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Ultimate Guide To CNC Machining Tolerances

CNC Machining Tolerances

If you are using a CNC machine, then part tolerance is something you cannot ignore.

A reason this guide explores everything you need to know about CNC machining tolerances.

So, keep reading to improve your parts’ accuracy and consistency during CNC machining.

What Are Tolerances In CNC Machining

Figure 1 CNC MachiningCNC Machining

Tolerance is a level of dimensional deviance in CNC machined parts instead of the original measurement. In fact, we can also call it dimensional accuracy.

That means that we can use it to express the part’s minimum and maximum dimensional limit. Having this in mind, the part is considered within tolerance.

Common Types Of CNC Machining Tolerances

A quick peek into some common examples includes;

· General Tolerances

This kind of tolerance is typically useful when applied in geometrically cylindrical or circular parts. We cluster their standard tolerances into four centered dimensionally on their range.

The tolerances limits are fine, medium, coarse, and very coarse. International standards like EN 2286 and ISO 286 define the dimension brackets’ tolerance limit range.

· Unilateral Tolerances

As the name suggests, only one part direction can have a tolerance. This variation can either be more or less of the actual part size.

Let’s take an example of a metal rod that has to be machined to fit on a product. Sure enough, we will have a metal rod out of dimensional range even by a fraction of an mm if not properly machined.

Such errors can be easily avoided by ensuring proper unilateral tolerances. So yeah, allowing a 0.05mm maximum deviation gives it a 45+0.00/-0.05mm tolerance range.

You realize after doing the math that 45mm and 44.95mm are the maximum and minimum diameter values permissible for the shaft. The upside of all this is the ease of inspecting the part.

The key here is having a fixed upper value with dimensions on one side varying. You can now sort of increase the inspection speed by standardizing a go gauge, simple right?

· Bilateral Tolerances

Here’s a simple explanation of this tricky concept. The key here is to have the part’s final measurement vary on the true profile or the nominal side.

Think about it. Having a bilateral tolerance of 45+0.05/-0.05mm simply implies a minimum of 45.05mm and a maximum value of 44.95mm.

What does this imply? That the nominal value used is 45mm.

· Limit Tolerances

For a dimension, its maximum and minimum value form the limit tolerance. This means that when the part is joined to the final product, these values will fit exactly in the component’s designated points of contact.

An example is a dimension with a limit tolerance of 10 to 10.5mm. As a rule of thumb, these two boundaries are where the final values will lie.

Considerations When Determining CNC Machining Tolerances

We follow a general product grade in case a customer does not specify certain tolerances.

There is no need to fancy all up and everything. The key is to create a nice product with minimum retooling costs.

Here is the interesting bit. Getting to know the features we critically consider before settling on CNC Machining Tolerances.

Figure 2 CNC Machining tolerance

CNC Machining Tolerance

· Choice Of Tight Tolerance

In some situations, we need to specify to the manufacturer the level of variance tightening. This is a great idea when we are dealing with prototypes that need perfect machining for testing.

We also require CNC Machining when producing electrical components. This is due to their ability to operate within the minimal error margin of +/-0.0005.

· Cost Of Tight Requirements

We associate high expenses with requests for tighter tolerances beyond the basic 0.1mm. The simplest explanation we can give is the strict actions needed to comply with standards.

In addition to that, we have manufacturers lacking CNC Machines with the required precision. When we use expensive equipment, it is high grade with support for lower variances.

· Inspection

Why is an inspection necessary after we machine a part using CNC? It is simply our basis for Quality Assurance Policies.

The major setback we have with unnecessary tolerances is the number of resources needed for inspection. The worst-case scenario is having manufacturers without the required grade of inspection machines.

Geometric Dimensioning And Tolerancing (GD&T) In CNC Machining

Figure 3 GD and TGD and T

When we think outside the box, defining Geometric Dimensioning and Tolerancing appears to differ from the rest. We actually cannot use it on the base dimension to define specific variances.

On the contrary, it uses specifications to assist us in indicating tolerances. That said, we define a component’s geometric trait using symbols in GD&T.

It’s a really cool way of expressing the parts datum reference, its tolerance, and modifiers. You will actually be surprised by the ease of such coded specifications having learned all the symbols.

How Type Of Material Affects CNC Machining Tolerances

Tolerance can be a headache but the choice of material can greatly ease that. We have certain features in materials that determine their level of achievable tolerance.

Let’s delve into some of these characteristics in detail;

· Heat Stability

Can heat stability really affect metals in CNC machining tolerance? Yes, it does but with very minimal impact compared to non-metals.

A practical example is the buildup of heat during machining that leads to material deformation. Sure enough, the process where the part is applicable is already restricted by this deformity.

· Abrasiveness

When we carry out CNC machining on rigid parts, the cutting tool really wears out pretty fast. That means achieving specific tolerances is a tall order.

Specifically, the downside of it all is the reduced accuracy due to variations in the cutting tool. Again, we are forced to regularly change the cutting tool when we machine abrasive materials.

· Hardness

When it comes to tolerances, rigid and tough materials achieve them with ease compared to soft materials. This means that we will take a longer processing time to tolerate soft parts.

That way, chances of changing the dimensions of the part after contact with the tool are reduced. Now you definitely have a tool with impeccable tolerance.

Tips To Achieve Tighter Tolerances In CNC Machining

Figure 4 CNC Machining Metal Parts

CNC Machining Metal Parts

When machining parts, it’s next to impossible that we would lack some form of variation. Make no mistake, a lot of precision is needed to achieve a cool part.

Tighter tolerances are currently blowing in CNC Machining. There’s just only one problem, the additional cost.

Fortunately, we can cost-effectively offset and account for the additional cost using various approaches. The win-win strategies include;

· Consider Final Inspection In Design

First, measuring tolerances on difficult parts is an uphill and daunting task. Yes, it is possible but we will require specialized equipment for precision.

In fact, parts containing smaller features are the hardest when we manufacture and inspect them. Fortunately, the right set of tools can guarantee tight tolerances with precise inspection and measurement.

· Part Tolerance Optimization For Specific Applications

We can guarantee a proper fit by having minimal deviations. We have an allowable tolerance for different materials during CNC Machining (+/- 0.05mm for metal and +/- 0.010mm for plastics).

If we opt for tighter tolerances, then the production cost will definitely rise. We can limit it to only the necessary component surfaces while meeting the design requirements.

That means that we have mitigated additional costs. However, we opt for standard tolerances if the need for tight tolerance is a luxury.

· Consider Perpendicularity And Parallelism

We definitely must prioritize these two especially when we are dealing with multiple components. The whole point is that any slight misalignment blows over time.

This tends to dent the part’s credibility and viability over time. That means that the part quality will also be substandard.

· Align Machinability Of Material With Tolerance Expectations

Normally, we design parts to provide specific properties based on the material. We can see how the advantages work magic at the end-use application.

However, we need to consider how the part’s machinability is impacted by such properties. Basically, softer materials tend to have a hard time holding tighter tolerances.

This is due to the low durometer they possess. On the contrary, it is very hard to mill hardened steel compared to plastics.

Factors Impacting CNC Machining Tolerances

Tolerances normally ensure we create bespoke and precise parts. But some factors can scale it down big time.

Let’s break down some factors that impact tolerances

· Cost

For starters, tighter tolerances result in increased costs. We can have precise yet not-so-tight tolerances as a way of cost-cutting.

· Machining Method

There’s one major factor we keep in mind while handling tolerance…method of machining. That said, it is easier if we select methods that are more precise than the rest for a quality end product.

· Finishes And Plating

We have to keep in mind that finishes and plating and add bits and pieces of material on the part surface. So yeah, they alter the part dimensions in such a way that a different tolerance is required.

Unequally Disposed of Tolerances in CNC Machining

Unlike normal tolerances, this type of Tolerance is actually contrasting. This means that they vary from the true profile allowable unequally in opposite dimensions.

We indicate the directional limits in the specifications of each part. For example, we may have upper and lower limits such as 5.99 or 6.09.

Either way, the most important aspect is one. That the distribution of the lower and upper limits is not equal to the given dimensions.

Understanding The International Tolerance (IT) Grade In CNC Machining

Simply put, it’s the maximum difference in size between a part and its basic size. We use this to improve the precision to pre-manufactured parts.

Additionally, we can use it on parts with threads and contoured surfaces. These parts are usually symmetrical and rotational in nature.

How Lower Deviation Compares To Upper Deviation In CNC Machining Tolerances

Upper deviation simply put is the difference between the basic size of a part and its maximum size. On the other hand, the lower deviation is basically the difference in the basic size of a component and its minimum achievable size.

Figure 5 Confirguring CNC MachineConfiguring CNC machine

How Machining Type Impacts CNC Machining Tolerances

Tolerances are greatly impacted by the machining methods we use. This is because some machining methods tend to be extremely precise compared to others.

Sure enough, in CNC Machines we apply the screw machining method. We basically have a part fed using a disc cam through a guide bush.

This way, the part moves around the tool as opposed to the other way. This means that we have allowed extreme precision by reducing the deflection and vibration of the part.

We can utilize this method fully on plastic parts phenolic. Their tremendously great dimensional tolerance ensures we can achieve +/-0.05” of tolerances.

How Tighter Tolerances Increase Production Cost In CNC Machining

Having tighter tolerances is great. Until you realize it impacts both the cost and quality of the part.

Sure enough, it is expensive when we perform high-tolerance machining. This is because expensive machining tools are required with the process running at a snail’s pace.

Now, when it comes to part inspection, it’s usually an extensive and superior CNC procedure. Again, we are talking about operating on a very small window of error like fractions of a millimeter.

Still, when all is set, the failure rate of these parts also blows its fair share of damage on the cost. With such high failure rates, the cost of production and inspection drastically shoots.

ISO 2768 In CNC Machining Tolerances

This is a standard we use for angular and linear tolerances. Its upside is minimizing inconsistencies while at it determining the average machining tolerances.

For simplified drawings using precision levels, we have two major categories;

· Angular and Linear General Tolerances

A look into this category gives us four distinct precision levels. Very coarse (v), coarse (c), medium (m), and fine (f). We can as well call them in DIN ISO 2768 as the four tolerance classes.

· Geometrical Tolerance

We use this to define precision-level features for tolerance classes H, K, and L. Let’s take an example of a part drawing labeled ISO 2768-mK.

The first part shows us its medium tolerance range. The second part shows us it is in tolerance class K.


With the information in this guide, you can certainly achieve the required CNC machining tolerance.

At KDM, we guarantee precise and accurate machining depending on your unique requirements and specifications.

More Resources:

What Is CNC Machining – Source: Thomasnet

Machining Tolerances – Source: Fractory

Understanding Machining Tolerances in CNC – Source: Protolabs

Machining Tolerances – Source: Universal Technical Institute


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