How to choose a coarse thread screw

These woodworking screws are characterized by large thread spacing and deep thread grooves, which provide a strong fastening. There are two types of course threads, depending on how the thread grooves are inserted, and each type has different applications and roles. Let's take a look at the different types of course threads, how to select one, and the criteria for length.

What is a coarse thread screw?

A coarse thread screw  is a type of wood screw (screw) for woodworking characterized by its strong fastening force. Unlike ordinary screws, coarse  thread screws have a pointed tip and deep, wide threads that bite into wood and other materials as they are driven in.

Coarse thread screws are very similar to common screws, such as slim screws. Both are screws with pointed tips and threads, but coarse thread screws  are most notable for the depth of their threads. They have coarser and deeper threads that catch the flesh of the wood into which they are driven, allowing for a quick and strong fastening. Compared to nails, coarse thread screws are more resistant to pulling force and can be driven directly into wood without a pre-drilled hole like screws. It is suitable for fastening by driving into wood with an impact driver or the like.

Select by type

Half thread screw

半ねじ

Shape

Grooves are carved from the tip to the halfway point, after which the shape is straight

Uses

For gluing wood

Caution

After a long period of time, thinning of the wood may leave a gap between the coarse thread screw and the wood in the unthreaded part of the semi-threaded type, resulting in rattling. In addition, when the screw heads are removed due to age-related deterioration, the wood may become unjoined.

Universal screw

全ねじ

Shape

Threads are carved on the entire length from the tip to the screw head

Purpose

Reinforcement when gluing with half-threaded screws.

Caution

Wood cannot be joined properly with full threads. This is because the entire course thread does not always dig through the wood with the same strength, and the digging force near the screw head is stronger, while the digging force near the tip is smaller. If it is absolutely necessary to use full threads to join two pieces of wood together, the pieces should be bonded together or secured tightly with clamps or a vise before driving in the course threads.

Selecting a Material

The material of the course thread body has a great influence on its strength. Using soft coarse thread screws tends to cause problems such as crushed screw heads and distorted shafts. Select a course thread that is suitable for your purpose.

Steel screw (made of iron)

スチール(鉄製)

Zinc plated and called 'Uni-chrome' or 'Chromate'.

Features

Inexpensive, but prone to corrosion

Purpose

For indoor use where strength is not required.

Stainless steel screw

全ねじ

For joining hard wood or using long coarse thread screws

Features

Corrosion resistant and rustproof

Purpose

When used outdoors and strength is required

Selecting by Length Criteria

Course threads are available in a variety of lengths, from about 25 mm to 150 mm. If you want to join two pieces of wood together, you may be wondering what length to choose. Here, we will explain the standard length of course threads. When joining with coarse thread screws, it is essential that the coarse thread screws not only penetrate the attachment (the member to be installed), but also that they bite firmly and sufficiently into the wood (the wood substrate) beyond it. In this case, the length of the course threads necessary for sufficient penetration should be 'the thickness of the object to be attached + 20 mm or more' as a rough guide.

However, if the thickness of the installation is greater than 20 mm, exceptional measures should be taken. In this case, use a coarse thread screw that is at least twice as long as the thickness of the object to which it is attached.

Summary

A coarse thread screw is a basic component that provides a strong bond to wood. Select the appropriate type of product, taking into account the thickness of the mounting material and wood to ensure an effective joint.