I moderate a few knitting machine groups on Ravelry, and we often get questions from people wanting to buy a knitting machine. This is a copy of an explanation I posted, with a few minor tweaks.
Firstly, you need to decide what thickness of yarn you want to knit with the most. 99%* of machines are a single gauge:
3.6mm is called "fine gauge" and it knits laceweight up to fingering
4.5-5mm is called "standard gauge", and can knit from light fingering up fine/sport weight yarn
6-7mm is called "mid gauge", and can knit fine/sport weight up to medium/worsted yarn
8-9mm is called "bulky" or "chunky" gauge and knits light/DK yarn up to bulky weight yarn
The gauge is determined by measuring the pitch of the needles. As you can see, there is overlap with all gauges. Knitting in the middle range of the machine (tension 5-6) is the most successful. Knitting with the wrong yarn/wrong gauge can produce holey knitting or stiff knitting.
There are two main types of knitting machines - punchcard and electronic. This refers to how the machine selects needles to create patterns, and how the pattern can be manipulated. Casting on and off, shaping, and cables are still done by hand, and the carriage is still moved physically by hand.
Machines can have plastic or metal beds. Plastic bed machines (with the exception of Passap) are more basic – any patterning is done manually, and ribbers are not usually available for them**. Bond/USM machines fall into this category.
Punchcard machines use a plastic punched card to read the pattern in, so your pattern repeat is limited to the width of the punchcard (usually 24 stitches, but there are other variants). You can vary the pattern by reversing or turning the card around, but to make any other changes you’d need to punch a new card.
Electronic machines use a mylar sheet or a connection to a computer (or built- in patterns) to do their patterning, so you can have up to 200st wide patterns (depending on machine). Depending on the machine, the pattern can be scaled, flipped and rotated, or parts of it isolated to create a new pattern.
You can get motors for some machines, of course – they are usually mounted on a rail above the carriage (for Japanese machines) or at the right hand end (Passap). They are very expensive, and only the Silver ones are still in production, so the others are hard to find and harder to get repaired. Motors just speed up the straight bits of knitting – everything else is still done manually.
Standard gauge machines (4.5mm) are the most commonly available. They mostly come with lace carriages (sometimes sold separately). Lace carriages work by bending two needles together to transfer a stitch – you can still create lace by hand, of course. Brother machines work by using a lace carriage in conjunction with the main carriage. Silver/Knitmaster use a second carriage that replaces the main carriage.
Garter carriages are separate electrical devices that sit on the machine bed and can create purl and plain stitches in the same row. They are slow, and they only work on Brother standard gauge machines.
Bulky/chunky gauge machines do not have lace carriages. Any lace patterning has to be done by manual manipulation - the needles are just too sturdy to work with a lace carriage - and there is no garter carriage for these machines. You can get garter bars for both gauges - it’s a big long comb you transfer the knitting to, so that you can turn it around and rehang it. But it’s quite time consuming for more than a few rows.
Knitting machines can do slip, tuck (brioche), fair-isle and knitweave (a contrast yarn is woven into the knitting on the purl side), as well as lace. Newer machines can do punch-lace – knitted a bit like fairisle, with two yarns, but one is thick and one is thin, and the thin yarn is knitted alone in certain areas. Newer machines can also do plating and intarsia.
It’s also possible to buy a ribber for most Japanese machines – this is a separate bed that fits on the front of the main bed at a right angle to the main bed. A ribber can be used to create various ribs, racked ribs and double-bed jacquard patterns.
I wouldn’t pay much more than about £50 GBP for a Brother punchcard machine (model 836 upwards) and £100 GBP for a Brother electronic. These machines can go for ridiculous amounts on Ebay – some sellers will take no account of depreciation, nor the fact that spares are almost impossible to buy.
Brother stopped production in 1996, and Toyota even earlier, so bear this in mind with the electronics - once they die, you need an electrical engineer to diagnose and repair them, and electrical spares can be hard to come by. The punchcard machines seem to last forever, though - there’s not much to go wrong with them. Your main problem will be replacing bent needles and the spongebar (though it is possible to repair your spongebar if you have to).
Silver (also called Silver Reed/Singer/Knitmaster/Empisal) are still in production, so getting spares isn’t so bad. Some of the older Knitmaster machines couldn’t take a lace carriage (the needles were too strong) so check before you buy.
Passap machines (also no longer in production) use a different patterning system. Every needle has a corresponding pusher, and the position of the pusher in combination with a setting on the lock/carriage determines what is done to the stitch on that needle. Passap machines do fantastic double-bed work, but Japanese machines do better single-bed work.
Buying a knitting machine is a bit like buying a computer – decide how much you can afford, and buy the best machine you can get for that money. If you think you’re likely to want a ribber or other accessories in the future, try and get a machine that can be added to.
Other useful reading:
Angelika's Yarn Store including chart history of knitting machines
*Brother made a convertible plastic bed machine, which has both 4.5mm and 9mm beds.
** The original Bond also had a ribber, but it wasn't very successful.