① nominal size and actual size
A, nominal size: is the name specified in the standard size is ideal for users and manufacturers hope to gain size and order sizes indicated in the contract.
B, actual size: is the actual size obtained in the production process, which is often greater than or less than the nominal size. This phenomenon known as deviation greater than or less than the nominal size.
② deviations and tolerances
A, deviation: during the process of production, because the actual size nominal size required is hard to reach, which is often greater than or less than the nominal size, so the standards provided for in the actual size and there is a difference between nominal size allows. Difference is positive, it is called positive, when the difference is negative, it is called negative deviation.
B, tolerance: standard set out in positive and negative deviations sum of the absolute value is called tolerance, also called "tolerance zone".
Deviation is directional, that is, "positive" or "negative," said tolerance is not directional, therefore, deviation value is called "tolerance" or "negative tolerance" name is wrong.
③ delivery length
Delivery length, also known as user requirements or contract length. Standard delivery length in one of the following provisions:
A, usually the length (also known as non-specific cut length): any length within the length specified in the standard, there is no fixed length requirements are known as the usual length. Standards such as seamless pipe: seamless hot-rolled (squeeze, expansion) seamless steel tubes 3000mm~12000mm; cold drawing (rolling) seamless steel pipe 2000mmm~10500mm.
B, scale length: specified length should be in the usual length range, is called for in the contract of a fixed length. But in practice are cut out absolutely exact length is unlikely, so the standard specified lengths and outlines the positive value.
Seamless structure standards are:
Cut lengths seamless pipe production yield decline was larger than usual length of seamless steel pipe producers proposed fare increases are reasonable. Increases businesses differ, based on general price 10% or so.
C, multiple lengths: multiple lengths should typically range in length, single length and structure to be given contracts total length a multiple of (for example, 3000mmx3, which is 3000mm number of 3 times, with a total length of 9000mm). In practice, should be based on the total length and allow positive 20mm, plus every single length should be cut allowances. Structural seamless steel tubes, for example, provides cutting allowance: outside diameter ≤ 159mm 5~10MM; >159mm od 10~15mm.
If standard when there is no length deviations and cutting allowances should be set by supply and demand and indicated in the contract is being negotiated. Times as long as with the specified length, will bring to the production yield has decreased so manufacturers proposed a fare increase is reasonable, its fare increases with length increases are basically the same.
D length, scope: the scope of usual length range in length, when a user requests a range of fixed length, the need indicated in the contract.
For example, typically of length 3000~12000mm, and the range specified length for 6000~8000mm or 8000~10000mm.
Visible range than the fixed length and a double length length loose, but usually increases the length a lot, also brings to the production yield reduction. So manufacturers proposed a fare increase is justified, 4% or so on its fare increase in the base price.
Black seamless steel pipe wall thickness
Seamless steel pipe wall thickness not everywhere the same, in its cross section and longitudinal tube objective phenomenon of seamless steel pipe wall thickness range, namely wall thickness. In order to control this heterogeneity, in seamless steel tube wall thickness specified in the standards does not allow indexes, General wall thickness tolerances of less than 80% (after consultation by both supply and demand).
Circular seamless steel tube of varying cross section diameter on the phenomenon, that is, there is not necessarily perpendicular to the minimum and maximum diameter outer diameter, the maximum outside diameter and ovality is the minimum outside diameter difference of (or not). In order to control the roundness, provided for in the existing standard indicators allowed by the roundness, 80% of the General provisions of the agreement does not exceed the tolerance for the outside diameter (after consultation by both supply and demand).