Recommended SPI for say 2-3mm veg tan or chrome

[chrisash]

New to machine sewing, wondered what most people use as SPI for general purpose leatherwork using either say 1.5mm x 2 thickness veg tan or chrome leather, I was thinking between 9-12 SPI but welcome thoughts using either 69-138 thread

[Wizcrafts]

I generally sew between 5 to 6 stitches per inch with thread sizes 69 through 138. But, some jobs may tolerate closer stitches while others are better off with longer stitches, like 4 to the inch. A lot of this depends on the size of the thread and needle. The thinner the leather the shorter the stitches should be to prevent loose threads or thread causing flexing of the work.

You mentioned 1.5 to 2mm thickness. I strongly recommend against using #138 thread in such thin leather. You may not be able to hide the knots! The most I would use would be #92, if that.

Thread sizes vs needle sizes.

• #69 thread, top and bottom: #18 (110) needle
• #92 thread, t&b: #19 or #20 (120 or 125) needle
• #138 thread, t&b: #22, 22.5, or #23 (140, 150 or 160) needle

Those sewing much shorter stitches are using thinner thread and needles to match. A custom boot maker sewing purely decorative stitches might do so using #33 bonded thread and a short #9 or #10 needle on a roller foot machine. The next bonded thread size up is #46. This thread can be sewn using a #14 needle. Such a thin thread and needle will allow closer stitches, like 8 to 12 per inch. But, each stitch will be 50% weaker in breaking strength than #69 thread. The biggest concern with short stitches is the risk of filigreeing the leather.