Joining Wood #2 - Dovetail Angles & the Through Dovetail

This past weekend, I hosted enthusiastic young woodworker, Matthew Liebenberg of Toolcraft, for an intense short course on Hand Cut Through Dovetails.

I firmly believe that it is imperative for sales people in the South African woodworking trade to have proper hands on experience and comprehensive knowledge of the tools that they are advising their customers on, as well as the correct techniques for using these tools efficiently, and this is the reason why Matt made the pilgrimage to Swellendam to join me in the workshop!

During our time together, the age old question of dovetail angles arose. What are 1:8, 1:7, 1:6, 1:5, 1:4 slope angles and why does it even matter?  He also asked what angles were specifically used on furniture of the 18th century? (among very many other questions).

Thinking about Matthew having these questions, it became clear to me that many of our fellow woodworkers have most probably also asked these same questions.

So, in the interests of historical woodworking academia, this week we get into the why, how and where of the Dovetail Joint, joint angles and the importance of the joint pertaining to cabinetmaking. 

The Through Dovetail Joint has an incredibly long track record, and the artifactual evidence shows us that this woodworking joint is truly ancient and most probably predates written history. Ancient Dynastic Chinese and Pharaonic Egyptian woodwork both offer up examples of dovetail joints. Over the centuries this joint has been known by many other beautiful names, such as Culver Tail, Fan Tail and Swallow Tail.

In recent years, some American woodworkers have coined the names Condor Tails and Eagle Tails for the very large dovetail joints used on workbench end vises. I really enjoy these names and have decided to call my very large tails of the type, the Bateleur Tail, because the splay of the Bateleur Eagle's tail is a remarkably apt description of the shape of a large dovetail joint. (And it's a proper African eagle, unlike the Condor which is actually a vulture, and I refuse to name my joints after a vulture!).

Dovetail Joints for woodworking use trapezoidal shaped projections on one board, which are designed to fit exactly into the corresponding trapezoidal cut-outs in the mating board. Technically, the joint is a variation of the Finger or Box Joint, but has far superior long-grain to long grain joint strength, more glue surface, resists warping way better than a finger joint and allows excellent seasonal cross grain expansion and contraction. The nett effect of the trapezoidal shape of the joints enables them to lock in one plane and it is this that makes the dovetail joint far superior for drawer making (Tails on the sides of the drawer) and case work that is subject to wide temperature and humidity changes.

Very early dovetail joints were rather crude by more modern standards and were often re-enforced with hand cut nails. As the joint became more refined over the centuries, the need for additional fasteners fell away.

Early dovetail joints were almost always exposed in both planes, however with the advent of the use of fine veneers to embellish patrician styles of furniture case pieces, these joints became more hidden. It was at this stage of the development in furniture making that the need to hide at least one plane of the joint, led to the joints that we now know as Stopped, Half Lapped, Half Blind, Hidden and Secret Mitred Dovetails. It is interesting to note that once dovetail joints became widely used in cabinetmaking, it was possible to use thinner stock and achieve excellent and strong construction, most notably in the making of drawers.

Today, hand cut dovetails on a piece of furniture are regarded as being the quintessential cabinetmaker's joint and are thought to be the reserve of specialist cabinetmakers. 

The truth about the joint is not as romantic. Any woodworker with a good eye and three or four accurate, sharp & well set tools can easily achieve beautiful dovetail joints.

All it takes is a good grasp of the concept of the joint, absolutely square and flat boards, good chisel and saw technique and a little initial patience. 

Let us use the Through Dovetail as our example joint. Historically, the joints were chosen at a reference ratio for a given piece. This was 1:5, 1:6, 1:7, 1:8 and so on. How this works is very simple. Cabinetmakers did not have easy access to mathematical instruments such as protractors in the 18th century, and a ruler and story stick were the only measuring devices that were generally employed.

These rulers (usually a two foot folding ruler) were in Imperial scales. 

The way to achieve a 1:8 slope angle, is to draw a line exactly 8 inches long. then draw a line 1 inch long starting at an end of the 8 inch line and projecting at a right angle (90º) to it. This will give you an obvious "L" shape. The final step is to connect the open end of the 1" line to the open end of the 8" line, and you have your tail angle!

In the case of a 1:6 ratio the short line will be 1" and the long line will be 6" long and so on. Simple, easily repeatable, and no special mathematical instruments are needed. For those of you who won't sleep until you know the actual angles of these historical ratios, here they are:

• 1:4 = 14.04º
• 1:5 = 11.31º
• 1:6 = 9.46º
• 1:7 = 8.13º
• 1:8 = 7.13º

It is common for woodworkers to say that we should use a 1:6 slope for softwoods and a 1:8 slope for hardwoods. I am not at all convinced by this and I think that it might be more a case of good marketing rather than the actual physics of the joint.

If you have the chance to get really close to museum quality 18th century furniture, you will notice that the drawer sides and fronts range widely in wood species. Softwoods, Hardwoods, soft Hardwoods and hard Softwoods and everything in between are used. You will also notice that the angles do not conform to a definitive standard at all. In the more patrician 18th century pieces that I have had the privilege of examining, it appears that the slope angle of the tails, as well as the tail widths and pin widths, are laid out more for their aesthetic value than being based on a set of predefined rules.

On Saturday, Matthew had the remarkable opportunity to closely examine a massive 18th century linen press with hidden compartments that is currently valued at R1,5 Million Rand. He learned more about dovetails and furniture construction in that 15 minutes, than he would ever have learned from Pinterest, Instagram and Youtube. The retired curator of the Drostdy Museum, Mr. Johan Kriek, was kind enough to give Matthew the rare opportunity to examine this cabinet and other exquisite furniture pieces in the collection in detail!

In all the casework pieces that we investigated, the dovetails were laid out in harmony, were well balanced and were pleasing to the eye. This is the point that I wish to stress here: dovetails are the most beautiful of all woodworking joints, we should lay them and cut them so that they please us, and not base them on strict rules. As long as the angles of the tails create a pleasing trapezoid, the joints are equally balanced across the board, and the pins are narrow and neat, you will have a beautiful and very strong joint that will test the centuries well.

Regarding the Youtube stars, who show you how to cut perfect dovetails in three minutes and less, I suggest that you ignore them. Woodworking for pleasure, accomplishment & pride, is not a competition, it is an eons old art form. Take you hand cut joints at your own pace, lay them out so that they please your eye, cut them well and crisply and you will enjoy them for many years.

Young Matthew has successfully achieved the Through Dovetail Joint, in both hard and soft wood. His task is now to join two small boards together with two tails and three pins, every day for the next month.

Matthew Liebenberg's first three fully hand cut Through Dovetail Joints

Joint #1 & #2 are Tulip Poplar and Joint #3 is Hard Maple

Hand cut joinery is about co-ordination of hand and eye, a sense of balanced design perspective, and a desire to do one's finest work. It is amazing that the more you practise, the luckier you get. Matthew's daily results and his personal critique of his own joints will be his best teacher!

Until next week, let's make shavings, not dust...