Explanation of Sheet Metal Tolerances

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Publish Date: December 2, 2022

In the 3D design of parts, the design dimension of parts needs a total amount and range that can be changed.

The designers of parts need to consider the range of product dimension changes when designing.

If the dimension variation range of the part is not appropriate, such as if the sheet metal tolerance is too small.

Then, a more precise parts manufacturing process will be required to ensure the precision of the workpiece.

Only a few manufacturers have such technology, and the production cost will greatly increase.

If the tolerance range is too large, the quality of parts cannot be guaranteed.

Therefore, a reasonable tolerance is to calculate the appropriate variation range of the part size.

What Is Sheet Metal Tolerance?

Sheet metal tolerance is a range that allows the design dimensions of parts to change.

The tolerance range is the upper and lower limit of the variable workpiece design size.

The tolerance zone of sheet metal tolerance refers to an area limited by upper and lower deviations.

Loose tolerances have a wider tolerance zone, while strict tolerances have upper and lower limits of a smaller range.

A strict tolerance range also means that the dimensions of the workpiece are more precise.

Why Do We Need Sheet Metal Tolerance?

During workpiece processing, slight differences in metal plates will lead to differences in final products.

The thickness, purity, texture, age, and processing method of materials will affect the quality of sheet metal processing.

Allowing a certain tolerance range can make the workpiece fit better.

It can also reduce the production cost, provided that it is controlled within a reasonable range.

Because too precise size requires more professional technology and equipment, it usually takes longer to complete.

Therefore, in part design, the use of reasonable tolerances has an important impact on the size of parts.

Relative Definitions of Tolerances

There are many forms of tolerance in sheet metal processing. Tolerance can be used for the length, width, and thickness of parts.

Tolerances can be set for wall thickness, bends, curls, countersinks, hems, holes, slots, notches, tabs, etc.

The parts not only have dimensional tolerance but also have differences between the actual form or mutual position of the points, lines, and surfaces constituting the parts and the form and mutual position of the ideal geometry.

This difference in form is called form tolerance, and the difference in mutual position is called position tolerance, which is collectively called tolerance of form and position.

Dimension tolerance: referred to as tolerance for short, it refers to the absolute value of the difference between the maximum limit dimension and the minimum limit dimension.

Or the difference between the upper deviation and the lower deviation. It is the allowable variation of size.

Position tolerance: refers to the total variation allowed by the position of the associated actual feature to the datum.

The positional tolerance is an error that limits the geometric relationship between the measured feature and the datum feature.

According to their different geometric relations, position tolerance can be divided into orientation tolerance, positioning tolerance and runout tolerance.

Tolerance grades: refers to the grades that determine the accuracy of dimensions.

The international standard is divided into 18 grades.

The larger the allowable variation range (tolerance value) of size, the smaller the processing difficulty.

IT01 to IT4 - for the production of gauges, plug gauges and measuring instruments.

IT5 to IT7 - for fits in precision engineering applications, IT8 to IT11 - for general machining.

IT12 to IT14 – for sheet metal processing or stamping, IT15 to IT16 – for casting, general cutting, etc.

IT17 to IT18 – for tolerance of plastic molding dimension, general outline dimension of surgical instruments, cold working, and welding dimension.

Tolerance symbol: the dimension tolerance is an absolute value without a sign.

Limit deviation=limit dimension - basic dimension, upper deviation=maximum limit dimension - basic dimension, lower deviation=minimum limit dimension - basic dimension.

When the basic dimensions are the same, the smaller the dimensional tolerance, the higher the dimensional accuracy.

The dimension tolerance is equal to the difference between the maximum limit dimension and the minimum limit dimension or equal to the difference between the upper deviation and the lower deviation.

How to Determine Sheet Metal Tolerances?

Part tolerance refers to the dimensions without tolerance indications in the design and manufacture of parts.

It also refers to those dimensions that are not included in the dimension chain and have no direct impact on the fit properties.

From the definition of tolerance, part tolerance has an important influence on the dimensional fit of parts in mechanical design.

If the part tolerance cannot be selected properly, the part design dimension chain will be incomplete.

In the process of tolerance marking, some parts with lower accuracy requirements can use general tolerances.

If the requirements for precision are high, the tolerance shall be specified in detail in the mechanical design.

Ensure that the part tolerance can meet the actual needs.

The function of a part determines the size, shape, location, and other requirements of its corresponding elements.

The selection of tolerance grade should ensure the design and quality requirements of parts.

The processing cost, product performance, function, service life, and fuel consumption should also be considered.

Forming or bending+/- 0.508 mm (0.020")
Bend to hole or feature+/-0.254 mm (0.010")
Diameters with inserts+/-0.0762 mm (0.003")
Angularity+/- 1°
Holes+/-0.127 mm (0.005")
Edge to edge±0.127 mm (0.005")
Edge to hole±0.127 mm (0.005")
Hole to hole±0.127 mm (0.005")
Hole to hardware±0.254 mm (0.010")
Edge to hardware±0.254 mm (0.010")
Hardware to hardware±0.381 mm (0.015")
Bend to hole±0.381 mm (0.015")
Bend to hardware±0.381 mm (0.015")
Bend to edge±0.254 mm (0.010")
Bend to bend±0.381 mm (0.015")
TOLERANCE GUIDELINES

How to Do Tolerance Analysis?

The methods of tolerance analysis are mainly one-dimensional and three-dimensional.

The one-dimensional method does not need to buy software, so the cost is low, while the three-dimensional method costs more.

There are also two different methods for one-dimensional tolerance analysis, one is the worst case and the other is the root mean square method (RSS).

The second method belongs to the category of statistical methods, while the limit method is relatively simple.

Upper dimension limit USL: 10.2+10.2+10.2+10.2+10.2=51

Lower dimension limit: 9.8+9.8+9.8+9.8+9.8=49, so the fluctuation range of dimension D is 49~51

The limit method is the direct accumulation of each size boundary, while the statistical method is to consider the probability of each size to calculate the probability of each size after accumulation.

If we want to use the probability method to analyze, we need to know the respective probability of each dimension.

The following is the distribution probability of dimension A. If it is a stable process, then it should be a normal distribution.

Then we need to know the overall distribution, and we need to know the two parameters of the normal distribution, the mean and the standard deviation.

The standard deviation describes the discrete state of a distribution. It is a measure of the average dispersion of a group of data.

The standard deviation is large, indicating that there is a large difference between most values and the average value.

The small standard deviation indicates that the difference between most values and the average is small.

After knowing the mean value and standard deviation, we can see the distribution of this dimension.

As shown in the figure above, the mean value is 10 and the standard deviation is 0.067.

If two dimensions are accumulated, the mean value is the same, and the standard deviation is different, then the cumulative distribution is completely different.

The results will be different if the distribution state of dimension fluctuation is different.

The original definition of tolerance is the way of limit definition, which can not describe a distribution well.

Two parameters are required to describe a distribution, mean value and standard deviation.

In order to associate with the original interval tolerance, another parameter - CPK needs to be introduced.

To simplify the description, we assume that the center does not shift, CP=CPK.

As shown in the figure below, with a tolerance range and CP, you can know the standard deviation. Add the mean value and the normal distribution can be determined.

The following table shows the sigma level corresponding to CP (CPK). CP (CPK) 2 means 6 sigmas, and CP (CPK) 1.67 means 5 sigmas.

When we know CP (CPK), we can get the sigma level, and we know the normal distribution.

Therefore, the mean value, tolerance range, and CP (CPK) should be known during tolerance analysis.

If we know the distribution of all dimensions in the dimension chain, we need to calculate the distribution of the total variation.

We need the calculation formula of RSS(Root Sum Square), that is, the square of the standard deviation of the normal distribution of the overall standard deviation is equal to the square sum of the standard deviation of each sub-distribution.

Therefore, the standard deviation of each dimension=the sigma level corresponding to the tolerance/CP, as shown in the figure below σ expresses the standard deviation.

σ²= (tolerance/process sigma) ²

Stacking different σ² is the total distribution of the overall standard deviation σ²

Finally, an excel template can be used to implement the analysis process.

Fill the relevant parameters of each dimension into the table in the template to get the stacking results of the overall standard deviation.

Conclusion

This blog introduces the basic knowledge of sheet metal tolerance and how to conduct tolerance analysis.

The design of parts is becoming more and more complex, and correspondingly, the tolerance is becoming more and more strict.

In order to achieve sheet metal tolerance in part design, manufacturers need to use precise machines for production.

ADH has 20 years of experience in sheet metal processing machine manufacturing.

Our products include press brakes, shearing machines, fiber laser cutting machines, etc.

If you need to buy a sheet metal processing machine, you can contact our product experts to get detailed information.

FAQs

What Is Sheet Metal Flatness Tolerance?

Flatness is a concept of form tolerance. The symbol of flatness is a circle (○), which is an index limiting the variation of the actual circle to the ideal circle.

It is a requirement for a circular contour of parts with cylindrical surfaces (including conical surfaces and spherical surfaces) within a normal section (a plane perpendicular to the axis).

What Is Sheet Metal Standard Tolerance?

A standard tolerance is any tolerance listed in an international standard to determine the size of the tolerance zone.

The standard tolerance is divided into tolerance grades, tolerance units and basic dimensions.

Generally speaking, standard tolerances are divided into 18 grades.

For parts with a certain size, the higher the standard tolerance level, the smaller the standard tolerance value, and the higher the accuracy of the size.

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