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Chapter: Mechanical : Metrology and Measurements : Thread Measurement

Thread Measurement

Threads are of prime importance, they are used as fasteners. It is a helical groove, used to transmit force and motion. In plain shaft, the hole assembly, the object of dimensional control is to ensure a certain consistency of fit.



Threads are of prime importance, they are used as fasteners. It is a helical groove, used to transmit force and motion. In plain shaft, the hole assembly, the object of dimensional control is to ensure a certain consistency of fit. The performance of screw threads during their assembly with nut depends upon a number of parameters such as the condition of the machine tool used for screw cutting, work material and tool.


·       Form measurement includes


·       Screw thread measurement


·       Gear measurement


·       Radius measurement


·       Surface Finish measurement


·       Straightness measurement


·       Flatness and roundness measurements


Screw Thread Measurement


Screw threads are used to transmit the power and motion, and also used to fasten two components with the help of nuts, bolts and studs. There is a large variety of screw threads varying in their form, by included angle, head angle, helix angle etc. The screw threads are mainly classified into 1) External thread 2) Internal thread.



Screw Thread Terminology


Fig 3.3 Screw Thread


· Pitch


It is the distance measured parallel to the screw threads axis between the corresponding points on two adjacent threads in the same axial plane. The basic pitch is equal to the lead divided by the number of thread starts.


· Minor diameter:


It is the diameter of an imaginary co-axial cylinder which touches the roots of external threads.


· Major diameter:


It is the diameter of an imaginary co-axial cylinder which touches the crests of an external thread and the root of an internal thread.


· Lead:


The axial distance advanced by the screw in one revolution is the lead.


· Pitch diameter:


It is the diameter at which the thread space and width are equal to half of the screw thread


· Helix angle:

It is the angle made by the helix of the thread at the pitch line with the axis. The angle is measured in an axial plane.


· Flank angle:


It is the angle between the flank and a line normal to the axis passing through the apex of the thread.


· Height of thread:


It is the distance measured radially between the major and minor diameters respectively


· Addendum:


Radial distance between the major and pitch cylinders for external thread. Radial distance between the minor and pitch cylinder for internal thread.


· Dedendum:


It is the radial distance between the pitch and minor cylinders for external thread. Also radial distance between the major and pitch cylinders for internal thread.


Error in Thread


The errors in screw thread may arise during the manufacturing or storage of threads. The errors either may cause in following six main elements in the thread.


1) Major diameter error


2) Minor diameter error


3) Effective diameter error


4) Pitch error


5) Flank angles error


6) Crest and root error


1) Major diameter error


It may cause reduction in the flank contact and interference with the matching threads.


2) Minor diameter error


It may cause interference, reduction of flank contact.


3) Effective diameter error


If the effective diameter is small the threads will be thin on the external screw and thick on an internal screw.


4) Pitch errors


If error in pitch, the total length of thread engaged will be either too high or too small.


The various pitch errors may classified into


1. Progressive error


2. Periodic error


3. Drunken error


4. Irregular error


1) Progressive error


The pitch of the thread is uniform but is longer or shorter its nominal value and this is called progressive.


Causes of progressive error:


1. Incorrect linear and angular velocity ratio.


2. In correct gear train and lead screw.


3. Saddle fault.


4. Variation in length due to hardening.


2) Periodic error


These are repeats itself at regular intervals along the thread


Causes of periodic error:

1.     Un uniform tool work velocity ratio.


2. Teeth error in gears.


3. Lead screw error.


4. Eccentric mounting of the gears.


3)Drunken error

Drunken errors are repeated once per turn of the thread in a drunken thread. In Drunken thread the pitch measured parallel to the thread axis. If the thread is not cut to the  true helix the drunken thread error will form.

Fig 3.5 Drunken Error


4) Irregular errors


It is vary irregular manner along the length of the thread.


Irregular error causes:


1. Machine fault.


2. Non-uniformity in the material.


3. Cutting action is not correct.


4. Machining disturbances.



Effect of pitch errors


·        Increase the effective diameter of the bolt and decreases the diameter of nut.


·        The functional diameter of the nut will be less.


·        Reduce the clearance.


·        Increase the interference between mating threads.


Measurement of various elements of Thread


To find out the accuracy of a screw thread it will be necessary to measure the following:


1. Major diameter.


2. Minor diameter.


3. Effective or Pitch diameter.


4. Pitch


5. Thread angle and form


1. Measurement of major diameter:


The instruments which are used to find the major diameter are by


·        Ordinary micrometer


·        Bench micrometer.


·        Ordinary micrometer


The ordinary micrometer is quite suitable for measuring the external major diameter. It is first adjusted for appropriate cylindrical size (S) having the same diameter


(approximately).Thisge processsetting’is. Afterknown takias‘ micrometer is set on the major diameter of



· Bench micrometer


For getting the greater accuracy the bench micrometer is used for measuring the major diameter. In this process the variation in measuring Pressure, pitch errors are being neglected. The fiducial indicator is used to ensure all the measurements are made at same pressure. The instrument has a micrometer head with a vernier scale to read the accuracy of 0.002mm. Calibrated setting cylinder having the same diameter as the major diameter of the thread to be measured is used as setting standard. After setting the standard, the setting cylinder is held between the anvils and the reading is taken. Then the cylinder is replaced by the threaded work piece and the new reading is taken.

Fig 3.6 Bench Micrometer

· Measurement of the major diameter of an Internal thread


The Inter thread major diameter is usually measured by thread comparator fitted with ball-ended styli. First the Instrument is set for a cylindrical reference having the root of spring under pressure. For that the new reading is taken.



2. Measurement of Minor diameter


The minor diameter is measured by a comparative method by using floating carriage diameter measuring machine and small V pieces which make contact with the root of the thread. These V pieces are made in several sizes, having suitable radii at the edges. V pieces are made of hardened steel. The floating carriage diameter-measuring machine is a bench micrometer mounted on a carriage.



Fig 3.7 Measurement of Minor diameter


· Measurement process


The threaded work piece is mounted between the centers of the instrument and the V pieces are placed on each side of the work piece and then the reading is noted. After taking this reading the work piece is then replaced by a standard reference cylindrical setting gauge.



·        Measurement of Minor diameter of Internal threads


The Minor diameter of Internal threads are measured by


1. Using taper parallels


2. Using Rollers.


· Using taper parallels


For diameters less than 200mm the use of Taper parallels and micrometer is very common. The taper parallels are pairs of wedges having reduced and parallel outer edges. The diameter across their outer edges can be changed by sliding them over each other.


· Using rollers


For more than 20mm diameter this method is used. Precision rollers are inserted inside the thread and proper slip gauge is inserted between the rollers. The minor diameter is then the length of slip gauges plus twice the diameter of roller.


3. Measurement of effective diameter


Effective diameter measurement is carried out by following methods.


1. One wire,


2. Two wires, or


3. Three wires method.


4. Micrometer method.



a) One wire method


The only one wire is used in this method. The wire is placed between two threads at one side and on the other side the anvil of the measuring micrometer contacts the crests. First the micrometer reading dl is noted on a standard gauge whose dimension is approximately same to be obtained by this method


b) Two wire method

Two-wire method of measuring the effective diameter of a screw thread is given below. In this method wires of suitable size are placed between the standard and the micrometer anvils. First the micrometer reading is taken and let it be R. Then the standard is replaced by the screw thread to be measured and the new reading is taken.



c) Three-Wire method


The three-wire method is the accurate method. In this method three wires of equal and precise diameter are placed in the groves at opposite sides of the screw. In this one wire on one side and two on the other side are used. The wires either may held in hand or hung from a stand. This method ensures the alignment of micrometer anvil faces parallel to the thread axis.




Best wire diameter is that may contact with the flanks of the thread on the pitch line. The figure shows the wire makes contact with the flanks of the thread on the pitch.


Hence best wire diameter,


4. Pitch measurement


The most commonly used methods for measuring the pitch are


1. Pitch measuring machine


2.                 Tool   maker’s   microscope


3. Screw pitch gauge


·  Pitch measuring machine

The principle of the method of measurement is to move the stylus along the screen parallel to the axis from one space to the next. The pitch-measuring machine provides a relatively simple and accurate method of measuring the pitch. Initially the micrometer reading is near the zero on the scale, the indicator is moved along to bring the stylus, next the indicator adjusted radially until the stylus engages between the thread flank and the pointer ‘K’ is opposite Measuring Machine line in its index mark a small movement is necessary in the micrometer and then the reading is taken next. The stylus is moved along into the next space by rotation of the micrometer and the second reading is taken. The difference between these two- measured readings is known as the pitch of the thread.


• Tool makers microscope




Worktable is placed on the base of the base of the instrument. The optical head is mounted on a vertical column it can be moved up and down. Work piece is mounted on a glass plate. A light source provides horizontal beam of light which is reflected from a mirror by 900 upwards towards the table. Image of the outline contour of the work piece passes

through the objective of the optical head. The image is projected by a system of three prisms to a ground glass screen. The measurements are made by means of cross lines engraved on the ground glass screen. The screen can be rotated through 360°. Different types of graduated screens and eyepieces are used.


·        Applications


Linear measurements.


o Measurement of pitch of the screw. o Measurement of pitch diameter.


o Measurement of thread angle. o Comparing thread forms.


Centre to center distance measurement.


Thread form and flank angle measurement



· Thread form and flank angle measurement


The optical projections are used to check the thread form and angles in the thread. The projectors equipped with work holding fixtures, lamp, and lenses. The light rays from the lens are directed into the cabinet and prisons and mirrors. The enlarged image of thread is drawn. The ideal and actual forms are compared for the measurement.


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