Reading Engineering Drawings and Tolerances

Reading a drawing to make a part

When a maker takes an engineering drawing to the workshop, the numbers on it matter as much as the shape. Each dimension shows the intended size of the part, called the nominal size. But no part can be made perfectly, so the drawing also gives a tolerance: the amount the size is allowed to vary and still be accepted. A tolerance sets an upper limit and a lower limit, and a finished part is only good if its measured size falls between them. This module covers reading dimensions and, above all, working with tolerances, so that a part comes out right first time.

Tolerance words

Learn these terms before reading any dimension. They are the words a tolerance question needs.

Match each dimension term to its meaning

Upper limit against lower limit

A tolerance is really two numbers. Working out both from the nominal size and the tolerance is the skill this topic tests.

What does a tolerance tell you?

A dimension on a drawing is marked with a tolerance. What does the tolerance tell the maker?

• How much the size is allowed to vary
• What colour the part should be
• Which material to use
• That the part must be exactly one size with no variation

Why tolerances matter

Tolerances exist because two things pull against each other. Parts must fit and work together, so their sizes cannot wander too far; but making a part to a very tight tolerance takes more time, more skill and more precise machines, so it costs more. A good drawing sets a tolerance just tight enough for the job and no tighter. When you answer, name the two limits, say a part is accepted only if it lies between them, and explain that a tighter tolerance means better fit but higher cost. Limits, acceptance, then the fit-against-cost trade-off: that is the full picture.

Pick the true tolerance facts

Select every true statement about tolerances.

• A tolerance gives the allowed variation in a size
• The upper limit is the largest acceptable size
• A part is accepted if it lies between the limits
• Every part can be made to the exact size with no variation
• A tighter tolerance is always cheaper to make

Order how to check a part

Put the steps for checking whether a finished part is acceptable in the correct order.

• Read the nominal size and tolerance from the drawing
• Work out the upper and lower limits
• Measure the finished part
• Compare the measurement with the limits
• Accept the part if it is within tolerance

Complete the tolerance facts

The intended size shown on a drawing is the _____ size. The allowed variation in a size is the _____. The largest acceptable size is the _____ limit. A part is accepted only if it lies between the two _____.

One shaft, one tolerance

Suppose a drawing shows a shaft with a nominal size of 50 millimetres and a tolerance of plus or minus 2 millimetres. Adding the tolerance gives an upper limit of 52 millimetres, and taking it off gives a lower limit of 48 millimetres. So any finished shaft between 48 and 52 millimetres is accepted. A shaft measured at 51 millimetres is within tolerance and passes. A shaft at 53 millimetres is too large and is rejected, and one at 47 millimetres is too small and is also rejected, even though both are close. Work out the two limits first, then simply check whether the measurement sits between them.

Tap the parts within tolerance

A part must be 50 millimetres with a tolerance of plus or minus 2 millimetres, so the limits are 48 and 52. Tap the TWO measured parts that are within tolerance.

• a part measuring 49 millimetres
• a part measuring 51 millimetres
• a part measuring 55 millimetres
• a part measuring 46 millimetres

Work out the upper limit

A part has a nominal size of 50 millimetres and a tolerance of plus or minus 2 millimetres. What is the upper limit, in millimetres? Add the tolerance to the nominal size.

Judge each finished part

Read each situation and choose the best answer.

• A drawing shows a hole of 20 millimetres with a tolerance of plus or minus 1 millimetre. What is the smallest acceptable size?
• Two parts must fit together every time on a production line. Why do they need tolerances?
• An engineer specifies a very tight tolerance. What is the likely effect on cost?

Build a tolerance rule

Choose the word for each gap to complete one clear rule about tolerances.

Explain reading drawings and tolerances

A friend thinks every part just has to be made to the exact size on the drawing. Using what you have learned, explain how tolerances work and why they are used.

• Explain what nominal size and tolerance mean
• Explain how to find the upper and lower limits
• Explain how to decide whether a part is within tolerance
• Explain why tolerances matter for fit and for cost