Basically, finger jointing connections are longitudinal joints of solid timber, the ends of which are glued together using interlocking profiles of wedge-shaped tines of the same width and length.

The term finger joint is defined in DIN standard 68140-1 and specified in other DIN/EN standards depending on the area of application. However, this standard is only binding for load-bearing joints, so some of the finger joints used in furniture construction do not comply with European and national standards.

Use of finger joints

Finger joints can be used wherever short, elongated workpieces made of solid wood (e.g. mouldings, planks, squared timber, boards) need to be extended.

This can be the case when good pieces from wood optimisation that have been cross-cut using optimisation cross-cut saw are to be reassembled into uniform workpieces. Finger joints also play an important role in timber engineering, as it would be almost impossible to realise large timber structures without workpiece extensions.

The need for a finger joint results from the fact that it is not possible to glue solid wood permanently on the face side due to the wood structure. By milling interlocking profiles, the contact surfaces between the workpieces are exclusively longitudinal wood and can therefore be glued together without any problems.

The finger-jointing of construction timber is subject to special regulations in Germany and the EU. Only specially certified companies are permitted to produce finger-jointed goods for use as construction timber (e.g. glues laminated wood, solid structural timber) (so-called "glue authorisation").

Forms of finger-jointed joints

If the workpieces to be joined do not have a square cross-section, a basic distinguishing feature is the direction in which the prongs are joined.

Vertical joint

The term "vertical jointing" can be used to refer both to the alignment of the tines on flat boards and to the position of the workpieces when milling tines with package systems. The boards are stacked on edge and are therefore vertical.

The tine profile is visible across the entire width of the workpiece.

Advantages:

• Many finger jointing lines are designed as package systems, i.e. they mill the workpieces in stacks next to each other in cycles. With workpieces standing on edge, considerably more workpieces can be accommodated in one package and therefore processed in one work cycle.
• The gluing surface is significantly larger compared to other joints, making it the most durable type of finger joint

Disadvantage:

• The finger joint is clearly visible on the surface.

Applications:

Primarily for construction timber or particularly inexpensive finger-jointed goods, as a centre layer for window scantlings

Horizontal finger joint

The tine profile is only visible on the narrow side of the workpiece (the tines are therefore aligned horizontally when the boards are lying flat), when the profile is milled with package systems, the boards lie flat or horizontally next to each other.

In order to prevent half-cut tine profiles and chipped areas on the surface, horizontal tines are usually designed as "shoulder tines". Milling tools adapted to the thickness of the workpiece are used. These only mill the tine profile up to a few mm below the workpiece surface. Smooth glue joints are created at the top and bottom (usually with synchronised scoring saws).

Advantages:

• Depending on the profile used, the joint is only visible on the surface as a transition between different wood structures; the joint itself forms a straight, closed joint.
• Due to the flat workpieces, it is possible to use very high-capacity systems that work completely in throughfeed mode

Disadvantages:

• Due to the above-mentioned manufacturing process, horizontal joint the capacity of most production plants.
• Tools and equipment of the plant are more complex and therefore more expensive
• The durability of the finger joint is lower than with vertical finger jointing due to the smaller gluing area

Application:

Primarily for furniture panels and other workpieces in the visible area

Standards for finger joints:

• DIN EN 385 (2007-11) Terms - Structural timber finger joints
• DIN EN 387 (2002-04) Terms - Glued laminated wood universal finger-jointed joints
• DIN 68140-1 (1998-02) Terms - Softwood finger-joint for load-bearing components

Image sources: Weinig/GreCon (Figure 1), Dirk Bartens