Dimple Countersinking Tools
Dimple countersinking is accomplished by using
male and female dies. The female die, shown in
figure 13-14, contains a spring-loaded ram that
flattens the bottom of the dimple as it is formed. This
prevents cracks from forming around the dimple. The
forming of a dimple is a combined bending and
stretching operation.
A circular bend is formed
around the hole. As in any bending operation, the
tension force at the upper side of the bend (break)
creates the radius at the junction of the two
surfaces-the top side of the sheet and the downward
bent inner wall of the dimple depression. The stretch
occurs around the hole as it is displaced from its
original position and relocated at the bottom of the
dimple. The female die must have a slightly larger
cone diameter than the corresponding dimension of
the male die. This allows for material thickness and
relieves the bending load at the break in order to avoid
circumferential cracks around the boundaries of the
dimple. As a further safeguard, a slight radius is made
on the female die at the junction of the top face with
the dimple depression.
Dimpling dies are made to correspond to any size
and degree of countersunk rivet head available. The
dies are numbered, and the correct combination of
dies to use is indicated in charts specified by the
manufacturer. Both male and female dies are
machined accurately and have highly polished
surfaces. When you dimple a hole, place the material
on the female die and insert the male die in the hole to
be dimpled. The dies are generally brought together,
forming the dimple by a mechanical or pneumatic
force.
As newer aluminum alloys were developed to
increase shear and tensile strength, they became more
difficult to form, since these alloys are harder and
more brittle. These aluminum alloys are subject to
cracking when formed or dimpled cold. For this
reason, it is necessary to use a hot dimpling process.
The application of hot dimpling to the more brittle
materials helps reduce cracking. The heat is applied
to the material by the dies, which are maintained at a
specific temperature by electrical heaters. The heat is
transferred to the material to be dimpled only
momentarily,
and none of the heat-treat
characteristics of the material are lost.
There are several models of dimpling machines
used in the Navy, from the bulky floor models to
portable equipment. One of the most popular portable
types is shown in figure 13-15. Basically, it has three
units:
the dimpling control unit, the dimpling
squeezer, and the thermo dimple gun.
The dimpling control unit is a small compact unit
designed to regulate dimple die temperatures,
prepressure, dwell time, and final forming pressure.
Figure 13-14.—Dimple countersinking.
13-10
