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I've just bought a diamond wheel-dresser which is a steel rod with a
block of steel at the end with a 1.5" x 1/2" face with coarse diamonds
stuck to it.
Anyone know how the diamonds are attached?

They are usually set in a copper alloy of sorts.

And how rugged are these
things to having the diamonds knocked off?

Unless yours is one of the plated variety (it would likely be nickel colored
if so), you don't have to worry about knocking the diamond off, it's not
just glued on the surface. It is held captive by being poured in the
matrix.

Can you just push it against a wheel at any angle? I mean with the
leading or trailing edge of the dresser hitting the wheel first? Or
must you be careful to get the centre of the dresser to hit the wheel
first?

It's your dressing tool (they're called a cluster diamond), so use it as you
wish, but if you want it to last, make sure only the diamond comes in
contact with the wheel. If you introduce it at a wonky angle, you may hit
the matrix instead of the diamonds, and that has the potential to screw up
the mounting such that you could lose diamonds on the periphery of the tool.

Should the dresser be held against the tool rest, or can you
just hold it freehand?

While you may be able to use it freehand, a rest helps control the cutting
action. It's also a little safer, If you slip freehand, you can end up
tangled with the wheel. You can believe me when I tell you that hitting a
running wheel with your hands can ruin a good day.

Are light "cuts" advisable? I've been a naughty
boy and used the wheel for aluminium. I have stood to one side, just
in case, however.

Don't horse it----but insure that it makes contact well enough to actually
do something to the wheel. Introduce it to the wheel, then work it
sideways, slowly feeding inwards until you have the surface of the wheel as
you like it. Keep it moving side to side to avoid creating grooves in the
wheel, which can actually ruin the dressing tool.


Also. I've just bought a new slower speed grinder (1425 rpm - 4-pole
on 50Hz) with a gray GP wheel and a white alumina wheel. Which would
be the best for touching up carbide-tipped lathe tools?

Neither one.

While I don't recommend them, aside from diamond or CBN, only a silicon
carbide wheel will perform with any degree of satisfaction on carbide.
Aluminum oxide (both of your wheels are aluminum oxide) is far too soft to
be effective.

If you want really good results, buy a diamond wheel, and run it wet.
Otherwise, a green silicon carbide wheel will work. The dust is
hazardous------causes silicosis-----and the finish such a wheel leaves
leaves a great deal to be desired. Silicon carbide (green wheel) isn't all
that much harder than carbide, so it tends to club it off instead of cut it
cleanly the way a diamond wheel does.

Way back, I just used the grey wheel that was in the grinder without a
thought. jack

And it did a poor job of almost everything, didn't it?

Harold