wrote:
>On 11 Apr 2007 at 8:23, Ian White GM3SEK wrote:
>
>< snip, snip >
>
>> Drill exactly the right size of pilot hole (not too big, not too
>> small)
>
>Which brought to mind a question that I've been dealing with for
>many, many years: "What is the correct size pilot hole for self-
>tapping sheet metal screws?" I have always measured the screw at its
>narrowest body diameter (i.e. not at the pointy tip) using a vernier
>caliper with jaws that will fit to the bottom of the thread. [An old
>machinist I knew always called it a "very near caliper" because the
>measurement obtained was "very near the correct dimension."]
>
>Anyhow, is there a chart like the tap-drill sizes vs. screw sizes
>that I've been missing all of this time?
It varies with the type and thickness of the metal, and the type of
screw as well. The only method you can completely rely on is to try a
range of different sizes, in small steps (eg 0.1mm / 4 mil), and find
out which one actually does give the best results. Then write it on the
box of screws.
--
73 from Ian GM3SEK
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