Hi,
when whipping rope ends is there a general rule when choosing size of twine?
E.g. “If you have 6mm hemp-rope, use 1mm waxed whipping twine.”
Also, is common sewing needles a good choice or is it recommended to use dedicated needles?
Whipping needles seem a bit expensive to be honest…
I’m a bit of beginner so pardon my simple questions. 
Thank you.
You’ll need a sailmaker’s needle, and a sewing palm. Instead of the palm, you can press the needle against a hard surface while holding the rope, to get it through. Pliers for pulling through may help as well. It is far too hard for a sewing needle.
Except in doing some for-the-heck-of-it lateral whipping
of webbing --so to preserve its flatness, as opposed to my
usual folding it into a ‘Z’/‘S’–,
I’ve never used a needle for whipping.
Rather, I prefer to tie multiple strangle knots, and so on,
hauling them tight with other tools.
E.g., in whipping 6mm rope (hemp or other), I’d likely use
some commonly found nylon fishing line in a 7-wrap
strangle knot with a few-wrap sailmaker’s whipping finish
on the inside (i.e., away from end).
(Somewhere on this forum I’ve explained and shown this.)
I’ve found the good discussion about whipping; it is here:
http://igkt.net/sm/index.php?topic=2225.msg17672#msg17672
I reiterate/summarize : the commonly published methods for
whipping are IMO far in excess of what is practically needed;
fairly simple methods with readily available materials suffice
to effect a good whipping (QED) ! (E.g., in Cape May, South
Jersey USA (yeah, some like the “south” bit; officially "New “),
one can get discarded monofilament fishline out of a big bin
collector for recycling, and this stuff is great for “small stuff”
of 1/2” diameter and less. Mason line is pretty available in
discard at construction sites (in bright colors --pink, green,
orange, yellow–, or not-so-bright), and hardware stores,
and works well over a similar range of ropes. (One can used
twin whipping cord to some success over larger diameters
–having the same number of wraps, but with doubled cord.)
And again, in my awareness around the maritime area of
Cape May, most ropes seem to be UNwhipped unless by
black electrical tape. (Perhaps I would see more of the
published fancy whipping were I to see more yachting
cordage?) The two can be combined : in some cases where
black tape has wanted to unstick, I’ve put on some whipping
to prevent that --and keep the tape working (e.g., bike tape
on handlebars).
Btw, for really small stuff --1/8" say, and for cable-hauling
polyester (lubricated) solid tape–, I’ll make sure the whipping
is begun by inserting it through the material, as I don’t
trust being able to get sufficient gripping friction from the
tension of whipping (since it too must be small and can
only sustain so much force) --lest pulling out a tail end
through some knotted part will strip the whipping.
And another consideration is this :: to what extent is whipping
stressed/taxed/worn? As noted, in the common practice of
cordage usage I’m aware of, whipping seems a forgotten art
or at best left to the vagaries of tape --which will work at least
for a while, and will show its failing by a flapping, unwinded
end, and can easily be renewed.
I suspect that in general one will have little issues with whipping
and can do well with occasional inspection; where needed, one
can redress a failing whipping (whose failure will have what sort
of consequences? --unlikely anything much severe, just annoying).
In this respect, one might favor putting on whipping that can
suffer a cut somewhere without that losing it all --simply do
two whippings, say; or use a method that is in essence a
sequence of binding knotting (West County whipping, e.g.).
And what method might work well with one strong slick
whipping material could be problematic with material that
is weaker and frictive (e.g. nylon monofilament can be
hauled tight in a multi-strangle knot; cotton string cannot
get nearly so tight, for overcoming friction and for want of
strength :: they need different methods.
HAVE FUN EXPLORING, EXPERIMENTING!