"Super-Liesl" - A P2-Controlled CNC Linear Knitting Loom
Christof Eb.
Posts: 1,194
Hi,
This is about another fun project. Again it is about a knitting machine controlled by P2. I am very much fascinated, what you can do with yarn. And I was always fascinated by automated machine tools like CNC lathes.
This time the goal is to make a machine, that can make scarfs with knitting patterns.
I don't kow, if I should name this machine "Super-Liesl" or better "Cordula"?
"Cordula", because "she" is doing cordels right now. Well, I do hope, that she will learn much more. But at the moment I am quite happy, that she has mastered this first critical step, because I didn't know, if this would work at all….
"Super-Liesl" would be, because "she" is a big automated Sister of "Strick-Liesel" the German name of "Knitting Jenny".
Strick-Liesl is a small form of a Knitting Loom. Which can be circular or linear. While knitting machines normally use many very special type of needles with flaps to form the stitches, with a loom, you need to do the stitches with an additional moving needle. The loom has one peg for every stich. The two rows of the loom can be used in two ways. Either you can create something hollow like a hat or you can make a fabric with plain stitches (rechte Maschen) and purls (linke Maschen). Purls than use the 2nd row of pegs.
Top view:
The machine consists of a P2 "Kiss Board" (Running Taqoz Forth), a "CNC-shield" with step motor controllers and the mechanics.
There are 3 axis XYZ and one rotating axle to turn the moving needle. The X-axis has about 54cm of possible travel to accommodate long looms. The orange coloured loom is mounted on a sled (X-axis), which has to have a long slot under the loom to let pass the new fabric. X uses a acme thread with pitch 8mm.
Y and Z are two linear actuators with ball screws, which I bought ready made. They have 5mm pitch.
The rotating axle is for the needle tool. The idea was to make it in a way, that in can handle both rows of pegs. So the needle has an angle of 30 degrees to horizontal. -It was a great question, if it would be possible to handle the yarn with such needle. The needle is an old 2mm steel needle for fine hand knitting. It has a good balance of stiffness. If there is a crash, the needle bends and can be bended back later. - Has happened....
The Y-Z table holds a second tool, which feeds the yarn around the pegs. It is just a tube with 6mm inner diameter. The red peg on the right hand side of the picture acts as a weight to give tension on the yarn and also this weight will pull back the yarn as needed, when the tube or the sled moves.
Each row of stitches needs 3 separate operations:
- Feed fresh yarn around the tops of the pegs (this operation will allow variations for patterns later).
- Do the stitches on every peg: Lift the old sling over the new yarn and the head of the peg towards the middle of the loom.
- Push down the new sling to the bottom of the peg.
There are two major difficulties in the process:
The yarn has it's own will and life. A human knitter does use his eyes very much to check and correct, while the machine must work bindly.
Knitting is not at all forgiving. If the yarn splits up or if the moving needle did not find the loop at all or if the needle takes both new and old slings, there will be an issue from now on for the next rows too. So you need very high reliability.
Status is at the moment, that Super-Liesel can do rows of 3 stitches. After 8 rows the weight, which provides tension onto the yarn has to be shifted manually. Reliability seems to be quite good. 150 stitches without failure. Speed is low, about 1 minute for one row of 3 stitches. Tested with yarn 2.5mm and 4mm.
A product:
A little video of the workings. 
Have fun!
Christof
Comments
....Video added.
So now it can do stitches on the back row, which shall give purls one day. The pattern is the classic "Strick-Liesl / Knitting Jenny" thingy now.
There are a some inefficient movements in this video, so timing could be improved somewhat. I had to swap the order of the stitches. Here some braided yarn is used, which does not split so easily.
It is hard to believe, how difficult it was to make the step from the one sided to this double sided work.
Most difficult is the fact, that the vertical position of the two slings on the peg is not well defined. The slings move along the peg. And they are not well separated. Perhaps I will be forced to create pegs with separate areas for the two slings.
A step further. A yarn brake which has also ability to pull back the yarn.
This type of yarn brake I find fascinating, because it has the ability to regulate the yarn tension and it "uses" the Euler-Eytelwein-formula. The yarn goes over circular hills with valleys between them. At the position of a valley there is a weight (here: some steel wire) on the yarn. If you put tension onto the yarn, this will lift the weight and this in turn will lower the angle of contact between yarn and hill. With less angle there is less friction. https://en.wikipedia.org/wiki/Capstan_equation There are 5 brakes in series, which add up their friction.
Here we want to have just a little bit more friction force, than the force of the weight of the pull back device - hook and peg. If the hook is pulled up fully, the yarn is pulled through the brakes.