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MAXNOVO MACHINE Tips : Modern Machining Processes are exceptionally accurate. However, no machines are capable of exact precision. Variances can be caused by a variety of factors from the part material to the machining process used. For this reason, engineers assign machining tolerances to parts during the design process. The international tolerance grade refers to the grade that determines the accuracy of the size. The international standard stipulates that it is divided into 20 classes, from IT01, IT0, IT1, IT2 to IT18. The larger the number, the lower the tolerance class (machining accuracy) and the allowable range of tolerance (tolerance) The larger the value), the less difficult the processing.
Tabe Of Contents
3.1 Turning
3.2 Milling
3.3 Planing
3.4 Grinding
3.5 Drilling
3.6 Boring
3.7 CNC Machining Center
3.8 Laser Cutting
3.9 Water-Jet Cutting
3.10 Electrical Discharge Machining (EDM)
6.1 Consider the Material Used
6.2 Consider the Application of Your Parts
6.3 Find the Right CNC Machining Service
6.4 Use High-performance Machine Tools
6.5 Maintain the Accuracy of Work Holders
6.6 Don’t Ignore Parallelism and Perpendicularity
General Speaking, Tolerance grade, it is also known as machining tolerance, refers to the allowable deviation or variation in dimensions and measurements of a manufactured part or component during the machining process. A tolerance is the total permissible variation of size. where the tolerance is the difference between the limits of size. Different machining processes may have specific tolerance requirements based on factors like the material being machined, the complexity of the part, and the intended application. How much do you know about the "Machining Tolerance" used to express machining accuracy ?
Machining tolerance, it is also known as dimensional accuracy, is the amount of acceptable variance in the dimension of a part. This is expressed as a maximum and minimum dimensional limit for the part. Parts are considered to be within the tolerance if their dimensions fall between these limits. If the part’s dimensions fall outside of these limits, however, these parts are outside the acceptable tolerance and considered unusable.
For designers, determining the appropriate tolerances for a part is an essential task in preparing a design for an order. However, it can be difficult to determine appropriate tolerances for a part, especially parts that are made of non-metallic substances. To develop appropriate machining tolerances for your designs, understanding standard manufacturing tolerances and the tolerances that certain materials and machining processes are capable of will be essential. For this reason, we’ve created some machining tolerance guidelines to help you determine machining tolerances for your nonmetallic parts.
International Tolerance Grade, also known as IT Grade, identifies what tolerances a specific process can produce for a given dimension defined in ISO 286. It establishes the magnitude of the tolerance zone or allowed amount of part size variation for internal and external dimensions. There are totally 18 grades of tolerance, the smaller the International Tolerance Grade number, the smaller the allowable tolerance zone, the higher the precision level. The IT Grade is used to indicate how precise an industrial process is. In designing, the engineer will determine a key dimension and relative tolerance for a particular feature. The tolerance for a particular IT grade can be calculated through the formula as below. The calculation formula can help engineers to know what IT Grade is necessary for producing a part with certain specifications. If the manufacturing process can’t reach the grade, you should consider another method.
T : it is the Tolerance in micrometers [μm]
D : it is the geometric mean Dimension in millimeters [mm]
ITG : it is the IT Grade, a positive integer
One thinks of D as being the key dimension on the part and T as being the required tolerance on that key dimension. The Larger the ITG, the Looser the Tolerance .
IT Grade refers to the International Tolerance Grade of an industrial process defined in ISO 286. This grade identifies what tolerances a given process can produce for a given dimension. How precise are common CNC machining processes ? Different International Tolerance Grade numbers represent different precision and suitable varying processing techniques.
IT Grade 1-4 : very precise, usually used for Gage making and precision work, precision measuring tools
IT Grade 5-16 : suitable for cutting operations like turning, milling, boring, grinding and sawing.
IT Grade 12-16 : suitable for manufacturing operations like pressing, rolling & other forming operations.
Here are some common machining processes and their associated tolerance grades :
According to the different functions of the product parts, the required processing accuracy is different, and the selected processing form and processing technology are also different. This article introduces the processing accuracy that can be achieved by several common processing forms such as turning, milling, planing, grinding, drilling and boring.
3.1 Turning
The work-piece rotates, and the turning tool performs linear or curved movement cutting in the plane. Turning is generally carried out on a lathe to process the inner and outer cylindrical surfaces, end surfaces, conical surfaces, forming surfaces and threads of the work-piece.
The turning precision is generally IT8~IT7, and the surface roughness is 1.6~0.8μm.
(3.11) Rough turning strives to use a large cutting depth and a large feed to improve turning efficiency without reducing the cutting speed, but the machining accuracy can only reach IT11, and the surface roughness is Rα20~10μm.
(3.12) Semi-precision turning and finishing turning should use high-speed and small feed and cutting depth as much as possible. The machining accuracy can reach IT10~IT7, and the surface roughness is Rα10~0.16μm.
(3.13) Using high-precision diamond turning tools and high-speed precision turning non-ferrous metal parts on high-precision lathes, the machining accuracy can reach IT7~IT5, and the surface roughness is Rα0.04~0.01μm. This kind of turning is called "mirror turning".
3.2 Milling
Milling refers to the use of rotating multi-edge cutting tools to cut the workpiece, is a highly efficient processing method. It is suitable for processing flat surfaces, grooves, various forming surfaces (such as splines, gears and threads) and special shapes of molds. According to the same or opposite direction of the main movement speed and the work-piece feed direction during milling, it is divided into down milling and reverse milling.
The machining accuracy of milling is generally up to IT8~IT7, and the surface roughness is 6.3~1.6μm.
(3.21) The machining accuracy during rough milling is IT11~IT13, and the surface roughness is 5~20μm.
(3.22) The machining precision during semi-finishing milling is IT8~IT11, and the surface roughness is 2.5~10μm.
(3.23) The machining precision during finishing milling is IT16~IT8, and the surface roughness is 0.63~5μm.
3.3 Planing
Planing is a cutting method that uses a planer to make a horizontal and relatively linear reciprocating motion on work-piece. It is mainly used for shape processing of parts.
The precision of planing is generally up to IT9~IT7, and the surface roughness is Ra6.3~1.6μm.
(3.31) The precision of rough planing can reach IT12~IT11, and the surface roughness is 25~12.5μm.
(3.32) The precision of semi-precision planing can reach IT10~IT9, and the surface roughness is 6.2~3.2μm.
(3.33) The precision of precision planing can reach IT8~IT7, and the surface roughness is 3.2~1.6μm.
3.4 Grinding
Grinding refers to a method of using abrasives and abrasive tools to cut off excess material on the work-piece. It is a finishing process that is widely used in the machinery manufacturing industry. Grinding is usually used for semi-finishing and finishing, the accuracy can reach IT8~IT5 or even higher, and the surface roughness is generally 1.25~0.16μm.
(3.41) Precision grinding surface roughness is 0.16~0.04μm.
(3.42) The surface roughness of ultra-precision grinding is 0.04~0.01μm.
(3.43) The surface roughness of mirror grinding can reach below 0.01μm.
3.5 Drilling
Drilling is a basic method of hole machining. Drilling is often carried out on drilling machines and lathes, and can also be carried out on boring or milling machines. The machining accuracy of drilling is low, generally only reaching IT10, and the surface roughness is generally 12.5~6.3μm. After drilling, reaming and reaming are often used for semi-finishing and finishing.
3.6 Boring
Boring is an internal diameter cutting process that uses tools to enlarge holes or other round contours. Its application range is generally from semi-roughing to finishing. The tools used are usually single-edge boring tools (called boring bars).
(3.61) The boring accuracy of steel materials can generally reach IT9~IT7, and the surface roughness is 2.5~0.16μm.
(3.62) The machining accuracy of precision boring can reach IT7~IT6, and the surface roughness is 0.63~0.08μm.
3.7 CNC Machining Center
Machining centers combine various machining processes like milling, drilling, and tapping. Tolerance grades for machining centers can vary widely depending on the specific operations performed.
3.8 Laser Cutting
Laser cutting uses a high-powered laser beam to cut through materials. Tolerance grades for laser cutting depend on factors like material thickness and the laser's power but are generally in the range of IT8 to IT16 for linear dimensions.
3.9 Water-jet Cutting
Water-jet cutting uses a high-pressure jet of water and abrasive materials to cut through various materials. Tolerance grades for water-jet cutting are typically in the range of IT10 to IT16 for linear dimensions.
3.10. Electrical Discharge Machining (EDM)
EDM uses electrical discharges to shape the work-piece. Tolerance grades for EDM can range from IT7 to IT13 for linear dimensions.
It's important to note that these tolerance grades are just general guidelines, and specific tolerance requirements may vary based on the engineering specifications, the intended use of the part, and industry standards. Engineers and manufacturers should carefully consider the application and design requirements when determining the appropriate tolerance grade for a machining process to ensure the desired level of precision and functionality.
The qualities of a part’s materials must be taken into consideration when defining part tolerances. The designer must define the characteristics of the material being used and take into consideration how each of these characteristics may affect the ability of the material to be machined and the acceptable tolerance. It is also important to consider what type of material you will be using when choosing the machining process for manufacturing, as some materials are incompatible with certain machining operations. Just a few of these characteristics are defined below :
4.1 Abrasiveness :
Certain materials that are very abrasive can be hard on the tooling process. Phenolics like G10/FR4, G11, GP03, and any glass laminates fall into this category. Because of the abrasiveness of the material, these materials can affect the tolerances of the design as they wear down the cutting machinery.
4.2 Heat Stability :
Some non-metallic materials, especially plastics, tend to warp in the presence of heat. This limits the types of machining processes that are acceptable and affects the tolerances of the part.
4.3 Hardness and Rigidity :
Soft, flexible materials are generally more difficult to machine to specified tolerances because of their ability to change dimensions. Polyisocyanurate, polyurethane, and XPS foam all fall into this category. As a result, extra measures may be needed to cut the material to fit tolerances.
There are many factors to take into consideration when determining tolerances. These include the following:
5.1 Machining Type :
The method of machining used will significantly impact the possible tolerances for the finished part, as some processes are more precise than others. The specifics of how machining processes affect tolerances are discussed in more detail further down this page.
5.2 Material :
Materials behave differently under stress, and some are easier to work with than others. These material properties must be taken into account when determining tolerances. The specifics of how materials affect machine tolerances are discussed further down this page.
5.3 Plating and Finishes :
Any plating or finishing processes should be taken into account when determining part dimensions and tolerances. While plating and finishing add small quantities of material to the surface of a part, these small amounts still alter the dimensions of the final product and should be taken into account before production.
Cost: Tolerances should be precise but never tighter than necessary, as tighter tolerances are more expensive to achieve. If your part will work with a three-decimal-place tolerance, do not make it a four-decimal-place tolerance.
It is also important to remember to double-check tolerances. Old part specifications that you wish to reuse may be using tolerances that are unnecessarily tight or may have tolerances that have been transcribed incorrectly. Even new part specifications may contain errors. Taking an extra few minutes to double-check existing tolerances on old and new projects can help avoid retooling costs in the future. When these factors are considered, and tolerances are used correctly, engineers can rest assured that their parts will fit properly when the manufacturing process is complete.
Effective tolerances are crucial in CNC machining, as tighter tolerances incur additional expenses. For instance, creating a part with a specific shape and tight tolerance can involve several cutting operations with different tools, resulting in more machine usage time and higher costs. But if you approach the production process correctly, you can frequently account for or offset these costs. Here are some tips for achieving optimal tolerances.
6.1 Consider the Material Used
Standard machining tolerances are usually +/- 0.005” for metal parts and +/- 0.010” for plastic parts. However, some parts may require even tighter tolerances to guarantee an appropriate fit. Dimension precision can be challenging for some materials and easy for others. Remember that some raw materials expand and contract when exposed to different temperatures or moisture levels. Therefore, you should define a new tolerance based on this factor.
6.2 Consider the Application of Your Parts
Not all parts require a tight tolerance design. The tolerance level needed for the machining part is frequently determined by its intended application. For example, creating parts that do not combine with others requires less milling precision. Given how much more costly it is to achieve tight tolerances, it is generally not used if it is not required.
6.3 Find the Right CNC Machining Service
Customers may achieve the appropriate tolerance by locating a reputable CNC machining service. Then, you can discuss your requirements with a manufacturing specialist and determine the optimal tolerances for the project. Generally, part designers specify tolerances before submitting manufacturing requests to CNC machining services, saving production costs and time.
6.4 Use High-performance Machine Tools
The dimension variations in a work part might result from improper cutting tool usage, tool deflection, or a dull cutting edge. Tool deflection often occurs on long-ended features such as long shafts and deep holes. Furthermore, dull cutting tools place your parts in an unfavorable position and affect the precision of the spindles.
6.5 Maintain the Accuracy of Work Holders
Work holders are important in ensuring that CNC machining tolerances are as required. They help keep the part in place as it is machined and serve as reference points for locations.
6.6 Don’t Ignore Parallelism and Perpendicularity
Don’t ignore parallelism and perpendicularity when considering tolerances, especially when working with multiple parts. Parallelism and perpendicularity are especially important for assembly because even slight misalignments can lead to larger misalignments over time, ultimately affecting the overall quality of the part.
