Anyone building something like this should consider that this could be dangerous for machines and maybe even for people if the machine comes loose or the swing frame holding it fails. Please indulge me a few thoughts about stresses within the frame.
Below is an interesting picture to ponder. The machine is balanced, top forward. It looks like the swing frame and machine are happy and content. They're not. There is a lot of force being contained, hidden within the frame. This is, in fact, the most stressful position that a sewing machine and swing frame can assume.
(Frame Under A Lot Of Stress)
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Imagine for a moment being behind the frame in the picture and gripping the top edge (as shown) of the machine with your fingers, thumbs placed on the bottom of the frame. Then, while you're holding it, someone takes the clips loose. A lot of pressure would be exerted by the machine trying to fall out. Your fingers would have a lot of pressure on them. Machines weigh commonly between 30 and 40 pounds, much of this acting like a lever the length of the machine's height. Many people would have a hard time holding the machine this way. An equal force also exists at the bottom edge, only in the opposite direction. The machine wants to twist and fall.
These forces must be conducted through the frame, where the corners are trying to shear apart. The very greatest stress point is where the forces of both the front and the back of the frame combine, where the uprights are attached. The twisting force at these two points can be great.
A welded mitered corner is essentially solid metal. A bolted corner is no stronger than the bolt(s). And they loosen. Especially where the uprights meet the bottom of the frame, I was concerned doing anything else.
All I can recommend in good conscience is what has been proven here. I'd feel terrible hearing of anyone hurting themselves or their machines because this device wasn't strong enough to contain the mechanical stresses that it must.
With these words of concern and caution aside, I'd love to see alternatives that work as safely. QB people are among the most creative and resourceful people I've ever seen. Just please be cautious and careful.
Continued soon...
John