Tolerance in Measurement

You may have heard that a particular precision measuring tool has a certain tolerance number, for example, ±0.001mm like the Mitutoyo Micrometer 293-340 series have. You can find it as well on the other precision tools. What is it actually?

Tolerance can be defined as the total allowable amount by which a measurement may vary. It is the difference between the maximum and minimum limits.

As an example, a dimension may be given as 1.525″ ±0.002″ which means that the manufactured part may be 1.527″ or 1.523″, or anywhere in between these limit dimensions.

For manufactured parts, to be fit together properly in an assembly, their tolerances should be within an acceptable range.

Instead of so much advancement in technology, there is always some sort of uncertainty associated with a measurement depending upon the least count of the measuring instrument.

To manufacture very accurate parts, good machining instruments is required which increase both time and cost. Thus, a compromise must be made between cost and tolerance. If a dimension is specified, in millimeters, as 5 ±0.02, the part will be acceptable to use if the measurement lies between 4.98mm to 5.02mm but beyond this limit, parts would not be acceptable.

Tolerance Vs Allowance

Tolerance and allowance are two important engineering terms and they are often confused with each other. Tolerance is the allowable variation in the dimension of a part being manufactured while an allowance is the difference between the dimensions of two parts which are going to be mate together to form an assembly.

Allowance specifies the fit i.e. looseness and tightness of the two mating parts. Allowance can be positive or negative depending on whether the shaft size is larger than the hole size or lesser than the hole size respectively.

Tolerance is linked with the basic dimension of the part being manufactured limited by the least count (the smallest measurement that a device can measure). While allowance describes how two parts will fit together. For a good fit between two parts, a deliberate amount of allowance is always given.

Tolerance in Linear Measuring Tool

Linear measuring instruments can be defined as those instruments that measure length, thickness, height, etc. Some of the linearly measuring instruments are;

  • Ruler
  • Vernier caliper
  • Micrometer Screw Gauge

Etc.

Each of these instruments has a lower limit below which they cannot measure the dimension; this is known as the least count of the instrument. Formally, the least count can be defined as the smallest measurement that an instrument can offer.

Tolerance is linked with the least count of the instrument. Smaller the least count, lesser will be the tolerance and greater the accuracy. But to improve accuracy, better instruments have to be used hence the cost increases.

So, we have to make a compromise between cost and accuracy. From the above instruments, screw gauge is the most accurate, but it also has the highest cost amongst the above instruments.

Accurate instruments require better machining which increases the manufacturing cost. No measurement can be a hundred percent accurate, there is always some sort of uncertainty associated with it which depends upon the least count of the measuring instrument.

Example:

Vernier caliper has usually a least count of 0.01mm while the least count of screw gauge is 0.001mm. As the least count of screw gauge is smaller than the least count of the Vernier caliper hence screw gauge is more accurate, lesser the tolerance offered by it.