A "rope making machine" is a gadget that twists several strands of thread into one larger one. It has a couple of advantages over using standard manufactured thread. One - if you make your own rope you have virtually unlimited control over the range of sizes. Two - the product of such a gadget looks a lot more like like real rope than ordinary thread does.
If you think about it a second you'll realize that just twisting three pieces of thread together will not make a piece of heavier thread. As the individual strands are twisted together, two things happen: they get twisted around their own axes, and they get twisted around each other. If they're simple twisted together, the individual strands will get twisted so tightly that, eventually, they probably will break. If they don't break, they'll be under such tension that, as soon as the ends of the finished piece of rope are released, they'll untwist themselves.
The rope making machine is a fairly simple gadget that simultaneously twists the three strands together and twists them individually, so the tension is equalized. The machine has two components: a single wheel (called the "looper") at one end, and three geared wheels (called the "whorls") at the other. In most modelers' setups the looper is clamped to something solid (e.g., the edge of a workbench), and the whorls are provided with some sort of mechanism that allows them to move closer to the looper as the rope is spun up. One final component, called the "top," is a movable, bullet-shaped gadget with three grooves in it. It moves along between the looper and the whorls as the rope is spun; the three strands run through the grooves in the top, which keeps the pitch (the angle at which the strands intersect) constant. An old, pre-twentieth-century ropewalk essentially worked the same way.
When I was working on the Heller Soleil Royal (my most-regretted model project ever - but don't let me get started on that one) I discovered that the longest pieces of line in the ship, the main topgallant backstays, would have to start out as pieces of silk thread about twenty feet long. I mounted the looper and whorls on two sawhorses, which I moved gradually across the floor of our basement as I made the backstays. As I remember, the job of making that one piece of rope (starting from the deadeyes on one side of the ship, up to the topgallant masthead, and down to the deadeye on the other side), took something over an hour. Great activity for listening to a ballgame on the radio.
The best explanation of how to make a rope making machine that I've encountered is in Dr. Longridge's Anatomy of Nelson's Ships. He was building a model on 1/48 scale; his machine incorporated a miniature railroad track that ran the length of his back yard. Lesser projects obviously don't need to be that big - nor do they need to be motor-powered, as Dr. Longridge's was. One beauty of the process: if it works at all, it works perfectly. I don't think anybody can distinguish between the rope Dr. Longridge made on his super-sophisticated contraption and the stuff I made with my Lego-based one.
Model Expo is currently marketing a pre-assembled rope making machine that's simplified in several ways: it has no means of moving the two end components, and it has no top. It struck me initially that it couldn't work. I read the instructions, which ME kindly offers online. Apparently the thing will only function in one direction - such that the individual strands are being unwound as the the looper spins them together. I guess that's mechanically sound if you aren't worried about the direction of the lay. I still question whether a really good-looking piece of rope can be made without a top, though. I think I'll hang onto my Lego version.