Finding the right needle size in the Haystack pt.1
Review part two of three of Stuart’s guide to thread (part one) and needles.
There are far too many different sewing needles. Big ones, small ones; sharp ones, blunt ones; silver ones, gold ones. Tens of thousands of different kinds of sewing needles. And there are probably a half-dozen different names for any one sewing needle. So how do you get “the right” sewing needle? This article may give you enough information to search through the haystack for the proper needle size, system, and point for your application. Or it may give you a headache. We shall see.
I am not going to talk about hand sewing needles, special coatings, ganged needles (household double and triple needles) and specialty forming needles (hemstitch, pin-point, chenille, felting, etc.), but we will still have a huge amount of material to cover.
Let’s start with the basic anatomy of a machine sewing needle. You have probably seen this diagram twenty times before.
Your machine’s needle bar grasps the shank. The point pierces the fabric. Between them is the blade. A lockstitch needle has a long groove on one side of the blade and a scarf scooped out of the other side of the blade. A chainstitch needle will have long grooves on both sides of the blade. The thread will run down the long groove(s) and through the eye.
The three critical dimensions are the diameter (thickness) of the shank, the diameter of the blade, and the distance from the butt end of the shank to the top of the eye. The needle thread loop (needed by the bobbin hook or bottom thread loopers) forms immediately above the eye of the needle, and it must form at a fixed location relative to the body of the machine. So the butt-to-eye distance is critical, and the shank diameter determines if you can even get the needle into the machine in the first place. The distance from the eye to the tip of the point is, perhaps surprisingly, not a critical dimension although it does affect how the needle performs, as do the overall taper of the point and the shape of the very tip. The third critical dimension (blade diameter) is the subject of the next section.
Blade Diameter (The Size)
The “size” of a needle is the thickness of the blade. Some needles have tapered blades, other needles are enlarged around the eye, but some part of the blade will have a uniform diameter and this is what gets measured. Needle size affects two things: how large a hole is made in the fabric and how large a thread should be used. As a general rule, use the smallest needle and thread that are suitable for the job.
“Will the thread fit through the eye?” That is too simple a question about needle size. You will get stitching problems if the thread is either too big or too small for the needle. At first glance, many of these problems look like issues with poor tension adjustment or thread quality. The ideal thread diameter is about half the needle eye diameter. It should be small enough to slide freely in the long groove on the blade when pulled, but big enough not to buckle inside the long groove when pushed. The needle-to-thread size relationship has considerable tolerance, but you need to be aware that you can’t use a fine thread with a heavy needle just because it will fit through the eye.
Needle system design (see next section) also affects proper thread size. For example, topstitching needles have larger long grooves and eyes to accommodate a heavier thread than a normal needle of the same size.
There is a “home recipe” that works pretty well to evaluate a needle and thread combination without using any charts, graphs or mathematics. Take the needle out of the machine and thread it. Hold the thread taut, with your hands about a foot apart and the needle between them. Tilt the thread halfway between vertical and horizontal and twirl the needle around the thread briefly. Stop twirling. If the needle does not slide down the thread as the twirling stops, it is too small for the thread. If the needle just falls down the thread, it is too large. The needle should slide smoothly, but with some slight drag from the thread.
Once upon a time, there were hundreds of needle sizing systems. By one estimate, there were roughly 4,000 sizing systems, with up to 15 sizes per system. Every manufacturer had a sizing system, and some manufacturers had two or three! Fortunately, only two systems are still in wide-spread use: the Singer system and the metric system. Needles are usually marked with both sizes, so checking for the desired size is easy.
The metric needle sizing system, introduced in 1953, is very straightforward. The diameter of the needle is given in hundredths (percentage) of a millimeter. For example, a Nm 100 needle is 1 millimeter in diameter, while a Nm 50 needle is half a millimeter in diameter. (At least one source has confused Nm with nanometer, but nanometers are 10,000 times smaller!)
The Singer system, which the Japanese industry also adopted, uses somewhat arbitrary numbers. In the smaller sizes, each successive size is pretty close to 0.05 millimeters larger than the size before, but the increment is larger for the larger sizes. You can use either system, but it’s easier to read the actual size right off the metric size.
Here is a table of standard Singer and metric sizes covering light through extra-heavy sewing. I have come up with a recommendation for corresponding Tex thread sizes in the third column, but you (or your sewing contractor) should determine which needle size works best for your specific combination of fabric, thread, and seam. The fourth and fifth columns contain the extremes of possible thread sizes, based on various assumptions of needle system and thread construction* and you should not expect to work at the extreme values without testing.
[How annoying, Stuart's chart won't format. Until I can figure out why, see the 22 kb pdf]
There are some sewing machines designed for light-weight materials only, such as the Juki DDL-8700A or -5550A models. Needle size 75 is about the upper limit for these machines. The most common medium-duty sewing machines can work down into the light-duty range and up to about needle size 100 or 110. It takes a heavier machine to use larger needles. It’s not just a matter of power, either. The parts under the needle plate need to be compatible with the larger diameter needles and threads. The hook-to-needle timing and spacing is affected by blade diameter, and the raceway between the hook base and the basket (bobbin case holder) must be large enough to let the thread slide freely. Most heavy-duty machines have an upper limit somewhere around needle size 160.
As a matter of fact, Schmetz has introduced a line of needles (FHS) specifically designed not to require any hook spacing changes when changing needle sizes. General purpose sewing machines usually don’t require such changes, but the machines used in shoe manufacturing apparently have tight enough working tolerances to require a mechanic’s attention when changing needle sizes in the Nm 70 to 120 range. Schmetz is marketing this needle design as a production time-saver for that particular application. So it is possible you will run into a medium-duty machine that won’t reliably stitch with a small needle unless the hook spacing is changed (and obviously, changed back when the original needle size is restored).
Needles larger than size 160 or 180 are for extra-heavy sewing. Only the heaviest of sewn products (e.g., leather shoes, saddles, industrial cargo slings) would require the corresponding thread. Ordinary heavy-duty machines can’t handle them. So if you plan to topstitch denim with T-240 thread or larger, be aware it’s going to take a specialty machine with greater capacity than a normal “denim” heavy-duty machine. In fact, needle systems for conventional machines don’t offer sizes this large, as the blade would be larger than the shank that holds the needle in the machine!
There is a super-heavy range above Nm 250, but it’s not relevant in practice. Even within the monster-sized Singer class 7, almost all models are limited to Nm 250. I could not find a needle larger than Nm 300 (Singer 29) in production from any of the major needle manufacturers (and those were for sack-sewing and bag-closing machines). A Nm 300 needle is 3 millimeters across (significantly more than 1/16 of an inch), and wouldn’t look out of place in a box of nails.
While the metric and Singer needle sizes are used almost everywhere, there are a couple of niches where other needle sizing systems survive. The two main exceptions I encountered when researching this article were blindstitch (which has mostly converted to metric/Singer) and machine lace embroidery. If you have an archaic machine manual or engraved plate that uses some other sizing system, CTSUSA has a size cross-reference chart that may help. Industrial Sewing Machine has a similar cross-reference that might be easier to read. You probably aren’t making machine lace embroidery, but if you are, you may find Dotec’s small cross-reference handy.
*[footnote] Readers who valiantly waded through my article on thread sizes may wonder how we relate needle diameters to thread sizes, when thread sizes obstinately refuse to provide actual thread diameters. The answer is painful. First we have to go from blade diameter to recommended thread diameter. The eye of a needle is smaller than the blade which contains it. Depending on needle system design, that might be 40% to 60% of the blade diameter, and half of the eye diameter is the ideal thread diameter. Once we have a target thread diameter, we have to convert to a thread size. This requires an estimate of the thread’s density or specific volume, and doing some algebra (convert diameter to cross-sectional area to weight to thread size). Then, if you are sensible, you will cross-check against various thread, needle and machine makers’ recommendations (which vary widely) for sanity.