I hope this isn’t too basic a question, but here goes. I’m about to assemble some cabinets with fixed shelves in rabbeted dados. In a FWW article, Building Bookshelves to Last, Jeff Miller suggested using confirmat screws to strengthen the joint. My question is, if I glue and screw the shelves do I also need to clamp them? The cabinets are fairly wide, so I’d need to purchase some additional clamps, which I’d like to avoid right now if possible (although having more clamps is always a good thing).
One other question, would it be ok to dry fit the cabinet and pre-drill the holes for the screws? The advantage would be to make the assembly go faster, but at the risk of the holes possibly not lining up perfectly.
FWIW, I’m following the suggestion of Steve Latta (Do An About-Face on Cabinets) and have already assembled the face frames. Unfortunately, I didn’t see his article soon enough, so the cabinet sides will be attached to the frame with biscuits, not rabbets/grooves, but from my test dry fits I don’t think that will be a problem.
Replies
Your question raises a lot of questions... Specifically:
- Plywood shelves or solid wood shelves? How thick?
- How thick are the cabinet sides and how deep is the dado?
-"Fairly wide cabinet" - How long are the shelves? Are they just supported at the ends or from the back as well?
(As long as your dry fit is good, can certainly predrill holes which will help with alignment during glue up. Consider using glue with longer set time for what sounds like a complicated glue up.
In general I try to use screws to hold things together rather than to pull them together, so I would always get things clamped together then add the screws. Guarantee you will use those new long clamps many times in the future!)
- the shelves are 3/4" oak plywood with solid oak wood edge banding on both the front and back. The front edge banding extends a 1/4" below the shelf, i.e., it's 1" thick
- the cabinet sides are also 3/4" oak plywood
- the dado is 3/8" deep
- the shelves are 41" long
- no support on the back (hence the back edge banding), but I will screw the 1/4" cabinet back to the shelves
- I'm planning on using Titebond II because of it's longer set time
I realize the shelves exceed the recommended length according to the Sagulator, but they won't be holding a lot of heavy items.
I am not opposed to clamping the cabinet, but you're right about it being a bit complex. I'm thinking of gluing the cabinet together on it's side (Ed Pirnik has a video on this), turning it face down onto the face frame (but not gluing it to the face frame) to make sure everything is aligned and square, clamping it, then adding the screws. I'll glue and clamp the face frame the next day. I'm still pretty new to woodworking and am getting pretty comfortable with a lot of the processes. Glue-ups, though, give me nightmares.
If you're using screws you don't need to also clamp. Dry fit and pre drilling for screws is a good idea. Biscuits to attach face frames is fine.
I would put 1 1/2"-2" front edges on 41" shelves for stiffening. Plywood that length won't offer much support on its own.
If you need additional clamping, you can use twisted rope for clamping. Just wrap a length of rope around the cabinet near a shelf location (protect the cabinet edges) and tie it securely. Then place a stick between lengths running across the front and back and twist them together. This will squeeze the ends together clamping the shelves in place while you drill and insert the screws.
I use Bessey 3/4" pipe clamps. They are only $20 each. I originally got 4 with 30' of pipe cut up. 4 at 2'. 4 at 3' the rest at 4'. I also got 4 couplers. It gives you options of many lengths. I have upgraded to another set of pipe and 6 more Bessey clamps. When using long runs up to 8'n the do bow but they still hold great.
I agree with bilyo - for shelves that long I would add a piece of thicker wood at the front - or in a dado partway back to add some stiffness. Having said that, just screwing that cabinet back to the shelves will add a bit of stiffness... When you say "edge banding" that usually means about veneer thickness but since it extends below 1/4" that I am guessing it is thicker and would add stiffness as well.
Agree about pipe clamps being very economical, the other economical option are Harbor Freight 48" Bar Clamps - pretty light duty but would work for this purpose and sell for maybe $16 a piece.
The edge band in front is 3/4" wide by 1" thick and the one in the back is 3/4" by 3/4". I thought about using a thicker piece in front but let my aesthetic sense override my common sense. Hopefully, I won't regret it...
Recent shop tip:
https://www.finewoodworking.com/2023/03/20/hide-steel-tubes-in-thin-shelves-to-prevent-sagging
I don't think there's much in the way of evidence to support any perspective.
Some thoughts below but tl:dr - it probably doesn't matter if you clamp, screw or do neither.
Glue does not really need clamping so much as it needs the faces to be joined to be in very close proximity. Clamping is simply one way to achieve this. Screws are another. It can be achieved with tight fitting joints too but simple rub joints are also extremely strong and require vast amounts of force to pull apart.
Remember that testable mechanical strength does not necessarily equate to failure in practice. Joint failure has more to do with design and loading than the glue strength as really the glue has very little to do in a well-designed piece. It is there to keep things in place, not provide much in the way of mechanical strength in a carcass.
Take yours for instance - the glue is simply preventing the shelves from pulling out of the dadoes, and preventing the sides from bowing out. The amount of force applied to the joint is minuscule as most of the load-induced bowing is prevented by the sides being of reasonable thickness and the shelf is prevented from pulling out by the other supporting side - the main force the glue has to counter is the small force exerted if the shelf bows. Clamped or not, if the joint is well-fitting, it will last. Most of the value of clamping is closing that well-fitting joint accurately when it is also filled with glue, and I bet your cabinet will be quite sturdy at the dry-fit without any fixings or glue at all.
As to whether the screws add much to the strength of the joint, again it's hard to determine.
When the glue is holding, they add nothing as the glue is so much stronger, but given you can easily hold a cabinet together for a good while with only screws (Ikea I'm looking at you) then clearly they might add some additional resistance to failure in future. It is possible to envisage a circumstance in which the glue in a dado fails but screws keep things together preventing the problem spreading to other joints, but it's not all that likely to be an isolated problem. The sort of things that cause a glue joint to fail tend to be mechanical issues elsewhere - you can't break a polo (lifesaver) mint in only one place after all - the factors causing the failure of one joint almost always also cause failure of another, and such will usually render a piece unsteady even if screwed.
All that having been said, there are a great many antiques held together with nothing more than glue and the odd nail. This is of course subject to survivorship bias as we only tend to see the furniture that did not fall apart, but 'back in the day' screws were too expensive to use for joinery and nails were also pricey so restricted to where they were the best or only option to do a job.
The screws will pull the joint together w/o the need for clamps. The trick is holding everything in perfect registration to do your drilling, a perfect application for clamps.
Perhaps half the clamps you would need to actually glue it up will do the trick as you can unclamp areas as you add the screws.
To cherry pick from the above:
Screws are great, and you likely don't need clamps. But clamps are always handy, and you'll need them sooner or later. I've never seen confirmation screws used on anything but particle board or mdf. I don't know anything about them. I'd use regular screws, but that's me.
Screwing the back to the shelves is a very good thing.
I take it the face frame covers just the top, bottom, and sides, not the shelves?
41 inches is too long for plywood shelves. They'll sag even with a very light load, over time. Instead of 3/4 x 1" strips on the front, make it at least 3/4 x 1-1/2", preferably a little more. You'll be happy for that extra stiffness, along with the screws and glue into the cabinet back.
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Sadly, the shelves are already made, finish applied and the entire cabinet is ready for assembly, hence the questions about glue-up. No, the face frame does not cover the shelves - they are recessed 1/4" from the face frame around the sides of the cabinet.
I guess you're never too old to learn from your mistakes. One thought I had was to add a support piece under the back of the shelf. Still might not prevent sagging in the front though.
I mentioned that I'm new to woodworking. I've been slowly replacing all of the woodwork in my bungalow from paint grade to stained oak over the past few years - basically trim carpentry. The last part of this project was to build a surround for a zero clearance fireplace I had installed. Pic attached of the progress so far (sorry the pic is sideways). A cabinet will go on each side of the fireplace.
Finally, as for sagging over time, I'm 71 so it will be a race to see if they sag before I kick the bucket. :)
It would be very easy to cut and pre-finish 1-1 1/2" stiffeners and then screw them to the center (lenghtwise) of each shelf. I highly recommend it. Since your shelves have finish on them, it would be almost pointless to use glue. Just use 4-5 screws per shelf.
One trick that has worked for me for longer shelves, if the undersides aren't showing, is to rout a half inch groove for most of the length, drop in a piece of ready rod and fill it up with 2 part epoxy. When that sets up it is as stiff as you'll probably ever need it.
I really appreciate everyone's suggestions. I found a local supplier (Chicago area) of 1/4" square steel bars that I will have powder coated black (or I might just spray paint them myself). Those will be epoxied inside the 1/4" overhang of the edge banding in the front of the shelf. I will also attach a 3/4" thick by 1 or 1 1/2" high cleat to the underside of the back of the shelf (which was actually in my original design) - I can also screw into it from the back of the cabinet. Hopefully, those two remediations will be able to handle any sag without compromising the aesthetics of the shelves. Let me know what you think.
Edit: an alternative would be to get a 1/4" or 3/16" inch thick by half inch or so steel bar, route a groove in the middle of the shelf and epoxy in the bar. A bit less work overall.