Many members,
although aware that a tap is used to cut a thread in a hole, may not know the
correct way to actually use one.
Taps
are made from hardened and tempered cast steel and, in the smaller sizes, are
extremely brittle and easily broken. Where a broken tap could jeopardise the
whole job, it is good practice to drill the hole slightly larger and accept a
shallower thread, if the nature of the work permits.
Taps normally
come in three types, Taper (which is used for the first cut), Second and Bottom.
Often only the Taper and Bottom taps are used quite satisfactorily but if the job is
critical and would cause great problems if it went wrong it might be advisable
to use all three taps (if they are available).
After drilling
the hole, select a tap wrench, that will not give too much leverage and tighten
it on the taper tap so that it will not come loose when in use. Never attempt to
tighten the wrench with the tap in the hole, it will almost certainly result in
a broken tap. Enter the tap in the hole and turn forward and backward, taking
great care to keep the tap square to the face and in line with the drilled hole.
If the surface is not large enough to test this squareness with a try square,
get the assistance of another person to sight the tap from the side; the
operator himself can sight from the front. The backward turn is to break the
cuttings and ease the next forward turn. All material except cast iron and brass
require the use of a cutting oil. Lubricating oil eases the process but does not
assist the cutting action. The value of the use of cutting oils and pastes has
to be experienced to be appreciated.
If the hole to
be tapped is a through hole, the taper tap may be screwed straight through and
the second and Bottom taps will not be required. If the through hole is a long
one, however, then difficulty may be found in screwing the taper tap straight
through, because the tap will tend to jam; if 'you persist then
tap will break. As soon as the taper tap begins to jam, remove it and use the
Second tap, and when that tightens use the Bottom tap. Repeat the use of
Taper, Second, Bottom taps until completion.
If this
procedure is adopted, tapping will be made easier and the danger of breakage
considerably reduced.
The same
procedure is adopted with blind holes, great care being taken not to screw the
tap hard against the bottom or again it will be broken. Before completing the tapping of a
blind hole it may be necessary to clean out the swarf from the
hole several times, and at the same time wipe the tap clean. Short blind holes
present further difficulties. To start and finish a thread in a short distance
requires great care, since it may not be possible to use the taper tap at all,
as this will merely ream out the hole. Starting with the Second tap is of course
more difficult, especially to start it square. To overcome this, the job should
be left in situ after drilling and the tap fitted into the drill chuck,
carefully pulling the drill spindle forward and backward. Always maintain an
even torque or twisting force, and a better 'feel' can be obtained with small
taps if the job as well as the tap and wrench is held in the hands.
Broken taps
Taps broken when tapping holes can be a problem and are most difficult to
remove. Tap extractors are available and are effective on large sizes. It is
sometimes possible to break out the tap with a small chisel or punch or even
screw it backwards out of the hole. On some occasions the broken piece may be
driven out from the other side, especially if it is the end of a taper tap.
Depending upon the job, it may be possible to soften the tap by heating with a
fine blowpipe flame and then carefully re-drilling. If, however, the tap is
broken in cast iron, great care must be taken with the heating, as local
heat may crack the casting.
See also
'Austin Seven Threads' researched by Peter Kemp and which
appeared in Focus, Jun 2001 pp20-25.
This article, written by Malcolm Watts, originally appeared in CA7C Seven Focus
Mar 2002 pp24-25.
Taps
are made from hardened and tempered cast steel and, in the smaller sizes, are
extremely brittle and easily broken. Where a broken tap could jeopardise the
whole job, it is good practice to drill the hole slightly larger and accept a
shallower thread, if the nature of the work permits.