Spiral Versus Fluted Dowels

Is either one better? The question sparks a discussion of how dowel joints work. March 20, 2007

Which is better, spiral or fluted dowels, for an exterior cope and profile door slab? Please give your reasoning.

Forum Responses
(Architectural Woodworking Forum)
From contributor R:
I'd call some of the dowel companies and get their take on the subject. They might have a tech you could talk to, otherwise you don't know who is answering your question or what their credentials are. I don't see anything in my Architectural standards book and you might also try the Forest Products Laboratories in Wisconsin site and look in their wood textbook. Lots of great info by a nonprofit research group.

From contributor M:
I was told by Chicago Dowel that they hold tighter tolerances on their spiral dowels than they do on the fluted ones, therefore they recommended that I use the spiral for my doors. But in keeping with contributor R's advice, give them a call and see what they have to say.

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
I believe that the answer to this question is similar to the answer to the question "Which is better... Ford or Chevy pickups?" That is, with a proper sized dowel and hole (not too shallow), proper grooving or fluting, and proper amount of adhesive, both will work very well.

More important than fluting or spiraling is the size... especially, not too large. In addition to size, we used to always put the dowels in an oven prior to use to dry and shrink the dowels slightly so they will fit into the hole with room for the adhesive and then they will expand in the hole with the moisture from the adhesive... but not expand too much. I have seen a lot of people that do not assure that the dowels are very dry or dowels that are too tight... they have problems.

The real discussion about dowels can also focus on their purpose. For side grain joints, their only purpose is alignment. The small increase in surface area that the dowel gives compared to the total area of the joint is so small that the dowel does not contribute to overall strength. Further, a good to excellent edge joint will be stronger than the wood itself without dowels, if the joint is properly manufactured.

When dowels are used on end grain, the end grain glue joint itself is quite weak. The dowels in this case provide considerable strength in such a joint. The dowels can equal the strength of mortise and tenon joints.

Note that the withdrawal strength of a dowel can be increased by lengthening the dowel. The shear strength can be maximized by using a dense wood species (hard maple) and a long dowel, so that it is the wood being joined that actually fails and not the dowel when in shear.

From contributor B:
The fluted dowels I've seen are pretty heavily fluted, leaving very little surface area between the flutes. Most spiral dowels I've seen on the other hand have 1/8" to 1/4" surface area between the spirals. The difference may be minimal, but this leads me to say that technically the spiral would be stronger because it has more surface area to grab available long grain on the side of the hole.

From the original questioner:
Thank you. We currently are using fluted dowels, but I think I will try some spiral dowel pins. Our door parts are run on a CNC and have tight tolerances for edge fit (profile and cope) and dowel holes as well. I actually called Chicago Dowel before I posted on here and was told there wasn't much difference and it was up to the user.

This issue came about due to the fact that some of our doors came apart after being on the job at the top and lock rail. It was determined after the fact, through the use of a moisture meter, that the customer had allowed the doors to get wet, then after the surface was dry, painted them, thus sealing the moisture in.

Even though the customer voided the warranty, we are trying to do everything on our part to make sure this doesn't happen again, including the possibility of changing glue, dowels, and even undersizing our door panels another 3/16" beyond what we normally undersize them. It was determined that the panels in the door had absorbed enough moisture to swell the panel and put pressure on the wet joints, thus causing the door to come apart.

Ford vs. Chevy pickups? Not a problem. I drive a Dodge.

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Contributor B raises a good point and that is that good adhesion is only possible (with standard woodworking adhesives) if the two pieces of wood are 0.002 to 0.006 inches apart. With a heavily fluted dowel, there can be an absence of wood within the indicated range, leading to a weak dowel joint.

From contributor J:
I did a lot of research about dowel construction before making the move from mortise and tenon. Some good points here about dryness, amount of glue and fit of dowels.

Another important thing for exterior work is what the dowels are made of. Beech, birch and maple are commonly what dowel pins are made of in the US and probably the worst choice for exterior work. Oak dowel pins are common in Europe for exterior door and window work. I tried to get some of the mentioned companies over here to make white oak pins but they will only sell oak dowel rods that are just too sloppy for any kind of joinery.

We get spiral fluted (fluted not grooved) pins made from Robien (Locust) from a company in Germany (see www.spilker.info). They claim this is the most rot-resistant non-tropical wood and harder than oak with more elastic strength. These are the best fitting pins we have ever used. For interior work we use Saunders Brothers pins. In our case the fluted seem to fit better than their spiral grooved. I will give Chicago Dowel a try. The Saunders fit is not very good. For gluing we use a dowel tip on our Pizzi glue pistol and a hydraulic press for assembly.

From contributor M:
I hit the Spilker website and saw it was only in German. I take it you use the 16mm pins in your doors. How do you go about ordering those from them? Is it a hassle to have them shipped from Germany?

P.S. I currently use 3/4"x4" dowels for my doors. What's everyone else using?

From contributor J:
We use 16 X 160 or a double row of 10 X 100 depending on edge profile, thickness and width of stile. On large sliders or windows, sometimes a triple row. All the shops I toured building doors with CNCs were using shorter 10mm pins, but a lot more of them. I think there is a depth limit for dowel drilling at least with the Homag machines.

Open the website with Babelfish or something similar to get a semi-translation. It's no hassle to have them shipped, just costly. If you want to order, it's best to email them. Most Germans speak English or can at least read it. It's been a couple years and I donít remember the cost exactly, but I think it figured out to an extra $8 per entry door and about $5 per average size window. Seemed very expensive until I put it in that perspective.