Originally Posted by
pbraun
The question is this: why does the 9mm fit and work? She will take the 9mm back, but the 1/4 foot is a little wider overall and is working well.
I will look in a little bit since I have both a 7700 and an Elna which has the 9 mm setup.
I suspect that the reason the 9mm works on the 7700 is that the 1/4" is from needle drop to right edge guide of the foot in both cases. So the needle is still going down into the needle plate in the same location and the right edge of the fabric is still being guided at 1/4" away from it. It happens to be a coincidence (and a fortuitous one) that the shank height and shape of both the 7mm and 9mm machines are identical, which lets you use the 9mm foot holder and foot.
The reason the 9mm foot holder won't work with the 7mm foot is that the "gripper" that holds onto the feet is almost assuredly different. I do recall that from yesterday's sewing on both my machines.
It sounds like your LQS shop simply forgot that the 7700 was a 7mm machine (I seem to recall it's been discontinued for a while now) and they sent you the foot holder and foot for an 8200/8900.