GlossaryKind of term: Manufacturing termDimensional tolerance
What is a dimensional tolerance?
A dimensional tolerance is the permitted variation of one dimension, given as limits, as deviations such as 20 ±0.05 or as a tolerance class such as H7.
A dimensional tolerance is the difference between the largest and the smallest permitted value of one dimension. It sits beside the dimension as a symmetric deviation (20 ±0.05), as upper and lower deviations (20 +0.1/0), as limits, or as an ISO tolerance class (⌀12 H7). A tolerance class pairs a letter for the position of the tolerance zone relative to the nominal size with a number for the IT grade, whose width grows with nominal size: h6 at ⌀25 is 0 to −0.013 mm. Dimensions without a tolerance of their own fall under the general tolerance.
What does the standard say about a dimensional tolerance?
ISO 286-1 sets out the system of tolerance classes, IT grades and fits. ISO 14405-1 defines the linear size of a feature of size; by default it is the two-point size, measured between two opposite points. Under ISO 8015 the independency principle applies: the size tolerance does not limit form, so a shaft can measure within h6 everywhere and still be bent. If the designer wants size to limit form as well, the envelope requirement Ⓔ sits beside the dimension. Under ASME Y14.5 the envelope applies by default. Dimensions hold at the reference temperature of 20 °C.
How does a dimensional tolerance work in practice?
The tolerance drives the choice of operation and gauge. A bore in class H7 rarely comes from drilling alone; reaming, boring or circular milling with a finishing pass are common, depending on diameter, material and quantity. A shaft in h6 may be turned to size on a good lathe or may need grinding. With tight tolerances measurement uncertainty counts too: as a rule of thumb, choose a gauge several times more accurate than the tolerance, and ISO 14253-1 governs how uncertainty is weighed when accepting or rejecting. The temperature of part and gauge matters for large dimensions and narrow zones.
What is a dimensional tolerance often confused with?
A dimensional tolerance is not a geometric tolerance: one concerns size, the other form, orientation, location or run-out. A tolerance class such as H7 is not a fit either: the fit is the pairing of a hole and a shaft, such as H7/g6. And a dimensional tolerance is not the general tolerance: that applies only where no tolerance of its own is given.
How do you see a dimensional tolerance in Blake?
In Blake the tolerances from the drawing sit with the part as proposals, each one to confirm or correct. If you adjust a time in the estimate because of a tight dimension, Blake records that correction with the person, the moment and the reason. More on estimating.
Frequently asked questions
What does H7 mean on a bore?
H7 is a tolerance class from ISO 286. The H puts the lower limit on the nominal size, the 7 is the IT grade that sets the width of the zone. At ⌀12 the bore may be up to 0.018 mm larger than nominal.
Does a tighter tolerance make the part more expensive?
Often, but not in every case. If the operation you run anyway holds the tolerance, you mainly notice it in measuring. If it calls for an extra operation, lower cutting data, a different gauge or more checking, the cost goes up. Judge it per dimension.