Regardless of what you’re manufacturing, CNC machine tolerance is an important factor to consider throughout your processes. But to understand everything required of you during this manufacturing process, you must first understand what tolerance is, its types, and how to measure them.
What Are Tolerances?
Die tolerance of CNC machines refers to the amount of allowance allowed for every manufactured part. It determines the acceptable limit within which the part in question can go or stretch from the expected parameters without compromising its quality.
See it as some form of allowance, like what we have in tailoring. When making clothes, tailors or designers give room for error, which they call allowance. This allowance makes it so that if the cloth in question is too tight, it can be adjusted to fit the wearer.
In engineering, tolerance is the permissible degree of error that allows manufacturers to deviate from the original design, giving room for modifications without losing the plot. This tolerance plays a huge role in the assembly process.
Calculation and Expression of Machining Tolerances
For the proper understanding of CNC machining tolerances calculations, let’s explore common terminologies you may encounter.
Before machine parts production, the manufacturing team always has a stipulated size for each item. This size is called the basic size. However, it often changes during the actual manufacturing process. So, the basic size is a starting point, describing the intended dimension of the design and making room for tolerance along the way.
The actual size is the dimension arrived at after the manufacturing process is completed. Unlike the basic size, which operates more like guesswork, the actual size of any manufactured part is more practical and realistic. The designers, at this point, try to make sure there is little difference between the basic size and the actual size, making them as close in dimensions as possible.
In manufacturing, limits are the boundaries that determine how big or small the parts or components of a manufactured product can be. There are two types of limits: upper limits and lower limits. The upper limits refer to the largest size a part is allowed to be, while the lower limit is the smallest size a part is allowed to be. Any part that falls outside these limits cannot be used.
The term “deviations” is the measurement of how much a part is different from the intended size. It helps manufacturers determine whether a part is bigger or smaller than their projections. So, while the basic size allows for variations during manufacturing, the deviation tells us how much different the final product is from what was originally planned.
This deviation is of two types: upper and lower deviation. The upper deviation measures how much bigger a product is from its planned size. To calculate this, you subtract the upper limit (i.e., the largest size a part is allowed to be) from the basic size.
Similarly, the lower deviation measures how small a part is allowed to be from its intended size. You can calculate this type of deviation by subtracting the lower limit (the smallest size a part is allowed to be) from the basic size.
A datum is a line of reference used for calibrating measurement equipment. It is to tolerances of CNC machines what a line of the equator is to geography. Both are imaginary lines. So, to measure the components needed for manufacturing, you must start at a specific point. This datum is the point manufacturers tend to use as their starting point. The aim is to ensure that everything is in the right place and is the right size.
Maximum Material and Least Material Requirements
Maximum Material Condition (MMC) and Least Material Condition (LMC) are two important things to consider in machining tolerances. MMC occurs when the manufactured part has the maximum allowed material, while LMC occurs when it has the least.
To explain how we can accurately calculate typical CNC machine tolerances, let us use a screw with a diameter of 15mm for reference. But then, we will also need to set the upper and lower limits of this screw. So, let’s say that the upper limit is 10mm and the lower limit is 6mm. Next, we subtract the lower limit from the upper limit to get the tolerance, expressed below:
Tolerance (t): Upper Limit – Lower Limit
t: 10 mm – 6 mm = 4 mm
From the above, we can say that 4mm is the acceptable range within which the diameter of our screw can vary.
Common Types of Tolerances in CNC Machining
While there’s an endless list of machining tolerances, these top the chart:
Standard tolerance, also known as general tolerance, refers to the regular tolerance allowed for common machine parts. So, whenever an engineering drawing does not make provisions for specific dimensions, the manufacturers turn to these standard tolerances.
In die praktyk, hierdie standard CNC machining tolerances abide by international regulations, and each region has its specific guidelines. For example, in Europe, the standard is ISO 2768, while in the United States, the go-to is ASME Y14.5. These standardized tolerances help to simplify the machining process and ensure consistency and efficiency throughout, even where the engineering drawings do not provide for specific requirements.
Bilateral tolerance concerns itself with symmetry and allows variations from different angles. Unlike unilateral tolerances, where the allowed deviation can only go in one direction, bilateral tolerance can play on both(positive and negative) sides of the base dimension.
Geometric Dimensioning and Tolerancing (GD&T)
This is a more complex system when compared to standard tolerance. While the standard dimension and tolerance cover the basic shape of a machine part and its tolerance, GD&T takes it a step further by not only specifying the dimensions but also providing the exact geometric characteristics those dimensions apply to.
To properly understand profile tolerance, we must first identify a profile line. A profile line is a line that runs across the cross-sectional area of a workpiece, ultimately outlining its shape.
The profile tolerance itself determines the acceptable range for this line, and it is measured in dimensional units like millimeters or inches. So, unlike other tolerances, this one regulates not only the size and dimension of a part but also its shape and curve to ensure it meets the strict design criteria.
Orientation tolerance concerns itself with how the shape of a workpiece can deviate from the original plan. This deviation is not measured in degrees, as one might expect when dealing with angles, but rather in dimensional units like millimeters or inches.
As its name implies, form tolerance deals with the physical features of a piece. So, here, instead of dimensions, we’re talking about the expected flatness tolerance on a CNC machine. It can also extend to its roundness or straightness. The goal is to ensure that the physical form of the manufactured part looks like the design specifications. Like oriental tolerance, we’re also dealing with millimeters or inches here.
In CNC machining, limit tolerance serves as a guide for what the acceptable dimensions are. Limit tolerance uses a two-dimensional scale to define what the upper and lower limits of a part’s dimension may be.
The upper limit is the maximum acceptable measurement, while the lower limit represents the minimum. See these limits as boundaries— anything measured between them is considered within the acceptable range. For instance, a limit tolerance of 0.55 to 0.65 mm means the upper limit is 0.65 mm and the lower limit is 0.55 mm. If the final measurement falls within this range, the part meets the precision criteria.
This tolerance addresses any shift in the specific features of a machine part from its intended position. It involves understanding how some elements may naturally deviate from their prescribed location.
So, for example, if a flat metal plate is to have a hole in a specific position. The location tolerance of CNC machines will allow a range of deviations from this position.
Unilateral tolerance provides that allowed deviations can only occur in one direction. That is, these deviations can only be an increase or decrease from the original dimension and not both.
Runout tolerance defines the allowed variation that can occur as a feature revolves around a central point. So, for example, we are manufacturing a spindle or a wheel, and we need it to rotate smoothly without wobbling or excessive deviation. Runout tolerance comes in to ensure that, as the wheel, for instance, completes a full rotation, any variation in its position stays within the allowed limits.
Unequally Disposed Tolerances
Unequally disposed tolerances are typically referred to as the U modifier, and it is represented by the letter ‘U’ enclosed in a circle. They come into play whenever the creation of a specific piece will require an unequal unilateral tolerance.
Factors to Consider When Choosing CNC Machine Tolerances
When it comes to choosing the appropriate CNC machine tolerances for your manufacturing system, you must consider many things. Each of these factors plays a special role in determining whether or not your manufactured product has met the laid-down requirements it should meet. Here are some factors you may consider before choosing CNC machine tolerances:
1. Do You Need Tight Tolerance?
Whether or not you should go for tight tolerance will depend on the specific requirements of your manufacturing project. Now, on the face of it, it may make sense to go for tight tolerance so you can get better accuracy.
But note that this decision will come at a price: higher costs and longer production hours. So, evaluate whether or not your production can go on without tight tolerance before making this (expensive) decision.
Striking a balance between tolerance and costs is important for you to manage your resources effectively. Most manufacturers look for the best tolerance that matches their budget, and that is an approach you may choose to take.
3. Machining Methods
The type of machine you use during manufacturing can affect the tolerance of your finished product. So, what you can do is check the tolerance levels of your machines before starting. Ask yourself, ‘What tolerance can a CNC machine do?’ Then, use the answer as a guide or reference point to determine what the best tolerance for your production is.
4. Surface Roughness
Different manufacturing materials have different types of surface textures, which can affect the typical CNC machine tolerances. So, when aiming for tight tolerance, you may want to consider the level of surface roughness you’re working with. That way, you can anticipate difficulties and act accordingly. For example, polished surfaces are much easier to work with than rougher ones.
Quality control is an important part of any manufacturing process, and this also covers CNC machine tolerance. What this inspection does is to check and see if the final products align with the specified tolerances. Now, choosing tight tolerance means the time required for an inspection will understandably be longer, and vice versa. So, bear that in mind before making your choice.
How to Find the Right Tolerance
Here are some ways you can choose the right tolerance for your CNC machining:
Use a Reputable CNC Machining Company
Outsourcing your project to a reliable CNC machining company can help take this problem from your hands. By relying on professionals, you can get advice on the best-suited tolerances for your manufacturing process. Plus, a good professional would have state-of-the-art equipment to ensure that you get the best results. A reputable company should also have a CNC machine tolerance chart that will make the process easier and faster.
Calculate the Tolerances Yourself
You, too, can calculate the best tolerance to use. But first, you must define the intended function of the part you plan to manufacture. This will lay the foundation for deciding what CNC machine tolerance you want to adopt.
Next, consider how the functionality of that part aligns with the tolerance you have in mind. Some productions may demand tighter tolerance, while others may work well with looser tolerance without compromising standards.
Once you have determined all these, you can then use the standard general rules that apply to tolerance calculations.
Use International Standards for Machining Tolerance
By following internationally recognized benchmarks, you can ensure that your CNC machining is compatible with industry standards. So, check which international standards fit your project requirements and your specific industry. For example, if your outfit is in Europe, stick to the ISO 2768. But if you’re in the US, the ASME Y14.5 is more acceptable.
Tips for Tighter CNC Machining Tolerance
Here are some guidelines to help you achieve tighter CNC machining tolerances:
Use Quality CNC Machines
Investing in superior-quality CNC machines is one of the many ways you can achieve tight tolerances. So, when choosing machines, check for their features, precision, and reliability. This will ensure that the manufactured product will meet the required tolerance levels.
Choose the Right Cutting Tools
Selecting the right cutting tools is also another way to achieve tight tolerance. Depending on the material type and the machining requirements, these tools can influence the accuracy of the final product. So, look out for specifics like cutting speed, coatings, tool geometry, etc.
Get Good Material Suppliers
The quality of the raw materials you use will directly affect your CNC machining outcomes. So, to get the best materials, partner with reputable material suppliers known for delivering high-quality materials.
Do Machining in a Temperature-Controlled Environment
Temperature differences can affect the dimensions of your machined parts. So, carrying out your machining in a temperature-controlled environment can influence the impact of your temperature fluctuations. Where the temperatures are consistent, you can get stable machining conditions and, in some way, predict the accuracy of your final product.
Hire Skilled Labour
Skilled workers also play a role in achieving tight tolerances. So, when employing machine operators, look for experience and those with a deep understanding of CNC machine tolerances. Their expertise will greatly affect the outcome of your manufacturing process.
Balance Tool Holders
Regularly balance your tool holders to reduce vibrations during operation, as these vibrations can affect the accuracy of your CNC machining process. But with balanced tool holders, you can get smoother cutting operations and achieve your desired tolerance.
Maintain and Calibrate Your CNC Machines Regularly
Regular maintenance and calibration of your CNC machines will guarantee consistent performance. So, add routine maintenance to your schedule to catch early address wear and tear of your machines and extend the lifespan of your equipment.
Implement Strong Quality Controls
Earlier, we spoke about how quality control is important to CNC machining. Throughout your manufacturing process, make room for robust quality control measures. For example, you can conduct regular inspections and rigorous testing to spot errors at an early stage. Also, you can try recording your CNC tolerance machine feedback over time to help you keep track of your activities and implement better quality controls.
Importance of CNC Machining Tolerance
CNC machining tolerance is more than just a technical term in the manufacturing process. It plays a big role in determining the quality, functionality, and reliability of your finished work.
In CNC machining, where precision is everything, tolerance is the thin line between making errors and producing excellent work. Understandably, every manufactured work will have some errors, which is why we make provisions for them.
However, tolerance comes in as a safeguard against too many deviations from the master plan, as too many deviations can compromise the integrity of the final product. Deviations generally have severe consequences, but CNC machining tolerance can help avoid these errors entirely.
The manufacturing process is not complete without machining tolerance. To ensure your products meet the recommended tolerance levels, you should consider the guidelines we discussed in this guide. But if you’re still unsure about what steps to take, we can help make our work easier. Contact us today!