When measuring, one uses an instrument with a calibrated scale. The measuring device can be rigid or adjustable (caliper gauge). When gauging, one compares the shape or a measurement with a gauge that embodies a desired measurement. The gauge is rigid.

In thread cutting, both functions are used. They are used in both main applications.

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Measuring threads with the aid of thread gauges and thread templates

Thread gauges are used to check the quality of a manufactured thread. Threads are standardised and so gauges can be used. Gauges usually have a good side and a reject side. It is marked with a red ring.

Thread templates are used to determine the pitch of a thread. The nominal dimension and the pitch are used to determine a thread. To measure the nominal dimension, you need a caliper gauge. 

With the data of the pitch and the nominal diameter, thread tables can be used to determine which thread type and which thread size is involved. Nuts and bolts can be assigned in this way. Even before recutting defective threads, one should know exactly which thread type is to be machined.

How do I determine a thread?

Thread determination: To determine an unknown thread, two pieces of information are necessary: the pitch and the outer diameter for a bolt thread, and the inner diameter for a nut thread. The pitch of a thread is the distance from one flank to the other in mm for metric systems or the number of threads on an inch for inch threads.

Praktische Gewindebestimmung Am Oldtimer Gewindebolzen mit einer Gewindeschablone

Thread determination: external thread

First, measure the diameter of the thread with a caliper (also known as a vernier caliper). With digital calipers, you can choose between inch and metric units. The diameter consequently tells you whether it is a metric or inch thread. Then the thread template (also called thread gauge) comes into play. Please check the individual thread templates until one of the templates can be fitted exactly into the threads. Then read off the desired pitch printed on the thread template. Now the thread slider comes into play, which contains all the information about the individual thread types. Here you can then compare the measurement results with the data for the thread types and select the appropriate thread type.

Determining the thread: internal thread

If the external thread is relatively easy to determine with the help of a thread gauge, this is more difficult with internal threads. If you do not want to destroy the workpiece, the only thing to do without considerable technical effort is to roughly measure and try. To do this, proceed as follows: Measure the core dimension of the thread with the outer legs of the caliper. This should correspond to the core hole drilled before tapping. In the example, we measure a diameter of 23 mm. The formula for determining the core hole diameter is:

dk = D-P (core hole diameter = nominal diameter pitch)
or: nominal diameter = core hole diameter + pitch

Now look for a suitable core hole size in the core hole table of the calipers. Here you should bear in mind that the design of the core holes often does not correspond exactly to the specifications, but is often made larger. According to the formula, there are two possibilities for the present example with metric units M 24 x 1 or M 25 x 2 or one possibility in the inch range: G 5/8 x 14 thread/inch. A selection is to be made by trying with corresponding screws.

Tabellenbild Bolzendurchmesser und Daten zum Kerndurchmesser und Fasendurchmesser