I visited a small local spining mill a while ago, and have since started to wonder how the different types of commecially-prepared rovings are made.
In that small mill, cleaned fleece was carded to make ropes that could immediately be sold for handspinning, or could be spun by the mill's spinning machine. Or, they could be fed into the pin-drafter. Pin-drafting machines use carded input. They use a couple of rows of really nasty teeth to straighten out the roving, which is then coiled up into a nice-looking rope. The pin-drafted rovings could be sold or spun as well.
I suspect that a lot of small mills in North America have pindrafting like this; they can
produce carded preps and pindrafted roving, which some will call
“combed” or “worsted”, although it’s not really if you are a purist. Mills
like Jagger in Maine, Morro in CA, and the small mills around me here
in western Canada have pindrafting equipment of this type, some of it "vintage"! You can handspin a “semi-worsted” yarn from this type of prep but not a
“true worsted”, again if you are a stickler for technical details. That's because of the initial carding step. True "worsted" prep goes straight from fleece to combing, no carding at all involved.
The combed prep you get from big distributors like Louet is different. True “worsted” or “topmaking” factories use a combination of “gilling
boxes” and combing machines to produce even more aligned fibers.
There’s no carding done, not even as a prepatory step, like in
pindrafting. I've never seen this type of equipment, and Googling it hasn't made me any wiser in terms of how it functions or what it looks like.
It used to be that only long-fibered wools were combed - in the UK, special "longwool" sheep were bred to support this industry - but nowadays even shorter, crimpier fibers can be combed. Merino is not a longwool, but it is put through this process to give smooth tops.
There are some smaller worsted mills still operating in the UK.
I’m hunting for “topmaking” mills in America, but there are not that
many. There’s the Maine Top Mill and Chargeurs in Jamestown SC, and
that’s all I can find on Google. It seems this was once a thriving
industry in the US, but it died out early in the 1900’s.
Finally, here is a cute Popular Science magazine article from 1891, which was written when the wool processing industry in the US was booming. It gives a nice overview of the then-recent developments of the machinery, and how processing was done.