Needle damage and fitting

The typical stitching needle used in footwear production and how to identify damage while ensuring correct fit in the machine.

by Ray Wisken

A damaged needle – whether bent, blunted or burred, or one that is poorly fitted in the stitching machine – will inevitably cause problems with the processes involved in manufacturing footwear. It is therefore essential that the operator is familiar with this small but very technical part of his or her machine, and is able to check that needles are fit for purpose both prior

to beginning work and during production. This information will help members of the factory team and departmental heads to ascertain if the needle may be the cause of common stitching problems.

Needle types

The first consideration for needle selection is the type of point, the choice of which is governed by the type of material being stitched and the final appearance of the seam to be achieved.

Leathers and heavily-coated textiles (such as the synthetic materials used in sports footwear) are usually best stitched with ‘cutting’ point needles – for example, narrow wedge types – whereas textiles and lightweight coated materials are best stitched with ‘puncturing’ point needles (often called ‘round’ or ‘cloth’ points). With leathers, the lower friction generated with cutting points will prevent too much heat damage to the threads. Conversely, it is preferable that textile yarns are ‘pushed apart’ by the needle during stitching rather than cutting through yarns, which would weaken the material (hence the use of cloth point needles with these materials).

Dense or harsh materials will also blunt needles, so regular needle changes will be required. Blunt needles will cause unsightly stitch-hole formation and create more heat damage to the thread. The narrow wedge and round point needles, and the stitches they produce are indicated below.

Stitches produced by narrow wedge and round needles

Parts of a needle

The parts of the stitching machine needle are shown in figure 1.

Figure 1: Parts of a needle

Butt: the end of the needle furthest from the point. This is tapered to allow contact with the stitching machine’s needle bar for correct fitting.

Shank: The section of the needle that fits into the needle bar and is the largest diameter part of the needle (to give support). The diameter of the shank and the needle length are decided by the stitching machine’s needle system. How the needle fits in the needle bar determines the ‘scarf’ position in relation to the ‘hook’.

Shoulder: The part of the needle between the shank and the ‘blade’. It is tapered to give strength to the blade.

Blade: This extends from the shoulder to the ‘eye’. As it passes through the material being stitched, this section is subject to friction.

Long groove: An indentation that runs down the length of the blade on the opposite side to the scarf. The top thread sits in this groove during stitching to allow for movement. The depth of the groove must match the thread diameter to provide control without restriction.

Scarf: This is the thinnest part of the needle, and is located above the eye and opposite the long groove. The scarf allows the hook to pass close to pick up the thread loop.

Eye: The hole which extends through the blade from the long groove to the bottom of the scarf. Different eye shapes can give different advantages during stitching. The eye must be very smooth, as the thread passes through it many times.

Point: This is the point of contact between the needle and the material, and point design must be selected according to the material to be stitched. The correct shape and point provides the most suitable penetration of the material. There are two main types: a ‘cutting point’ for leather and a ‘round point’ for cloth.

Damage to the needle point

Figure 2: Damage to a needle point

Checking for needle point damage (burr): The point of the needle may be damaged in normal use or by hitting hard surfaces. Burrs appear most commonly on the point of the needle (figure 2), and these can cause damage to the material and thread. Damage may be visible, or felt by running fingertips or finger nails over the needle.

Checking if a needle is bent: The suspect needle should be removed from the stitching machine and the shank rolled on a flat surface. This will reveal if the needle is bent.

Needle policy: Factories should have a policy to cover the correct use of needles. For example, if a needle breaks, a system should be in place to account for all the broken parts.

Correct fitting of the needle

As indicated in figure 3, the flattened area of the scarf needs to face the hook. If the needle is not fitted properly, problems can affect both the thread and stitch quality.

Figure 3: Correct and incorrect fitting of a needle

How can we help?

Please email systems@satra.com for assistance with stitching processes in footwear production.

Publishing Data

This article was originally published on page 16 of the February 2018 issue of SATRA Bulletin.