Countersinking And Dimpling; Machine Countersinking; Dimpling; Rivets And Sheet Thickness - Van's Aircraft RV-8 Construction Manual

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5.5 COUNTERSINKING AND DIMPLING

Flush riveting requires that a flat or "mushroom'' set be used and that the skin around the
rivet hole be countersunk either by dimpling or machining. Be sure to remove any vinyl
coating before machine countersinking or dimpling the parts. Dimple countersinking will
be simply referred to as dimpling for the remainder of this manual.
For AD3 rivets, a total material thickness between .016 [.4 mm] and .032 [.8 mm]
must be dimpled. Material thickness between .032 [.8 mm] and .040 [1.0 mm],
should be dimpled, but a countersink may be used if necessary. Finally, for a thickness
of .050 [1.3 mm] and above the material must be countersunk.
For AD4 rivets, .050 is the minimum thickness that may be countersunk.
Metal thicker than .040 is difficult to dimple, so it is common practice to machine
countersink any material thickness that is more than .040. There are a couple of
reasons for this. First, although RVs are designed so that a countersunk joint is
acceptable, the interlocking nature of a dimpled rivet line is stronger. Second,
dimpling leaves a thicker edge for the shop head of the rivet to form against,
meaning that the metal deforms less and there is less chance of "working" rivets later.
This recommendation is meant as a guideline, not an absolute rule. There may be
instances where countersinking 0.032 stock makes the job easier, and certainly can
be used. One example is when a sheet is sandwiched between two other sheets.
In this case, the sheet receiving the rivets manufactured flush head is dimpled, the
sheet underneath is machine countersunk to receive the dimple, and the third sheet
is left full thickness. When countersinking the second sheet you must go slightly
deeper than you would for a rivet. See Figure 1.
There are a few instances on RVs where it is considered acceptable to enlarge holes
when machine countersinking. Primarily this is done to countersink for the
installation of flush screws. It is also done where multiple layers are already riveted
together, and on material that is too thick to dimple, such as a wing spar flange.

MACHINE COUNTERSINKING

Machine countersinking removes metal and is done with a stop countersink tool and a drill motor
be used in areas where the skin thickness is sufficient. CAUTION: Check the machine countersink depth frequently since
variation in part shape, hand pressure, and tool sharpness will cause the depth to vary.
100°
0.020
0.032
0.032
0.032
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EXAMPLE 1. TOP SHEET
TOO THIN. COUNTERSINK
ENLARGES HOLE.
FIGURE 2: RIVETS AND SHEET THICKNESS
05-08 RV-ALL 2 04/15/13
PAGE PAGE REVISION: REVISION: DATE: DATE:
DIMPLED SKIN
0.032
EDGE OF
MACHINE COUNTERSUNK
HOLE GIVES POOR
EDGE SUPPORT FOR
RIVET HEAD AND
BODY
0.032
MACHINE CTSK
OK WHEN HELD
BETWEEN SHEETS -
GOOD SUPPORT OF
RIVET SHOP HEAD
FIGURE 1:

DIMPLING

Machine countersinking can only
.
AD3 RIVETS
TYPICAL
0.050
0.040
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EXAMPLE 2. MINIMUM
EXAMPLE 3. MORE THAN
ACCEPTABLE THICKNESS
ADEQUATE THICKNESS
OF TOP SHEET.
OF TOP SHEET.
Where the skin thickness is insufficient, the countersunk hole for the rivet head enlarges the original rivet hole and no longer
supports the shaft of the rivet. Only a portion of the rivet head is now contacting the skin, so it cannot achieve its design strength
which is based on full head contact. See Figure 2, Example 1.
When using the minimum acceptable thickness, the full rivet head is supported and the original hole is not enlarged as shown in
Example 2. Example 3, shows not only the fully supported rivet head, but also contact with the rivet shank.
Use the appropriate rivet or screw as a gauge when you machine countersink. Stop when the rivet or screw is flush. For a dimpled
skin riveted onto a machine countersunk surface the countersink must be slightly deeper as mentioned earlier. Proper depth is .007
deeper than when the rivet head is flush. This depth correction corresponds to seven "clicks" on a microstop countersink tool
indexed in .001 inch increments.
When countersinking for a #8 (or larger) screw, the countersink cutter is removing so much material that it can easily "chatter"
resulting in a rough surface finish and an out-of-round countersink. A good process for obtaining a smooth finish is to set the
microstop countersink tool .005 inch short of the final depth, make the first cut at a slow speed applying heavy pressure, readjust
the microstop to the final depth, and make the final cut at high speed applying light pressure.
DIMPLING
The dimple dies can be used in most tools traditionally used for riveting such as hand and pneumatic squeezers and special
dimpling tools such as one commonly referred to as a C-frame, which is used for dimpling in the middle of large sheets.
It is fully acceptable, and common practice, to use a rivet squeezer (hand or pneumatic) to dimple substructure that will be covered
up, since the dimples will not be visible.
The best exterior finish quality possible starts with well formed rivet dimples. When dimpling any rivet hole that will show on the
exterior it is best to only dimple with a C-frame or other tool that can produce crisply formed, quality dimples. Most rivet squeezers
(particularly a hand squeezer) cannot deliver comparable results especially on the large dimples required for flush screws.
When dimples are not fully formed, the aluminum skin around the perimeter of the dimple (approx. .5 radius from hole center) will
have a dished shape. Once you learn what to look for, this can easily be detected with your eyes by evaluating the reflection in the
skin surface. When viewing the reflection of an overhead light source across the top of a dimple, the only place the reflection should
be distorted is inside the dimple. The skin should look clear and distortion free all around the perimeter of the dimple. Under-formed
dimples are a rather common problem. Fortunately it is difficult to "over do it" by hitting the arbor too hard when using a C-frame
tool for dimpling, that is unless extreme force is used which may cause localized stretching of the material.
When correct technique is used dimple dies will scuff the surface of the skin within the entire die contact area. This scuff mark
indicates the dies have fully seated together with the aluminum materiel tightly sandwiched between them. On the contrary if only a
circular ring (formed by the dimple die outer diameter) is present on the skin surface the dimple has not fully formed. The amount of
force required to accomplish this varies, depending upon material thickness and dimple size. Thin material like .016, and small
dimples require much less force than .040 material and a #8 screw dimple.
It takes practice to learn good technique. The goal is learning how to judge when a dimple is well formed. When using a C-frame
tool listen for the sound produced when striking the arbor. The sound of fully seating dies is different from the sound of not fully
seating dies. This sound varies depending upon the type of hammer used, but there is always a noticeable difference.
The dimple countersinking process stretches the metal around the perimeter of the hole being dimpled. It is very important that
holes drilled to final size be well deburred beforehand to reduce the likelihood of cracking. The bigger the dimple, the more
stretching occurs and the greater the chance of cracking.
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