How to Read Vernier Protractor

Vernier protractor or vernier bevel protractor is an important tool in angle measurement. Not only for measurement but also for layout, it can be a decisive tool for accuracy and precision in which the resolution of the vernier protractor reaches up to 5 minutes.

The same as the analog vernier caliper and vernier height gauge, a vernier protractor employs a vernier scale to magnify the main scale’s reading so that it provides that great resolution.

If you are going to measure the angle of a working edge, in which two surfaces are available for the blade and stock to stick onto, a vernier protractor comes as the suitable measuring tool for that. In contrast, if you are going to measure the angle of slope or surface level, you need an inclinometer.

This protractor model solves the limitation of a regular semicircle protractor by having a movable arm that measures 360 degrees. Aside from that, the measurement can be taken although the blade rotates clockwise or counterclockwise. And it comes with some locking mechanisms.

Reading any vernier model can be tricky. But, it’s basically the same with reading the vernier caliper. For someone who wants to learn it, this page will take them through the steps of how to read the vernier scale. At the end of the step, they will add the vernier scale reading and the main scale reading.

Steps of Taking the Vernier Protractor Reading

  1. The side surface of the object to measure should be cleaned properly.
  2. Don’t forget to clean the vernier protractor as well if there is debris or water or etc.
  3. Avoid using dirty hands. It may cause degradation to the instrument as it is made of metals. Better off using a pair of gloves.
  4. Attach the surface edges of the object to measure on the stock and the blade of the protractor.
  5. Read the main scale.
    1. Measurement in a clockwise direction (vernier scale travels in clockwise direction): Observe where the vernier scale’s 0 marking stops. The main scale reading is the number on the left just before that 0 marking.
    2. Measurement in a counterclockwise direction (vernier scale travels in clockwise direction): The main scale reading is the number on the right just before that 0 marking.
  6. Read the vernier scale.
    • Measurement in a clockwise direction: Observe every graduation (both main scale and vernier scale) on the right side of the “0” marking of the vernier scale. Keep observing until you notice two lines are exactly coinciding. Note how many divisions it is from the zero marking to the two coinciding lines.
    • Measurement in a counterclockwise direction: Observe every graduation (both main scale and vernier scale) on the left side of the “0” marking of the vernier scale. Keep observing until you notice two lines are exactly coinciding. Note how many divisions it is from the zero marking to the two coinciding lines.
    • Multiply the divisions you obtain on the vernier scale with 5 minutes.
  7. Add both main scale and vernier scale reading. The value hence obtained is the reading of the vernier bevel protractor.

Sample Calculation

The main scale readings are graduated in degrees of arc.

On the vernier scale, there are 24 divisions in total with 12 divisions on the right side and 12 divisions on the left side of the center zero. These divisions are marked from 0 to 60 minutes of the arc; this magnifies that each division is equal to 5 min of an arc. It is also the reason for multiplication by 5 to get the reading of the vernier scale; before adding it to get the final reading.

The above-mentioned 12 divisions are equally spaced in the span of 23° of the main scale. It also means that each division of the vernier scale would be equal to 1/12th of 23°.

Now, for the sample calculation

  1. The reading on the main scale as per the picture is in front of ‘0’ of Vernier scale i.e. 51° as per the picture.
  2. The reading corresponding to 45 on the Vernier scale is directly coincident with one of the main scale reading. It should be kept in mind that this reading is in minutes and according to conversion in degrees is required before summation.

Now, summing both the readings we get:

Total reading = 51° + 45’ = 51° 45’

If you want the final result in degree, convert the minute to a degree unit, then add them.

Total reading in degree = 51° + ( 45/60 × 1° ) = 51.75°

The Least Count of Vernier Protractor

Like micrometer, the least count of vernier Protractor is evaluated as follows:

The 12 divisions of vernier scale = 23 divisions of the main scale

So,

1 Division of vernier scale = 23° / 12

1 Division of vernier scale = 1.916°

Since we know that

1° = 60 minutes

Hence,

1 Division of vernier scale = 1.916° × 60 minutes = 115 minutes

Now, the least count of Vernier protractor is given by the following formula

Least Count (LC) = Value of two main scale divisions – Value of Vernier scale division

Hence,

Least Count (LC) = 2° – 1.916° = .084°

Or, converting in minutes (multiplying with 60, as 1 degree = 60 minutes)

Least Count (LC) = 120 minutes – 114.96 minutes

Least Count (LC) = 5 minutes

Important Tips for Accurate Readings

  • Care should always be exercised to store the Vernier protractor in safe and cool storage with a layer of oil to avoid unnecessary damage to the surface/body from corrosion.
  • It should be kept in mind that any fluctuation in temperature during the measurement may result in minor variations in the reading of the Vernier protractor.

Last Thought

Those are the steps how to read vernier protractor. You need to master how to read its vernier scale. After finding the number, you have to add the vernier scale and the main scale reading to find the final result. If you require accuracy of about 5 minutes, there is no choice except vernier bevel protractor. The best vernier bevel protractor that we recommend is from Mitutoyo which you read here.