The botton hole attachment most often used with the Featherweight model 221/222 has the part number 160506 stamped on it.If you have the complete attachment there will be multiple drop-in "cams" that govern the size and style of button hole you are performing when you use the attachment.The embossed image on the back of the cam shows you the style and size of the button hole the cam produces.
There should also be a chrome metal plate about four inches long having a slotted blued steel strap sticking out one side of the chrome plate. There is a thumb screw that should be there as well. It is used to secure the feed-dog cover plate. This pate covers the feed-dogs of the machine while providing a smooth surface for the fabric to move around on. The Featherweight is not a zig-zag machine, therefore the fabric has to be the thing that moves. In use, don't try to direct the fabric or hold it down allowing the fabric to be free to move as directed by the attachment.
The attachment is secured to the machine just like the original presser foot, only a whole lot bigger. Unlike the standard foot the button hole attachment also has a lever that I call a "pickle-fork" that straddles the needle setting screw as the attachment is installed on the machine. As the needle bar travels up and down the screw drives the lever and powers the attachment.
Please practice the use of the attachment before you attack something for real. A secret with the attachment is that you sew around the button hole twice. The button hole will be better than many found on modern machines. Modern machines and their button hole function work like they learned from Arthur Murry. Step-side-step-side-step-step. The Singer attachment sews the botton hole in one flowing movement that you fall in love with. When done sewing the button hole you must cut the hole in the fabric for the button to go through still but I'm sure there is a app' for that!
Once you get fairly good with the attachment you might want to try adjusting the zig-zag stitch wider for the first time abound the hole and then reduce the width of the stitch for the next time around and change the color of the thread to a complementing thread color for a two-tone button hole that can be very striking.
But oiling your button hole attachment comes first. If metal touches metal inside the attachment and they move against each other, put a drop of oil on it.
