True­ness of gauge

To ensure that a bolt rotates precisely within the thread it is necessary for both the thread dimensions of the bolt and the nut to offer enough “play” for the fastening process. This “play” is reflected in the trueness of gauge.

In practical use, trueness of gauge is guaranteed via precisely tailored tolerance systems, which also incorporate different surface coatings. If surface protection is to be applied, the threads of bolts should be produced smaller and those of nuts correspondingly larger, to ensure functional capability.

Characteristics of thread gauging

This procedure is undertaken using a DIN EN ISO 1502 gauging, which often results in problems occurring in practice, although these can be avoided if a number of aspects are taken into account:

  • “Used” gauges should be employed for the outgoing inspection, as the dimensions of “new” gauges are lower than the thread maximum (see DIN EN ISO 1502).
  • The thread should be inspected prior to application of the surface coating. If the thread gauge is difficult to apply - for example in the case of scaled or blasted surfaces - a drop of oil is permitted and helpful.
  • In the case of zinc flake coatings the surface may be scratched/loosened by the thread gauge, resulting in the “jamming” of the ring. Cleaning and re-application is recommended in this case.

A further problem occurs in the gauging of internal threads: as nuts in the bath in particular are “retaining parts”, this leads to concentration in the thread base. Although gauging may not be possible, the nuts can still be fastened securely. However, according to DIN EN ISO 10683 2014-10 ordering parties and coaters may negotiate other procedures. Regarding this, in the "Inspection of threadability of nuts with zinc flake coatings" guideline the Deutscher Schraubenverband introduced an alternative test pin that is already in use and is intended to be incorporated into a VDA regulation.

Funktionsprüfung „Lehrung von Gewinden“

Mit diesem Verfahren wird festgestellt, ob Schraube und Mutter präzise ineinander passen. Im Ergebnis liefert die attributive Prüfung eine grundsätzliche Aussage – nämlich „in Ordnung“ (i. O.) oder „nicht in Ordnung“ (n. i. O.). Zusätzlich durchgeführte qualitative Prüfungen können darüber hinaus auch konkrete Messwerte angeben.