Question
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.
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.
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.
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.
P.S. I currently use 3/4"x4" dowels for my doors. What's everyone else using?
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 don’t 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.
