Phil,
I placed the uprights in the center of the frame to best match the front to back center of gravity. It keeps it as balanced as possible when tipped 90 degrees forward or back. While some of the newer machines, such as the Singer Touch & Sew series for example, have the pillar favoring the back of the bed, most of the old machines have the pillar centered. This seemed the best position for the most machines to have the best center of gravity.
David,
There are two main reasons for the length of the uprights on the swing frame.
First, it offers a good method of holding the frame when placing it into or removing it from the base. If held below the pivot shafts, one might get hands caught up and maybe pinched between the pivot shafts of the frame and the base. Held like ski poles, it allows easy gripping of the frame, with fingers safely out of the way.
Secondly, when the frame is in the base, the tall uprights allow easy turning of the machine. In particular, it allows easy turning of the machine 90 degrees backward, with spool pins pointing toward you, by simply pulling forward on one of the metal uprights. They serve as good handles for control. I'd definitely miss them if they weren't there.
It's funny that you mention weights. I didn't intend to post anything until further along, but I have been working with experimental weights to fine tune the balance. The uprights are the best place to put them.
I appreciate all comments on the workstand. Thank you both for your inquiries into the reasoning for its design.
John