Tolerance is a very important factor to consider when creating parts via CNC machining or any form of manufacturing. The tolerance determines how precise the final parts are, and how accurate they are to the engineering drawings.
They also determine how well components in an assembly fit in with each other. CNC machining is one manufacturing process that achieves tight manufacturing tolerances consistently.
However, there are many still factors to consider when specifying tolerances for cnc machined parts in your engineering drawings.
In this article, we’re going to examine tolerances and the factors you have to consider when selecting tolerances.
First, let’s start with the definition.
What are CNC Machining Tolerances?
The tolerance of a cnc machined part is simply the amount of variation allowed in the dimension of the final manufactured part. This indicates the level of precision required for the part
Engineers specify tolerance by setting two values known as the upper and lower limits of the part’s dimension. These represent the maximum and minimum dimensions acceptable for the part respectively.
For the part to be accepted, its dimension must fall between these two values. If not, it will be rejected for scrap or rework.
Types of CNC Machine Tolerance Systems
Engineers communicate the tolerances they want for their products through their engineering drawings and plans. The CNC Machinists interpret these tolerances from the drawings and creates the required accuracy for the customer.
They can use a wide variety of acceptable methods and systems for this. Some of these systems include:
1. Bilateral Tolerances
This is usually specified with the plus/minus sign (±). It means that the final dimension of the part is allowed to vary either above or below the basic dimension specified for the part.
The variance from the basic dimension can be either positive or negative. For example, let’s say you want a rod of length 25mm, and you apply a tolerance of +/-0.01mm to the rod.
This means that the maximum size permissible for the rod is 25.01mm, while its minimum length is 24.99mm. Anything outside of these values will be rejected.
2. Unilateral Tolerances
Unilateral tolerances are specified with either the plus (+) or minus (-) sign. They specify that the final dimension should only vary in one direction from the base direction.
This system is very helpful, especially in the manufacture of holes and shafts. For example, let’s say the engineer specifies a hole with a dimension of 40mm and a tolerance of -0.2mm.
It means the maximum size of the hole is 40mm with a minimum size of 38.2mm.
3. Limit Tolerances
Engineers use limit tolerances to specify a range in which the dimensions of the manufactured part should fall into. These tolerances specify an upper and lower limit without specifying a base dimension.
For example, you can have a cylindrical part with a dimension of Ø23mm – 23.5mm. This means the diameter of the final piece should fall between the set limits of 23mm and 23.5mm with a tolerance of 0.5mm.
4. Geometric Dimensioning and Tolerancing (GD &T)
Geometric Dimensioning and Tolerancing is a system of notations and symbols engineers use in communicating manufacturing information.
The information the GD&T systems communicates is far more than tolerance information. It provides information on the part’s geometry and how to check its tolerance.
It also provides information on the part’s fit and relationship with other components in the assembly. It is one of the most thorough tolerance systems, and its often used in drawings and plans for CNC machining precision parts.
What are ISO Tolerance Standards for CNC Machined Parts?
ISO tolerance standards are internationally recognized systems of limits, fits, and tolerances. These systems help simplify drafting technical plans and diagrams as they provide standardized tolerances for the parts.
Two widely used ISO standards for evaluating CNC machined parts are the ISO 2768 and ISO 268 standards. ISO 286 specifies a metric system of limits and fits for mating machine parts, while ISO 2786 is mainly concerned with linear and angular dimensions.
Factors that Affect Tolerances
There are several factors that can affect the final dimension of a part and whether it strays in or out of the tolerance band. These factors include:
√ Machining Method
The machining method determines how accurate the final part will be after machining. To achieve tight tolerances, you’ll have to use precision machining methods to get your desired accuracy.
In some cases, the part might even need additional processes like lapping or honing to achieve the tolerances. For most CNC machining processes, the standard tolerance available is 0.005” (0.127mm).
However, there are CNC machining processes that can offer tolerances lower than that.
√ Material Choice
Mechanical properties of the material, like the hardness and heat stability, significantly determine the part’s tolerance. For example, softer materials are easier to machine, but they’re more prone to deformation, which can be a problem.
Also, with heat-sensitive materials, you must account for temperature expansion when setting tolerances and machining the part.
Precision machining produces highly accurate parts and achieves tight tolerances. However, it raises production costs. So, balancing quality with cost is a big part of determining the correct tolerances to select for your part.
You have to evaluate the type of fit and function required by the part. If you can get away with using a higher tolerance band for it, then there’s no need to splurge on more expensive manufacturing methods.
√ Surface Finishes
Surface finishes can either involve the additional removal or addition of material to the part’s surface. You have to consider this when dimensioning the part and choosing its tolerances.
If not, the finishes will change the part’s geometry and take it out of the tolerance range required.
√ Inspection Methods
Your part’s final dimensions are only as accurate as the tools used in measuring them. This emphasizes the need for proper quality control methods.
For non-precision parts, calipers, gauges, and micrometers may be used to verify the part’s tolerances. However, to verify tight tolerances, better inspection methods like Optical microscopes, comparators, and CMM machines are needed.
How to Choose The Right Tolerance for CNC Machined Parts?
There are many factors to consider when selecting the right tolerance for your machined parts. Among all these, the primary factor is cost.
High-powered CNC machines can achieve extremely tight tolerances, but they are also very costly to run. In addition, there is a high part rejection rate which can increase production costs
So, you should consider the intended application of your machined part before selecting its tolerance. If you want to save on production costs and reduce lead times, you can even remove tolerances on non-important parts that do not mate or interfere with others.
Also, if you can use a looser tolerance for the part, then you should avoid using tight tolerances. You can get more insight about this from professional machine shops.
The machinists and production engineers will create plans on how to apply tolerances to the part cost-effectively.
Using the right machine shop is key to achieving tight tolerances consistently and cost-effectively. As an experienced manufacturing shop for CNC machined parts, RapidDone is undoubtedly the best vendor for your project!
Our shop floor boasts powerful 3, 4, and 5-axis machines capable of rapidly delivering even the most exacting tolerances. We also have a talented array of engineers and machinists with experience in several industries manning these machines.
If you have any questions about CNC machined part tolerances or want to learn more about our CNC machining services, please talk to us here.