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Lee - you are right about the ellipse being traced. This device is a Trammel of Archimedes and is actually used in drafting and woodworking to describe an ellipse. It's also used as a toy called a "nothing grinder" . Note the diagram on the rhs of the above wikipedia page.I am still trying to get my head around the sewing machine! This page of animations isn't an elaborate hoax is it?Matthew
Yes, it's an ellipse and I believe the locus of the mid point between the pivots on the two sliders is a circle.
In the sewing machine the "shuttle" (the thing that holds the lower spool) is not permanently attached to the machine at all. What happens essentially is that the mechanism creates a big loop in the upper thread on the bottom side of the cloth, then it "throws" the shuttle with the lower thread bobbin through the loop. After that the threads are tightened up before the next stitch is begun.On some sewing machines you wind the lower thread onto a thing that looks a lot like the shuttle in a weaving machine rather than the more typical bobbin arrangement.
That sounds possible - but two things make me think that other methods are used. Firstly, that would create a real stitch which if you cut one loop would hold on the next, or next but one; but machine sewing will run if you break a stitch (OK subsequently discovered this is only old and bad commercial machines) making me think it is more complex than a real stitch . Secondly the mechanism for "throwing" doesnt tally with memories of my mums sewing machine - i thought it worked as loops through loops. Will seek out or draw diagrams.
Yep agree with that as well - its a really cute device. In a hand wavy way; I guess the centre of the locus must be at the cross over point, cos what ever shape the mid-point traces must be symmetrical on both axes, and it seems that at any point of the rotation you could imagine two isosceles triangles formed by the midpoint, the device centre, and each pivot point. the distance device centre to midpoint thus remains constant (ie circle) and radius is equal to half the distance between the two pivots.
Nice set of animations there, JP.Not sure I understand this one though:-Steam engine Principle-