Buttonhole Sewing Machine

Overview

A buttonhole sewing machine is a single-purpose industrial head that stitches and cuts a finished buttonhole in one short cycle. The operator loads the garment under the work clamp, presses the foot pedal, and the machine clamps the fabric, sews the two parallel rows of stitching, sews the bar tacks or bartack at the ends, cuts the slit, trims the threads and lifts the clamp. A skilled operator runs one buttonhole in a few seconds. The machine replaces the slow and uneven hand buttonhole and gives every hole on a shirt front or a jacket the same length, the same stitch density and the same cut.

Two families cover almost all production. The straight or shirt buttonhole is a plain bar with square or rounded ends, used on shirts, blouses, light trousers and bedding. The keyhole or eyelet buttonhole has a round eye at one end, which gives a shank button room to seat without distorting the hole, used on jackets, coats, suit trousers and other tailored goods. The two types need different cams, knives and feed patterns, so a head is built for one family even though the running gear is shared.

Zig-zag needle swing

The buttonhole edge is finished with a dense zig-zag, classed as a 304 zig-zag lockstitch. The Needle Bar reciprocates up and down from the Main Shaft and at the same time rocks side to side, so each stitch lands on the alternate edge of the cut line and the thread wraps the raw edge. On a mechanical head the swing comes from the Zig-Zag Swing Cam through the Swing Link. On an electronic head the Bite Stepper Motor sets the bite width for each stitch, which lets the operator change density and width from the panel. Below the throat plate the Rotary Hook catches the needle loop and passes it around the Bobbin to set a lockstitch. The Take-up Lever then pulls up the slack so the knot seats in the fabric.

Thread path and tension

Needle thread runs from a cone on the Thread Stand, through the Thread Guide eyelets, into the Tension Assembly, past the Check Spring and the take-up, and down to the needle. The Tension Disc pair grips the thread under a load set by the Tension Nut. Tension matters more on a buttonhole than on plain seaming because the dense zig-zag must lie flat and the knot must pull to the underside, not show on the face. At cycle end the Tension Release Solenoid solenoid opens the discs so the trimmer can draw thread free without snapping it.

Cutting the buttonhole

The opening is slit by a chisel Cutting Knife ground to the buttonhole length, driven down by the Knife Bar against the Cutting Block anvil. The Cut Cam or a Pneumatic Cylinder drops the knife once per cycle. Timing of the cut is a setting. Cut-before-sew slits the cloth first and the zig-zag then finishes the already open edge, which suits eyelet work where the eye must be cut cleanly. Cut-after-sew stitches the closed buttonhole first and cuts last, which keeps light woven fabric from fraying while it is sewn. The knife and block wear, so both are reground or replaced when the cut edge starts to leave whiskers.

Gimp cord and the eyelet

A tailored keyhole buttonhole is raised and firm because a gimp cord runs inside the zig-zag. The Gimp Device feeds the cord through the Gimp Guide along the cut line, the zig-zag stitches wrap over it, and the Gimp Clamp holds the start while the Gimp Cutter severs it at the end. The cord gives the buttonhole a rounded, corded edge that resists stretching, which is why a coat buttonhole holds its shape over years of use. Straight shirt buttonholes are usually sewn without gimp.

Clamp, feed and finish

The Clamp Plate pair pins the fabric flat on each side of the cut line while the Feed Stepper Motor traverses the Feed Bracket along the buttonhole length, one increment per stitch, riding a Linear Guide Rail and Ball Screw. The Clamp Lift Cylinder raises the clamp at cycle end for loading and unloading. When the rows and end tacks are done the Trimmer Knife shears both threads close to the cloth. The whole sequence is run by the Control Board reading the Shaft Encoder on the Servo Motor so it can stop the needle up and fire the cut and trim at the right shaft angle, with the operator selecting the pattern at the Operator Panel.

Applications

These machines sit on shirt lines, jacket and suit lines, jeans and workwear lines, and in uniform and home-textile plants. A shirt factory runs straight buttonhole heads in a row with feeders for front placket and cuff holes. A tailoring plant runs keyhole heads with gimp for coats and trousers. Because each head is set to one length, density and cut style through its pattern, output is consistent across a long run and a buyer gets the same buttonhole on the first garment and the ten-thousandth.