Standard GB/T1804-m & ISO 2768-1/2


GB/T1804 is a Chinese standard. The “-m” indicates the tolerance class.

This standard is equivalent to ISO 2768.

Detail Information of GB/T 1804-2000
Description : General tolerances Tolerances for linear and angular dimensions without individual tolerance indications
Sector / Industry: China National Standard
Classification of Chinese Standard: J04
Classification of International Standard: 17.040.10
Date of Issue: 2000-07-24
Date of Implementation: 2000-12-01
Older Standard (superseded by this standard): GB/T 1804-1992; GB/T 11335-1989
Quoted Standard: GB/T 1800.1-1997; GB/T 1184-1996; GB/T 4249-1996; GB/T 6403.4-1986
Adopted Standard: ISO 2768-1-1989, MOD

Drafting Organization: Mechanical Engineering Research Institute
Administrative Organization: National product size and geometry specifications Standardization Technical Committee
Proposing organization: National Machinery Industry Bureau
Issuing agency(ies): State Quality and Technical Supervision


This standard specifies the satellite broadcast system integration down converter technical requirements and methods of measurement.

For the same measurement uncertainty can ensure that any equivalent measurement methods may also be used. Dispute should be based on this standard shall prevail.

This standard applies to radio and television satellite broadcast system integrated down converter development, production, use, and operation and maintenance.

ISO 2768

ISO Tolerances

According to DIN ISO 2768-1

General tolerances for linear measures and level squares with four tolerance classes are useful for simplifying drawings.

By choosing the tolerance class precision levels common in workshops should be taken into account.

If smaller tolerances are needed or bigger ones are more economical,
then these tolerances are indicated next to the nominal size.

Table 1 Limits for linear measures

Table 1

For nominal sizes below 0,5 mm the limit measures are to be indicated directly at the nominal measure.

Table 2 Limit measures for radius of curvature and chamfer height

Table 2

Table 3 For nominal masses below 0.5 mm, the dimensions of the boundary shall be given directly at the nominal size.

Table 3

ISO 2768 – m or general tolerance ISO 2768 – mFor nominal sizes below 0.5 mm the limit measures are to be indicated directly at the nominal measure. If general tolerances according to ISO 2768-1 are valid,
the following has to be inserted in the title box, i.e. for tolerance class medium

For new designs only the general tolerance according to DIN ISO 2768-1 should be valid. The limit measurements of the tolerance classes m and f of DIN ISO 2768-1 are identic with those of DIN 7168-1.

ISO Tolerances

According to DIN ISO 2768-2

DIN ISO 2768-2 is for simplifying drawing and fixes general tolerances in three tolerance classes for form and position.
By choosing a special tolerance class exactly the precision level common in workshops should be taken into account.

If smaller tolerances are needed or bigger are more economical these tolerances should be mentioned directly according to ISO 1101.

General tolerances for form and position should be used while the tolerance principle according. to ISO 8015 is valid and while this is mentioned in the drawing.
This tolerance principles says that no opposite relation between measure, form and position tolerance exists (principle of superposition).

General tolerances for form and position are valid for form elements for which form and position tolerances are not indicated individually.
They are applicable for all characteristics of the form elements accept cylinders, profiles of any line or surfaces, inclines, coaxiality, position and total movement.

Table 4.1

Table 4.2

Table 4.3



Mechanical and physical properties of engineering alumina ceramics


Mechanical and physical properties of engineering alumina ceramics

Pertti AuerkariVTT Manufacturing Technology

ISBN 951-38-4987-2ISSN 1235-0605

Copyright Valtion teknillinen tutkimuskeskus (VTT) 1996


Chemical and thermal stability, relatively good strength, thermal and electrical insulation characteristics combined with availability in abundance have made aluminium oxide Al2O3, or alumina, attractive for engineering applications. Much of its traditional use is in classical refractory service. However, the present work is limited to grades of alumina that qualify for structural engineering, ie. Topoly crystalline grades with at least 80% (mostly at least 90%) Al2O3 and no open porosity. In practice impermeability at room temperature requires that the total porosity is less than about 6% (Ryshkewitch 1960, Richerson 1982).

Alumina has several allotropic forms, but only the usual type or α-alumina is considered here. It has an internal crystal structure where the oxygen ions are packed in a close-packed hexagonal (cph) arrangement with aluminum (and other metal) ions in two-thirds of the octahedral sites. Alumina does not deviate much from stoichiometry but even small levels of impurities can influence high-temperature diffusion rates greatly. Alumina-based high-strength ceramic alloys are also available but not considered here. Alumina has a melting temperature of about 2040℃, but impurities and alloying elements form secondary phases that can melt at considerably lower temperatures.

Engineering grade polycrystalline alumina products are usually made by sintering alumina powder at high temperature (>1300℃). The manufacturing route limits the component and section size that can be produced in reasonably full density. The manufacturing process is also a major source of the initial defects that through fracture toughness will limit the strength of alumina components in service. As a consequence, strength of alumina is not a strict material property but dependent on stressed volume (or stressed surface, if surface defects dominate). The size dependence of strength makes larger components relatively weaker, and this is further amplified by the difficulty of sintering large pieces to an equivalent end density with small ones, and by generally higher maximum residual stresses in larger components.

While strength via the inherent brittleness is an important design-limiting factor, most engineering alumina is used primarily for its other functional qualities. This frequently creates an optimization task in design, when functionality needs to be maximized without compromising mechanical integrity. This requires also consideration of the manufacturing, because the high-temperature manufacturing processes including joining and coating operations control both the material properties and residual stresses in the final product.

Alumina is here graded into two main groups, the first of high-alumina grades with at least 99% Al2O3 (Table 1) and the second of alumina grades between 80% and 99% Al2O3 (Table 2). These main groups can be further divided into subclasses according to type, purity and intended service (Morrell 1987).

The difference between the grades is mainly in the amount of impurities and some deliberate alloying agents such as sintering aids. Alloying with other oxides does not necessarily impair the mechanical properties, but on average the best mechanical and other properties are seen in high purity grades of alumina. The first group of Table 1 is generally characterized by high density (> 3.75 g/cm3), high sintering temperatures in manufacturing (1500 – 1900 ℃) and relatively good mechanical performance. The lower grade alumina of Table 2 are cheaper to produce and therefore attractive for purposes where the properties are sufficient.

Table 1. High-alumina engineering ceramics (grades A1 – A4, at least 99% Al2O3) and their characteristics (Morrell 1987).


Table 2. Engineering alumina grades A6 – A9 (80% ≤ Al2O3 ≤ 99% as requirement) and their characteristics (Morrell 1987).


This work aims to describe the physical and mechanical properties of engineering alumina for the purpose of materials characterization under wide-ranging stresses and temperatures. Particular applications are anticipated in evaluating the complex deformation and stress histories as well as the residual stresses of dissimilar joints and coatings involving alumina as one component. Therefore, emphasis lies in the temperature dependence of the relevant properties that determine the performance after manufacturing processes and later in service.

The glass phase’s influence on alumina ceramics

The content of glass phase has an important influence on the production and quality of Al2O3 ceramics.

The glass phase plays a role in the bonding of the Al2O3 ceramic slabs, which together with the dispersed crystalline phases, can fill the pores in the ceramic and make the ceramic materials compact.

The glass phase can also inhibit the growth of the crystal, and prevent the crystal form change when the temperature changes.

The glass phase also has a negative effect on the ceramic. The mechanical properties of the crystal phase is lower than that of the crystalline phase, and the thermal stability is poor, and it will begin to soften at lower temperatures.

Glass structure is loose, often in the structure of the gap filled with some metal ions. Under the action of the electric field, the polarization can be easily produced, so that the insulating property of the ceramic material is decreased, and the dielectric loss is increased.

α- Al2O3,β- Al2O3,γ- Al2O3

Alumina ceramics is a kind of ceramic material with alpha Al2O3 as the main crystal. The Al2O3 content is generally from 65% to 99.99%.

Industrial alumina is composed of bauxite (Al2O3 – 3H2O) and diaspore preparation, the purity requirements is not high, generally prepared by chemical methods.

Fused corundum is used as the raw material to add carbon in the arc furnace in 2000 ~ 2400℃ melting process, also known as artificial corundum.

Alumina has many homogeneous crystal. According to the studies reported that there are more than and 10 kinds of variants, but there are three main types, namely,

γ- Al2O3,β- Al2O3,α- Al2O3.

Alumina crystal transformation relations, such as the following figure, its structure is different, so its nature is also different, in the high temperature of 1300 ℃ or more almost completely transformed into α- Al2O3.

α- Al2O3, belongs to the spinel type (cubic) structure, the oxygen atom shape is cubic close packed, the aluminum atom fills in the gap. Its density is small. And unstable at high temperature, poor mechanical and electrical performance, does not exist in nature. Because it is a loose structure, it can be used to manufacture porous materials.

β- Al2O3 is a Al2O3 aluminate minerals content is very high. Its chemical composition can be approximately represented by RO and R2O – 6 Al2O3 – 11 Al2O3 (RO refers to the alkaline earth metal oxide R2O, alkali metal oxide), which is composed of alkali metal or alkaline earth metal ions such as [NaO] – and [Al11O12] + spinel type unit layer overlapping piled up oxygen ions arranged closely set up accumulation of Na + completely contained loose in perpendicular to the C axis stacking plane, can quickly spread in the plane, showing the ionic conductivity.

α- Al2O3, belonging to the three party system, the unit cell is a pointed diamond surface, in nature, there are only Al2O3, such as natural corundum, ruby, sapphire and other minerals. Al2O3 structure is the most compact, low activity, high temperature stability. It is the most stable crystal form in the three forms, and the electrical properties are the best, which has excellent mechanical and electrical properties.