And I am still around
A conventional "plain" sewing machine controls the stitch length by the movement of the "Feed dog" that pulls the material from underneath in short strokes. Best you study an actual machine to see how it works the length of stroke can be adjusted by a knob or lever in the sewing head. A few "walking foot' or "needle feed" machines also feed from the top but that is another story.
A typical plain sewing machine operates at around 5000 rpm (equaling 5000 stitches per minute), however quilting is thicker than say shirt fabric and it is normal to reduce the speed to between 3500 and 4500 RPM. You reach a point where thread breakages become an issue, there is too much friction and the needle can become so hot it will melt the thread.
However a frame quilting machine head mechanically or CNC driven does not have mechanical feed dog. To control the stitch length you need to match the feeding speed to the stitching speed using a ratio that that gives the right stitch length.
These days most quilting machine designs use a moving head in an XY frame, the fabric in its frame stays still, mechanically considerably more complex, the benefit being a greatly reduced machine footprint.
The machine will be heavy with a large moving mass even for a small design, This means that unlike a manually driven machine where the material is stationary when the needle penetrates the fabric the machine will be moving and so will the needle, this can be alleviated a little by a the foot on the machine that holds the fabric down the "Presser foot" but there will be some movement. in spite of the movement the system will work as long as the stitch to feed ratio is not too big. Typically the stitch length at 3500RPM can be around 3mm. Longer stitches will require slowing the machine down.
For any given material, thread and machine there will be a sweet spot, Its not unlike metal cutting in that respect.
On The Mach 3 side the objective will be to maintain a constant feed / stitching speed ratio. at any feed rate, if this is not done the stitch length will vary.
I cannot imagine Mach could handle moves on a stitch by stitch basis except at slow stitching speeds? However it would be a good feature for things like "Back tacking" at the start and end of a line of sewing. Some sewing heads can also trim the threads off at the start and end of a line of stitching that would have a subroutine (It has to back up the line it just did)
Lastly sewing machines are not perfect there are too many variables thread will break. A means of reversing the program back to where the break was and a little past it will be required.
It's been a while since I thought on this I wonder if Mach4 is better equipped to handle CNC sewing?
Regards
john McNamara
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