In article .com,
"flybynightkarmarepair" wrote:

The objective is to end up with a hole which after dimpling is #30,
i.e. 0.1285. Others have noted that dimpling enlarges the hole. So,
MY strategy is what you alluded to earlier - drill slightly undersize.
Big enough to admit the cleco, but smaller than you want the hole to
end up after dimpling.

My procedure for flush riveted skins is to drill #40 and cleco with
3/32", take everything apart, deburr, put it back together, drill (or
ream - its a little marginal, but you can ream to #32 from a #40 pilot
hole, and it leaves less of a burr - notice I'm NOT deburring a second
time) with #32, take it apart, dimple, cleco with 1/8, rivet. I'm an
inexperienced builder, so this anal retentive procedure makes up for my
lack of skills and confidence.

I also have a punch that simultaneously punches and dimples. You can
get them from Aircraft Spruce and others, but be aware that the
American made punch costs more than a whole ChinCom Roper-Whitney
knockoff punch set. And they are not actually that useful - you can't
use them for matched hole tooling for instance.

I would suggest looking at the hole with a 10X magnifier and look for
small cracks. The best dimpled holes are those that are drilled,
deburred, then dimpled with a good set of dies and somethhing to hold
the dies. The combination punch/dimpler leaves a fair number of stress
raisers in the hole and can be susceptible to fatigue.

--
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