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The amount of tap oversize required for the plating process is calculated from the plating coating thickness .
When it is considered that the diameter of the part of the screw increases when the thickness of the plating is added, it is the "effective diameter", which is the diameter of the slope (flank) part of the thread.
When plating is applied to a internal thread with a full-angle angle of 60°, such as a metric thread or a unified thread, the calculation prediction can be made with the 3-sided ratio of the right triangle = 1:2:√3.
The amount of plating thickness added to the effective diameter of the screw is twice the plating thickness on one side. Since it is a diameter, it is 4 times on both sides. Therefore, 4 times the plating thickness is a guide for the oversize when selecting a tap.
At OSG, there is a "STD" that means "standard" to aim for screw accuracy equivalent to grade 2 in tap accuracy notation. Oversized taps have a notation such as +1 or +2 on the case to indicate that fact.
For each STD+1, an oversize of 15 μm (micrometers) is used for pitches of 0.6 or less, and 20 μm for pitches of 0.7 or more.
For example, if the plating thickness is 5, the required amount of tap oversize is 4 times 20 μm, which is STD+1. If the plating thickness is 10, the required tap oversize amount is 40 μm, which is a guideline for selection of STD+2.
* When selecting a gauge for oversize, it is important to consider "what do you want to inspect?" For example, if you want to inspect that the internal thread has been machined +0.03 mm larger than the 2nd grade, use a "2nd class +0.03" plug gauge.
As a standard stock product at OSG, the gauge for JIS grade 2 metric screws is "grade 2 + 0.03" for plugs. In the ring "2nd class - 0.03". In terms of ISO grade, only the plug gauge is available in "6H + 0.03".
In the course of a meeting with the customer, we may need oversized taps and gauges that are not in the standard lineup.
In that case, please consider making taps and gauges with special products.