Blind Dadoes Versus Dowel Joinery

      A discussion of the efficiency and accuracy trade-offs involved in switching from blind-dado and screw construction to dowel joinery. August 8, 2008

Question
We've been using blind dado construction done nested base for almost a year now. I have an extremely repeatable system but there are a few things that I feel add to the assembly time. First is that we still have to manually drill for the confirmats in the horizontal part to eliminate splitting, and second is dealing with finished ends. After visiting the AWFS show last year and spending some time with the manufacturers of dowel drill and insert machines I keep thinking that would reduce my assembly time. From what I can see it adds a minimum of 30% more cycle time to the nest on the CNC to cut blind dadoes, plus the added tool wear and dust collection issues. If any shops have made this change, please give me some of your thoughts pro or con. Am I wrong in thinking that the CNC operator could bore the horizontal parts in the time it takes to run the next sheet?

Forum Responses
(Cabinetmaking Forum)
From contributor R:
First off, I would say that maybe you need to look at the reasons you are doing things the way you do. Personally, I see no reason that you have to use the confirmats at all. A blind dado joint with a little glue and a couple of 18ga x 1 1/4" staples should be plenty strong. Many times we over-engineer our products for no reason other than to satisfy our own need to "build it stronger." Think how fast your process would be with no confirmats... No boring, no screwing together. Just squirt some glue in the right spots and tack together. If you don't think it will be strong enough, do a small sample cabinet and have fun destroying it after the glue has cured. Another change in your process might be to handle the cabinets more gently until the glue has set. You also might want to use hot melt on the backs to keep the cabinets square until the glue sets.

All that said, dowels can be very efficient as well, but you will need some very expensive equipment to make it work well, namely a dowel boring and insertion machine and a case clamp.

At the different shops I have visited I have noticed a few commonalities at different levels. Small CNC shops doing less than 30 cabinets per day use the staple screw method like True 32. Shops in the 30-60 cabinet per day range are using blind dado or confirmat construction (not both). Larger shops making 100+ cabinets per day are using dowel construction. Keep in mind this is only based on my personal observation and by no means is a statistical study. These would all be euro style construction using CNC router and or beam saw/ptp for panel processing.



From contributor M:
The short answer to your question is that you can gain time in the assembly process using confirmats instead of gluing and stapling. There are other benefits as well.

As far as time to drill while the sheet is nesting, it all depends. How many parts are on the sheet being nested, how fast is your nest, how many parts will have to be drilled? If you have a sheet full of stretchers and nailers, your operator will have to remove the sheet, load, sort, and then drill. This may be a lot to do before the next sheet is finished. I just have the operator sort and let someone else do the drilling. I would be interested to see if that works for you.

Contributor R, that is a good observation that you make. I would say that this industry is slow to change, and that may be why you haven't seen more confirmats. The beauty of this system is that it bridges the gap and is a good choice for small and large volumes alike. And you don't have to invest in case clamps. Dowels and case clamps work well when you have many boxes the same size and you can put several boxes in the same load... and even better, have two clamps with very little down time. Clamps are difficult for the low volume shop to justify because they are doing so many different sizes and tying the machine up for 1 or 2 boxes.

With confirmats, you have the alignment advantages of dowels and the speed of screws, and you don't have to monkey with a case clamp. If assembly becomes the bottleneck, you put drivers in more hands. With a clamp, it is only as fast as the operator and the glue.



From contributor R:
If he was using blind dado and staple/glue only it seems that would be the fastest way to go because there would be no secondary operation to do. The operator could be edgebanding rather than boring horizontal holes.


From the original questioner:
Thanks for all of the feedback guys! I didn't mention in my first post that the majority of our work is commercial and the specs call for an AWI standard which requires dowels and/or confirmats. Our residential work on plywood boxes presently gets stapled and screwed (#8x2"), but we still machine for blind dado for ease of position and location.


From contributor M:
Stop dados are permitted in Premium grade, as are dowels, European assembly screws, and fully concealed interlocking mechanical systems. There are other requirements for this grade. Please refer to (400A-T-12).

Contributor R, the consensus is "secondary operation = bad." It is a tradeoff. Those that do butt joints and staples consider blind dados a secondary operation... not essential and a non-value added step. But you do have alignment advantages with dados, so there is a payoff. The same is true with confirmats. You have even more accurate alignment with these guys, not just vertical, but positionally as well (front to back). With stopped dados that are machined on a flat table, you must make the dado long enough to accept the tenon. Unless you radius the tenon to fit the slot (talk about additional operations). You don't have this as much with confirmats. They are as perfect as your machining.

I recently showed my guys how flipping the stock before they cut it was costing us serious time. It would take them 5 seconds to flip a piece. Not too bad, but we had 5,000 parts to cut, and it would cost us almost 7 hours. If I had the option of finishing the job in 21 hours or 28... I hope you see what I mean. This may not be so dramatic with 30 cabinets a day, but then again, it all adds up. I would much rather be paying a guy to build much needed cabinets for the shop than flipping stock. I need to do a time study on stopped dados vs confirmats. What am I saying? I can't even get cabinets for the shop!



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