Engineering GCSE, Second Edition by Mike Tooley

Author:Mike Tooley
Language: eng
Format: epub
ISBN: 978-1-136-40647-8
Publisher: Elsevier

Figure 2.52 A tap wrench

Taps are relatively expensive and should be looked after carefully. High-speed steel ground thread taps are the most expensive. However, they cut very accurate threads and, with careful use, last a long time. Carbon steel cut thread taps are less accurate and less expensive and have a reasonable life when cutting the softer non-ferrous metals. Whichever sort of taps are used, they should always be well lubricated. Traditionally tallow was used, but nowadays proprietary screw-cutting lubricants are available that are more effective.

External threads are cut using split button dies in a die holder as shown in Figure 2.53. One face of the die is always marked up with details of the thread and the maker’s logo. This should be visible when the die is in the die holder. Then the lead is on the correct side for starting the cut. Screw A is used to spread the die for the first cut. The screws marked B are used to close the die until it gives the correct finishing cut. This is judged by using a standard nut or a screw thread gauge. The nut or gauge should run up the thread without binding or without undue looseness.

Test your knowledge 2.46

Give TWO reasons why a tap must be started square in its hole.

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