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CORRECT USE OF TAPS
A guide to successful machine tapping
•
Use the correct tap to suit the application
•
Select the correct tapping drill size
•
Direct an adequate supply of the recommended lubricant to
the cutting area of the tap
•
Make sure the workpiece is securely held.
•
Use a tapping attachment suited to the application and align
the tap with the hole.
•
When using a machine without lead screw feed, hand feed
the tap until sufficient engagement produces self feed.
•
When using a machine with lead screw feed, set the lead to
correspond with that of the tap. This also applies on two and
multi start taps.
Preparation of Holes
A good hole is a pre-requisite of a good thread. Some of the factors
which contribute to inferior threads are:
(a) Out of round holes. The thread will be correspondingly out of
round.
(b) Poor surface finish in the hole.
(c) Size of the hole. A holewhich is too small will cause overloading
of the tap with the possible breakage.
(d) Hard spots and abrasive surfaces in the cored holes. These
holes should be pre-drilled.
Percentage Thread Depth
For general purpose work a thread depth of 75% is recommended.
A drill size equal to the minor diameter of the tap produces a 100%
thread depth. This practice is normally recommended fot the following
reasons.
(a) 100% thread depth requires excessive power to turn the tap,
with consequent possible breakage.
(b) 100% thread depth is only 5% stronger than the normal depth
of 75%.
(c) Even a 50% thread depth still produces a thread stronger than
its mating bolt.
