Leather threads for sewing machines: selection and technical features

Working with natural or artificial leather is fundamentally different from sewing ordinary fabrics, and the quality of the final product here directly depends on the correctly selected accessories. Leather thread for sewing machines must have increased tensile strength, abrasion resistance and the ability to withstand significant tension in the seam. An error in choosing consumables can lead not only to rupture of the product during operation, but also to breakdown of expensive sewing equipment.

Unlike cotton analogues, specialized threads have a special twist structure and often undergo additional chemical or mechanical processing. This is necessary so that the seam is smooth, without thickening, and does not rub against the sharp edges of punctures in dense material. Professionals know that saving on quality thread when working with leather is a direct path to defects that cannot be corrected.

In this article we will analyze in detail the types of threads, their markings, the features of working with them on industrial and household equipment, and also consider the typical mistakes made by craftsmen when choosing consumables for leather products.

Basic types of threads for working with leather

The sewing accessories market offers several main categories of threads, each of which has its own physical properties and scope of application. Understanding the differences between the two is a basic skill for any technologist or leatherworker. The choice of a specific type depends on the thickness of the leather, the type of sewing machine and the aesthetic requirements of the seam.

The most popular option is considered nylon threads. They are monofilament or twisted thread made of polyamide fibers. Their main advantage is high strength and elasticity, which allows the seam to “breathe” along with the material. However, nylon is sensitive to high temperatures arising from needle friction, and therefore requires the use of silicone oil or wax treatment.

The second common type is reinforced threads. Their core consists of durable fibers (often lavsan or nylon), and the shell is made of cotton or viscose. This structure provides a high breaking load, and the cotton braid protects the internal fibers from overheating when rubbing against the needle and material. This type is most often recommended for heavy industrial machines.

Deserves special attention waxed threads. Wax impregnation (paraffin, beeswax or synthetic analogues) performs a dual function: it holds the villi together, preventing frizz, and serves as a lubricant, reducing friction. This is critically important for the skin, as it reduces the risk of damage to the edges of the puncture and facilitates the passage of the thread through dense layers.

⚠️ Attention: Never use regular cotton threads for sewing items made of genuine leather that are subject to stress. The cotton quickly rubs against the sharp edges of the leather sections inside the seam, which leads to a sudden rupture of the product.

There are also threads from Kevlar and other ultra-strong fibers, but their use is justified only in specific cases, such as the production of protective equipment or equipment for extreme conditions. For standard leather production tasks (bags, belts, wallets), high-quality nylon or reinforced thread is quite sufficient.

Marking and decoding of thread numbers

Confusion about thread numbers is one of the most common problems that crafters encounter. Different manufacturers use different numbering systems, which often leads to errors when purchasing materials. To avoid situations where the purchased thread turns out to be too thin or, conversely, does not fit into the eye of the needle, you need to understand the basic metrics.

The most common is the metric system (Nm), where the number indicates the length of thread in kilometers contained in one kilogram of weight. For example, thread Nm 40 means that 1 kg contains 40 km of thread. The rule here is: the higher the number, the thinner the thread. However, for leather, another system is more often used, based on thickness or conventional numbers (for example, LL, LH).

In Russia and the CIS countries, threads of the brand LL (lavsan polished) and LH (lavsan cotton). The number after the letter indicates the linear density. There is also the American Tex system, which shows the weight of 1000 meters of thread in grams. In the Tex system, the rule is the opposite of metric: the higher the number, the thicker the thread.

For clarity, here is a table of the correspondence of popular thread numbers used in leatherworking:

Thread type	Metric number (Nm)	Linear density (tex)	Application
Thin leather (gloves)	70 - 90	15 - 20	Decorative seams, fine haberdashery
Average thickness (wallets)	40 - 50	30 - 40	Wallets, covers, light bags
Thick leather (bags)	20 - 30	50 - 70	Backpacks, belts, belts
Very thick (saddles)	10 - 15	100+	Saddlery, heavy harness

When choosing, pay attention to the country of origin, since Chinese, Turkish and Russian numberings may differ even with the same names. Always check the actual thread thickness with a micrometer or by visual comparison with a standard if you are working with a new supplier.

Selecting a needle and setting up a sewing machine

Choosing the right thread is only half the battle. The second half is to properly set up your sewing equipment. Skin needle has a specific sharpening of the tip, reminiscent of a blade or spear, which allows it not to tear the fibers, but to cut the material, creating a neat puncture. The use of needles with a round point (for knitwear) on the skin is unacceptable.

The diameter of the needle must correspond to the thickness of the thread. If the eye of the needle is too narrow, the thread will experience excessive friction, heat up and melt or break. If the eye is too large, the thread will “walk,” causing skipped stitches and inconsistent tension. The optimal ratio is when the thread occupies about 60% of the eyelet area.

Adjusting thread tension requires a special approach. The upper tension is adjusted by the shuttle discs, and the lower tension is adjusted by a screw on the shuttle. For thick threads, the presser foot pressure should also be increased so that the material moves evenly under the needle. Insufficient pressure will cause the upper thread to pull the material, creating loops.

It is important to monitor the needle temperature. When working at high speeds with synthetic threads (nylon, nylon), the friction can be so strong that the thread begins to melt before it leaves the material. Using needles with titanium coating or Teflon coating helps reduce friction and dissipate heat.

⚠️ Attention: If you hear the characteristic smell of burning plastic or see melted knots on the underside of the seam, stop work immediately. This is a sign of overheating of the needle, which will lead to weakening of the seam and damage to the thread structure.

Waxing threads: technology and necessity

The process of waxing (or waxing) the thread is a critical step when working with natural leather, especially if you are using twisted threads that are not factory impregnated. Wax fills between the fibers of the thread, making it monolithic, smooth and more tensile.

In addition, wax serves as an excellent lubricant. When passing through thick skin, the thread experiences enormous resistance. A dry thread can simply get stuck or tear the material, while a waxed thread easily slides inside the puncture channel. Wax also helps fix the nodules, preventing them from spreading over time.

For waxing, you can use special shoe wax, paraffin wax, or even regular beeswax. The thread is pulled through the wax block with little force so that it warms up from friction and penetrates deeply into the structure. After this, it is recommended to slightly warm the thread with a hairdryer or pass it through a hot cloth so that the wax is distributed evenly and hardens in the desired shape.

How to replace special wax?

In an emergency, you can use a regular candle (paraffin wax), but it is best to avoid scented candles as the chemical additives may adversely affect the skin or thread in the long term. The ideal option is specialized shoe wax, which does not stick and does not stain your hands after it cools.

There is an opinion that modern synthetic threads do not require waxing. This is only partly true. The factory “polish” (hence the name LL) really makes it easier to pass, but an additional layer of wax will never be superfluous when working with particularly thick or tanned leathers. This extends the life of both the thread and the sewing machine.

Typical problems and solutions

Even experienced craftsmen encounter seam defects. Most often, problems are associated precisely with the mismatch of the thread with the type of material or machine settings. Let's look at the most common symptoms and methods for eliminating them.

If the thread is constantly is torn during the sewing process, there may be several reasons. First, check the sharpness of the needle - a dull point does not cut through the skin, but pushes the thread, causing it to break. Second, inspect the thread path: burrs on the guides, needle plate, or hook may cut the thread. Thirdly, it is possible that the thread is simply too thin for the chosen thickness of leather.

Problem looping below or above usually indicates a tension imbalance. If the loops are from below, loosen the top tension or strengthen the bottom. If the loops are on top, do the opposite. Looping can also occur when the stitch is too long on a thin thread or when using a thread that does not match the needle number.

An uneven, “walking” seam is often a consequence of poor quality of the thread itself. Cheap threads may have uneven twist, bulges, or “beardiness” (loose fibers). Such defects get stuck in the material, disrupting the rhythm of the shuttle. Using high-quality branded threads (for example, Gutermann, Coats, Anchor) minimizes this risk.

FAQ: Frequently asked questions

Is it possible to sew leather with regular #40 sewing machine thread?

Technically, it is possible to lay a stitch, but such a seam will be extremely short-lived. Ordinary threads do not have sufficient tensile strength and quickly saw through the sharp edges of the skin cut. For a time estimate this is acceptable, but for the operation of the product - absolutely not.

Which thread is better: nylon or lavsan?

Lavsan (polyester) is more resistant to ultraviolet radiation and high temperatures, it “floats” less when the needle heats up. Nylon (polyamide) is more tensile strength and more elastic, but is afraid of the sun and can stretch. For products that will be used outdoors (saddles, harnesses), lavsan is better. For bags and clothes, where the elasticity of the seam is important, nylon is often chosen.

Why does the thread break only on the front side?

This is a classic sign that the thread is not having time to slip into the hook or is rubbing against the edge of the hole in the needle plate. Check whether the gap between the needle and the plate is too large, and whether the needle number matches the thickness of the thread. The problem could also be that the top tension is too strong.

Do I need to lubricate the thread with oil or is wax sufficient?

For most tasks, high-quality waxed thread is sufficient. Silicone oil is used on industrial high-speed machines where the friction is so great that the wax burns. At home or on low-speed machines, oil can stain light leather, so wax is preferable.