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
Figure 4-15.--Hot dimpling kit.
boundaries of the dimple. As a further safeguard, a
This same unit is used with both the hot dimpling
slight radius is made on the female die at the junction of
squeezer and the thermo dimple gun.
the top face with the dimple depression.
The hot dimpling squeezer is designed for use
Dimpling dies are made to correspond to any size
where stationary squeezer operation is impractical or
and degree of countersunk rivet head available. The
impossible. It is capable of working all material gauges
dies are numbered, and the correct combination of dies
up to and including 0.091 of an inch. The squeezer is
to use is indicated in charts specified by the
designed to dimple in areas that are inaccessible to
manufacturer. Both male and female dies are machined
other types of equipment. Electrical heaters
accurately and have highly polished surfaces. When
independently warm male and female dies. The heaters
you dimple a hole, place the material on the female die
produce a short heat-up and recovery time. The male
and insert the male die in the hole to be dimpled. The
die is adjustable to provide the maximum squeeze on all
dies are generally brought together, forming the dimple
gauges of material. The unit also has a cooling feature.
by a mechanical or pneumatic force.
The thermo dimple gun is used to dimple in the
As newer aluminum alloys were developed to
center of panels and in those areas otherwise
increase shear and tensile strength, they became more
inaccessible to stationary dimpling equipment. When it
difficult to form, since these alloys are harder and more
is being used on the aircraft, the thermo dimple gun
drives the dimple from the exterior while the female die
when formed or dimpled cold. For this reason, it is
and dolly bar are used on the inside. The thermo dimple
necessary to use a hot dimpling process. The
gun is air-cooled. This eliminates the need for
application of hot dimpling to the more brittle materials
cumbersome heat-resistant gloves. This tool is small,
helps reduce cracking. The heat is applied to the
compact, well balanced, and easy to handle.
material by the dies, which are maintained at a specific
temperature by electrical heaters. The heat is
Before adjusting the control unit for dimpling, you
transferred to the material to be dimpled only
should refer to the equipment manufacturer's dwell
momentarily, and none of the heat-treat characteristics
time chart. When you set up any dimpling equipment,
of the material are lost.
follow the step-by-step procedure outlined in the
operating and maintenance manual supplied with the
There are several models of dimpling machines
equipment. Since equipment types vary, it is
used in the Navy, from the bulky floor models to
impractical to specify a standard procedure; however,
portable equipment. One of the most popular portable
there are four general requirements of a dimple, and by
types is shown in figure 4-15. Basically, it has three
examining each, it is possible to denote improper
units: the dimpling control unit, the dimpling squeezer,
setting up of equipment.
and the thermo dimple gun.
1. Sharpness of definition. It is possible to get a
The dimpling control unit is a small compact unit
dimple with a sharp break from the surface into the
designed to regulate dimple die temperatures,
dimple. Two things control the sharpness of the break:
prepressure, dwell time, and final forming pressure.
the amount of pressure and the material thickness.
4-9
