Understanding Tolerances in Engineering

Why in the hell doesn't this fit properly? All they needed is to make it the exact same size as the other one!

When designing or manufacturing something, different kinds of technical drawings are usually used. The shapes and sizes are determined by the drawing, even though there is a small detail which causes some issues:

You can never manufacture something in an exact size

It might sound odd, but you just can't. Ever tried to cut something to an exact length? Got it right? By what accuracy, you might have tools which have 1mm accuracy but.. what about 0.001mm?
That's really small and you might think it doesn't even matter. It may matter.

A tolerance is what we call the limits of acceptable difference in size, length, distance etc., I might have designed an object with a hole. The hole may be drawn to have a diameter of 5 mm, but with a tolerance that it can be 0.2 mm smaller or larger.
Now imagine the hole is designed for a rod, 5 mm of diameter. It might have similar size tolerance, 0.2 mm smaller or larger. Sounds good, right?

Still having fun?

Now let's imagine how the object is manufactured. The hole is within the tolerances, 4.87 mm of diameter. Within the tolerance.

However this time, the rod happens to be 5.15 mm of diameter.

It is within the tolerances.. but now we have difference in size. You can't fit a bigger rod into a smaller hole without causing some damage unless you use special methods of attaching the rod to hole, like heating up the area around the hole to cause it to expand and to be able to enter the rod.

However this will lead to the situation where the rod will be stuck in whatever position it was attached to the hole in.

_{Wikipedia Commons}

This is why we need to work on the tolerances

There are plenty of different tolerances and they have strong standards, at least ISO standards are commonly used. However you can find all of these in Google, so it's no point of adding all the tables in the post. _{The tables are usually something like this}

There are three different types of fits with tolerances. But I've been taught that it can be simplified into two: Either to make a big enough clearance that everything fits together easy and maybe give a possibility for the objects to move.

For example, you might have a hole to have a tolerance of +0.1 to +0.2mm and the rod has tolerance of -0.0 to -0.3mm. Thus always when both are within the tolerances, the hole will be larger than the rod. Yay!

The other way is that there is pressure. The tolerances are designed to give slight or great hold, meaning different kinds of removable plugs (which require some strength to remove) or stronger fits, which cause the materials to permanently stick together.

In some of these cases, thermal expansion is used to attach the items to each other. You heat up the other one and/or cool down the other one. When the temperature difference normalizes, the items will be attached to each other.

This is quite often seen when adding bearings to a axle.

This is just a very light touch in tolerances

There are tolerances in shape (no round object is 100% round etc.) or in other things, like surface roughness. Even in size tolerances there is much I have not touched, but I tried to keep this.. interesting?

Meh, good luck with that.

_{This and first picture from Pixabay!}