Last week I sent out an order of 400 small cabinets that were assembled with the pre-glued dowel pins. The next day I got a call from the customer wanting to know why most of his cabinets are falling apart. The dowels were inserted with a dowel inserter, and set up in a case clamp. The ends of the dowels that were sprayed by hand were more than adequately wet - so much so that the green glue ran a lot of times. Has anybody else learned the hard way about pre-glued dowels? I, for one, am voting for traditional wood glue.
From contributor A:
Same experience here, the cabinets were fine until we unloaded the truck. They did not survive the truck ride too well though.
All our cabinets came apart at the vertical joints. I think I'm going to experiment with glue for the verticals. I did like inserting the dowels with the machine. I also liked that we could stockpile parts with the aqua pins. If you pre-insert a dowel with conventional glue, it's going to seep out of the hole and dry, and then you wouldn't be able to get the sides flush.
The Miller Dowel operates differently than a traditional pin dowel. In fact, a more accurate description of the product is a modified treenail. This is, more or less, a functional tapered peg. The design relationship between the multi-stepped dowel and the multi-stepped drill bit give the MD an overall net tapered effect, while eliminating any actual taper or conical shape. The significance of this modification is that pegs with a smooth taper have a wedge effect that tends to split wood, plywood, MDF, etc. Furthermore, if a tapered peg backs out in the slightest amount, it loses all surface tension in an instant and fails.
Pin (cylindrical) dowels do not normally split materials because they don’t wedge. There is a predefined differential in size between the dowel and its pilot hole, usually snug but not enough to blow out. A weakness of traditional dowels is the lack of mechanical withdrawal resistance. The fibers provide substantial shearing resistance, and the glue provides withdrawal prevention…hopefully.
One advantage all dowels have over screws is their anchoring performance in end grain. When a dowel is properly glued in end grain, the adjacent long grain fibers of the dowel and the long grain fibers in the pilot hole create a very secure hold. Screws, while sturdy in cross grain situations, are weak in end grain. The simple explanation is that screw threading severs the fibers in which it is attempting to get purchase. This is much weaker. Furthermore, strip out from over turning must be noted as problem as well.
A setback with pin dowels is they do not anchor well in cross grain. Think about the separation you have seen in doweled rail/stiles, chair legs/rails, drawer face/drawer side, etc. Dowels will often fail in the side grain component and hold like hell in the end grain.
The Miller Dowel is designed with a diameter change in the side grain surface component. When the pilot hole is looked at it appears to have a countersink in the hole; this is what the head of the MD seats on. This MD feature eliminates the problem of glue failure in this component because the withdrawal resistance comes from the head’s clamping effect (like a nail or screw head).
I think that addresses the principles of our design for the most part. The Miller Dowel SR system is a modified version of our original setup. With the SR, the continuous stepped drill bit has been separated into two bits. The top 1/3 has become the through-bore tool. The bottom 2/3 is the edge-bore tool. The bits are on 10m shanks and sold as sets.
A production advantage is that when inserting the MD, will fall through the top piece and into the substrate. This means that the fastener itself will align the components. It is basically RTA. If a shop has robotic assembly equipment, then the MD SR will not apply. However, if a shop has boring equipment, but no auto assembly, then this could be worth looking at.
Joints created with Miller Dowels are stronger than pin dowels or screws. The MD’s are self plugging, so they save a step and cost compared to screws. Also, plugs can fall out, MD’s cannot. As explained, in many applications a MD joint is stronger than a screwed joint, especially in end grain and MDF. Miller Dowel Joinery incorporates the flexibility and durability of wood joinery for the same amount or less labor as other fasteners. On top of all this, they cost less than some screws including confirmats. As mentioned, there are drawbacks for certain applications One feature of screws is that they can be backed out and reinserted (to an extent), and this may be critical in a design. A MD can be drilled out, but it is destroyed in the process of course. Also, the MD is an exposed connection, which doesn’t work in all designs. . In regard to automated assembly, we are at least a year away. But I hardly think this should pigeon hole the Miller Dowel SR as a hobbyist tool.