error:- In this case, the zero of the circular scale is above the reference line of the main scale. This micrometer has a negative zero error.

To calculate the negative zero error, note the division of the circular scale that coincides with the reference line. Subtract the coinciding division from the total number of divisions on the circular scale. To obtain the zero error multiply the difference by the least count and assign a negative sign to it.

 

15. What precautions should be taken before or while using a micrometer

  1. Micrometer caliper should never be used as a snap gauge
  2. Clean the micrometer before using
  3. Micrometer should never be stored with its measuring anvils closed
  4. All of the above

Answer.4. All of the above

Explanation:-

Precaution of micrometer

Maintenance of a micrometer in good condition and assurance of measuring accuracy are possible only if the user observes the rules of good practice. Some of these rules are:

1. A micrometer should never be stored with its measuring anvils closed, because flat surfaces wrung together for any length of time may corrode. To prevent this, leave a small gap between the anvils.

2. A micrometer should be oiled in only one place, the micrometer screw, and only with light oil. Using heavy oil, which can become gummy, makes the micrometer insensitive and may cover up an error in the screw. If a micrometer is stored for any length of time, or where it is likely to must, it should be covered with a light film of oil and wrapped in oiled paper.

3. A micrometer caliper may be traversed several hundred-thousandths by holding the frame and rolling the thimble along the hand or arm in several strokes. Holding the thimble and twirling the frame to open or close a “mike” is frowned upon by good mechanics, probably because it causes excessive wear of the screw.

4. A micrometer caliper should never be used as a snap gauge. The development of many types of special-purpose gages has limited the old practice of trying to make one gage or a few general gages do all of the work. Using a micrometer as a snap gage is not practicals it does not hold its dimension well enough even with the clamp ring, and there is the danger of springing the mechanism in gaging a slightly oversize part.

5. Accuracy of measurements made with a micrometer depends to some extent on the measuring pressure used. The “feel” necessary to produce the correct measuring pressure between the anvils is acquired by practice and comparing measurements with precision gage blocks or some other accurate standard. Whenever a machine operator or inspector has an opportunity, he should compare his “feel” with that of someone else to be sure that he is using the correct pressure. A gage block or accurate plug gage can be used to check any difference.

6. Before a micrometer is used, it is customary to wipe it off and to pull a piece of paper between the anvils in the manner

7. The micrometer screw should run freely with no play at any point in its travel. This adjustment is made with the nut, which causes the threaded sleeve to tighten on the screw.

8. A micrometer screw that binds or has loose points along its travel cannot be connected with the adjustable nut. It must be returned to the gage maker. This condition is caused by abuse or uneven wear.

9. The micrometer mechanism should be cleaned whenever it becomes gummy, contains an abrasive grit, or is to be adjusted. Any good cleaning agents, such as gasoline or benzine, are satisfactory. For most purposes, a non-inflammable solvent is equally satisfactory and is safer to handle.

10. The micrometer screw should be checked with a precision gage block in at least four places other than zero to verify its accuracy. The procedure consists simply of measuring a selected group of blocks ranging between zero and one inch.

11. On all the latest micrometers the thimble On can be slipped in relation to the screw. On the earlier models, the anvil was adjustable. In adjusting a micrometer to read correctly, the thimble is not set to zero when the anvils are in contact but is set at some dimension to distribute the error. For example, if a micrometer screw had an accumulating error of .0003 inch in the length of its travel and it was set correctly at Zero, it would be off .0003 inch at one inch. However, if the caliper were set correctly, in the middle of its travel it would be .0001s inch under at zero and .00015 over at one inch, which is a much better condition. It is customary to consider the size of the work measured with the micrometer and to set, if possible, the base somewhat near the mean. From this explanation, it is obvious that because the micrometer does not return exactly to zero when the anvils are in contact does not mean that it is not adjusted properly.

12. When the faces of the spindle and anvil become warm and are no longer flat and parallel to each other, the error should not exceed .0002 inch on a micrometer which is graduated to control measurements to a limit of .001 inch, and should not exceed .00005 inch on a micrometer which is graduated to control measurements to a limit of .0001 inch. Measuring the ball at several points over the surface of the anvils will show up any error in parallelism. Parallelism can be tested by means of two different-size balls mounted in an aluminum holder.

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