Knitting machines were developed throughout the 20th century in order to speed up what was otherwise a very time-consuming process for producing clothing. Initially they improved upon an operation ostentibly involving two knitting needles, by using moving carriages past a row of small moveable hooks, and then allowed for those pins to be selected by a computer program. Each step represented an order of magnitude decrease in human labour-time needed to make a given garment.
Linear Knitting Machines
These are best suited for producing garments with large surface areas, especially with chiefly rectangular surfaces. For example, scarves are easily made in one job, sections can be sewn together to produce sweaters, or complex digital patterns can even produce one such garment in a single run (see below).
Thanks to the efforts of Knitic, who were hacking on old floppy-disc-controlled Brother linear knitting machines, some free software was created to enable general programming of this type of machine. Since then, the OpenKnit project has gone ahead and created a completely open hardware design for a mostly-3D-printable machine.
Circular Knitting Machines
These are more appropriate for producing garments that involve tube sections where a seam sewn along its length can cause discomfort, e.g. socks, stockings, winter hats and sleeves for other garments. Their utility, while arguably narrow, is very powerful since they can produce socks in minutes with a skilled operator, and socks/hosiery are generally the type of garments that wear out fastest.
Their theory of operation is relatively simple, but as far as I know there are no CNC designs in the public domain, although there is a lot of information to search through and many images of actual machines with long-expired patents.
While the 100-year-old steel machines look like they could be very difficult to replicate in a basic fab-lab due to their intricate construction, the existence of more recent 'toy'/'hobby' versions, constructed with chunky plastic, seems to indicate that this could be possible on your average RepRap Prusa-Mendel, so long as small size is not an initial requirement.
As far as I have seen from these modern plastic versions, their hooks are not designed to be locked in place (as in the 8.5minute sock video, when turning a heel), so some modification may be needed to add that functionality. After that, it may be possible to add digital control of individual hooks as with the above linear machines. In the near term, it would be very helpful to see the internals of one of these machines (preferably a high-end one), so am appealing for anyone out there to keep an eye out for any broken-down Addi Express machines, so that we can avoid 'reinventing the wheel' from scratch. - Andy D