Swiss Turning – When Tolerances Count

A Swiss Lathe is a modern turning machine that offers multiple levels of tolerance to achieve what regular lathes cannot. Machinists use lathes to perform cutting, sanding, and shaping operations. They use these processes to cut metals and nonmetals and create different parts out of them. These parts may be fragments of a larger mechanism such as an engine.

What sets a Swiss-style lathe apart from regular lathes is its Computerized Numerical Control (CNC) capabilities. This modern capacity gives machinists more access to precise cutting options for more accurate parts. This capacity also exists in other machines, which may confuse machinists looking for CNC machines.

It may help machinists differentiate between the machines available to them by understanding where Swiss lathes come from. Here is an overview of the machine’s etymology and its overall functions.

Turning Center vs Lathe

What is a Swiss Lathe? KSI Swiss
What is a Swiss Lathe? KSI Swiss

Horizontal CNC turning centers and lathes may resemble each other, which is understandable because they are related machines. Lathes are 2-axis machines, which came before Swiss turning centers. Turning centers evolved from lathes by including 3-, 4-, and 5-axis capacities for creating parts.

This extra capability is the primary difference between a turning center and a lathe. Despite this distinction, there is no formal differentiating factor between the two. However, machinists may refer to a lathe when their project involves a simpler machine. They may call for a lathe when they only need to perform turning operations between the X and Z axes.

Meanwhile, a machinist may call for a turning center when they require extra operations. These machines integrate drilling, milling, and sub-spindle capabilities for more precise processes than they can achieve with a lathe. These procedures are possible because the turning center includes a Y-axis.

Another difference between a CNC turning center is how it utilizes computer processes to operate. Controlling processes with a computer allows more precision than a manual process. With a computer, the machinist can move all axes at the same time, thus achieving the required amount of precision that a project requires.

Despite these differences between the machines, some machinists may refer to these two terms as the same machine and use them interchangeably. In particular, a Swiss-type lathe tends to utilize CNC capabilities to achieve tight tolerances, thus combining the operations associated with turning centers and lathes into one machine.

Swiss-type lathes achieve tighter tolerances than regular lathes because of their rigidity. Their guide bushing holds the materials being processed while the Swiss machine tooling cuts through it.

A regular lathe applies tool pressure away from where the machine holds the materials. Meanwhile, a Swiss-type lathe holds the material as close to the tool as possible. Holding the material right up against the tooling gives a machinist greater dimensional accuracy to help them achieve maximum tolerances.

CNC Machining Tolerance Guide

Swiss Turning - When Tolerances Count - KSI Swiss - Measuring Tolerances
Close Up of Micrometer

A CNC screw machine requires different tolerances to conduct all operations needed to create certain products. In general, tolerance refers to the measurements that signify how precise a manufacturer wants a certain part.

A smaller or tighter tolerance means a manufacturer needs more precision to produce a part. Meanwhile, larger or looser tolerance requires less tolerance. Parts that need tighter tolerances may not work optimally if a manufacturer produces them with the wrong tolerance.

A specific part will require a specific tolerance. CNC machining utilizes computer programs and electromechanical devices when creating parts. These elements automate the processes, making precision an integral factor to consider.

Precision varies between the different parts involving CNC machining. These different parts require various degrees of accuracy to create. As such, a designer must specify the tolerance suited to the exact part they make.

Different tolerances offer various results. Here is a general CNC machining tolerance chart as a guide to the different dimensional tolerances for specific linear dimension ranges (in mm):

Linear Dimension Range (mm) Dimensional Tolerance Class
Fine (f) Medium (m) Coarse (C) Very Coarse (v)
Between 0.5 and 3.0 ±0.05 ±0.1 ±0.2 -
Between 3.1 and 6.0 ±0.05 ±0.1 ±0.3 ±0.5
Between 6.1 and 30.0 ±0.1 ±0.2 ±0.5 ±1.0
Between 30.1 and 120.0 ±0.15 ±0.3 ±0.8 ±1.5
Between 120.1 and 400.0 ±0.2 ±0.5 ±1.2 ±2.5
Between 400.1 and 1,000.0 ±0.3 ±0.8 ±2.0 ±4.0
Between 1,000.1 and 2,000.0 ±0.5 ±1.2 ±3.0 ±6.0
Between 2,000.1 and 4,000.0 - ±2.0 ±4.0 ±8.0

Machinists use numerical values to measure lathe machining tolerances. The “±” symbol usually precedes these numbers to determine the final product’s variable length. For instance, a part measuring 1.5mm in height may need ± 0.05mm tolerance. So, the final product must measure between 1.45mm and 1.55mm in variable height to pass quality inspections.

Types of Lathe Tolerance

A specific tolerance may work better than another when manufacturing any part. Here are common lathe tolerances associated with CNC machining.

Standard Tolerance

Standard machining tolerances are for parts that machinists fabricate the most. Such widely fabricated parts may include pins, pipes, and threads. Since these parts are common, machinists may have already assigned standard machining tolerances ideal for creating them.

Generally, machinists in milling services offer ± 0.1mm tolerances. They have these tolerances in place in case their customers do not specify tolerance levels for the parts they commission the machinists to produce.

Depending on their protocols, machinists may follow a CNC machining tolerance chart like the one from the previous section. Various bodies for international standards tend to set the ranges of standard machining tolerances. These bodies include:

  • The International Organization for Standardization (ISO)
  • The American Society of Mechanical Engineers (ASME)
  • The American National Standards Institute (ANSI)

Bilateral Tolerance

Word or phrase Bilateral in a dictionary

Bilateral tolerances involve variations between the given dimensions. These variations may either be negative or positive, as the “±” symbol indicates. The actual tolerance a machinist applies when producing a part may deviate from a given dimension by a minuscule level.

For example, a machined part 0.1mm shorter or longer than the specified measurement if the given bilateral tolerance is ±0.1. This variation in measurement usually applies to a specific part’s exterior dimensions.

Geometric Dimensioning

This machining tolerance type involves more thorough processes than other systems. Geometric dimensioning and tolerancing is a CNC machining tolerance highlighting measurements and appropriate deviations. Machinists tend to use geometric dimensioning and tolerancing for parts with extremely precise dimensions.

These tolerances also specify a machined part’s geometric characteristics. So, a machinist would know about a final product’s concentricity, flatness, and true position regarding the CNC machine.

Unilateral Tolerance

Unilateral Tolerances - KSI Swiss
Unilateral Tolerances - KSI Swiss

Machinists usually implement unilateral tolerances when they design parts that go into another. These tolerances allow deviations in one direction only, meaning the variation can only be either negative or positive.

For instance, a unilateral tolerance may look like +0.00mm / -0.05mm. This given tolerance means that a part can only be smaller by 0.05mm and must not exceed the specified measurement once finished.

So, a part calling for 1.5mm can measure at least 1.45mm once finished. Should this part exceed 1.5mm, it cannot go into its intended position, which would affect the final item.

Limit Tolerance

This CNC machining tolerance involves a range of values. Limit tolerances are basically the limit of measurement indicating that a final part will work optimally as long as it falls between the given value range.

For instance, a 13mm-13.5mm limit tolerance specifies that a part must measure between this range once a machinist finishes making the part. The upper limit is 13mm and the lower limit is 13.5mm.

How To Choose Tolerance

Tight Tolerance - KSI Swiss

Choosing the right CNC machining tolerances involves part assessment. A machinist must determine at what point will dimension or size variation affect a part’s overall function and performance once they make it. In general, a machinist must select the best method to ensure that a part remains optimal.

Determining the limits requires close assessment of the type of part being created. For instance, critical components in engines may require higher degrees of precision than other parts. The final product may experience serious consequences should the machinist apply any deviation from the given geometric forms.

Meanwhile, other parts may allow a degree of freedom in precision. These types of parts that allow slightly larger deviations in their manufacturing processes may give machinists more liberty in manufacturing their parts.

Here is a closer look into the various ideas that machinists may consider when they choose between CNC machining tolerances.

Identify Part Functions

A machinist will adjust to their clients’ needs for their parts. In other words, clients requiring parts to be made must communicate with their machinists during the creation process. Everyone involved must have a clear understanding of the part’s function, so the machinist can apply the right tolerances.

For example, as mentioned earlier, parts that need to fit with one or many components may need tight tolerances to optimally produce. Without a specific tolerance, CNC machining services may proceed with standard tolerances that do not indicate a client’s unique designs.

A single part may also require a tighter tolerance in one of its structures. The different tolerances that a single component may require add to the importance of understanding the part’s function. In cases where different sides may need various precision levels, the points where a part must fit into holes tend to need the highest precision. After all, the higher the precision, the tighter the tolerance.

In essence, specific parts require a specific tolerance for proper designing. Understanding what a part will be used for can help determine the degree of accuracy a machinist needs when producing a part.

Once a machinist knows a part’s function, they can adjust accordingly. They may not need a tight tolerance when creating parts that do not combine or fit into another, thus requiring less milling accuracy.

CNC machining providers can also save money by understanding the functions of the parts they create. For instance, machinists may opt-out of tighter tolerances in instances where they are unnecessary. Relatedly, CNC machining providers can avoid charging clients more money than they require for their services.

Consider Costly Tight Tolerances

The cost of high tolerances - KSI Swiss
Price vs quality, the cost of high tolerances.

Regarding a tighter tolerance, CNC machining tools can cause wear. Thus, tighter tolerances tend to be costly. Tools may produce a single part in the beginning without problems. However, the same tool may begin failing if a machinist must produce multiple versions of the same part. This production affects precision consistency.

Machinists may have to replace their machining tools often if a client orders a part in thousands. In addition, the machinist may adjust machining speeds. This adjustment may boost production time, thus increasing the cost.

Another factor that influences high tolerance machining’s high costs is its speed. High tolerance machining works much more slowly than other tolerances. Like adjustments in production time, delays affect the costs that a CNC machining service provider may require to complete a part.

Meanwhile, the tools capable of high tolerance machining can be more expensive than others. Using the machines themselves may require specialized operations to use, adding to costliness.

Tighter tolerances may also need wider CNC inspection measures than loose tolerances. Tight tolerances work with millimeters, which are small enough in and of themselves. As such, the window for error is small. This risk boosts costliness. Relatedly, the failure rate with high tolerance machining is high, influencing the cost of producing parts that require such tolerances.

Inspect Parts

Inspecting Tolerance - KSI Swiss

Parts with extremely tight tolerances require extra time for an inspection to determine their exact tolerance. Machinists may also need specialized equipment to assist with this verification. These tools can measure specific parts’ tolerance, so machinists can adjust accordingly.

Determine the Chosen Manufacturing Methods

Connecting with the previous strategy, a machinist will identify the CNC machine with a suitable tolerance for the parts to be created. CNC machines vary in their capacities to produce different kinds of parts. Some machines can handle one thing that another cannot.

As such, a machinist must assess the manufacturing methods they are ready to provide. Once they have set expectations on their capacities, they can easily identify the tolerances they can handle.

Some cases may complicate the process and require machinists to conduct manual operations on a part. These additional operations serve as preparation for the machining process that may not achieve the fine tolerance a part requires.

Assess the Chosen Material

Bar Stock Tolerances
Different materials have different tolerances.

Machinists adjust to their client’s choice of materials when selecting a machining tolerance. Different tolerances have unique effects on various types of materials. Likewise, different materials have unique characteristics that result in the various effects of tolerance on them. Common material characteristics that machinists consider include:

  • Abrasiveness: The coarser a material is, the tougher they are on cutting tools. As such, rough materials can wear out a CNC machine faster than smooth materials. Coarse materials can also affect how a machine achieves a specific tolerance. Machinists may have to adjust the cutting tool, resulting in less accuracy when making a part. Constant machining on abrasive material may eventually require the technician to change the tool. As such, the machining process may get delayed.
  • Hardness: Hard materials tend to be better for tight tolerances than soft materials. Soft materials risk changing dimensions when cutting tools touch them because of their natural disposition to expand and warp. As such, machining soft materials against tight tolerances requires machinists to adjust their processes to ensure the final product remains optimal.
  • Heat Stability: Machinists who work with non-metals must also adjust their operations when machining specific materials. Heat tends to build up during the machining process because of the rapid motions involved in the entire operation. As heat builds around the material, it may lose shape. As such, this quality restricts the kinds of processes a machinist can apply to a particular material.

Be Sure To Choose the Right Tolerance

Product Engineers Decide Part Tolerances - KSI Swiss
Product Engineers Decide Part Tolerances.

Engineers must know how to find the right tolerance and know that they selected the correct tolerance for a particular product. Knowing the right tolerance ensures that a designer understands how much room there is for deviation and variation in a part’s given tolerance.

Besides getting a good idea of the best tolerance for a particular product, understanding how much leeway a part has in the machining process can also influence how fast the production time will go. Likewise, the CNC machining company can craft a price that suits the services they would render.

Getting an estimated price may also help clients avoid paying more than they should accidentally. Some cases involve CNC machining service customers spending more money than required, thinking they asked for the best quality a service provider can offer.

As such, communication between machinists and customers is imperative when it comes to finding the right tolerances. Together, the CNC machining service provider can adjust to their client’s goals to assess the best tolerances for the parts they require.

Most of the time, a client’s engineers may specify the tolerances they need for a particular project. Once they submit these specifications to a CNC machining company they trust, the production process goes smoothly.

Should a client not specify their preferred tolerances for a specific project, machinists automatically apply standard tolerances. Although these tolerances allow some leeway that may not immediately be detectable, these minuscule changes may influence the final product’s performance.

Conclusion

Machinists looking for new machine tools to improve their operations may benefit from a Swiss lathe. This machine offers more tolerance options than standard lathes, which may expand a CNC machining company’s services by allowing them to create more precise parts for more industries.

Tolerance is an important factor when creating parts in the machining industry. Machining different materials with a Swiss lathe may be easier than using a regular lathe because of the CNC capabilities it offers. With automated features, machinists have better control of the tolerance that a material may need.

Choosing the right tolerance for a material involves understanding the final product’s function. Knowing what product must be created helps machinists adjust the tolerances they apply to materials to achieve the part they need. Machinists would do well to communicate with their clients regarding the project in question.

Consider getting a Swiss lathe from KSI Swiss. Our world-class CNC automatic lathes are ideal for any machining company looking to expand operations with modern processes. Get in touch with our representatives to determine the best product for your business.

References

https://www.rapiddirect.com/blog/cnc-machining-services-tolerances/

https://www.ctemag.com/news/articles/swiss-made

https://www.protocase.com/resources/tolerances/cnc-machining.php

https://fractory.com/cnc-machining-tolerances/