lab report:Screw Thread Measurement

TITLE:
Screw Thread Measurement.

OBJECTIVE:

1. To measure the Major Diameter, the Minor Diameter and the Effective Diameter of a Screw Plug Gauge using Floating Carriage Diameter Measuring Machine and Horizontal/Vertical Profile Projector.
2. To compare the reading between 2 measuring machine.
3. To compare the reading with the standard value.
4. To evaluate the reading. 

APPARATUS:

1. Floating Carriage Diameter Measuring Machine
2. Set of calibrated screw thread measuring wires (B8)
3. Set of Metric Cylindrical Standards (M24)
4. Vertical Profile Projector
5. Horizontal Profile Projector
6. Set of Core Prisms ( Steel Prisms)

THEORY:

This is a projection apparatus that can produce an enlarge projection shadow of an object. The projection methods of examination methods are well adapted to the examination of form tools, profile gauges, press tools, gear teeth, screw threads, etc. Sizes of the object may be checked by direct measurement on the enlarged shadow and subsequent division by the multiplication factor. The magnification factor is accurate and that the design of the apparatus permits maximum latitude in holding and adjusting the object.

• Major Diameter

The major diameter can be measured with a micrometer, caliper or steel rule. Major diameters are generally the first numbers found in thread designations. A 1/2-10 Acme thread for example, has a major diameter of 500 in. Care must be taken to measure the major diameter on a section of the screw thread that is not worn. A worn portion will measure smaller (or larger if burrs have been rolled up) than the original major diameter. Therefore, it is good practice to measure the major diameter over the least used section of the screw.

• Minor Diameter

The minor diameter can be determined by direct measurement on an optical comparator or by measuring the depth of the thread with a depth micrometer and subtracting twice the measured depth of thread from the major diameter. When using a comparator to measure the minor diameter, remember that the reflected image is reversed (except on modern, image correcting comparators). This means that the bottom of the shaft is shown at the top of the screen. Often oil from the shaft runs down and collects on the bottom of the thread grooves increasing the shadow image. If the oil is not removed, a false (oversized minor diameter) reading will result.

• Effective Diameter 

The pitch diameter is the diameter at which the thread tooth and the thread space are equal. To accurately measure the pitch diameter requires an optical comparator or thread wires. The optical comparator is the easiest to use as the measurement can be directly made and no mathematics are necessary. The disadvantage to the optical method is that the screw must be physically removed from the machine and taken to the comparator. Also, many small shops may not be equipped with a comparator. Measurement over thread wires is an attractive alternative to the comparator for measuring pitch diameter. These measurements can be made directly on the screw. Thread wire measurements are quite accurate, however, they require the use of mathematical formulas along with thread form and pitch information to translate the measurement results into the pitch diameter. The mathematical formula can be found in the Screw Thread Standards for Federal Services Handbook H28 or other engineering handbooks dealing with threads. Roton's application engineers can help you with a library of computer software which does all the complex calculations in seconds. Contact our application engineers for more details and on-line assistance with wire measurements and calculations.

PROCEDURE:
Floating Carriage Diameter Measuring Machine

• Major Diameter

1. The position of the fiducial indicator and the micrometer anvils so that the gauge to be measured is between or within the range of the micrometer
2. A suitable Standard Cylinder was placed between the centres. The suitable Standard Cylinder should have diameter within 1mm of the actual major diameter of workpiece. Time for temperature stabilisation and centralisation of the fiducial indicator was allowed and the reading of the micrometer drum “R1”.
3. The standard was removed and was replaced with the workpiece to be measured. The micrometer drum reading “R2”was been noted.
4. The Major Diameter Dmajor of the workpiece is been evaluated.

• Minor Diameter

1. The position of the fiducial indicator and the micrometer anvils so that the gauge to be measured is between or within the range of the micrometer
2. A suitable Standard Cylinder was placed between the centres. The selected prisms was been inserted between the standard and the anvils. The hanging prisms should be vertical. The reading on the micrometer drum “R3” was been noted.
3. The standard was removed and was replaced with the workpiece. The prisms was been inserted between the thread grooves and the anvils, and the micrometer reading “R4” was noted.
4. The Minor Diameter Dminor of the workpiece is noted.

• Effective Diameter

1. A suitable Standard Cylinder was placed between the centres. The suitable Standard Cylinder should have diameter within 1mm of the actual effective diameter of workpiece.
2. The wires selected were inserted in positions between the Standard Cylinder and micrometer anvils. The wires are selected accordance with N.P.L. recommendation. The reading on the micrometer drum “R3” was noted.
3. The standard cylinder was replaced with the Screw Plug Gauge. Note the micrometer reading “R2” with the wires inserted in the thread grooves.
4. The Effective Diameter Deff of the Screw Plug Gauge was evaluated

Profile Projector

A briefing session will be conducted prior to students carrying out the measurement on the specimen. Please take note of the procedures for setting up the specimen on the adjusting table. Adjust the magnification to ensure the shadow obtained is sharp and clear. Movements can be made through micrometer adjustment. Parameters to be measured on the specimen are:

• major diameter
• minor diameter
• effective diameter

SAMPLE CALCULATION:

• Major diameter

Dmajor = D1 + (R2 – R1)

           = 24 + ( 3.154 – 3.124 )

           = 24.030 mm

• Minor diameter

Dminor = D1 + (R4 – R3)

           = 24 + (8.092 – 12.524)

           = 19.568 mm

• Effective diameter

D3 = D1 + (R6 – R5)

      = 24 + (4.072 – 7.286)

      = 20.786 mm

P = 0.86602(3.0) – 1.820

    = 0.778 mm


Deff = D3 + P

       = 20.786 + 0.778

       = 21.564 mm

• Percentage error

     For Major Diameter of the Floating Carriage Diameter

Percentage error = 24.030 – 24mm x 100%

                                                24

                          = 0.125%

     For Major Diameter of the Profile Projector
Percentage error = 23.992mm – 24mm x 100%
                                                 24 

                          = 0.033%

DISCUSSION:
From this experiment, we have study about to measuring screw thread by using 2 types of machine which are known as floating carriage diameter measuring machine and the profile projector. By using floating carriage measuring machine, we have measured the internal diameter for the screw thread. The major diameter value is 24.030 mm, the minor diameter value is 19.568 mm and the effective diameter value is 20.786mm. The percentage error for major diameter value compare with the standard value is 0.125%.

For the second machine, profile projector that was used to measure the screw thread. In this experiment two types of profile projector vertical and horizontal profile projector. The major diameter value is 23.992 mm and for minor diameter value is 19.875 mm and the effective diameter value is 22.041 mm. The percentage error for major diameter value compare with the standard is 0.033%.

There are many possible errors that can occur during the experiment. The possible errors involved include the miss alignment of the workpiece where the workpiece may not be placed at the correct coordinate or axis. There is no a fix location to place the workpiece and scale to align workpiece. Other possible errors are the position of the student’s eyes during the time the reading is taken may contribute to the error itself. Miss position measured surface may also be the possible error for this experiment. Temperature in the laboratory may also affect the metal size due to thermal contraction. So, measurement may vary.

There are necessary precaution which must be implementing during the experiment is done:

1. The measured surface must first be clean from all contaminant such as oil, dust, moisture and many more. This is to ensure the accurateness of each reading. 
2. The workpiece must be placed at the correct position where the projector can show the image clearly so that we can take the reading correctly.
3. The workpiece must be clamped rigidly so that it is not moveable during the experiment done. Other that to ensure the accuracy of the reading, this measurement is also taken to avoid or minimize error.
4. The person who takes the reading must be in right position to avoid parallax error.

CONCLUSION:
From the experiment, we have learned how to use both Profile Projector and also Floating Carriage Diameter Measuring Machine to measure the profile and the dimensional of the workpiece ourselves. Both machines were successfully used to measure the Major Diameter, the Minor Diameter and the Effective Diameter of a Screw Plug Gauge. After measuring we were able to compare the readings between both measuring machines. The value we obtained was closed to the standard value. In conclusion, all the objectives were achieved successfully therefore the experiment was success.

RECOMMENDATION:
The measured surface must first be clean such as from oil, dust, and moisture. This is to ensure the reading is accurate. The workpiece must be placed at the correct position where the projector can show the image clearly so that we can take the reading correctly. The workpiece must be clamped rigidly so that it is not move during the experiment done. The person who takes the reading must be in right position to avoid parallax error.

REFERENCE:
1. http://www.engineersedge.com/screw_threads_chart.htm
2. http://www.threadcheck.com/the-three-wire-method-of-measuring-pitch-diameter/technicalinfo/
3. http://en.wikipedia.org/wiki/Screw_thread
4. http://www.mfg.mfu.edu/cyberman/quality/metrology/dimension/htm