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Machining Tolerance Guide: Tolerance Grades Achievable by Different CNC Equipment

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We deal with cnc machining every day. Do you know what precision tolerance grades can be achieved by turning, milling, planing, grinding, drilling, and boring? Tolerance grade refers to the level that determines the accuracy of a dimension. The national standard stipulates 20 grades, from IT01, ITO, IT1, IT2 to IT18. The larger the number, the lower the tolerance grade (machining accuracy), the larger the allowable range of dimensional variation (tolerance value), and the easier the machining. Product parts require different machining accuracy depending on their function, and the selected machining methods and processes also vary. This article introduces the machining accuracy achievable by several common machining methods such as turning, milling, planing, grinding, drilling, and boring, and what the tolerances are in the field of machining .

Machining Accuracy Overview

Machining accuracy is primarily used to describe the degree of precision in product manufacturing. Both machining accuracy and machining error are terms used to evaluate the geometric parameters of a machined surface. Machining accuracy is measured by tolerance grades; the smaller the grade value, the higher the accuracy. Machining error is expressed numerically; the larger the value, the greater the error. High machining accuracy means low machining error, and vice versa.

There are 20 tolerance grades, ranging from IT01, IT0, IT1, IT2, IT3 to IT18. IT01 indicates the highest machining accuracy of the part, while IT18 indicates the lowest machining accuracy. Generally, IT7 and IT8 are medium-level machining accuracy grades.

No machining method can produce absolutely accurate actual parameters. From the perspective of the part’s function, as long as the machining error is within the tolerance range required by the part drawing, the machining accuracy is considered to be guaranteed.

What is tolerance?

In short, tolerance is a measurement that indicates the level of precision you want to achieve in a part. Specifically, CNC tolerance indicates the degree of variation allowed in the final dimensions or measurements of a custom-machined part.

In CNC machining workshops, parts are measured numerically, typically indicated by a ± sign. For example, you might specify a tolerance of ±2.550 inches for a part with a length of 0.001 inches. This indicates that the length of the manufactured part is variable, with measurements between 2.549” and 2.551”. If a part with a height of 1.5 inches requires a tolerance of ±0.005”, the final part should fall within the range of 1.495” and 1.505” to pass quality inspection.

The standard or non-standard tolerances in the drawings indicate the level of precision that CNC machining service machinists should use when producing parts. The smaller the number (which is a stricter tolerance in manufacturing), the higher the required precision. The larger the number (also known as a looser tolerance), the lower the precision you need.

What is the most stringent tolerance for CNC machining ?

Want to know the absolute limits of CNC precision? Achieving ultra-tight tolerances seems impossible. Want to explore the truly achievable limits and how? The tightest CNC tolerances aren’t a single number. It depends on the complexity of the machine, materials, and parts. Generally, with top-of-the-line CNC equipment and rigorous process control, precision below +/- 0.001 inches (+/- 0.0025 millimeters) is achievable ( something typically impossible with standard equipment ) .

Therefore, defining the “most stringent” tolerance for CNC machining is not easy, as it is not a fixed value. It actually depends on a variety of factors. First, the machine tool itself must be considered: its age, maintenance, and inherent performance. A brand-new, high-end five-axis machine tool can naturally achieve tighter tolerances than an older, poorly maintained three-axis machine tool. Second, the material being machined must be considered. Some materials (such as certain grades of aluminum or steel) are more stable and easier to machine precisely. Other materials (such as certain plastics or special alloys) may be more difficult to machine due to factors such as thermal expansion or tool wear. Furthermore, the geometry of the part also plays a significant role. Complex features, very thin walls, or deep cavities may be more difficult to maintain tight tolerances.

Therefore , manufacturing and inspecting parts with extremely small tolerances, even below +/- 0.001 inches (approximately +/- 0.0025 millimeters), requires meticulous planning, the use of appropriate cutting tools, skilled machinists, and robust quality control processes. Thus, while there is no universal “most stringent” tolerance applicable to all situations, achieving extremely high precision is absolutely possible with the right methods and equipment. The key lies in understanding the various variables and pushing the limits of possibility for each specific project.

Manufacturing characteristics and machining accuracy of common CNC machining equipment

Different CNC machines can manufacture different types of parts with varying degrees of precision. Parts with complex designs and different precision requirements may require a combination of different machines to manufacture. Below is a list of the various CNC machines commonly used by Elimold and the precision levels they can produce.

Ordinary lathe

A lathe is a machine tool primarily used to machine rotating workpieces using a cutting tool . Common lathe machining accuracy is generally IT7~IT8, with a surface roughness of Ra0.8~1.6µm. Initial turning aims to improve turning efficiency by using a large depth of cut and feed rate without reducing the cutting speed, but the machining accuracy can only reach T11, with a surface roughness of Ra10~20µm .

Semi-finish turning and finish turning should use high speed and small feed rate and depth of cut as much as possible. The machining accuracy can reach IT10~IT7 and the surface roughness is Ra0.16~10um.

High-speed precision turning of non-ferrous metal parts on a high-precision lathe using a finely honed diamond cutting tool can achieve a machining accuracy of IT5~IT7 and a surface roughness of Ra0.01~0.04um. This type of turning is known as “mirror turning”.

CNC lathe

CNC lathes and turning centers are high-precision, high-efficiency automated machine tools. Equipped with multi-station turrets or power turrets, these machines possess a wide range of machining capabilities, capable of processing complex workpieces such as straight cylinders, inclined cylinders, arcs, and various threads, grooves, and worm gears. They feature linear interpolation, circular interpolation, and various compensation functions, demonstrating excellent economic benefits in the mass production of complex parts. CNC lathes utilize digital signals and programmable automatic control to control the position, angle, speed, and force signals generated during equipment operation and machining processes; that is, they automatically machine the workpiece according to a pre-programmed machining program. CNC lathes typically achieve turning precision up to IT6 and surface roughness up to Ra1.6µm.

Conventional milling machine

A milling machine is a machine tool that uses milling cutters to process various surfaces of a workpiece. Typically, the milling cutter’s main motion is rotation, while the movement of the workpiece and the milling cutter is the feed motion.

  • Milling can typically achieve a machining accuracy of IT7~IT8, with a surface roughness of Ra1.6~6.3um.
  • During rough milling, the machining accuracy is IT11~IT13, and the surface roughness is Ra5~20um;
  • The machining accuracy during semi-finish milling is IT8~IT11, and the surface roughness is Ra2.5~10um;
  • The machining accuracy during precision milling is IT6~IT8, and the surface roughness is Ra0.63~5um.

CNC milling machine

CNC milling machines, also known as Computer Numerical Control (CNC) milling machines, utilize digital information to control mechanical motion and the machining process. These machines offer strong adaptability and flexibility in part processing, capable of machining parts with particularly complex contours or difficult-to-control dimensions, such as mold parts and shell parts; they can also machine parts that are difficult or impossible to machine with conventional machine tools, such as complex curved parts described by mathematical models and three-dimensional curved surface parts; and they can machine parts that require multiple machining operations after a single clamping and positioning. These CNC milling machines typically achieve the following levels of precision:

  • The milling precision in CNC milling can generally reach IT6-IT8, and the surface roughness is Ra0.6-5um.
  • The positioning accuracy of linear motion coordinates is 0.04 mm, the repeatability is 0.025 mm, and the milling accuracy is…
  • 0.035mm.
  • Ultra-precision machining currently achieves dimensional and positional accuracy of 0.01~0.3µm, shape and contour accuracy of 0.003-0.1µm, and surface roughness of Ras 0.05µm for steel parts and Ras 0.01µm for copper parts.

Drilling machine

Rotary beds are usually divided into several different types, and Elimold commonly uses the following types.

A radial drilling machine is a type of drilling machine in which the radial arm can rotate and move up and down around the column, and the spindle box usually moves horizontally on the radial arm. It refers to a machine tool that mainly uses a drill bit to process holes in a workpiece. Usually, the rotation of the drill bit is the main motion, and the axial movement of the drill bit is the feed motion.

A bench drill, or bench drill for short, is a small drilling machine that can be placed on a worktable with a vertically arranged spindle. Bench drills typically drill holes with a diameter of 13mm or less, generally not exceeding 25mm. Spindle speed is usually changed by altering the position of the V-belt on the pulley, and spindle feed is manually operated.

Drilling is a fundamental method of hole machining. Our drilling operations are often performed on drilling machines and lathes, but can also be performed on boring machines or milling machines. The machining accuracy of drilling is relatively low, generally only reaching IT10, and the surface roughness is generally 12.5~6.3um. After drilling, reaming and boring are often used for semi-finishing and finishing.

grinder

A grinding machine is a machine tool that uses grinding wheels and abrasives (such as grinding wheels, sandbags, oilstones, and abrasive compounds) to grind the surface of a workpiece. It can process various surfaces, such as planes, internal and external cylindrical surfaces, conical surfaces, and helical surfaces. Through grinding, the shape, surface precision, and finish of the workpiece are brought to the desired level; it can also perform cutting operations. Commonly used grinding machines include hand-operated grinding machines, surface grinding machines (fine grinding), and cylindrical grinding machines. This type of equipment can generally manufacture parts with the following precision requirements.

  • Grinding accuracy of ordinary grinding machines: For steel plates of 160x160mm, flatness 0.008mm~0.004mm, surface roughness Ra0.8um; for steel plates ≤250x250mm, flatness 0.025mm~0.01mm, surface roughness Ra0.8um; for steel plates of 630~1000, flatness 0.08mm~0.05mm, with reference tolerance grade IT5~IT7, surface roughness Ra0.16~1.25um.
  • The grinding accuracy of precision CNC grinding machines is as follows: steel plate 0~500mm, flatness 0.004mm, roughness Ra0.2um. High-precision grinding uses oilstones, but these can only remove a few micrometers of material.
  • Ordinary cylindrical grinding machines have a roundness of 3µm and a surface roughness of Ra 0.4µm; high-precision cylindrical grinding machines can achieve a roundness of 0.1µm and a surface roughness of Ra 0.01µm.

Gear hobbing machine

In metal cutting machine tools, the machine tool used to process gears or worm gear teeth is called a gear hobbing machine. Currently, the most widely used gear hobbing machine is the CNC gear hobbing machine. CNC gear hobbing machines are suitable for batch, small batch, and single-piece production of cylindrical gears and worm gears, as well as drum gears with certain parameters. They can also continuously index and hob short spline shafts with a length less than 300 mm for 6 or more teeth using a spline hob. Furthermore, sprocket hobs can be used to hob sprockets. When machining cylindrical gears, both climb milling and conventional milling can be used, employing axial feed (vertical feed) to machine the full tooth width. CNC gear hobbing machines use radial feed to machine ordinary worm gears. The adjustment and machining methods for CNC gear hobbing machines when machining spline shafts and sprockets are the same as when machining cylindrical spur gears. Its machining accuracy can reach grade 4-5, according to GB10095-88.

planer

A machine tool that uses a cutting tool to carve flat surfaces, grooves, or shaped surfaces of a workpiece. This type of equipment is characterized by its versatility, simple structure, and reciprocating motion. Its machining accuracy can reach IT8~IT7, and the surface roughness Ra of the machined surface is 1.6~6.3µm.

EDM machine

Electrical Discharge Machining (EDM) is a type of machining equipment primarily used for electrical discharge machining. It is widely used in the manufacture of various metal molds and mechanical equipment. EDM utilizes the electro-erosion effect generated by pulsed discharge between two electrodes immersed in a working fluid to remove conductive materials. It is also known as electrical discharge machining or electro-erosion machining, and its English abbreviation is EDM.

Electrical discharge machining (EDM) with perforation is based on the principle of electro-erosion and can process workpieces with complex shapes and hard materials. The surface roughness can be controlled between 0.63 and 2.5 μm using this method. The electrode material varies depending on the material being processed. During the process, the electrode consumes energy, leading to electrode wear. In actual EDM, many factors need to be controlled, such as the magnetic field energy and heat energy around the electrode. These interfering factors directly affect the required machining accuracy. Generally, the minimum discharge distance is 0.50 mm, the corner distance can reach 0.05-0.07 mm, the discharge depth is (30 ± 0.02) mm, the discharge accuracy is typically ±0.01 mm, and the surface roughness is Ra 0.8 μm.

Wire cutting

A wire EDM machine is a type of electrical discharge machining tool that uses a molybdenum wire to cut metal (especially hard materials and complex-shaped parts) through electro-erosion. The working principle of a wire EDM machine is to use a continuously moving fine metal wire (called the electrode wire, usually molybdenum or copper wire, commonly with specifications of 0.12, 0.14, 0.16, 0.18, and 0.20 mm) as the electrode to perform pulsed spark discharge cutting on the workpiece. (The instantaneous temperature of the pulsed spark discharge can reach 7000°C or higher; the high temperature causes the metal being cut to vaporize instantaneously, generating metal oxides that dissolve in the cutting fluid.)

Wire EDM uses brass wire with a cutting accuracy of ±0.02m and a processing speed of 5m/min. There is a wire radius R angle at corners (if using electrical discharge, the corner radius is approximately R0.1~RO.2mm). Many specifications of commonly used copper or molybdenum wire are available, including :

  • 0.20mm wire: Under normal processing conditions, the minimum machinable groove width is 0.25mm, and the minimum inner corner radius is between R0.11 and R0.13mm.
  • 0.15mm wire: Under normal processing conditions, the minimum machinable groove width is 0.20m, and the minimum inner corner radius is R0.08~R0.1mm.
  • 0.10mm wire: Under normal processing conditions, the minimum machinable groove width is 0.15mm, the maximum workpiece thickness is 25mm, and the minimum inner corner radius is R0.06~R0.08mm.

The smaller the wire diameter, the easier it is to break during processing, and the lower the efficiency. A Q0.1mm wire takes approximately four times longer to process than a medium 0.2mm wire, and twice as long as a Q0.15mm wire. The thicker the workpiece, the larger the internal corners require during processing. Oil-processed internal corners are smaller (insulating medium classification: pure water and kerosene), approximately 0.02mm, and are used for processing mold punches or applications requiring 0.10mm copper wire.

Wire EDM can achieve a maximum angle of 45°, a maximum thickness of 80mm, and an accuracy of ±0.002μm. With water as the medium, an oxide layer will adhere to the workpiece surface, with a single-sided thickness of 3μm. During use, the oxide layer on the mold core will peel off, causing dimensional changes and an error of approximately 0.05μm. Oil-based machining offers high precision and quality, and can process all conductive materials, but it is also more expensive.

CNC engraving machine

From a processing principle perspective, engraving machines are a combination of drilling and milling. Based on different engraving methods, they can be divided into two main categories: laser engraving and mechanical engraving.

  • Laser engraving: mainly used in the machinery industry, it uses CNC technology as the basis and laser as the processing medium. The physical transformation of the material under laser engraving irradiation is instantaneous melting and vaporization, which enables laser engraving to achieve the processing purpose; the processing accuracy of laser engraving machines is usually 0.05mm.
  • Mechanical engraving: also known as CNC engraving machine, is mainly used in woodworking, advertising, crafts and gifts , etc. The processing accuracy of mechanical engraving machine is usually 0.05m.

Laser cutting machine

Laser cutting involves focusing a laser beam emitted from a laser source into a high-power-density beam via an optical path system. This beam irradiates the workpiece surface, causing it to reach its melting or boiling point. Simultaneously, high-pressure gas, coaxial with the beam, blows away the molten or vaporized metal. As the relative position of the beam and the workpiece moves, a kerf is formed in the material, achieving the cutting purpose. Its optimal cutting accuracy is ±0.02mm, and the kerf roughness Ra < 6.5µm.

CNC engraving machine

A CNC engraving and milling machine is a type of CNC machine tool that can precisely perform various machining operations. Its machining accuracy is 0.002 μm, and its surface roughness Ra < 0.015 μm.

Bending machine

A bending machine is a machine that can bend thin plates. Its structure mainly includes a support frame, a worktable, and a clamping plate. Its bending accuracy is 0.3mm, with a maximum parallelism of 0.04 and a maximum perpendicularity of 0.2.

saw

Machine tools used for sawing various metal materials. There are three common types of saws: circular saws, band saws, and bow saws. Their machining accuracy is typically burr-free sawing, with a perpendicularity of up to 0.1 mm and a surface roughness Ra of 3.2~1.6 μm.

What are the ISO standards for machining tolerances?

Unsure which tolerance standards apply to your CNC parts? ISO 2768 is the primary ISO standard for general machining tolerances. If a tolerance is not specified, a default tolerance is provided. For more stringent and specific needs, the geometry and tolerances (GD&T) on the drawing are crucial. When discussing ISO standards for machining tolerances, ISO 2768 is the most common. This standard is very useful because it specifies general tolerances for linear and angular dimensions, as well as geometric tolerances (such as flatness, straightness, and perpendicularity) for parts made by metal cutting (machining) or forming. If specific tolerances are not indicated on the drawing, this is often used as the default standard. This helps avoid ambiguity.

In addition, ISO 2768 has different tolerance grades. For linear and angular dimensions, they are usually represented by letters, such as “f” (fine), “m” (medium), “c” (coarse), and “v” (very coarse). For geometric tolerances, grades such as H, K, and L are used. You can choose the appropriate grade according to your application requirements. However, it is important to understand that ISO 2768 is applicable to general tolerances. Geometric dimensioning and tolerance marking (GD&T) comes into play when you need very strict, specific control over certain features, or when the functional relationship between features is critical.

GD&T is a more complex symbolic language used on engineering drawings to define the nominal geometry of parts and assemblies, as well as the permissible deviations of features in terms of shape, orientation, position, and profile. Compared to simple general tolerances, it allows for more precise and explicit control. Therefore, while ISO 2768 provides a good benchmark, the critical dimensions and features of parts are usually directly annotated with specific tolerances and GD&T markings on the part drawing to ensure they meet design intent and functional requirements. At Elimold , we are proficient in interpreting and following ISO 2768 and detailed GD&T specifications in our machining processes.

How to achieve consistent CNC machining tolerances?

Professional manufacturers employ various methods to prevent tolerance variations between parts. Tool wear is frequently observed during regular part sampling and measurement by machinists; if the wear is severe enough to cause deviations from customer specifications, the tool will be replaced. To minimize appearance and measurement deviations, engineers communicate acceptable levels of variation through drawings and annotations. Customer-selected machining inspection programs, such as ASQC Z1.4 2008 Level 2 Zero Rejection, help manufacturers understand acceptable tolerance limits.

The Importance of Tolerances in CNC Machining

Tolerances are crucial in CNC machining for several reasons:

Precision and fitTolerances ensure that parts fit correctly during assembly. Without proper tolerances, even minute dimensional variations can lead to mismatched parts and functional failures.
quality assuranceTolerances allow for setting quality thresholds during the production of parts, ensuring that all parts are produced in accordance with specifications.
Cost controlTighter tolerances typically lead to higher production costs. This is because they require more precise tooling, longer machining times, and more stringent quality control. Setting appropriate tolerances allows manufacturers to strike a balance between precision and cost-effectiveness.
Material selectionDifferent materials respond differently to processing. Setting the correct tolerances helps accommodate the specific behavior of these materials, ensuring consistency in production operations.

Elimold is your professional CNC machining service partner.

Elimold ‘s standard prototypes and manufacturing tolerances conform to ISO 2768 ( metals follow ISO 2768-m, while plastics follow ISO 2768-c ) . Furthermore, we cater to specialized, high-precision needs. Simply specify your unique requirements on your drawings, and our advanced machining technologies will ensure precise compliance. Our large manufacturing facility in Shenzhen, China, also boasts numerous 3-axis, 4-axis , 5-axis , and 6-axis CNC machine tools. We also offer various surface treatments for our CNC-machined parts. In addition to CNC machining , we provide diverse manufacturing capabilities to customers worldwide.

Our CNC machining services also encompass quality inspection, material certification, and full-size inspection with reports. The full-size CNC inspection process relies on cutting-edge metrology and measuring tools and is crucial for ensuring that prototypes or final parts meet the precise specifications and tolerances required by your client. In other words, we will repeatedly check to ensure your requirements are met, and if not, we will work with you to deliver parts that do. If you have any questions about CNC machined parts tolerances or would like to learn more about Elimold ‘s CNC machining services, please feel free to contact a member of our manufacturing team.

in conclusion

CNC machining tolerances are a critical aspect of precision parts manufacturing, representing how closely dimensions approximate design specifications. Therefore, correctly understanding and selecting the appropriate tolerance grade is essential for ensuring part functionality, maintaining quality, and reducing costs. Appropriate tolerances can be achieved by considering material properties, machining methods, and the necessity of part inspection. This approach optimizes efficiency while minimizing costs. Whether it’s stringent tolerances for critical workpieces or more lenient tolerances for general workpieces, good tolerance design is a hallmark of success in any CNC operation. Elimold, as one of the most reliable CNC machining companies, has been producing precision tolerance parts and prototypes for customers in virtually every industry. If this is exactly what you need, our professional team can certainly meet your requirements!

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