①What are nominal size and actual size?
A. Nominal size: It is the nominal size specified in the standard, which is the ideal size that users and manufacturers hope to obtain, and is also the order size specified in the contract.
B. Actual size: It is the actual size obtained during the production process, which is often larger or smaller than the nominal size. This phenomenon of being larger or smaller than the nominal size is called deviation.
②What are deviation and tolerance?
A. Deviation: In the production process, since the actual size is difficult to meet the nominal size requirements, that is, it is often larger or smaller than the nominal size, the standard stipulates that there is a difference between the actual size and the nominal size. The positive difference is called positive deviation, and the negative difference is called negative deviation.
B. Tolerance: The sum of the absolute values of the positive and negative deviation values specified in the standard is called tolerance, also called "tolerance zone".
Deviation is directional, that is, expressed as "positive" or "negative"; tolerance is non-directional, so it is wrong to call the deviation value "positive tolerance" or "negative tolerance".
③What is the delivery length?
The delivery length is also called the user-required length or the contract length. The standard has the following provisions for the delivery length:
A. Normal length (also called non-fixed length): Any length within the length range specified in the standard and without a fixed length requirement is called normal length. For example, the standard for structural pipes stipulates: hot-rolled (extruded, expanded) steel pipes 3000mm~12000mm; cold-drawn (rolled) steel pipes 2000mm~10500mm.
B. Fixed length: The fixed length should be within the normal length range and is a fixed length required in the contract. However, it is unlikely to cut out an absolute fixed length in actual operation, so the standard stipulates the allowable positive deviation value for the fixed length.
Take the structural pipe standard as follows:
The yield rate of the production of fixed-length pipes is significantly lower than that of the normal-length pipes, so it is reasonable for the production enterprises to ask for a price increase. The price increase varies from enterprise to enterprise, generally about 10% on the basis of the base price.
C. Multiple length: Multiple length should be within the normal length range, and the contract should indicate the single length and the multiples of the total length (for example, 3000mm×3, which is 3 times of 3000mm, and the total length is 9000mm). In actual operation, the allowable positive deviation of 20mm should be added to the total length, and the cut margin should be left for each single length. Taking the structural pipe as an example, the cut margin is stipulated as follows: 5~10mm for outer diameter ≤159mm; 10~15mm for outer diameter＞159mm.
If there is no multiple length deviation and cutting margin in the standard, it should be negotiated by the supply and demand parties and indicated in the contract. The double length scale will bring a significant reduction in the yield rate of the production enterprise, just like the fixed length. Therefore, it is reasonable for the production enterprise to propose a price increase, and the price increase is basically the same as the price increase of the fixed length.
D. Range length: The range length is within the normal length range. When the user requires a fixed range length, it needs to be indicated in the contract.
For example: the usual length is 3000-12000mm, while the range length is 6000-8000mm or 8000-10000mm.
It can be seen that the range length is looser than the fixed length and multiple length requirements, but it is much stricter than the usual length, which will also reduce the yield rate of the production enterprise. Therefore, it is reasonable for the production enterprise to raise the price, and the price increase is generally about 4% on the base price.
④ What is uneven wall thickness?
The wall thickness of the steel pipe cannot be the same everywhere. There is an objective phenomenon of uneven wall thickness in its cross section and longitudinal tube body, that is, uneven wall thickness. In order to control this unevenness, the allowable index of uneven wall thickness is specified in some steel pipe standards, which is generally not more than 80% of the wall thickness tolerance (implemented after consultation between the supply and demand parties).
⑤ What is ovality?
There is a phenomenon of unequal outer diameters on the cross section of a circular steel pipe, that is, there are maximum and minimum outer diameters that are not necessarily perpendicular to each other. The difference between the maximum and minimum outer diameters is the ovality (or out-of-roundness). In order to control the ovality, some steel pipe standards specify the allowable index of ovality, which is generally stipulated to be no more than 80% of the outer diameter tolerance (implemented after negotiation between the supply and demand parties).
⑥ What is curvature?
The steel pipe is curved in the length direction, and its curvature is expressed by numbers, which is called curvature. The curvature specified in the standard is generally divided into the following two types:
A. Local curvature: Use a one-meter long ruler to measure the maximum curvature of the steel pipe and measure its chord height (mm), which is the local curvature value, and its unit is mm/m, and the expression method is such as 2.5mm/m. This method is also applicable to the curvature of the pipe end.
B. Total curvature of the whole length: Use a thin rope to tighten from both ends of the pipe, measure the maximum chord height (mm) at the bend of the steel pipe, and then convert it into a percentage of the length (in meters), which is the total curvature of the length of the steel pipe.
For example: the length of the steel pipe is 8m, and the maximum chord height is measured to be 30mm, then the curvature of the whole length of the pipe should be:
0.03÷8m×100%=0.375%
⑦ What is size deviation?
Size deviation or size deviation beyond the standard allowable deviation. The "size" here mainly refers to the outer diameter and wall thickness of the steel pipe. Usually, some people call size deviation "tolerance deviation". This term that equates deviation with tolerance is not rigorous and should be called "deviation deviation". The deviation here may be "positive" or "negative". It is rare that both "positive and negative" deviations are out of the norm in the same batch of steel pipes.
