Machining is becoming increasingly precise and new innovative technologies are constantly appearing on the market. Machining tolerance and margin are two key terms that engineers use to define the desired level of machining accuracy.
To make the most of new improvements in the machinery and manufacturing industries, it is important to understand the difference between these two concepts and how they work. In this article we present a comparison between abatement and tolerance, explaining them separately and highlighting their differences with practical examples.
What is tolerance in technology?
Tolerance is a concept in engineering design that quantifies unplanned variations in the dimensions of machined parts. As is known, even the most precise CNC machine cannot always produce a part with absolutely precise dimensions. There is always some degree of error, as no machine is perfect in an absolute sense.
The goal of engineers and machinists is to use their skills to reduce these deviations to an acceptable range. This range is defined by the tolerance, which defines an upper and lower limit on how far the dimension can deviate from its true value.
Importance of tolerances in production
The concept of tolerance is extremely practical and universal in machining because it provides a reference range for dimensional deviations that machinists can target.
For example, a professional machinist may choose to use a normal machine instead of a precision CNC machine with an accuracy of +/- 0.001 mm to produce a part with tolerance requirements of only +/- 1 mm. This would save costs, resources and time.
Additionally, tolerance calculation also provides the quality department with guidance on how carefully the dimensions of a part should be checked after machining. In quality control, engineers also regularly use advanced tolerance calculation techniques, such as tolerance stacking, to convey design intent.
Importantly, it is also a necessary prerequisite for the concept of “interchangeability”. In shops with large orders, each part must be similar to a certain tolerance level. Tolerance levels establish the basic standard for a part to be identical to its other counterparts.
Different tolerance strategies in technical drawings
Now let's delve deeper into the topic and discuss different tolerance strategies in technical drawings using examples.
Direct limits
With direct tolerance limiting, only the minimum and maximum allowable dimensions of a dimension are specified. This is one of the most popular tolerance methods in technical drawings and is preferred for its clarity and space saving. However, the base value of the dimension remains unknown, which can be annoying when necessary.
In the example, the figure showing the direct limit tolerances shows that this dimension can vary between 26.6 and 26.9.
Positive and negative tolerances
Positive and negative tolerances are another common tolerance method in technical drawings. They define tolerance as positive and negative deviation (±) from its base value. It's clear and informative, but it takes up a little more space than straight borders, which can sometimes make the drawing a little cluttered.
Within this system, tolerance can be unilateral or bilateral. A one-sided tolerance defines a deviation on only one side of the basic size. This means it is only positive or only negative. Bilateral tolerance varies on both sides of the base size and has positive and negative deviations.
What is an engineering subsidy?
Tolerance is the planned variation in the dimensions of mating parts to achieve the correct connection. It is a necessary design requirement that allows functional technical adjustments in mechanical assemblies.
The perfect example to explain is an axle and hub assembly. To achieve a technical fit between an axle and a hub, the designer defines a specific gap between the two parts. Positive clearance means there is a small gap between parts, while negative clearance represents interference between the mating surfaces.
Tolerances and technical adjustments
The clearance depends on the type of adjustment desired. There are three types of fits for hub-axle systems: press fit, loose fit and transition fit.
As shown in the figure above, the gap is mathematically the difference between the smallest hole and the largest wave. For clearance adjustment mapping, the shaft cannot be larger than 2.999 and the hole cannot be smaller than 3.000. Therefore, the clearance is +0.001, which means it is a loose fit.
It is important to highlight that both the axle and the hub have their tolerances, as shown in green. These tolerance bands are set for both parts and work hand in hand to maintain the correct clearance between both parts after assembly.
Importance of tolerance in production
As we know, tolerances for individual parts are chosen based on practical manufacturing limitations. For this reason, machinists devote special attention and resources to precisely fitted parts, as this also implies a precise tolerance range.
Therefore, tolerances and tolerances work hand in hand in mechanical assemblies. The confusion between the two terms mainly comes from this.
As mentioned earlier, tolerance is a design feature that defines the planned deviation in the dimensions of corresponding components. If both mating parts are not machined to the defined tolerance, the technical adjustment will not work correctly.
Permission vs. Tolerance: What are the differences?
Now let's return to the debate about room for maneuver versus tolerance. We can take a closer look at how latitude and tolerance differ and why they are so often confused with one another. The table below summarizes their differences.
| Description | Tolerance |
| The planned deviation in the dimensions of the corresponding parts. | Unplanned variation in a dimension due to errors and imperfections inherent in machining. |
| Intended for surface mating only – also includes special symbols and terminology on technical drawings to indicate the type of fit. | Specified for each dimension of a part in an engineering drawing. |
| Difference between the maximum shaft radius and the minimum hole radius (shaft-hub connections). | Tolerance is the difference between the maximum and minimum allowable dimensions of a part. |
| It can be positive (backlash) or negative (interference). | Indicate positive and negative deviations from the base measurement in both directions. |
Concluding
It is common and understandable to confuse tolerance and tolerance – they represent similar themes in engineering projects. However, it is important for designers and machining experts to understand their difference. Tolerance limits machining inaccuracy, while tolerance provides a reference point that can be used to achieve a specific technical fit. They are linked but still different.
