From plug gauges to gauge steels:
Everything you need to know about thread gauges

Test equipment or measuring device: The finer points of measurement technology

Before we get down to the nitty gritty, let's first take a closer look at the basics of measurement technology. This is all about determining whether a component reaches the required nominal dimension. As is so often the case, there are several ways to achieve this.

Of course, you can simply measure. To do this, you need a measuring device, i.e. a scale, a tape measure, a caliper gauge or similar. You then use this to compare the Dimensions of the test specimen and obtain a numerical measured value as a result.

Gauging, on the other hand, works differently. The result here is not a numerical value, but a categorisation into good or reject. In this way, you cannot determine the exact dimensions of a thread. The advantage of this method is that it is quite simple to carry out and minimises application errors. Where nothing is measured, there can be no measurement or reading errors. The result is clear and leaves little room for interpretation.

Fit, tolerance and co.: What does the gauge actually do?

You have probably noticed additions such as H7 or H8 on your taps or other tools. These are specifications for tolerance. This describes the values between which the exact Dimensions of a component may fluctuate so as not to impair the correct fit between the external and internal thread.

You can use a thread gauge to check whether this tolerance is adhered to, i.e. whether the thread is true to gauge. This ensures that you as a manufacturer supply flawless products and, as the system is standardised according to DIN, you are also on the safe side legally. In the professional sector in particular, it always makes sense to check the accuracy of the threads produced at regular intervals in order to prevent possible complaints from customers.

The system of thread gauges

Thread gauges can be used to check internal and external threads. For each thread, one tool is required to test the upper limit of the tolerance class and another for the lower limit. The two are differentiated by a red marking, which identifies the tool for determining the reject. In certain cases, both are mounted on one handle, in which case they are referred to as limit plug gauges.

Gauging internal threads with a plug gauge

To check an internal thread, you need a plug gauge. The thread plug gauge tells you whether the thread is too small and checks other attributes of the thread profile. If this check is negative, you can still rework the thread. This is not the case with the reject plug gauge: If the thread fails the test, it is too large and can therefore no longer be saved.

How to use a plug gauge

Ensure that the thread is clean and free of chips and dirt and work by hand without applying any particular force. The good plug gauge must be able to be screwed in completely in this way. However, the reject plug gauge must not be able to be moved more than two turns, otherwise the thread will be too large.

Special case limit plug gauge

According to DIN 2245, the limit plug gauge is a special version of a thread gauge for bores with a nominal diameter of 1 mm to 40 mm. In this range, both plug gauges required for testing are located on a common handle with a good side and a reject side, which is characterised by a red marking. In all other respects, however, they are used in exactly the same way as described above.

Smooth gauges for the internal thread

The core diameter can also be checked using limit plug gauges or good and reject plug gauges. In this case, these are smooth, cylindrical tools without threads. The good side must be able to be fully inserted into the hole, while the reject side must not fit in.

External thread gauging with a ring gauge

Testing an external thread with ring gauges is similar to using plug gauges. A distinction is made between a good thread ring gauge and a thread rejection ring gauge (again recognisable by the red marking). There is no equivalent to the limit plug gauge here; these are always two separate test devices.

How to use a plug gauge

First ensure that the thread is clean and chip-free. It must be possible to unscrew the good ring gauge completely without applying force. The reject ring gauge must not be able to be moved more than two turns, otherwise the thread is a reject.

Smooth gauges for the external thread

Smooth gauges are used to check the external diameter of the external thread. This must be carried out after tapping, as the diameter will still change.

Material and calibration: How to recognise quality

To be able to rely on the results of the test, you should ensure that you only use high-quality gauges from a renowned brand. The test equipment should be made of first-class material such as gauge steel. You are particularly safe if you also have a calibration carried out. In the BAER online shop, you can easily add such a calibration to your shopping basket under the article number "KAL01" when placing your order.

Thread gauges in the BAER online shop: Here you will find item no., price per piece and further information

Are you looking for reliable thread gauges to check your threads? At BAER, we always have a wide selection of products in stock for you. The right product type is available not only for the metric standard thread, but also for American standards such as the UNC thread and special cases such as the G pipe thread, the trapezoidal thread or the tapered Rc pipe thread. Our customer service is looking forward to your enquiry and order.