Cabinet Machining and Assembly Options in a CNC Shop

Accuracy and efficiency are the goals. How you get there can vary, depending on your machinery choices. December 7, 2008

We are a 5 man shop that recently added a CNC router. Primarily we build frameless cabinets for commercial projects. We are now faced with trying to standardize how we build boxes, and I'm looking for some feedback on how to assemble boxes. We are currently just screwing them together, but I'm prepared to switch to rabbets and dados, doweling, or any other manner that might streamline the process. I'm certainly not opposed to investing in the proper machinery to facilitate this, as we have quite a bit of casework business on the books. Thoughts?

Forum Responses
(Cabinetmaking Forum)
From contributor C:
Using some simple jigs, and a good assembly table, plus staplers to get things fixed together and indexed quickly, assembling with screws (Zip-Rs by Hafele) is probably the most effective and quickest of all your choices.

Butt joints only except for the dado slot for the backs, and your CNC work is faster and easier than with blind dados.

From contributor D:
A little more information would be helpful. Does your router have horizontal drilling capability or a method of drilling on another machine? Are you happy with the end product you have now? Do you use any glue in your joints or just the screws?

I always like to have some method of aligning parts - it makes a big difference in speed of assembly. We use dowel construction and I would not change for us, but as many have said, it requires a large investment. Hand inserting dowels is insane and even the dowel shooters are finicky at times. I would not want to be without our horizontal bore inserter. You probably could go to the blind dados. It would help in assembling the box, but in trade of more machining time. I am thinking if you have the horizontal drilling ability you might consider just pre-drilling for your screws or adding two dowels in each joint just for alignment. I always used a melamine glue with screws - just a small bead to lock the joint from moving around and adding additional strength. Too much will make a mess.

I am a big fan of trading machining time for assembly time. Let the machine do the work and increase your accuracy at the same time. And do yourself a favor. If you now tap the parts flush at the front, start offsetting your top and bottom .5mm to 1mm in from the ends. It's too hard to keep perfectly flush in a production environment.

From the original questioner:
Our router doesn't have horizontal drilling capacity. There are several local shops that use horizontal boring and/or insertion machines in conjunction with their routers, and it seems to work pretty well. It seems like what takes us the most time is getting the parts lined up, not the actual process of driving the screws in.

And yes, we are happy with the end product, but I'm just looking to save some time. Quite frankly, I think a cabinet that is screwed together is a stronger box than the cabinets I'm familiar with that are doweled together.

From contributor L:
We use a combination of dowels and confirmats, and we scream through assembly. Until you get a horizontal bore machine, program the router to drill through at 9.5mm off the edge. You'll be amazed at how much time it will save.

From contributor M:
Please do a search for "confirmats" on this site, as I have written quite a bit about it. In short, confirmats are great in that they serve as a metal dowel and clamp. We have not found it necessary to use dowels with confirmats, as contributor L has suggested. We get good alignment with just the confirmats. For use, it would be extra drills in the horizontals.

Some have talked about using cam locks and dowels for finished ends. I think this is an excellent idea.

If you have the money, look at CNC horizontal boring/dowel insertion machines. I would still use confirmats as much as possible. The nice thing about CNC horizontal boring is that you can place holes where you want them, like 37mm from the back edge.

We use a construction boring machine for our horizontals. It serves the same purpose, but has advantages and disadvantages. Advantage is that it drills all the holes in 1 cycle. Disadvantage is that it is on fixed 32mm increments. We just set this variable in our software and it is not an issue.

I would only use C-axis on your router for horizontals as a last resort. It takes too much time for 1 piece.

We also use 5mm dowels for our drawers. Most inserters will not handle 5mm dowels... I think it is 6mm and up. They are too small and give inconsistent results. It does take more time to machine the drawers, so be careful that it does not interfere with machining case parts.

Stopped dadoes require too much cycle time on the table, and excessive use of tooling.

From contributor L:
We actually use the combination of the two - dowels and confirmats - because it gives us the alignment instantly. The confirmats are to eliminate the case clamp. We do actually glue up quite a few cases, but we are seriously considering going to separate end panels to speed things up. Stepped dado is a lot of time on the machine and the wear is great, however, it does help the alignment issue out of the gate.

From contributor M:
We use a 7mm hole for the verticals. If there is any need for alignment, we do so with a slight tap of a deadblow. Do you glue your dowels? How many dowels/confirmats are you using in a base cabinet side?

From contributor R:
So you guys are saying it is faster to CNC the holes and cut out the part, then take your parts to another machine (horizontal boring), rather than have the CNC cut dados? I would have to see it to believe it. You have done time studies for this? I would think just the risk of mistakes at the construction boring machine or someone dropping a part would far outweigh the few minutes it takes a CNC to cut dados.

From contributor I:
We nest all our cabinet panels that require vertical machining on our Biesse FT. All parts that do not require vertical machining are cut on our beam saw, all horizontal boring is done on our Koch PTP, drilled and inserted. We produce labels both at our beam saw and our nester and all these labels are read at the Koch PTP. All unfinished ends have both dowel holes and pilot holes for screwing, finished ends only 8mm dowel holes. We have tried all the other methods mentioned and have done time studies, and yes this method is faster. Just the time we saved in assembly with alignment alone was enough to know that this is the method that works best for us.

From contributor M:
Yes, it is faster. About a month ago I decided to do blind dadoes on a small job. It was much slower, and still had to deal with alignment. The issues you bring up seem more hypothetical. Again, in my opinion, you spend too much time machining your dadoes and tenons. You use a small diameter bit, adds extra time and increases wear.

From contributor L:
We use 6 - 8mm dowels and 4 confirmats in our base cabinets. No, we don't use glue in the bases. We almost always glue wall cabinets. We also pocket screw the hanging rail on the cases. We are getting ready to dump the pockets and drill for dowels and confirmats on these too. We have no alignment issues. We cut, band, process in small enough batches that we don't need labels. Drawer strechers, nailers and sink base plates/stretchers are all the same size drilled in the same patterns, saving lots of time.

Yes, I have done a time study and our method is very fast. If you will order one cabinet from cab parts and analyze it, you will see what I am stating. Before I purchased our machines, I would order up to 40 cases from them on average and have the entire job standing in one day, by myself.

From contributor A:
Never underestimate the material handling cost. With one machine (a flat bed CNC) you can do all the construction milling with dados. You can also drill pilot holes in the dado slots if so desired. You can actually do it to a level where the assembly people do not even need a tape measure anymore. I'm working this plan with a client in Jamaica where skilled labor is hard to come by.

In order to keep the dados clean and gap free, you need to machine a slightly narrow (say 11/16) dado and also machine the top face of the mating part so that the small "shoulder" part is the same thickness. This gives you consistent results with inconsistent materials, and also provides a glue surface in melamine parts. Bear in mind that machining time is not the biggest savings you get when buying a CNC. Accuracy, quality and reduced assembly time are almost always a bigger savings.

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From contributor A:
The concept I talked about above can also be done for edgebanded cabinets by simply stopping the dados in the cab side, and notching the mating pieces. A touch of extra programming, but no additional machine time.

From contributor J:
A good dead flat assembly table is most important. I like to staple first to align everything flush, then pilot hole and countersink, then insert screws and all done. I use 1 5/8" Quickscrews.

From contributor K:
We dowel everything. We start with accurate parts cut on a slider, then straight to the bander, then to a pod-and-rail CNC. While those parts are running, we run the decks and spreaders on a CNC dowel inserter (8mm on 64mm centers are standard but we can locate them anywhere we want to). Everything is glued during assembly, then straight to the case clamp (buy a nice interlocking automatic case clamp or your guys will get tired of resetting the clamp locations manually). We used to use confirmats, but since we started dowelling everything, we will never go back. We also use the CNC dowel inserter to dowel our drawers and mitered doors together. A 50,000 investment worth every cent and more. As far as alignment issues, there are some variances. The only way to get true alignment is to nest everything at once on the same machine, but that's a whole different technique than what is feasible in a small custom shop that wants to dowel with multiple machines.