Engineering Tolerances: Types, Fundamental Rules, and Fits

tolerance in engineering

In the mechanical engineering and instrument manufacturing industry, the interchangeability of parts and components refers to that one batch of parts or components of the same specification can be installed on the machine without any selection or additional repair (such as benchwork repair) to meet the specified performance requirements.

In order to satisfy the interchangeability of parts in mechanical manufacturing, the dimension of production parts should be within the desired tolerance limits. This must stipulate a unified standard for the form, size, precision, and performance of a kind of part. Similar products also need to size reasonable grading, in order to reduce the product series, this is the product standardization. Thus, the concept of specified engineering tolerances and fit came into being.

What is Engineering tolerance?

Suppose a 100 mm metal rod is machined. Even if all the bars are intended to be machined into the same shape, not all the bar’s manufacturing accuracy can be machined to exactly 100.00mm because of the size and orientation of the bars. Although the design and fabrication sites have been working to reduce such deviations, they still cannot be controlled to zero.

engineering tolerance

This size and shape deviation basically fluctuates up and down with the target value as the center. Therefore, the upper permissible value and the lower permissible value of the upper permissible value relative to the target size are determined based on the use of the metal bar. The difference between these two values (allowable range) is known as the “tolerance”. 

In short, tolerance is the deviation of parts in the process of processing, measuring equipment precision because of the impact of the deviation must exist. It is the amount of variation allowed for any given dimension to achieve the proper function. Part dimensions within the desired tolerances range are qualified. Engineering tolerances include dimension tolerance, shape tolerance, and position tolerance.

Dimension tolerance

Dimension tolerance is the amount of variation allowed in a size. It’s the basics of engineering tolerance. The maximum allowable value is called the maximum dimension. And the minimum value is called the minimum dimension.

Tolerance is the absolute value of the algebraic difference between the maximum upper limit size and the minimum upper limit size, as well as the absolute value of the algebraic difference between the upper deviation and the lower deviation.

Tolerance is a numeric value without a plus or minus sign and cannot be zero. Within the case of constant basic size, the smaller the dimension tolerance, the higher the dimension accuracy. Specified tolerance indicates the requirement of manufacturing precision and reflects the degree of difficulty of machining.

Shape tolerance

(1)  Straightness

Straightness is the condition that the actual shape of a straight element on a part maintains an ideal straight line. It is also known as the degree of straight. The straightness tolerance is the maximum variation allowed by the actual line against the ideal line. That is, in the drawing given to limit the actual line processing tolerance allowed by the tolerance range of variation.

(2) Flatness

GD&T

Flatness is the representation of the actual shape of the plane elements of the part to maintain the ideal plane. This is commonly referred to as the degree of flatness. The flatness tolerance is the maximum amount of variation allowed by the actual surface against the plane. That is, in the drawing given to limit the actual surface processing tolerance allowed by the tolerance range of changes.

(3) Circularity

Circularity is the condition in which the actual shape of the elements of a part is equidistant from its center. The degree of roundness, as it is often called. The roundness tolerance is the maximum permissible variation of the actual circle against the ideal circle in the same section. That is, the variation range given on the drawing limit the machining tolerance of the actual circle.

(4) Cylindricity

Cylindricity refers to the point on the outline of the cylindrical surface on the part, and keeps its axis equidistant. The cylindricity tolerance is the maximum variation allowed by the actual cylinder to face the ideal cylinder surface. That is, given on the drawing, used to limit the actual cylindrical machining tolerance allowable range.

(5) Profile of a Line

The profile of a line is to represent the curve of arbitrary shape on the given plane of the part and keep its ideal shape. Profile of a Line tolerance is the permissible variation of the actual contour of a non-circular curve. That is, given on the drawing, to limit the range of variation allowed by the actual curve processing tolerance.

(6) Profile of a surface

Profile of a surface is a surface of arbitrary shape on the part to maintain its ideal shape. Profile of a surface tolerance is the permissible variation of the actual contour of a non-circular surface to the ideal contour. That is, given on the drawing, used to limit the actual surface processing range.

Position tolerance

 Position tolerance refers to the overall amount of changes allowed by the position of the particular element relative to the datum. It’s another important parameter of engineering tolerance.

(1) Directional tolerance

Directional tolerance refers to the overall amount of variation in the direction allowed by the reference in relevance to the particular elements. Such tolerance embodies parallelism, perpendicularity, and angularity.

(2) Location tolerance

Location tolerance is the full range of variations in a position that are allowed to correlate the actual elements to the reference. This kind of tolerance includes concentricity, symmetry, and position.

(3) Runout tolerance

A runout tolerance is a tolerance item that is given on the basis of a specific test method. Runout tolerance can be divided into circular runout and total runout. Tolerance of the above shapes and positions is collectively referred to as Geometric Dimensioning and Tolerancing (GD&T).

General tolerance

ISO 2768

In mechanical drawings, apart from the tolerance for certain dimensions and characteristics, those unspecified dimensions are generally required to follow certain standards. Taking our commonly used international engineering tolerance standard DIN ISO 2768 as an example, the general dimensional tolerance is m, the shape tolerance is K. And the marking method is ISO 2768-mK. The following is a table of linear dimension tolerance levels for reference.

Fundamental rules

Rule#1 Envelope rule

 This is a requirement that dimension tolerance and GD&T are related to each other. The actual tolerance of the dimension element with envelope rule shall comply with the max entity boundary. I.e. its outer function dimension does not exceed the max entity dimension. And its partial dimension does not exceed the min entity dimension.

Engineering tolerance rule#1

Rule#2 Independence rule

The principle of independence is that each size and shape given on the drawing is independent in position and should meet its own requirements. It is the basic principle that the relationship between dimension tolerance and form tolerance should follow. 

Rule#3 Tolerance of position rule

For tolerance of position, when dimensional elements are datum, S, L, or M must be specified in the feature control frame.

Engineering tolerance rule#3

 

Rule#4 Other than tolerance of position rule

For other than a tolerance of position, RFS applies with respect to the tolerance, datum reference, or both, where no modifier is specified. MMC must be specified in the feature control frame when it is appropriate and desired.

Engineering tolerance rule#4

Fits

In mechanical assemblies, the relationship between a hole of the same basic size and the tolerance zone of the shaft is called fit. As the actual size of the hole and shaft is different after assembly can produce a gap or interference. In the fit of the hole and the shaft, the algebraic difference of the hole size minus the shaft size is a gap when it is positive and a surplus when it is negative. 

tolerance fit

The coordination shall be classified into three categories according to the differences of the gaps or interference:

Clearance fit

The hole’s tolerance band is above the shaft’s tolerance band, and either pair of holes matched with the shaft becomes a fit with clearance (including a minimum clearance of 0). 

Interference fit

The hole tolerance band is below the axis tolerance band, and either pair of holes matched with the shaft becomes an interference fit (including a minimum gap of 0).

Overfit

The bore tolerance overlap on the shaft tolerance, allowing one pair of holes to fit on the shaft, either with clearance or interference fit. 

The essence of selecting the suitable tolerance level is to properly solve the contradiction between the operating requirements of machine components and the machining process and cost. The principle of choosing the tolerance level is to settle on a lower tolerance level the max as potential on the premise of meeting the application requirements of the parts.

The precision machining requirements should be coordinated with the production possibilities. That is, affordable process technology, assembly technology, and existing equipment ought to be used. However, if necessary, it is necessary to adopt strategies to enhance the accuracy of the equipment and improve the method to assure the accuracy of the merchandise.

It is very vital to pick out the acceptable tolerance level for the matching size. As a result of in several cases, it will determine the operating performance, service life, and dependability of the matching components. And at a constant time, it influences the parts manufacturing cost and production efficiency.

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