①Nominal size and actual size
A. Nominal size: The nominal size specified in the standard, the ideal size that users and manufacturers want to see, and 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.
② Deviation and tolerance
A. Deviation: In the production process, because the actual size is difficult to meet the nominal size requirement, that is, it is often larger or smaller than the nominal size, so the standard stipulates that there is a difference between the actual size and the nominal size. A positive difference is called a positive deviation, and a negative difference is called a negative deviation.
B. Tolerance: The sum of the positive and negative deviations specified in the standard is called tolerance, also called "tolerance zone".
The deviation is directional, that is, it is expressed as "positive" or "negative"; the tolerance is not directional. Therefore, it is wrong to call the deviation value "positive tolerance" or "negative tolerance".
③Delivery length
The delivery length is also called the length required by the user or the length of the contract. The standard has the following regulations on delivery length:
A. Normal length (also known as non-fixed length): Any length within the standard specified length and no fixed length requirement is called normal length. For example, the structural pipe standard stipulates: hot-rolled (extruded, expanded) steel pipe 3000mm~12000mm; cold drawn (rolled) steel pipe 2000mm~10500mm.
B. Fixed-length length: The fixed-length length should be within the usual length range, which is a fixed-length dimension required in the contract. However, in actual operation, it is impossible to cut out as much as the fixed-length length, so the standard specifies the allowed positive deviation value for the fixed-length length.
Take the structural pipe standard as:
The yield rate of pipes with fixed-length pipes is much lower than that of pipes with normal lengths. It is fair for manufacturers to request price increases. The rate of price increase varies from company to company. Generally, the price increase is about 10% on the base price.
C, double ruler length: the double ruler length should be within the usual length range, and the contract should indicate the single ruler length and multiples of the total length (for example, 3000mm×3, which is 3 multiples of 3000mm, and the total length is 9000mm). In actual operation, a positive deviation of 20mm should be allowed on the basis of the total length, plus a margin for the incision of each single ruler length. Take the structural pipe as an example, the allowance for cutting is stipulated: 5~10mm for outer diameter ≤159mm; 10~15mm for outer diameter >159mm.
If there is no double-length deviation and cutting allowance in the standard, it should be negotiated by the supplier and the buyer and indicated in the contract. The double-length standard is the same as the fixed-length length, which will bring about a substantial reduction in the yield rate of the manufacturer. Therefore, it is fair for the manufacturer to raise the price, and the price increase is basically the same as the fixed-length length.
D. Range length: The range length is within the usual length range. When the user requires a fixed range length, it must be indicated in the contract.
For example: the usual length is 3000~12000mm, and the range cut-to-length length is 6000~8000mm or 8000~10000mm.
It can be seen that the range length is looser than the fixed-length and double-length lengths, but it is much stricter than the usual length, which will also reduce the yield of production enterprises. Therefore, it is reasonable for the manufacturer to raise the price. The price increase is generally about 4% above the base price.
④Uneven wall thickness
The wall thickness of the steel pipe cannot be the same everywhere, and there are objectively unequal wall thicknesses in the cross section and longitudinal pipe body, that is, uneven wall thickness. In order to control this non-uniformity, some steel pipe standards stipulate the allowable index for uneven wall thickness, which is generally stipulated not to exceed 80% of the wall thickness tolerance (implemented after negotiation between the supplier and the buyer).
⑤ Ovality
There is a phenomenon of unequal outer diameters in the cross section of round steel pipes, 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 ellipticity (or non Roundness). In order to control the ovality, some steel pipe standards stipulate the ovality tolerance index, which is generally specified as not exceeding 80% of the outer diameter tolerance (implemented after negotiation between the supplier and the buyer).
⑥ Curvature
The steel pipe is curved in the length direction, and the curve degree is called the degree of curvature. The curvature specified in the standard is generally divided into the following two types:
A. Local curvature: Use a one-meter straight ruler to lean against the maximum bend of the steel pipe, and measure the chord height (mm), which is the value of local curvature. The unit is mm/m, and the expression is 2.5mm/m. . This method is also suitable for tube end curvature.
B. Total curvature of the total length: Use a thin rope to pull taut from both ends of the pipe to measure the maximum chord height (mm) of the bend of the steel pipe, and then convert it into a percentage of the length (in meters), which is the length of the steel pipe The full-length curvature.
For example: the length of the steel pipe is 8m, and the measured maximum chord height is 30mm, then the total length of the pipe should be:
0.03÷8m×100%=0.375%
⑦Size is out of tolerance
The size is out of tolerance or the allowed deviation is called the size exceeding the standard. The "size" here mainly refers to the outer diameter and wall thickness of the steel pipe. Usually, some people call the habit of dimensional out-of-tolerance "out of tolerance". The term equating deviation and tolerance is not strict, and should be called "out of tolerance". The deviation here may be "positive" or "negative". It is rare that both "positive and negative" deviations are out of line in the same batch of steel pipes.
