Understanding the "Fit Tolerances" for Precision Hole Notes

When performing "hole making", high-precision machining may be required depending on the application. In JIS, there is a "fit tolerance" that is used to specify high-precision dimensions for holes and shafts, and it is difficult to use if you are not used to it due to the high degree of subdivision.

What is a "precision hole"?

"Precision holes" are holes that have dimensional accuracy in microns and are used for a variety of purposes. In addition, "precision holes" are machined using a tool called a reamer to finish the hole after drilling, and the precision hole is machined in units of 1 micron. *1 micron = 0.001 mm

What is "Fit Tolerance"?

The "fit tolerance" is the "dimensional tolerance" specified for the hole and the corresponding shaft mating. The tolerance range is set in detail according to the "standard size classification" as well as the general tolerance, but the biggest difference from the general tolerance is that the "tolerance range" is represented by symbols. Plus, it's one-way tolerance. For example: (0/+0.0018)=10H7We call the above H7 and other symbols "tolerance zone class", the hole is represented by an uppercase letter like H7, and the axis is represented by a lowercase letter like H7.

The purpose of "fit tolerance".

"Fit tolerance" is used depending on the mechanical purpose of the part and the relationship between the hole and the shaft. It can be broadly divided into the following three uses.

1. If you select a "Tolerance Zone Grade" that is slightly larger than the standard size for the bore diameter and a "Tolerance Zone Grade" that is slightly smaller than the standard size for the shaft diameter, a gap will be created between the bore and the shaft. For mechanical use, it is used for detachable parts, bearings, etc.
2. A transition fit is a "tolerance zone grade" fit in which the hole and shaft either have a clearance fit or have an interference fit. It is used for mechanical purposes, such as high-precision dowel pins and parts to be disassembled using fixtures.
3. An interference fit is a fit in which the shaft diameter is larger than the bore diameter and the shaft and bore are engaged. For mechanical use, it is used to assemble parts into one piece by hot assembly.

The symbol for the "fit tolerance".

When using Fit Tolerances, tolerances can be specified with symbols such as H7, but why should you use symbols? It is basically similar to the "General Tolerance Table", on the grounds that the tolerance values are in microns, which is very precise; There is a fit of the hole and the shaft; And the "standard size classification" is also divided into very fine. Therefore, if it is expressed only by tolerance values, it is necessary to calculate whether the fit is a "gap fit", but if it is represented by a symbol, such as a fit of H7, H7, it is immediately known that it is a "gap fit".

Fit tolerances are not limited to holes and shafts

As indicated, the "fit tolerance" is generally considered to be the "dimensional tolerance" of the hole to the shaft, but it can also be used for the size of the mating part such as the bump. However, it is more difficult to achieve accuracy in microns in plane machining than machining holes and shafts, so be careful when using it.