In great pieces of woodworking you usually don’t find much in terms of parts that are screwed or nailed together.
Technical Reasons
Of course in some cases screws are unavoidable, for example if you want to be able to take an assembled piece of furniture apart again, for easier transportation. Or they might be used to affix hardware but rarely to permanently hold two wooden pieces together.
The reason why glue joints are preferable is quite simple: Wood moves.
Wood is comprised of tiny fibers, the wood grain. As temperature and — even more so — the moisture content of a piece of wood changes, it will deform. However, this deformation is not uniform. Wood expansion/contraction along the grain is basically non-existent. It expands and contracts significantly across the grain. By how much depends on many factors, but you could see, say, a 0.5% change of width over time. This doesn’t sound much, but keep in mind that 0.5% of 50″ is already a quarter inch! (The factors that influence wood movement probably warrant another post at some time…)
If you drive a screw into a wooden board, it will tap into the board and initially have a good grip on the wood. But then over time the wood will expand and contract, expand and contract, and your metal screw will not. Which means that over time every screw in a piece of wood will become lose and your fine piece of furniture will get wobbly…
Aesthetic Reasons
I don’t think I need to elaborate much on this. Well made dovetail joints on wooden furniture look awesome. Screw heads generally not so much…
Glue vs. Screws in MDF and Plywood
MDF and plywood don’t suffer from the anisotropic expansion/contraction I just described. In fact, that’s the very reason why plywood consists of thin layers of wood glued together such that the grain in each layer is perpendicular to its neighbors.
And MDF is barely wood at all. I would describe it as saw dust mixed with epoxy. (There are other reasons why screws in MDF might be problematic, tho…)
Also some people might tell you that if you are mostly working with MDF and plywood you are not really woodworking…
Let’s Make Some Glue Joints
A Word on Glue Joint Strength
Many factors can influence the strength of a glue joint and this is probably a topic for another post. For now, let’s just say that a well made glue joint is very strong. In fact, it is common for the glue joint to be stronger than the adhesion between the wood fibers in your work piece. While this sounds awesome it will come to haunt you if you ever have to separate two pieces of wood that are well glued together after the glue has fully set: You are more likely to split the wood to the left and the right of the joint and thereby ruin your work piece rather than split the joint itself.
The take-away from that is that we want to avoid ending up in a situation where we have to pry apart a fully set glue joint. And here is how we can avoid this:
Dry Fit
Before you ever apply glue to your work piece you want to do what is called a Dry Fit: This is putting the pieces together the way they are supposed to go together, but without applying any glue. A dry-fit allows you to
- Make sure all the pieces actually fit together the way they are intended to fit together: Did I make all the pieces correctly or is there one that I need to scrap and re-do? Do all the pieces make good contact or is there some sanding required somewhere?
- Confirm which surfaces you actually have to apply glue to. — There is no shame in marking those surfaces during the Dry Fit.
- Check that you can actually do all the glueing in one session. — Do I have enough clamps of the right size so that I can apply pressure to all the joints as needed? (No shame in actually trying to apply the clamps here.) Are all the glue joints clampable at once based on their location? — You might at this point decide to only make some joints in one session, and the rest in another, once the first set of joints has fully set.
- Get a feel of the complexity of your glue session. — Make sure you don’t bite off more than you can chew:
Know Your Glue
Open Time
Now that everything looks good in your Dry-Fit you are ready to take things apart again, apply glue to the joints and put the pieces together again with the glue in place and apply the clamps. — Well, congratulations you have just started a race against the clock.
One characteristic of glue is what it is called its “Open Time”. It tells you how long you have from squeezing the glue out of the bottle until the pieces you want to glue together have to be glued up, properly aligned and clamped. The Open Time varies from glue to glue and it is usually listed somewhere on the glue bottle’s label or available on the manufacturer’s web site, but typically we are talking maybe ten minutes here.
If your pieces are not in place before the Open Time is up, your glue joint might be severely weakened because no good bond will form.
That’s one of the reasons why the Dry Fit is important: If it takes you anywhere near ten minutes to assemble all the pieces even without the glue and the clamping, you probably ain’t gonna make it. In that case you probably want to postpone some glue joints you wanted to make until later, for a separate session.
Set Time
Besides the Open Time, Set Time is another thing you need to know about your glue. Set Time is how long it takes for your glued up and clamped pieces to bond together. This doesn’t mean that the joint has reached full strength, only that a bond has formed that is strong enough so that your pieces won’t come apart anymore easily.
For all practical purposes, I would consider the Set Time the bare minimum amount of time for which your joints need to be clamped. Longer is of course better.
The set time will depend on many things, like the type of glue, temperature, humidity, phase of the moon etc… Again a figure should be listed on the back of your bottle or available on the manufacturer’s web site. For a typical value I would say “well, maybe about 45 minutes”. — I usually leave my pieces clamped together much longer than this. Maybe a couple of hours. Clamp time could be too short, but it can never be too long. Unfortunately waiting for joints to set might mean that you have to stop working on a piece you were eager to finish. But that’s just woodworking teaching you patience…
To reach full strength a glued joint will take about 24 hours, but if you put large gobs of glue in a gap it might take longer.
Expanding vs. Non-Expanding Glue
Depending on how your glue is formulated, it might expand while it sets. Polyurethane based glues are the ones that tend to expand.
This has both advantages and disadvantages: Expanding glue will fill any voids in your joint if the fit is not perfect, thereby maximizing the contact surface in your joint, giving you a stronger bond.
On the other hand expanding wood glue will continue to squeeze out of a joint even after you have applied the clamps until the Set Time is over. So you cannot just apply the clamps and walk away. You have to baby-sit your work piece and keep wiping off excess glue (which is best done with a moist rag BTW) until the Set Time is over. If you don’t do this, you could be in for some sanding later (and glue tends to gum up sand paper if it is not completely cured). Also, if your piece is not securely clamped, the expanding glue might move the pieces which affects your fit. Another reason for babysitting…