What is ISO 2768? A Guide to Tolerance Standards for CNC Machining

Time to read: 8 min

Fabricating a product without deviations from the original design is extremely complicated. Even if you’re able to get an instance comparable to your design intention, it’s nearly impossible to achieve the same exact dimensions in a batch process.

That said, you can decide how much a fabricated product can deviate from the original intention in order to be accepted. In manufacturing, this range of acceptance is defined by tolerance limits. These tolerances represent the variations between nominal dimensions (the original intention of the design) and the maximum and minimum values of a dimension that still guarantees a fit. In other words, it’s a controlled margin of error.

For example, let’s say you specify a round solid bar of 100mm length with Ø50mm that will fit inside of a hole in another component. You place an order to fabricate 200 of these Ø50mm rounds bars and when you receive and measure them, you get values like Ø53mm, Ø47mm, Ø51mm, Ø49mm, with a lot of variation. The bars also vary in length and when you take a closer look, you realize they are not perfectly circular.

Can you still use them? If not, can you reject them and demand that the vendor redo them at no cost? How close to the Ø50mm should you really be?

ISO 2768: An International Standard

ISO 2768 is an international manufacturing standard that can not only help answer those questions but also minimize inconsistencies while accounting for manufacturing costs as well. Because the standard was created by an international committee, it puts you on the same page as companies all around the world to prevent misunderstandings. And because it’s the global industry standard, China Manufacturing parts’s CNC machining service adheres to ISO 2768 medium requirements. 

ISO 2768 is divided into two parts that aim to simplify drawings by defining precision levels as general rules:

1. General tolerances for linear and angular dimensions with precision levels defined as: f-fine, m-medium, c-coarse, v-very coarse
2. Geometrical tolerances for features with precision levels defined as tolerances classes: H, K, and L

For example, a drawing could be specified as ISO 2768-mK, which means it should meet the tolerances ranges for “medium” from Part 1 and tolerance class “K” from Part 2. By including the ISO 2768 specification, you are simplifying your drawing, and avoiding writing tolerances for every dimension and feature. The standard is made of general rules because there are exceptions when a dimension on a part needs a tighter tolerance than those set by ISO 2768. Such instances are normal, and not uncommon, so you should always check the drawing title block for general tolerance requirements and note any special part specifications or project requirements.

Applying ISO 2768 to a drawing

In order to understand the content of this standard and its parts, it’s best to use a real engineering example. Figure 1 shows a vehicle engine with a compressor for AC. The component that supports the compressor and connects it to the engine is our focus; we’ll call it the “compressor base”. Our compressor base prototype example will be made from an aluminum casting, then machined and drilled.

Once we’ve defined a 3D model with nominal dimensions, we identify which features need tight tolerances and which ones don’t so we can communicate these requirements on the drawing. We include differentiating levels of tolerance to effectively manage costs. If all dimensions require tight tolerances, then costs increase significantly due to more demanding tooling/fixtures, operator skills, and increased likelihood of scrap/rework requirements. (You can learn more about how tolerance drives manufacturing effort in our China Manufacturing parts, Inc. MasterClass). Delivery times also increase since each part needs a strict quality check to corroborate each dimension, especially when components have complex compound geometries that aren’t easy to quantify.

When you design a part, it’s important to focus on the main function of each feature. Some dimensions’ margin of error is critical to control since their purpose is to align to other parts. Other features have dimensions and locations that aren’t critical for alignment, so they can have wider tolerances during fabrication — it’s a tradeoff between accuracy and cost.

For our compressor base example, Figure 2 below shows which features require a tight tolerance and which ones do not. Keep in mind, this is an example that differs from other designs, but every designer should develop a proper classification based on their product’s function.

In our case, the drilled holes to connect to the Engine block and to the Compressor need to be aligned and positioned correctly, therefore their tolerance is in the fine category (see #1 and #2 in Figure 2). The contact surfaces between components are also important for alignment (#3 and #4). However, for this example, we’re able to use a medium tolerance since a more accurate machine roughness did not benefit alignment enough to justify the extra cost.

Because the purpose of the ribs is to add strength, their wall thickness can be defined with a less rigorous tolerance as long as it meets the lower limit (#5, coarse tolerance). The main body of the base was defined as very coarse tolerance (#6) and we define references planes or datums to control the rest of the dimensions (#7, fine tolerance since we will be dimensioning from these surfaces). Keep in mind that for other designs, features like ribs, fillets, and chamfers might require tighter tolerances, depending on their function.

It bears mentioning that other standards work with similar dimensional concepts, the most common of which is Geometric Dimensioning and Tolerancing (GD&T), which is related to ISO 2768 Part 2. Learn more about the basics of GD&T here.

ISO 2768 Part 1: Linear and angular dimensions

Table 1 shows the precision levels or tolerance class designation for linear dimensions. One application is the dimension between holes for our compressor base example (see Figure 3).

Table 1: Tolerance Classes – Linear Dimensions

In a similar way, Table 2 shows the tolerances for external radii and chamfers.

Table 2: Tolerance Classes – External Radii and Chamfers

And Table 3 defines the tolerances for angular dimensions. Notice the tolerances units on Table 3 are degrees and minutes as expected for an angular dimension. In the next section we will discuss “perpendicularity” whose units are actually length (mm) despite the fact that it controls two surfaces in an angle.

Table 3: Tolerance classes – Angular Dimensions

ISO 2768 Part 2: Geometrical Tolerances for Features

Part 2 defines the three tolerance ranges H, K and L. These are different from the fitting and clearance tolerance grades that also use letters and numbers. Similar to ISO 2768 Part 1, there are nominal ranges and deviations, but the difference is how we define those deviations.

For example, instead of defining an upper limit and a lower limit, in Figure 4 we define a region between two references (i.e. parallel planes), so the fabricated surface should lie between them. This may sound more complicated, but it makes sense when you realize that if you place a caliper to measure two rough surfaces, you’ll get multiple different values due to the roughness of the surfaces. We define datums to use as a reference for dimension to controlling how much deviation is acceptable. In Figure 2, we picked three perpendicular planes for the base compressor (datum A, B, C on Figure 1). 

Table 4 defines Flatness and Straightness tolerance classes. In our compressor base, the contact surfaces between compressor and base and the contact surfaces between base and engine are important, so their flatness will be specified in the drawing.

Straightness controls how much a surface varies within a specified line on that surface. Another use of straightness is for the axis of a part to control how much bend or twist is allowed.

Table 4: Straightness and Flatness Tolerances

As mentioned before, Perpendicularity has distance units (mm or in). Similar to Flatness, we define two planes separated by a gap equal to the permissible deviation in Table 5. We control the 90 degrees angle indirectly since we’re measuring whether the surface is in the permissible region or not. (see Figure 6)

Table 5: Perpendicularity Tolerances

Table 6 shows the tolerances for Symmetry and permissible deviations for two features on a part that are uniform across a datum plane.

Table 6: Symmetry Tolerances

And Table 7 corresponds to Run-out, which is the total variation that a surface can have when the part is rotated around a datum’s axis. Notice that the marked surface is on tolerance despite the fact that it is not perfectly cylindrical. 

Table 7: Run-out Tolerances

You may have noticed that there is no table defined for parallelism. This is because ISO 2768 Part 2 defines parallelism as equal to the numerical value of the size tolerance or the flatness/straightness tolerance, whichever is greater.

Conclusion and Next Steps

ISO 2768 covers some of the tolerance and geometric characteristics used in manufacturing, but there are more standards in Geometric Dimensioning and Tolerancing (GD&T), whose symbols are shown in Table 8. For those interested in the topic, we recommend you read more about GD&T and the ASME Y14 standard.

Several companies have been implementing a method called Model Based Definition (MBD) with the goal to increase collaboration by including all GD&T, tolerances and datum information in 3D models rather than 2D drawings. In theory, it’s possible to do so since some CAD software has tools to include these symbols and values as parametric information. It’s an interesting idea, but replacing 2D drawings with 3D models as the record of authority probably won’t be happening any time soon.

Regardless of how you implement ISO 2768 tolerancing, China Manufacturing parts, Inc. can understand and meet your requirements. Create an account and upload your part to see what China Manufacturing parts’s instant quote process can do for you.

Table 8: GD&T Terms

Type of control	Geometric characteristics	Symbol
Form	Straightness
Form	Flatness
Form	Circularity
Form	Cylindricity
Profile	Profile of a Line
Profile	Profile of a Surface
Orientation	Perpendicularity
Orientation	Angularity
Orientation	Parallelism
Location	Symmetry
Location	True Position
Location	Concentricity
Run-out	Runout
Run-out	Total Runout