Quantcast

Mortise and Tenon Joints

UKworkshop.co.uk

Help Support UKworkshop.co.uk:

Mike B

Established Member
Joined
21 Mar 2005
Messages
116
Reaction score
0
Location
Brymincham
Hi

I am planning on joining some oak rails (140 x 30mm) to some oak posts (55 x 60mm) to form a kind of bed frame arrangement, using mortise and tenon joints. I have been trying to do some research but as usual most of the descriptions appear to be a “what I did” and do not contain the “whys” or dimensioning decisions behind the designs. They do however raise wood movement concerns when using rails of this size.

From what I can gather there are 2 approaches for joints of this size :

1) a double tenon with a haunch in between, glue applied to only the tenons
2) a larger single tenon with a haunch either side, glue applied to tenon only

I am tending toward the first method as it appears to be stronger but am having to guess as to the best dimensions – I was thinking along the lines of 40mm wide tenons, 15mm clear at the top, and a 40mm central haunch. For tenon thickness I was thinking of either 16 or even 18mm
even though it breaks the “thirds rule” as there will be plenty of mortise wall either side. For tenon length, I was thinking of 25mm with 10mm for the haunch.

Should I allow for wood movement and if so how?? I could make the mortise slightly wider than the tenon but this seems daft if the tenon is glued…

Also, if the joint is to be held together by a central bolt instead of glued so it can be disassembled should I adjust any of the sizes?? E.g. make the tenon length shorter??

As I said these are my best “guesstimates” based on what looks right to my eyes but any advice on a more scientific approach to deciding tenon lengths, widths etc for max strength would be most welcome, as would any recommendations of a good text that describes the “whys” as well as “how to”??

Thanks
Mike
 

Chris Knight

Established Member
Joined
14 Jan 2004
Messages
6,641
Reaction score
1
Location
SE London - NW Kent
Mike,

Your proposed dimensions for option 1 sound fine although I personally would go for a somewhat longer tenon -say 38mm (one and a half inches) . The "thirds" rule isn't really a rule and is usually broken anyway - especially when people use 3/4 inch thick wood and take fright at the skinny appearance of a quarter inch tenon.

The size of tenons you propose will not move unduly and so wood movement should not be a problem.

I don't think anything needs adjusting for a situation where you use a bolt for a knockdown fitting, except as you indicate, you could shorten the tenons somewhat - I wouldn't go shorter than an inch however.

Do not make the mortise wider than a tenon - you need a nice fit so that it can be glued properly and to avoid the joint twisting. A close push fit is what you need to aim for - not one needing a hammer - the application of glue will cause immediate swelling and if too tight it is very hard to get the joint together.
 

tim

Established Member
Joined
5 Nov 2004
Messages
2,307
Reaction score
0
Location
Herefordshire
Mike

Whenever I have made beds I tend to make the tenon as large as possible assuming that the bed posts are sufficiently large.

My reasoning for this (and I am prepared for the fact that since I'm self taught it may fly in the face of teaching) is that since there is no glue and the bed bolt is doing all the work, the timber in both the mortice and tenon are free to move.

The tenon is a snug fit in the mortice (as Chris rightly suggests) and the bolt draws the tenon tight into the mortice. I'm not sure which areas you are planning to glue - I'm assuming just the M and Ts joining the legs to the headboard and footboard? Don't use glue on the side rails unless you never intend to move the bed again!!

Cheers

Tim
 
Top