How to choose mechanical parts surface roughness

How to choose mechanical parts surface roughness
The surface roughness is reflected parts surface micro geometric shape error,and is an important technical indexes, it is to check the main basis of metal parts surface quality, It choose reasonable or not, the directly related to product quality, service life and cost of production.
The mechanical parts surface roughness selection method of three kinds, namely calculation method, test method and the method of analogy. In the mechanical parts design work, applied the most common is the analogy method, this method is simple, rapid and effective. Application analogy method need plenty of reference material, all kinds of existing machinery design manuals provides comprehensive and documents. The most commonly used is and tolerance grade to adapt the surface roughness. Normally, the mechanical parts dimensional tolerance requirement is smaller, machinery parts surface roughness is smaller, but they also do not exist between fixed function relation. For example, some machines, instrument handle, the handwheel and sanitary equipment, food machinery on some of the mechanical parts’ modified surface, their surface request processing very smooth namely surface roughness is demanding, but its size tolerance requirement is low. Under normal circumstances, have dimensional tolerance requirement parts, the tolerance grade and surface roughness numerical between still have certain corresponding relationship of.
In some mechanical parts design manual and machinery manufacturing monographs, machinery parts surface roughness and mechanical parts dimensional tolerance relation of experience and formulas are introduced, and there are many list for reader’s selection, but as long as attentive reading, can discover, although take exactly the same experience calculation formula, but from the list of numerical is endless also and same, some still have great difference. This give not familiar with this situation people bring temptation. They also increased in the mechanical parts to choose the surface roughness of difficulty.
In the practical work, for different types of machines, its parts in the same size tolerance conditions, on the surface roughness requirement is differs. This is coordinates the stability problem. In the mechanical parts of the design and the manufacture process, for different types of machines, its parts with stability and interchangeability requirement is different. In the existing machinery parts design manuals, reflective basically has the following three types:
Class 1 is mainly used for precision machinery, to cooperate with stability is demanding, request parts in use process or by many assembly, after the parts of wear parts size limit not more than 10% of the tolerance values, which is mainly used in precision instruments, meters, precision measuring surface, extremely important parts of the friction surface, such as the inner surface of cylinder, precision machine tools of spindle of neck, coordinate boring of spindle of neck, etc.
The second type of mainly used in ordinary precision machinery, to cooperate with the demand is higher, the stability of the requirements of parts wear parts size limit not more than 25% of the tolerance values, the request has very good closed materials, its main application in such as machine tools, tools, and rolling bearing fits the surface, taper pin hole, and relative velocity higher surfaces such as sliding bearing with surface, gear teeth in mining, etc.
Category 3 is mainly used for general machinery, the requirement of mechanical parts wear limit shall not exceed 50% of dimensional tolerance values, no relative motion parts, such as interface box cover, sleeve, request tightly surface, and the key to the keyway face, Relative velocity is not high face, such as scaffolds hole, bushings, belt wheel when work surface, reducer, etc.
In this paper we will mechanical design handbook of table value of statistical analysis, old the surface roughness of the national standards (GB1031-68) into refer to adopt international standard ISO promulgated the 1983 new national standard (GB1031-83), adopt preferential selection of evaluation parameters, such as contour arithmetic average deviation of the Ra = (1) / (l) ∫ l0 | | dx y. And using the first series of preferential selection in Ra numerical is deduced, and the surface roughness Ra dimensional tolerance relation between the relevant for IT
Class 1: Ra p 1.6 Ra acuities 0.008 x IT
Ra acuities were 0.8 Ra acuities 0.010 x IT
Category 2: Ra p 1.6 Ra acuities 0.021 x IT
Ra acuities were 0.8 Ra acuities determination x IT
The first three categories: Ra acuities 0.042 x IT
The above three equation list, as table 1 and table 2 and table 3 shows.
Table 1 tolerance grade and surface roughness (used for precision machinery)
Table 2 tolerance grade and surface roughness (used in ordinary precision machinery)
Table 3 tolerance grade and surface roughness (used for general machinery)
In the mechanical parts design work, according to dimensional tolerance choose surface roughness value, shall, according to the different types of machines, choose corresponding table values.
Need explanation is, table 1 series adopts Ra values, which the old gb Ra limit for the first 2 series will be confronted with numerical values, conversion by and by. We use function value in the table by, as it is conducive to improve product quality and individual value adopt depend. JiuGuo mark tolerance grade and surface roughness corresponding table of contents and forms are more complex, on the same tolerance grade the same size piecewise same basic dimensions, hole and axis of surface roughness values are not the same, different types with numerical is not identical also, this is because the old appying the standard (59) GB159 tolerances numerical and the above factors concerned. The current new gb appying (GB1800-79) on the same tolerance grade within the same size subsection basic dimensions of standard of tolerance values are the same, this makes tolerance grade with surface roughness tables greatly simplified and also more scientific and reasonable.
In design, surface roughness destiny1 or must choose from reality, the comprehensive measure parts surface function and technology economy, can make a reasonable choice. Given in the table of tolerance grade and surface roughness available for reference in design.

By Murphy Lee
honestmen metal parts factory

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