What do we know about dowel strength?

Dowels may be becoming in more vogue again, but what about their strength? The problem with deciding whether one fastener is better than another is that fact that it is a fairly broad thing to test. It has to do with many differing factors: the diameter, length, and species of the dowel, the type of adhesive used, and or course the characteristics of wood being joined.

Most of the explorations into dowel strength have compared them to other joints using simple tests of joint failure. A good example is this experiment from Canadian Woodworking from 2010. They tested a T joint in solid oak by comparing: (i) a 3-dowel joint (⅜”×2″), (ii) a mortise and tenon joint, and (ii) a 2-biscuit joint. The biscuit failed at 325lbs of force, the M&T at 500lb, and the dowel-joint at 650lbs. An article, “Joint Success” in British magazine Furniture & Cabinetmaking showed the following failures (tested using a hand-pump hydraulic ram) is shown in the table below. None of the fixings failed.

Method	Specs	failure (psi)	What failed?
biscuit joint	1 × No.20	180	glue line + fracture of material parallel to biscuit slot
Dowelmax	5 × 10×50mm dowels	660	glue line
Domino	2 × 8×40mm tenons	400	glue line + partial fracturing along the grain
mortise and tenon	45mm depth	420	glue line
pocket hole		280	screw and glue line

Furniture & Cabinetmaking

Yet another test from the November 2006 issue of Wood Magazine “Wood Joint Torture test”.

Method	Shear test (lbs)	Pull-apart test (lbs)
mortise and tenon	1017	2525
Dowelmax	609	1866
Beadlock	541	1486
Domino	464	1170
biscuits	187	766

Wood Magazine (2006)

The reality is that actual testing of joints would likely have to include the use of different species of wood for dowels, the use of various glues, and the application to various types of connections, and target species. It would require a large study that also assessed the effect adhesive curing has on the strength of a joint. There is actually a load of commercial research into the area of fastener strength, which isn’t really that surprising. This is partially because it is the most commonly used joint in commercial furniture construction (largely due to the fact that it is inexpensive). With the resurgence of interest in multi-storey wooden buildings, there will no doubt be more interest in the use of dowels in timber engineering. Below are some examples of dowel strength in the literature.

In a 2022 article from the Warsaw University of Life Sciences [1], the authors investigated the influence of the type of invisible wooden connectors on the strength in glued corner joints for chipboard and MDF, which are commonly used in the construction of box furniture. They used (i) four wood grooved beech dowels, (ii) two beech Domino tenons, and (iii) a mixed arrangement of connectors. Results showed that dowels provided the highest strength over Domino tenons. They also determined that the use of MDF over chipboard increased the strength.

Many studies look at withdrawal strength, which isn’t exactly the sort of strength tests you see in peoples workshops. A 2020 study [2] looks at the withdrawal strength of 8mm plain and spiral dowels made of beech and oak. The authors investigated the influence of relative humidity, dowel structure and wood species on withdrawal strength. Spiral dowels were found to have a significantly higher withdrawal strength over plain dowels. The influence of the wood species was not found to be statistically significant overall.

Another study [4] looked at the joints made from hardwood plywood of 19 mm thickness, using beech and hornbeam multi-grove dowels in various diameters (6, 8 and 10 mm) and depths of penetration (9, 13 and 17 mm). The best results were for joints made with 8mm beech dowels penetrating 17mm into the joints. The 10mm diameter dowels failed, likely due to reduced edge thickness in the panels. In general strength does increase with dowel diameter. There are even studies which look at optimal dowel spacing. In one study [5] the author found that the optimal spacing of dowels in cabinet construction (MDF and particleboard) was 96mm. Spacings of 32mm and 64mm resulted in a reduction of the maximum load-bearing capacity per dowel due to over-lapping zones of influence by neighbouring dowels.

There are even studies that look at structural joints made with axially loaded glued-in hardwood dowels [3]. The authors of one study [3] look at various large plywood (Glulam) beam configurations held together with a series of 12mm by 120mm dowels. Withdrawal strength was found to be about 30 MPa (4351 psi), and overall joints have high strength properties (except the dowel joints do succumb to brittle shear failure of adhesives or wood members).

In all likelihood, dowels are a very good choice for holding together a joint in a piece of furniture. I mean in normal furniture there is never really going to be the excess loading applied in testing. You just have to make the appropriate choice for the material being used. For most circumstances, that’s likely a ⅜” diameter spiral dowel, made of a species like beech. It may be even possible to use them to join larger structures such as workbench bases, where multiple ½” diameter dowels, 3-4 inches in length could be used.

Further reading

1. ŚMIETAŃSKA, K., MIELCZAREK, M., “Strength properties of furniture corner joints constructed with different wooden connectors and wood-based materials”, Ann. WULS – SGGW, Forestry and Wood Technology, 118, pp.55-66 (2022)
2. Podlena, M., Böhm, M., Hýsek, S., Procházka, J., Černý, R., “Evaluation of Parameters Influencing the Withdrawal Strength of Oak and Beech Dowels”, BioResources, 15(1), pp.1665-1677 (2020)
3. Koizumi, A., Jensen, J.L., Sasaki, T., “Structural joints with glued-in hardwood dowels”, Joints in Timber Structures, pp.403-412 (2001)
4. Dalvand, M., Ebrahimi, G., Tajvidi, M., Layeghi, M., “Bending moment resistance of dowel corner joints in case-type furniture under diagonal compression load”, Journal of Forestry Research, 25, pp.981-984 (2014)
5. Tankut, A.N., “Optimum dowel spacing for corner joints in 32-mm cabinet construction”, Forest Products Journal, 55(12), pp.100-104 (2005)

Is there a future for dowels in furniture making?

Let’s talk about dowels. For years they have been the lesser cousins when it comes to joinery for furniture. They are often as maligned as butt-joints, but is the “dislike” warranted?

There are of course many ways to join two pieces of wood. For joining corners the most popular method is often dovetails – for aesthetics alone. Are dovetails always practical? You can make them by hand, or use a jig and router to speed things up, but they aren’t the be-all-and-end-all. Hidden joints are done in many ways – some people like using the tenons of Festool’s Domino system (the system is nice, bit pricey for the home user), others sliding dovetails (nice looking, but a lot of work for being mostly hidden), and still others use dowels.

The Stanley No.59 dowelling jig

Plain old dowels. The word dowel is an old English word analogous to “doule” which simply means a part of a wheel. It can also be traced its history in the Middle German language “dovel” which meant to plug. Now dowels have been around for quite a while. Viking longboats used trunnels (tree-nails or big dowels) to hold ribs together, and dowels were used to pin structural members joints together in buildings. However the use of dowels in furniture construction came as a late development, appearing in the early 19th century as an alternative to mortise-and-tenon joints.

For many years many woodworkers have shied away from them. This may be in part because of poor experiences with dowels in a certain self-assembly flat-packed furniture. But dowels are actually ideally suited to building with plywood, especially Baltic-birch. The multi-layered form of plywood is not well suited to the likes of dovetails, or mortise and tenon joints. It is also an ideal way of joining solid wood.

It also may be because the old-type dowel-jigs have always been somewhat mediocre. They weren’t really designed for placing dowels along the edges of large cabinet carcasses, or on mitre joints. The early ones, like the Stanley No.59 were versatile in application, but suffered from only being able to drill one hole at a time. The newer ones include the Veritas Dowelling Jig, which is good at some tasks, but not the most optimal solution , and the Woodpeckers Ultimate Dowelling Jig 2.0, one of their OneTIMETools… too short lived to form any sort of opinion on. But then there are what some consider the game-changers, the DOWELMAX and JessEm jigs. These tools let you put dowels in just about any configuration (each has its own pros and cons).

The Dowelmax…

or the JessEm?

Then there is the dowel pins – spiral-grooved or multi-groove. The grooves, or flutes allow air to escape and glue to fill the voids as the dowels are inserted. Without them, it is possible that the dowels would not be able to be inserted completely into the hole. There is no real definitive answer as to which has the better holding power, but both are infinitely better than smooth dowels (unless the dowels are to be used in through-dowel situations where they are exposed). The dowels are generally like pressed-beech biscuits, with the compressed wood in a ⅜” dowel expanding about 1/32″ (0.8mm) on contact with moisture in the glue, creating a tight fit.

Dowels offer a convenient, inexpensive way to make a robust joint. They can give incredible strength to mitre-joints, and make carcass joinery quite efficient. Whether you choose to use them is really up to you, but they offer a method of joinery that is both inexpensive and sturdy. But how strong are they really? Well we will tackle the concept of dowel strength in the next post.