CNC Milling Tolerances: Make Precision Parts

In the manufacturing industry, the precision of parts is indispensable. For example, in some industries, such as aerospace, automobile manufacturing and other fields with strict tolerance requirements, the tolerance is usually relatively small to ensure high precision and high quality processing. Therefore, to make extremely accurate parts, it is necessary to master the key factor of CNC milling tolerance, because it can determine the quality and accuracy of the final product. In this blog post on Longsheng Technology’s website, we will discuss tolerances in CNC milling and how to ensure the best production results.

Why is precision CNC milling important?

In order to achieve precision machining effect, CNC milling is a very necessary and important process. It can quickly process a variety of complex products in a short time, including tangible and intangible products.

Compared with traditional machining methods, precision CNC technology can provide high efficiency and precision in the manufacturing process. This means that through the use of CNC systems, it is easier to produce parts with high quality, control accuracy and repeatability, and enhance product quality, and CNC milling can reduce the loss of human error and improper operation in the manufacturing process, while also increasing output.

Secondly, CNC milling can improve the safety of products. CNC milling of high-precision parts, molds, tools, etc., can not only ensure the size and appearance, but also ensure the safety of products through corrosion resistance and strength and other performance control. For example, in the manufacturing of aerospace equipment, CNC milling can ensure the durability of complex systems on aircraft and spacecraft, thus ensuring flight safety.

CNC milling tolerance types

The tolerance precision of CNC milling determines the quality of the final product. The maximum allowed gap between the actual size and the design size is the error range of the parts. The following are the common tolerance types and standards that can be achieved in CNC milling

Dimensional tolerance

CNC milling can achieve a dimensional tolerance of ±0.005mm, the dimensional tolerance is the most basic type of tolerance in CNC milling process. It refers to the maximum allowable difference between the actual size and the design size of a part in manufacturing. The size of dimensional tolerance is related to the material, shape, size, machining process and required accuracy of the part.

Position tolerance

Position tolerances are also tolerance types that control the position and orientation of a part. Position tolerances include perpendicularity, parallelism, included Angle, measured length, measured Angle, measured direction, etc. Usually, position tolerances of ±0.01mm can be achieved by CNC milling

Fit tolerance

Fit tolerance refers to the maximum allowable deviation when assembling a part. Fit tolerance is determined by the shape, material, machining process and required precision and other factors. In the process of CNC milling, fit tolerance of H7-h6 can be achieved

CNC milling machining tolerance class

According to the requirements of machining method and assembly accuracy, appropriate IT tolerance grade values are selected for dimensional marking.

Linear dimension limit deviation value

Tolerance class	0~3	>3~6	>6~30	>30~120	>120~400	>400~1000	>1000~2000	>2000
F	±0.05	±0.05	±0.1	±0.15	±0.2	±0.3	±0.5	—
M	±0.1	±0.1	±0.2	±0.3	±0.5	±0.8	±1.2	±2.0
C	±0.2	±0.3	±0.5	±0.8	±1.2	±2.0	±3.0	±4.0
V	—	±0.5	±1.0	±1.5	±2.5	±4.0	±6.0	±8.0

The limit deviation value of the radius and height of the chamfer

Tolerance class	0~3	3~6	>6~30	>30
F	±0.2mm	±0.5mm	±1.0mm	±2.0mm
M
C	±0.4mm	±1.0mm	±2.0mm	±4.0mm
V

The limit deviation value of the angular dimension

Tolerance class	0~10	>10~50	>50~120	120~400	>400
F	±1°	±30′	±20′	±10′	±5′
M
C	±1°30′	±1°	±30′	±15′	±10′
V	±3°	±2°	±1°	±30′	±20′

No tolerance values are specified for straightness and flatness

Tolerance class	0~10	>10~30	>30~100	>100~300	>300~1000	>1000
H	±0.02mm	±0.05mm	±0.1mm	±0.2mm	±0.3mm	±0.4mm
K	±0.05mm	±0.1mm	±0.2mm	±0.4mm	±0.6mm	±0.8mm
L	±0.1mm	±0.2mm	±0.4mm	±0.8mm	±1.2mm	±1.6mm

No tolerance value is specified for perpendicularity

Tolerance class	0~100	>100~300	>300~1000	>1000
H	±0.2mm	±0.3mm	±0.4mm	±0.5mm
K	±0.4mm	±0.6mm	±0.8mm	±1mm
L	±0.6mm	±1mm	±1.5mm	±2mm

No tolerance value is specified for symmetry

Tolerance class	0~100	>100~300	>300~1000	>1000
H	±0.5mm
K	±0.6mm		±0.8mm	±1mm
L	±0.6mm	±1mm	±1.5mm	±2mm

No tolerance value for circular runout

Tolerance class	Circular runout tolerance value
H	±0.1mm
K	±0.2mm

Precision difference between CNC milling and CNC turning

CNC milling and CNC turning are two types of processes in online CNC machining. Although they can both be used to manufacture high-precision parts, they are different in terms of accuracy.

CNC milling tolerance

CNC milling precision can reach 0.005 mm, because CNC milling is a highly controllable processing mode, can be processed in a small size range of materials, relatively speaking, it is more suitable for processing the need for complex shape parts, in the manufacturing process to the workpiece for a variety of cutting and slotting, Tolerances can range from a few hundredths of a millimeter to a few microns

CNC turning tolerance

In contrast to CNC milling, CNC turning processes parts by rotating the workpiece and using a tool to cut material away from the workpiece. CNC turning is suitable for making cylindrical, conical, spherical and other rotating symmetrical parts. CNC turning can achieve a single tool or multiple tools at the same time, and the cutting path is usually along the axis of the workpiece. Because the workpiece rotates steadily, CNC turning can usually achieve higher machining accuracy. In high precision CNC turning, the tolerance range can be in the tens of a few millimeters to a few microns.

It should be noted that the actual machining accuracy not only depends on the machining method, but also is affected by many factors such as workpiece material characteristics, tool selection, machining parameters, equipment accuracy and operating techniques. In practical application, manufacturers will evaluate according to specific requirements and feasibility, and choose the appropriate CNC machining method to meet the required accuracy requirements.

Factors affecting the precision of CNC milling

In the process of CNC milling, many seemingly small factors may affect the accuracy of the whole process. Let’s talk about the factors that affect the precision of CNC milling and how to deal with them.

Machine tool accuracy

The geometric accuracy and dynamic accuracy of machine tools play an important role in CNC milling accuracy. The rigidity of machine tool, positioning accuracy, repeated positioning accuracy and the precision of mechanical transmission system will affect the precision of machining results.

Tool quality and wear

Tool geometry, size, quality and material all affect machining accuracy. If the tool quality is poor, the tool may bend, break or fail during machining. These problems can cause vibration, which can have a negative impact during the machining process. Therefore, the selection of high-quality cutting tools is an important step in CNC milling process. In addition, it is important to properly maintain and replace the tool during machining.

Cutting fluid

The quality and usage of cutting fluid is one of the important factors affecting the precision of CNC milling. The different substance content and proportion in the cutting fluid may have different effects on the thermal effect, chip removal, tool wear and surface quality during the machining process. In addition, factors such as the cleanliness of the cutting fluid, temperature and flow rate can also affect the accuracy and stability of CNC milling.

Workpiece clamping and positioning

The clamping and positioning of the workpiece on the machine tool is very important to maintain the machining accuracy. Poor clamping and positioning may lead to workpiece movement or deformation, thus affecting the accuracy of machining results.

Cutting parameter

Cutting parameters are also one of the factors affecting the precision of CNC milling. These parameters include tool feed speed, cutting speed and cutting depth. Selecting suitable cutting parameters can reduce cutting load and vibration and improve milling accuracy.

Programming and operation

CNC milling program writing quality, parameter Settings will also affect the machining accuracy. The programming needs to consider cutting force, tool wear, thermal deformation and other factors to maintain machining accuracy. Parameter Settings need to be optimized according to equipment characteristics, material characteristics, etc., in order to obtain the best machining effect and accuracy.

Spindle accuracy

Spindle is the core part of CNC milling. The spindle is responsible for rotating the cutting tool. If the spindle is not accurate enough, then the tool will wobble, resulting in a rough or inaccurate surface of the part. Therefore, the need to choose a high precision spindle.

Material quality

The hardness of the material is too high or too low, will have a bad effect on the precision of CNC milling. Especially some more difficult to process materials, such as titanium alloy, superalloy, more likely to cause problems in the process of processing, the need for manufacturers to do a good job in the process of processing process control and equipment debugging.

Selection and application of CNC milling tolerance level

See the table below for details:

Tolerance class	Application scope and examples
IT01	Used for ultra precision dimensional transfer benchmarks, such as ultra precision standard gauge blocks
IT0	Used for extremely precise dimensional transmission benchmarks and precision fitting dimensions that are particularly important in aerospace. For example, special precision standard gauge blocks, individual particularly important dimensions of precision mechanical parts, and calibration gauges for checking IT6 level shaft gauges
IT1	Used for precise size transfer benchmarks and extremely important precision fitting dimensions for high-precision measurement tools. For example, high-precision standard gauges are used for proofreading and verifying the dimensions of IT7 to IT9 grade shaft gauges, as well as for some particularly important precision mechanical parts
IT2	Used for high-precision measurement tools, particularly important for precision fit dimensions. For example, checking the dimensional manufacturing tolerances of the IT6 to IT7 level workpiece gauges, verifying the calibration plug gauges of the IT8 to IT11 level shaft gauges, and verifying the dimensions of some particularly important precision milling parts
IT3	Used for precision measurement tools, high-precision precision fitting of small-sized milling parts, and shaft diameter and housing aperture matching with C-grade rolling bearings. For example, the calibration gauges for inspection of IT8 to IT11 grade workpieces and the calibration gauges for inspection of IT9 to IT13 grade shaft gauges, the machine tool spindle that matches the particularly precise P4 grade rolling bearing inner ring hole (diameter up to 100mm), the journal of precision machinery and high-speed machinery, the shell aperture that matches the P4 grade centripetal ball bearing outer ring, and the precision matching of individual small size parts with special precision in navigation instruments in the aviation and marine industries.
IT4	Shaft diameter and housing aperture for precision measurement tools, high-precision precision fits, and P4 and P5 grade rolling bearing fits. For example, calibration gauges for inspecting IT9 to IT12 grade workpiece usage gauges and calibrating IT12 to IT14 grade shaft gauges, machine tool spindles that match P4 grade bearing holes (hole diameter>100mm) and P5 grade bearing holes, journals of precision and high-speed machinery, machine tool casing holes that match P4 grade bearings, diesel engine piston pins and piston pin seat holes, and reference holes or shaft diameters for high-precision (level 1-4) gears, Special precision aperture of instruments used in the aviation and maritime industries
IT5	Under regulations with very low requirements for fit tolerance and high requirements for shape tolerance, this type of tolerance level can make the fit properties relatively stable, equivalent to the highest accuracy in the old national standard. It is used for particularly important fit dimensions in machine tools, engines, and instruments, and is rarely used in general machinery. For example, the calibration gauges for inspecting IT11 to IT14 level workpiece usage gauges and calibrating IT14 to IT15 level shaft gauges, the machine tool box holes that match P5 level rolling bearings, the machine tool spindles that match E level rolling bearing holes, the journals of precision and high-speed machinery, the machine tool tailstock sleeves, high-precision indexing disc journals, indexing head spindles, precision screw reference journals, and the outer diameter of high-precision boring sleeves; The matching of precision holes in the main shaft instrument of the engine, the holes of level 5 precision gears, and the reference shafts of level 5 and level 6 precision gears
IT6	The mating surface has a high requirement for uniformity, which can ensure a relatively high mating property and is stable and reliable in use. It is equivalent to the old national standard level 2 shaft and level 1 precision hole, and is widely used in important mating in machinery. For example, the inspection of IT12 to IT15 workpiece gauges and the calibration of IT15 to IT16 shaft gauges are used as calibration gauges; The outer shell hole that matches the E-grade bearing and the machine tool spindle journal that matches the roller bearing, as well as the assembled bronze worm gear, outer diameter installation gear, worm gear, coupling, belt pulley, and cam journal in machine tool manufacturing; The supporting journal of the machine tool screw, the centering diameter of the rectangular spline, and the column of the radial drilling machine; The outer diameter dimensions of the guide components of machine tool fixtures, precision shafts in precision instruments, precision shafts in aviation and marine instruments, particularly important shafts in automation instruments, postal and telecommunications machinery, watches, outer diameters of cylinder liners in engines, crankshaft main journals, piston pins, connecting rod bushings, connecting rods and bearing shells; The reference hole of grade 6 precision gears and the reference journal of grade 7 and grade 8 precision gears, especially the top circle diameter of grade 1 or grade 2 precision gears
IT7	Widely used in general machinery, the application conditions are similar to IT6, but the accuracy is slightly lower, equivalent to the tolerance of the old national standard intermediate precision shaft or level 2 precision hole. For example, checking the measuring gauges for IT14 to IT16 level workpieces and checking the calibration gauges for IT16 level shaft gauges; The hole diameter of the assembled bronze worm wheel flange in the machine tool, the hole diameter of the coupling, pulley, cam, etc., the hole for the machine tool chuck seat, the rocker arm hole of the rocker drilling machine, the bearing hole of the lathe lead screw, the inner hole of the machine tool chuck guide component, the connecting rod hole and piston hole in the engine, and the positioning hole for the reamed bolt; Important parts in textile machinery, parts with high requirements in printing and dyeing machinery, precision matching inner holes in precision instruments, important inner holes in electronic computers, electronic instruments, and instruments, important inner holes in automation instruments, reference holes for precision gears of level 7 and 8, and reference shafts for precision gears of level 9 and 10
IT8	In mechanical manufacturing, it belongs to medium precision. In the manufacturing of instruments, meters, and watches, due to its small basic size, it belongs to a higher precision range and is widely used in agricultural machinery, textile machinery, printing and dyeing machinery, bicycles, sewing machines, and medical equipment. For example, checking the IT16 level workpiece gauge, the size fit of the bearing seat liner along the width direction, the general fit of the cross tooth shaft, pawl needle wheel, etc. with the clamp plate in the radio instrument,
IT9	The application conditions are similar to IT8, but when the accuracy is lower than IT8, it is used, which is slightly larger than the accuracy tolerance value of the old national standard level 4. For example, in machine tools, the outer diameter and hole of the shaft sleeve, the control component and shaft, the idle belt pulley and shaft, the shaft and bearing of the control system, the general matching parts in textile machinery and dyeing machinery, the internal hole of the oil pump in the engine, the internal hole of the valve guide, the fit between the flywheel and the flywheel sleeve, the general matching dimensions in automation instruments, the dimensions without specified tolerances for parts with higher requirements in watches, and the key width matching dimensions in single key connections, Matching dimensions of moving parts in typewriters
IT10	The application conditions are similar to IT9, but when the accuracy is required to be lower than IT9, it is equivalent to the 5-level accuracy tolerance of the old national standard. For example, the fit size on the bracket in electronic instruments and meters, the fit size between the insulation sleeve hole and the collector ring sleeve shaft in navigation instruments, the fit size of the riveted part in typewriters, the size that requires a general unmarked tolerance when the basic size is less than 18 mm in watches, and the size that requires a higher unmarked tolerance when the basic size is greater than 18 mm, the size of the fit between the oil seal retaining ring hole in the engine and the crankshaft belt hub
IT11	Widely used in situations with large gaps and significant changes that do not cause danger, it can also be used for low fitting accuracy and allows for large gaps after assembly, equivalent to the old national standard level 6 accuracy tolerance. For example, on machine tools, flange flanges and holes, sliding blocks and sliding gears, grooves, etc; Agricultural machinery, locomotive body components and stamping processing matching parts, unimportant parts in clock and watch manufacturing, dimensions without tolerance specified in tools and equipment used in watch manufacturing, rough active matching in textile machinery, lower required matching dimensions in printing and dyeing machinery, thread connection and rough dynamic connection in grinding machine manufacturing, gear top circle diameter tolerance not used as measurement reference, etc
IT12	The requirement for fitting accuracy is very low, and there is a large gap after assembly, which is suitable for parts with basically no fitting requirements. The size limit deviation with high requirements for unmarked tolerances is slightly smaller than the level 7 accuracy tolerance of the old national standard. For example, non matching dimensions and inter process dimensions, engine separation rods, unmarked tolerance dimensions for process equipment in watch manufacturing, limit deviations for unmarked tolerance dimensions in metal processing in the computer industry, and connections between wrench holes and wrench seats in machine tool manufacturing
IT13	The application conditions are similar to IT12, but slightly larger than the accuracy tolerance value of the old national standard level 7. For example, non matching dimensions and inter process dimensions, unmarked tolerance dimensions for machining parts and circular holes in computers and typewriters, and center distance between two holes
IT14	Used for non fitting dimensions and dimensions not included in the dimension chain, equivalent to the old GB level 8 accuracy tolerance. For example, in machine tools, automobiles, tractors, metallurgical machinery, mining machinery, petrochemicals, motors, electrical appliances, instruments, aviation and navigation, medical equipment, clocks, bicycles, sewing machines, papermaking and textile machinery and other mechanical processing parts, the limit deviation of unmarked tolerance dimensions
IT15	Used for non fitting dimensions and dimensions not included in the dimension chain, equivalent to the precision tolerance of level 9 in the old national standard. For example, for stamping parts, wood mold casting parts, and heavy-duty machine tool manufacturing, the dimensional limit deviation without tolerance indication when the basic size is greater than 3150 mm
IT16	Used for non fitting dimensions, equivalent to the old national standard level 10 precision tolerance. For example, the dimensions of casting parts in typewriters, the external dimensions of boxes in radio manufacturing, general external dimensions in surgical instruments, dimensions for bending and extension processing, dimensions for wooden parts in textile machinery, dimensions for plastic parts, dimensions for wood mold manufacturing and free forging
IT17 IT18	Used for non matching dimensions, equivalent to the tolerance of the old national standard’s accuracy level 11 or 12, used for plastic forming dimensions, general external dimensions in surgical instruments, and tolerances for cold work and welding dimensions

How does CNC milling ensure strict tolerance control?

Using effective measures and comprehensive application, can effectively ensure the strict tolerance control in CNC milling process, improve machining accuracy and part quality, CNC milling can ensure the strict tolerance control through the following ways:

Linear milling

Linear milling is suitable for machining angular parts and curved parts.

Operation process:
a. Clean the workpiece surface.
b. Determine the machining trajectory. Usually, CAD/CAM software is used for programming.
c. Select suitable cutting tools for milling.
d. Start processing.

Note:
a. To ensure that the machining process, the cutting force within the control range.
b. Ensure that the number and movement of cutting tools during milling are normal.
c. When machining the bottom surface, the direction of processing should be kept horizontal.

Surface milling

Surface milling is suitable for producing parts with strict plane, dimension and tolerance requirements.

Operation process:
a. Clean the surface of the piece and clamp it on the jig.
b. Take out a suitable tool and install it in the spindle.
c. Manually set machining track and cutting parameters.
d. Start processing.

Note:
a. Make sure you select the correct tool and handle size.
b. Ensure that the finish and tolerance control of the machined surface meet the requirements.
c. Check the tool and fixture for damage after each machining.

Back milling

Back milling is suitable for machining deep or high workpiece. This method allows for milling of multiple step forms in the same direction.

Operation process:
a. Clean the workpiece surface.
b. Determine the machining trajectory. Usually, CAD/CAM software is used for programming.
c. Select the appropriate milling cutter and install the clamp.
d. Start processing.

Note:
a. Make sure that the movement of the tool remains smooth while milling the bottom.
b. Maintain a moderate cutting feed rate during milling.
c. Ensure that when re-milling, the milling operation is restarted at a partial height.

Comparison of CNC milling tolerances with other processes

CNC milling is a computer numerical control technique that can accurately reproduce design drawings to produce high quality metal or plastic parts. In contrast, the traditional machining and manual process may have more errors and human interference.

If you need to produce a part that requires high precision, you may want to choose CNC milling. This ensures that each part has the same size and shape, thus minimizing possible assembly problems and other quality issues.

With the CNC milling process, your parts can achieve 0.005mm accuracy, making it the method of choice for many industrial parts. Compare it to other processes according to the following list:

Machining Technology	Tolerance
CNC Mliing	±0.005mm
CNC Turning	±0.003mm
CNC Drilling	±0.002
CNC Grinding	±0.001mm
CNC Wire Cutting	±0.02mm
CNC Laser Cutting	±0.05mm
CNC Stamping	±0.01mm
CNC Turn-milling Compound Machining	±0.005mm-0.01mm

These tolerance accuracy ranges are generally empirical values, and the actual machining results are affected by many factors, including machining material characteristics, machine tool accuracy, tool wear, machining parameters, etc. Therefore, in the specific manufacturing process, it is necessary to carry out machining tests and adjustments according to the actual situation to ensure that the tolerance requirements of specific parts are met.

Longsheng Technology: CNC milling strict tolerance

Need to build precision parts for your project? Then you may want to know about Longsheng Technology Company, our team has rich experience and expertise, can control the tolerance of various shapes and sizes of parts. No matter what kind of content customers need, we can meet their needs to ensure the accuracy and quality of parts.

We give some tolerance standards that may be encountered in CNC milling production for reference.

Our tolerance capacity table:

• Minimum line width: 50um
• Minimum aperture: 50um
• Minimum convex height: 100um
• Minimum surface roughness: Ra0.2um

At Longsheng Technology, we provide high precision CNC milling services to ensure that your products meet the required tolerance requirements. We have complete facilities and lean milling processes to ensure that we can produce as many high quality products as possible in the shortest possible time. If you need high quality CNC milling services, please do not hesitate to contact us. We will provide you with the best solution.

FAQ