How to Achieve and Maintain Tolerances Below 25 Microns

 

In machining, tolerance refers to the allowable variation of a parameter within a specific range. This parameter can encompass various measurable physical properties such as temperature, humidity, noise levels, stress, solar radiation, and speed, as well as physical dimensions like spatial measurements.

 

Tolerances define the acceptable range of actual parameter values. In engineering, tolerances can be seen as the control limits for errors. Tolerances typically have an upper limit, known as the maximum limit size, and a lower limit, referred to as the minimum limit size.

 

 

CNC Machining Tolerances

 

The term "tolerance" in CNC machining has two interpretations: one refers to the tolerances of CNC machines, and the other refers to design tolerances in CNC machining.

 

CNC machine tolerances indicate the level of dimensional accuracy that a machine can achieve while processing parts. The precision of CNC machining is extremely high, with some machines achieving a manufacturing precision of ±0.0025mm, which is about a quarter of the thickness of a human hair. However, different machines have varying tolerances, typically defined by the machine manufacturer. A commonly used average tolerance in the market is 0.02mm. CNC machining service providers also inform customers about the tolerances of their available CNC machines.

 

 

Design and Machining Tolerances

 

In design and machining contexts, tolerances refer to the allowable range of variation in part dimensions. As long as the part falls within this range, it can fulfill all functional requirements. In this scenario, tolerances are established by the part designer based on the part's functionality, fit, and shape. Design and machining tolerances are crucial for the fit and assembly of parts. For instance, the tolerance requirements for an electric motor's components are significantly stricter than those for a simple door handle. This is because the numerous and complex parts of an engine must fit together precisely. Typically, the corresponding tolerance notations will appear next to the applicable dimensions of the part.

 

By understanding and implementing proper tolerances, manufacturers can enhance the quality and functionality of machined components, ensuring successful assembly and operation in various applications.

 

Achieving tolerances between 25 to 50 microns is relatively straightforward. However, if tool wear causes changes to the workpiece surface, it may exceed tolerance limits even before the tool wears down. This raises the question of how to maintain these tolerances during larger production runs.

 

Tolerances below 25 microns present greater challenges, with many machining processes aiming to maintain stricter tolerances between 5 to 12 microns. These tighter tolerances are even more difficult to achieve in bulk manufacturing. Here are some tips for managing them:

 

 

 

Use 80% of the Tolerance Band

 

Due to the strict nature of tolerances, size adjustments are necessary during production. As tools wear, the machined surface may expand (outer surface) or shrink (inner surface). The stricter the tolerance, the more size adjustments will be required.

 

A key rule of thumb is to use the average value of the tolerance band as the standard when initially determining sizes and making adjustments related to tool wear. By using the average value, you effectively utilize only half of the tolerance band. All relevant dimensions of the machined parts will fall at the high point of the outer surface or the low point of the inner surface. Because the tolerances are very tight, operators may need to make size adjustments after processing only a few parts.

 

If your dimensions are close to the low or high end of the tolerance band (depending on the outer or inner surface), you can extend the time between required adjustments. I recommend setting the target size at around 10% of the tolerance limit. This approach can effectively double the time between size adjustments.

 

 

Minimize the Impact of Thermal Variations

 

This point is particularly relevant for turning centers but also affects machining centers. As the machine heats up, its components can elongate. When the machine is idle, these components contract. Changes in component size can lead to variations in machining dimensions. When maintaining strict tolerances, thermal-induced changes can cause significant issues. For example, certain turning centers may experience outer diameter contractions of up to 25 microns when components reach operational temperature.

 

A common method to reduce thermal variation is to run a preheat program when the machine is first powered on and whenever it remains idle for more than a few minutes. This may involve running the program without raw materials.

 

Thermal characteristics vary among machine manufacturers, with some machines performing significantly better than others. If you need your machine to maintain strict tolerances, this should be a crucial consideration when purchasing new equipment.

 

 

Consider the Impact of Left-Hand or Right-Hand Tools

 

The choice of tools affects the lifespan of the machine. This is particularly important for most rough machining operations, where the type of tool used transfers forces to the machine's bed. For most slant bed turning centers, this means using left-hand tools and running the spindle in the opposite direction (M04).

 

Using right-hand tools on such machines will cause the spindle to rotate forward (M03), pulling the cutting tool away from its support direction due to the shearing action of the machining operation.

 

This often pulls the turret away from the cross slide and the cross slide away from the bed, placing significant stress on the machine's moving parts and leading to accelerated wear. While machines may initially maintain tolerances easily, it becomes increasingly difficult or impossible over time.

 

By following these strategies, manufacturers can better achieve and maintain tolerances below 25 μm, enhancing production quality and efficiency.