Here are the most common stitches used in making tactical gear. The majority of the text was taken as highlights from the military and standard publications listed below. I’ve gone through these publications and taken what I think is important to know. This info was cross referenced with The Parachute Manual Vol 1 by Dan Poynter since this is the guide that the majority of DIY gear makers use.
The Parachute Manual Vol. 1 – Dan Poynter (referred to as PPM)
Standard Practice for Stitches and Seams – ASTM D 6193
General Repair Procedures for Individual Equipment – TM 10-8400-203-23
General Fabric Repair – FM 10-16
Preasure – The amount of force exerted downward on cloth by the presser foot. The presser foot holds the fabric in place while a stitch is being sewn. The pressure and feed dogs then work together to move the fabric forward to the next stitch. If the pressure is too light, the feed dogs move the fabric forward unevenly. The amount of pressure needed depends on the weight of the cloth. As a rule, the lighter the fabric the less pressure needed to move it. Before sewing on fabric whose weight differs from that of the fabric last sewn on the machine, test the pressure by sewing a seam on scrap material. Use the same type of fabric and the same number of layers that will be used in the repair. If one layer of fabric slips, if stitches are skipped, or it the fabric is pulled down in the bobbin area, adjust the pressure. Follow directions in the appropriate technical manual to adjust the pressure on each sewing machine.
Bartack – Is a series of zigzagged stitches used to strengthen stress points. It is a locked stitch to prevent it from unraveling. Special and expensive bar-tack machines are often used in industry to make bar-tacks.
301 Lock Stitch:
This type of stitch shall be formed with two threads: one needle thread, A, and one bobbin thread, B. A loop of thread A shall be passed through the material and interlaced with thread B. Thread A shall be pulled back so that the interlacing shall be midway between surfaces of the material or materials being sewn.
The majority of tactical gear is made using this stitch at 8-10 stitches per inch.
304 Zigzag Lock Stitch:
This type of stitch shall be formed with two threads: one needle thread, A, and one bobbin thread, B. This stitch type is exactly the same as stitch type 301 except that successive single stitches form a symmetrical zigzag pattern.
According to TM 10-8400-203-23, ½ inch to ¾ inch bartacks are 28 spi. ¾ inch and greater are 42 spi and 1/8 inch wide. I have read however read elsewhere that bartacks on PALS webbing are 48 spi. I am checking into this and will update when I get clarification.
Tension – The amount of stress put on a thread. When the tension between the top thread and the bottom thread is correct and balanced, the connecting link of each lockstitch is in the center. The stitches look the same on both sides. When there is too much tension on the top thread or too little on the bottom thread, the link is on the bottom. When there is too much tension on both threads, the fabric puckers and the stitches break. When there is too little tension on both threads, the stitches have too much play, the threads form loops, and the seam is weak.
Stitch Length (Stitch Count) – FM 10-16 states that stitch length is measured by the number of stitches per inch (spi). Most often short stitches are used on light fabric and long stitches on heavy fabric. The PPM describes this as stitch count and goes further to describe how to count the stitches. For a straight stitch, you start at zero from the first hole made by the needle and moving forward counting each successive penetration until you have moved one inch.The number of the final hole is your spi. Counting zigzag stitches is similar however you only count one side. The length of the stitch is determined by the movement of the cloth through the machine, depending on what type of machine you are using will determine how this is adjusted. Check your users manual.