We have a few things in play: Needle position, Feed length, feed direction, and feed direction modifier. Something that stays consistent no matter what is the needle piercing the fabric. The up/down motion is independent. The needle will go up and down, the thread will lock stitch, tough luck. The cams will advance in time with this up and down.
If we think of the sewing as an inhale (needle in the fabric) and an exhale (needle outside of the fabric), for me at least, it helps see what is going on.
Inhale: lockstitch process
Exhale: adjust
Stitch length is how far the feed dogs move the fabric on an exhale. Stitch width is the magnitude of needle position change while exhaling. While the needle is in the lock stitch process the most that can be done is the drop of the feed dogs and their lower position slide to the proper length get them into the right position to move the fabric.
It is really quite cool, and the timing is everything.
The stitch cams cams take that all into consideration simply by the mechanical timing. The cam will rotate a set number of degrees during the exhale.
In a C-cam you will notice that it has a couple of different features in comparison to some of the other cams. A more block like approach and a wider diameter.
The wider diameter allows for more drastic position changes between stitches. With the stitch width modifier, and the cam diameter we can traverse the entire width of the needle plate. That's a lot of real estate to work with in comparison to older machines. That means, for example, from one position the maximum change in one exhale can be 7mm.
With a little geometry lets take a gander at this. We know the amount of rotation is fixed in each exhale. It is mechanically linked to the last portions of a lock stitch. So in say 20 degrees rotation (I have not measured this yet, it is just a guess to get 18 inhales/exhale cycles in one whole rotation), you'll have another stitch. The smaller the radius of the cam (E or G cams), the smaller the cord across the cam. Which means mechanically you have a more drastic cut to make in the cam to change positions, in the 10 degrees during the exhale part of cycle. But make the radius larger, the total radius change will be the same but the rate of change/angle needed to get there will be smaller, which helps prevent jammed mechanics, and decreases the stress on the mechanic "reading" the cam.
Cool right?
This consistency probably assisted the original designers in making the first cams and improving designs.
I have been modeling these cams to get an understanding of how they actually work in relation to the machines. I hope to 3D print a few "blank" cams to test out my theories on behavior without having to have my hands in the machine, nor worry about me damaging the machine to figure it out.
If we think of the sewing as an inhale (needle in the fabric) and an exhale (needle outside of the fabric), for me at least, it helps see what is going on.
Inhale: lockstitch process
Exhale: adjust
Stitch length is how far the feed dogs move the fabric on an exhale. Stitch width is the magnitude of needle position change while exhaling. While the needle is in the lock stitch process the most that can be done is the drop of the feed dogs and their lower position slide to the proper length get them into the right position to move the fabric.
It is really quite cool, and the timing is everything.
The stitch cams cams take that all into consideration simply by the mechanical timing. The cam will rotate a set number of degrees during the exhale.
In a C-cam you will notice that it has a couple of different features in comparison to some of the other cams. A more block like approach and a wider diameter.
 |
| 3D model of a "blank" C-cam and a zig-zag cam for reference |
The wider diameter allows for more drastic position changes between stitches. With the stitch width modifier, and the cam diameter we can traverse the entire width of the needle plate. That's a lot of real estate to work with in comparison to older machines. That means, for example, from one position the maximum change in one exhale can be 7mm.
With a little geometry lets take a gander at this. We know the amount of rotation is fixed in each exhale. It is mechanically linked to the last portions of a lock stitch. So in say 20 degrees rotation (I have not measured this yet, it is just a guess to get 18 inhales/exhale cycles in one whole rotation), you'll have another stitch. The smaller the radius of the cam (E or G cams), the smaller the cord across the cam. Which means mechanically you have a more drastic cut to make in the cam to change positions, in the 10 degrees during the exhale part of cycle. But make the radius larger, the total radius change will be the same but the rate of change/angle needed to get there will be smaller, which helps prevent jammed mechanics, and decreases the stress on the mechanic "reading" the cam.
Cool right?
This consistency probably assisted the original designers in making the first cams and improving designs.
I have been modeling these cams to get an understanding of how they actually work in relation to the machines. I hope to 3D print a few "blank" cams to test out my theories on behavior without having to have my hands in the machine, nor worry about me damaging the machine to figure it out.
 |
| Modelled blank double layer C-Cam |
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