The overall goal when building an object is for the result of the work to correspond to the designer’s design.
Achieving perfect correspondence between material reality and the ideal geometric model is conceptually and physically impossible, and there will always exist in reality a deviation, a difference, with the model.
This deviation may be larger or smaller depending on the degree of accuracy and precision implemented in the fabrication process.
It is the designer, having an overview, who indicates in the drawing the maximum and minimum values tolerated for each dimensional characteristic of the object, these values, these ranges, are defined precisely by the name of “tolerances.”
Generally, each object has dimensions for which wider tolerances are allowed and other dimensions where tighter tolerances are required.
For example, the arm of an excavator will have wide tolerances, in terms of millimeters, for the maximum dimensions and narrower tolerances, in terms of hundredths of a millimeter, for the pin-bush mating surfaces.

Typically, by means of cutting, assembling and welding operations, tolerances in the range of millimeters can be met, so for dimensional features where tighter tolerances are required, additional machining is required.
These machining operations are carried out with the aid of special machinery, capable of removing material from the workpiece with great precision by the use of special cutting tools, generically called, precisely, machine tools.
The machine tools commonly used in carpentry are: drill, lathe, milling machine, boring machine, or machines that are a combination of these.
The size, precision, and degree of automation and control of these machines is extremely variable depending on the type of machining to be done.
Typically, these machining operations are performed by specialized companies that, by working for multiple carpentry manufacturers, are able to amortize and optimize the considerable costs incurred in purchasing and operating the machines.
In fact, very few carpentry manufacturers, usually the larger and more structured ones, have an in-house machine tool department.