Thread: joints for chairs

Chairs have to withstand considerable racking forces, so the joints need to be first class. Lap joints and dowels are certainly not in that category.

Mortice and tennons are not hard to master. I would recommend you get a mortice chisel the width you require (usually one third the width of the timber you are morticing) and make a few practice joints.

What WW said. The latest Australian Wood Review has a good article by Evan Dunstone on the forces affecting chairs, and the joints needed.

Just to reinforce what WW said - lap joints depend entirely on glue to hold them together - a M&T at least has some mechanical support when the glue fails, which is an important consideration for something like a chair. Sudden failure of a lap joint is likely to have rather unpleasant consequences, whereas M&T joint will usually give you a bit of warning that something is amiss when you sit down, allowing you to transfer your butt in time, to something more comfortable than the floor!

Dowels have some of the properties of M&T, just not as much - less glue surface, so quicker to fail under stress, & when the glue does fail, less material bearing un-planned loads - an awful lot of busted dowels have come past me over the years.

It's worth remembering that any glue will almost certainly fail at some time - not in your lifetime, maybe (if you are very lucky & work very meticulously), but inevitably, at some point in an item's life, & the more stress on the joint, the sooner it will be. I read a lot of brave words on these forums about how wonderful "modern" glues are, but don't take them too seriously. There are a lot of factors that decide how long a joint lasts, and 'modern' glues are just as prone to failure as less modern glues, at the hands of most w'workers.

(Says I, wondering what the lifespan of the laminated beams holding up my house is supposed to be.... )

Is it better or worse to have a through tenon?

Originally Posted by spongebrain

Are there any differences in strength between an open and a closed mortis?

Much depends! If glueing surfaces are of equal size, & both well-made, theoretically they ought to be the same. OTH, an open mortise can be flared & wedged, needing no glue (of course a closed mortise can be fox-wedged or pinned & need no glue too, but wedging a through-tenon is easier done). Then again, an open mortise implies you have chopped away more material, & so weakened that member more (which may be important, or may not matter at all).

Every woodworking joint is a series of compromises, & you have to decide which ones you can live best with - both types of M&T have their places...
Cheers,

Is it a good idea to wedge and glue?

Originally Posted by spongebrain

Is it a good idea to wedge and glue?

If you're the belts AND braces type it will give you added peace of mind....

Thanks for the help.

All the information above is good and accurate, however, I would think long and hard before committing to any particular form of M & T joint. I don't think I've ever seen a through mortice in a chair - they're usually reserved for heavy constructions such as in traditional timber frame dwellings and work benches etc.

Dry (as in no glue) M & T joints were employed by early joyners because glue wasn't on the table back then. They did use draw pegs though in place of glue. For the majority of chairs, a simple M & T will suffice without any additional wedges or pegs.

I would encourage you to investigate and invest in animal glue if you're going to be making chairs. Animal glue has proven the most suitable glue over hundreds of years. No modern glue can match its qualities - plus it's reversible (with heat/hot water) so chairs assembled with animal glue can be easily repaired rather than disposed of.

Originally Posted by spongebrain

Is it difficult to hand cut precise mortises? If I cut sloppy mortises, will hide glue still do the job?

Well, HG is a bit better gap-filler than many, but it is not very good practice to rely on thick glue-lines. Rule of thumb - the thinner the glue-line (all else being equal & provided it is continuous) the stronger the joint. Epoxies are usually quoted as the best gap-fillers, but there are lots of different epoxies.
Also, as WW says, think of the future - chairs are THE pieces of furniture that are most likely to need repair at some stage(s) of their lives. Getting things apart to do a proper job after good 'ol uncle Charlie has bogged those loose joints up real good can be true challenge. So if you are making something fancy that you expect to last a while, think about the bloke who might have to fix something - it might be YOU!

Chopping good mortises isn't dead easy - like most things it requires practice to do well. In fact, many think cutting dovetails is the acme of good joinery, but I would say it's a bit more difficult to make a really good M&T joint. The main difference is that the mistakes leer at you on dovetails, whereas the really important part is mostly hidden on M&T joints (at least until the joint fails). However, it's worth persevering, IMO, until you can do a reasonable job. (Then you can go & get a machine to do the job & you'll know how to get the best out of it. )

It's not as bad as I may be painting it - most people can do a pretty convincing job after a few tries. Besides being the most economical approach for a beginner, it's a much more pleasant & neighbourhood-friendly way to go . Drilling out most of the waste with a drill that is slightly narrower than the width of the mortise (Forstner bits are really useful in this application) not only reduces the effort, it can help keep the walls perpendicular & the depth even. Purists will tell you not to pre-drill, but I think it's easier & less discouraging to start that way, & gradually move into chisel-chopping only, after you gain confidence & skill. Don't expect perfect walls - the nature of hand-cutting means a few imperfections. With a decently-sized mortice, having about 80% close wood to wood contact on the cheeks is a good aiming-point. It's most important to keep everything square & the shoulders meeting neatly. So you see, there are actually more things to worry about for a good mortise than on through-dovetails.....!

Cheers,

IW and WW. What is your opinions of the loose tenon joints, by that I mean where you just make the mortise in both pieces to be joined and insert and glue the loose tenon into the mortises. Are they as strong as the conventional method of M&T joints or not.
It appear to be much easier to achieve a good joint using the loose tenon method. Just thought I would like to get an experts view on them.

I've never used loose tennons, but to my thinking, if I were to use them, I'd glue the tennon into the rail with epoxy or similar and treat it as if it were a normal tennoned rail and glue it into the leg with animal glue.

I just don't like the idea of a joint that can fail in two places!

Originally Posted by munruben

IW and WW. What is your opinions of the loose tenon joints, by that I mean where you just make the mortise in both pieces to be joined and insert and glue the loose tenon into the mortises. Are they as strong as the conventional method of M&T joints or not.
It appear to be much easier to achieve a good joint using the loose tenon method. Just thought I would like to get an experts view on them.

Munruben - I ain't no expert! Just an enthusiastic woodie who has been around for a few years.

I have rarely used 'slip tenons' except to repair occasional broken tenons (I would normally remake the part, unless it's a reasonably old & significant piece where I think everything possible should be preserved). But according to the protagonists, they are as good as the 'real thing', and so long as the glueing surface is the same (& the same in both joined members) there is no reason why they shouldn't be. If I had the gear, I would use them on 'modern' stuff, & I do use (router) biscuits when making frames for end-panels in kitchens or similar things that are not going to suffer any stress. However, I have a distinct dislike of screaming routers, & avoid them as much as possible (I own three of the damn things, too, but that's a long story!) so when I do use slips, I have to drill & chisel out the mortises. This is not such a bad thing - I have read that mortises made with dull router bits are baaad - the fibres get pounded & squashed & don't 'wet' with the glue properly, making a poor glue-joint, even though you may have a lovely clean fit.

I'm sure WW will champion 'proper' M&Ts in any furniture worthy of the name, & I prefer them too, out of sheer conservative nature & prejudice

Cheers,
Edit: There, WW went & beat me to it! So you would entertain loose tenons too, WW (in the 'right' circumstances)? I'm not sure there would be such an advantage in using epoxy for the rail part - I'm yet to be convinced tha it is going to be around as long as HG, and it is just as prone to failure as any other glue - the mere fact it needs mixing gives room for errors & less-than ideal quality. May as well use the same glue each side, IMO, and retain the removeability factor. Neither glue is weaker than the wood, when properly applied, after all........

I’ve made around 160 chairs in the last 3 years using loose tenons and haven’t had a complaint yet, including some demo chairs in the workshop that have received some terrible punishment.