Re: Taps for Wood -- Which kind?




John L. Poole wrote:

I recall from metal working days that taps come in sequence, e.g.
"taper", "plug", and "bottom".

plug is the same as bottom (at least in the UK) The sequence of three
is "taper, second, plug"

Second should be used for starting small holes in soft materials, such
as small holes in aluminium sheet or wood of almost any size. Taper
works fine if you do it right, but the small amount cut with each turn
makes it all too easy to strip the thread out while you're cutting it.

A coarse thread is essential, and some thread forms are mrginally
better than others. Whitworth (UK) or UNC (US) are good choices in hard
maple and the like. BSF, UNF and the typical metric threads are no use.
Dedicated wood threads are even better and important if you're using
softer timbers, such as beech. OTOH, Whitworth / UNC have much easier
availability, particularly if you're trying to make one component in
metal.

Other thread forms like Acme or buttress work great (especially for
clamps), but they're hard to cut with a tap or die, because of their
sharp corners. You might do well with a pair of taps (the first
taper/second tap also has the corners radiused) and cutting the male
thread with a single point tool instead of a die-set.

Some hard timbers, like hickory, are poor for thread cutting. They're
strong in bulk but they're too "stringy" to give a good thread form.
Others, like softer but fine-grained tropicals, are surprisingly good -
as any turner can tell you. It's worth reading Holtzapffel for advice
here.

Large threads in hard maple can usefully be lubricated on cutting, just
to keep temperatures down and stop the tap sticking (beeswax). It
doesn't seem to help cutting much, but it certainly reduces the turning
force.

Cyano afterwards is worth it too, particularly if you have vacuum
available (run the tap through again afterwards). Any thread in wood is
a bit of a "dog on its hind legs" situation and any help is worth
having.

.