PRODUCT MEASUREMENT TOOLS (GSR JERMAN)
T H R E A D I D E N T I F I C AT I O N
To identify an unknown thread you need to get two details: the pitch and the outer diameter for a bolt thread or the inner diameter of a nut thread. The pitch is the gap between the flanks for metric threads or the number of pitches per inch for inch threads.
First you measure the diameter of the thread with a caliper. The digital caliper offers inch and metric units. The diameter will show you if it is a metric or an inch thread. You continue with the pitch gauge. Please test the single sheets until one of them fits precisely into the thread. Then you can read the pitch of the thread on the gauge which is printed onto it. Now the thread slide is used which contains all details about the different thread types. You can com- pare the measured values with the data on the thread slide to find out the correct thread type. All measuring tools are described on the following pages.
If it is relatively easy to determine the external thread using the pitch gauge it is complicated to get the internal thread size type. If you don’t want to destroy the workpiece and avoid major technical effort you can just roughly measure and try out. The procedure is as follows: Measure the core size of the thread with the outside leg of the caliper. This should be the core hole size which has been drilled before the thread was cut. In our example we measure an external diameter of 23 mm. The formula to determine core hole diameter is:
dk = D – P(Core hole Ø = Nominal Ø – Pitch)
or: Nominal Ø = Core hole Ø + Pitch
M E A S U R E M E N T
Now you can check the core hole table of the thread slide to find a suitable core hole size. Hereby you have to regard that the core holes are mostly larger than the guideline. After calculating the example with the formula you get two possible metric sizes M 24 × 1 or M
25 × 2 or one inch size G 5/8 × 14 pitch/inch. A decision is made by trying out the appropriate screws.