These are used to guide, fold or preposition the materials as they are sewn. Superposed and lapped seam attachments are called“fell seaming” attachments because they fold a fabric edge during the sewing operation . Hemming attachments are also referred to as felling attachments.(Pdf)
Folding Device: Used to position the trim Just in front of the needle. Bias strip are applied by drawing the binding off a roll above or to the side of the machine and through a folder and tensioning device before reaching the needle.
Hemmers: Attachments may also be used to fold various types of edge finishes and trims, including hems. Hemmers (video) are needed for hemming tubular goods so the operator may be able to disengage the garment and quickly position another.
Narrow Hem Folder: Makes a double rolled hem of the type used on scarves and handkerchiefs and a folder to make a hem as narrow as this is easily contained within the space of a presser foot.
Lap fell Folder: The folder, which joins major garment parts together, the commonest one is used for jeans construction.
Binders: Many folders are available which add further items of self-fabric or other material to a garment. These come under binders. Fabric edges are frequently bound either as a means of edge neatening or create a decorative effect or both. These can also be bound seams(video) and piped seams(video)
Gathering And Shirring: Special attachments may be used for shirring, riffling, gathering (video) and tucking. Special attachments called a blade shirrer is used when the center ply of three plies of fabric is to be gathered while the outer plies of fabric is to be gathered while the outer plies are to remain smooth. Blade shirrer feed the fabric that is being gathered by pushing the fabric with the serrated free end of the blade.
Tucking: Knife tucking or pleat tucking is similar to shirring but with small knife-edge folds. Shirring (gathering) and tucking may also be done with special purpose machines, special presser feet and differential feeds.
Automatic Positioners: These a re attachments that have a tripper mechanism to stop the sewing and positioning action after a given length of sewing. These attachments are automatic stitch series mechanisms that are attached to a machine. E.g. bar-tack, buttonhole, button sewing and pocket sewing machines.