Introduction
During the fabrication and manufacturing process, there are variations between machined parts. Despite this, ensuring the proper functioning of the parts as designed is paramount. This takes us to the popular phrase with engineers: “Measure twice and cut once.”
Because the complexity of machined parts can vary widely, tolerances and precision engineering are inseparable. Tolerances, the margins of allowable error, keep engineering manufacturing moving with all the likelihood of imperfections.
Two of the most used types of tolerances in CNC machining are unilateral tolerance and bilateral tolerance. Considering the different applications of these tolerances, it is worthwhile to compare the meaning and relevance of each.
Let’s now jump into this detailed comparison of unilateral tolerance and bilateral tolerance as used in CNC machining for manufacturing processes.
What are Tolerances in CNC Machining?
First, what does tolerance mean in CNC machining?
Tolerance in CNC machining is the allowable variation in a part’s or component’s dimensions about the target dimension. The tolerances are typically on angular and linear dimensions.
Engineers and machinists use tolerance ranges for precise control of the acceptable limits of a part or component. Take a case where the basic dimension of a hole is 2.000 (+.000/-.002). The acceptable range of dimensions (tolerance zone) for the hole would be between 2.000 and 1.998. The upper and lower limits are 2.000 and 1.998 respectively.
The value after the ‘+’ sign gives the upper limit while the value after the “-” sign gives the lower limit.
What is Unilateral Tolerance?
Unilateral tolerance defines a situation whereby the room for variation from the basic dimension is in only one direction. The tolerance values could be in the positive or negative directions. A good example is where the tolerance range is +0.000/-0.002.
Here, the dimension of the machined part can only vary in the negative direction from the nominal dimension. The part can increase in dimension by 0.002mm in one direction only, the negative one.
Such unilateral tolerance is common where a bearing has to fit in another part. Dimensions of the bearing can only be smaller than the opening for the mating parts to be installed successfully.
What is Unilateral Tolerance?
There are instances when the tolerance varies in the negative and positive sides of the nominal value. Termed bilateral tolerance, this type allows for both negative and positive deviation from the nominal dimension.
Adjusting the above example to +/-0.002 demonstrates this more clearly. In this instance, the nominal size of the part can deviate upwards or downwards by 0.002mm to attain the upper and lower limits of the measurement. If the basic dimension is 50mm, unilateral tolerance gives an allowable range of 50.002mm to 49.998mm.
Note that this variation from the nominal size can also be unequally disposed i.e. not with equal variation. We term such presentations as unequal bilateral tolerances. The permissible variation from both sides of the nominal dimension is unequal.
What are the Differences between Unilateral and Bilateral Tolerance?
Dimension Variation
The first difference between unilateral and bilateral tolerance is the type of dimension variation accepted. While bilateral tolerance allows for variation both below and above the main dimension, unilateral tolerances are restricted to only one direction – either negative or positive.
Dimensioning Control or Preference
We could also differentiate the tolerances based on the level of control they have on part dimensioning. Unilateral tolerance applies where a variation to one side is preferable. For instance, machinists use unilateral tolerance when they want to make sure a shaft diameter does not exceed the diameter of a hole.
Applications of Different Tolerances
This takes us to the different applications of unilateral tolerance and bilateral tolerance (equal and unequal bilateral tolerance). Unilateral tolerances often go hand in hand with clearance and fits applications.
For bilateral tolerance, the most popular applications are general-purpose components. Small deviations from the standard dimensions on either the negative or positive side do not affect the component’s functionality. It does not matter whether they are unequally disposed tolerances or not.
Why are Unilateral and Bilateral Tolerance Important in Manufacturing?
Quality manufacturing and well-defined engineering tolerance are intertwined. Engineers use unilateral and bilateral tolerances to manufacture parts and products. There are several reasons why the manufacturer will seek to stick to unilateral and bilateral tolerances:
Money Savings
When they receive bilateral and unilateral tolerances in good time, manufacturers have the measurements ready. There is no form of guesswork during the manufacturing process. With unilateral and bilateral tolerances, the manufacturer already knows the level of precision the client needs. This element has a direct effect on money savings for manufacturing, particularly considering the reduced wastage of materials and money.
Enhanced customer satisfaction
With well-defined engineering tolerances, the manufacturer can produce better parts and products. The quality control and engineering teams use these specifications to meet the customer’s requirements. Products created following the defined tolerances are almost perfect. As long as the manufacturer can achieve strict tolerances, the client will always be happy.
Improve Teams’ Collaboration
In many manufacturing setups, the QA team and the engineers experience communication challenges. Often, the problem is that the engineers want a part or product produced, but the QA team doesn’t understand how.
Geometric dimensioning and tolerancing (GD & T) prevent this misunderstanding by providing information on the required precision level. Provided the teams involved in the manufacturing process understand the unilateral vs. bilateral tolerance issue, miscommunication is not a problem.
How do Material Properties Influence the Choice of Tolerance Type?
When determining what type and extent of engineering tolerance to use, an important consideration will be the material properties. Is the material plastic or metal? Every material has mechanical, physical, chemical, or electrical properties.
The following material properties are worth considering for unilateral vs. bilateral tolerance:
Content and Microstructure of the Material
During machining, less tool wear occurs in a material with a homogenous structure than in a heterogenous one. Consequently, a homogenous structure promotes tighter tolerances. The machinist wants to understand the material composition and choose materials with the right microstructure to attain accurate tolerances in CNC machining.
Material Hardness
Titanium and other hard materials wear the cutting tool fast. Due to this wear, the cutting tool can produce dimensional errors. These errors can depart from the specified unilateral and bilateral tolerances. Most materials are harder to machine due to their higher hardness. A worn-out cutting tool has problems producing a smooth surface finish. The resulting surface imperfections can hinder unilateral and bilateral tolerance.
Aluminum and brass are some of the most machinable materials. Getting tight tolerances with the materials is easier than with harder materials. This suitability for machining can come with a drawback – poor strength. Manufacturers usually have to balance between strength and tolerance levels for certain applications.
Thermal conductivity
This is an equally important material property when precise unilateral and bilateral tolerances are crucial in machining. The machining process generates heat due to the interaction between the cutting tool and the material.
Materials with poor thermal conductivity are susceptible to thermal expansions, which can negatively affect dimensions. Aluminum and copper are examples of materials with high thermal conductivity. They are easy to machine to accurate unilateral and bilateral tolerances. Stainless steel and other materials deficient on this property need extra cooling during machining.
Dimensional Stability
On this property, factors such as temperature and stress come into play. How does the material behave behind under temperature and other variations? A material will be capable of attaining tight unilateral tolerances and bilateral tolerances if it retains its size and shape despite the application of such external factors. Plastics are examples of materials with poor dimensional stability.
Choose Aria Manufacturing
A CNC machined part provider defines tolerance for a part. Unilateral and bilateral tolerances provide information for high-quality CNC machined parts.
The machined part provider should place the part within the tolerances. But as a user, you don’t need to know all the bilateral and unilateral tolerance jargon. You can identify a manufacturer that adheres to the requirements of unilateral and bilateral tolerances because their machined parts are consistently high-quality.
At Aria Manufacturing, manufactured parts always adhere to strict tolerance system requirements. Get in touch for more details.