I does sound like you are adjusting the foot pressure - very interesting that they refer to is a the vibrator - the reason you would do it at different needle positions is that would change the height of the feet. Tightening it when the feet are all the way up would result in the least pressure, and with the needle all the way down, the most pressure.

There is a regular spring loaded presser adjuster on top of the bar as well...

I wonder if this was an early attempt at a free-motion/dropfeed solution?

vibrator
 

Steve, In a way, you are correct. The "vibrator" is set by way of the thumb screw that is pointed out on the faceplate. The needlebar needs to be in the correct position when the thumb screw is tightened in order for the vibrator to function correctly. Try tightening the thumb screw when the needle point in just entering the fabric. What the vibrator does is to lift the presser foot from the fabric for an instant during the normal stitch cycle, while the needle is down. This allows the fabric to be rotated so that close curves and small circles can easily sewn. While treadling along, the fabric feels continuously free.
Johnm

interesting tech... I will have to get one of these to test that out!

[ATTACH=CONFIG]540204[/ATTACH]Took her for a "spin". On this VS sm, the "vibrator" refers to the up & down vibration of the presser foot as it lifts slightly during each stitch cycle. In my case, I had the needle just entering the fabric and the vibrator thumbscrew at the bottom of its slot when tightened. An artist I am not, but I can spin tight circles and curves.
Johnm

That is neat - wonder if I've got a machine that will do that . . .

Thanks, johnm, SteveH, and steihy for this information! The thread clarifies several questions I've had about my White VS machine and opens a whole new area of experimentation. Page 26 of my manual shows attachments for "etching"--clearly White's term for free motion embroidery. The attachments described include an "etching plate," which covers the feed dogs, and an etching foot. Instructions specify adjustment of the vibrator screw on the face plate. Alas, the attachments are not to be had within my budget, but there are many work-arounds. Looking forward to free motion quilting on my VS, such a joy to treadle.

stelhy If you got that face plate back on easily, my hat it off to you. Matching the peg on the main crank to teh cury V in the needle bar is always a challenge for me.