Questions Answered – Big Tenons Reduced When?

About Tenons

Paul, I noticed during your bed-build that one of the tenons used had a slant on one side. When does a tenon need to be more than a rectangle in appearance? When is a haunch tenon the best use of a tenon? Lastly when a wide board is used and a tendon required, Why is the tendon sometimes cut in two or three tenons instead of one long tenon? I think I need some schooling on when to use the various types of tenons.

Thank you

Boyce

Q 1) When does a tenon need to be more than a rectangle in appearance?

A 1) I wasn’t sure what the question was in this part. This might help, I don’t know. It depends somewhat on the tenon type and the application. In furniture and general joinery we employ double tenons and twin tenons. Double tenons are paired inline, one above the other, twin tenons are generally paired side by side. There is much information that confuses this; some woodworkers for instance take a twin tenon and describe it as a double tenon and double tenons as twin tenons. This may not seem to matter, but it makes a big difference if you ask someone to use one or the other.

DSC_0038 In door construction we generally rely on the double tenon to minimise the amount of stock we remove from the mortise hole. This of course was when we made doors with wide bottom rails such as shop, and business entryway doors and such. Mansions too had massive door rails. Since the advent of metal doors these doors have disappeared and so too the methods. Taking too much out to accomodate a full-width tenon weakens the door stile at this critical juncture, which is usually at the bottom and middle rail intersection of doors; removing the whole of the mortise means there is no integrity uniting the thinner inside and outside sections of the mortise. By leaving a section between the double tenons, we maintain integrity and strength.

DSC_0038 - Version 2Twin tenons on the other hand are side by side and can be large or small. We use them on thick doors, say 2 1/2” to 3 1/2” thick. This adds great strength to the door. We also use it for cross rails for wider material where a rail intersects a leg of a table or desk for instance.

When we use wide tenons and do not want to create a double tenon, we can reduce the width by adding what we call a secret haunch.This is the “slant” referred to above. The secret haunch, shown bottom, restrains the extended aspect of the rail and prevents any curl from occurring. Any shrinkage that takes place over the narrower tenon will be in the area of the sloping secret haunch and will not be so visible if shrinkage takes place.

 Sizing makes the determining factor. We rarely use such large tenons but when we do we have options. But don’t consider smaller tenons to be exempt from a double tenon feature. We did this in woodworkingmasterclasses here and it was much extra work that was worthwhile and will always be worthwhile. Joints are often hidden from view, concealed within surrounding wood, but they are not hidden to us even after they are inside. I think it is a most enjoyable thing to make any woodworking joint provided the whole process is completed by hand.

Q 2) When is the best use of a haunch?

A 2) The best use of a haunch is when it’s necessary. I have a written a treatise on Mortise and Tenons for Today’s Woodworker as part of a new book I am working on and the use of haunched M&T joints is covered there. We use haunches at the 90-degree corner to a frame. This allows the total enclosure of a tenon while constraining the full width of the tenon along the intersecting shoulder line of the tenon. Table construction relies on this joint in particular and so too workbenches and worktables. I think though that one of the most apt uses of the haunch is in door construction where a groove is used around the inside rim to house panels of different types. The groove for the panels necessitates a haunch so its a sort of symbiosis I like, if that’s not too far a stretch of the meaning of the word.

The Secret Haunch

Drawings from My Woodworking Journal by Paul Sellers

DSC_0038 - Version 3

 

11 comments on “Questions Answered – Big Tenons Reduced When?

  1. How do you adjust the length when using double tenons? Is there a ratio you try to keep in mind for length and thickness?

    • I have heard different maxims over the years and scale is important. In general, for joinery and furniture making, there is no need to increase tenon thickness more than 3/4″. Most exterior doors are never more than 1 3/4″ thick unless they are bigger than the standard max of 3′ wide and standard max of 7′ tall. A 1/2 to 5/8″ is standard tenon thickness as this leaves plenty of ‘meat’ on the inside and outside of the mortise hole. Top and bottom rails are always different with the top rail being often about half the width of the bottom rail and often more. The reason for this is you want the strength at the bottom to take the weight of what’s loaded on it from the top. Stiles are always narrow because you want the minimum amount of wood that can expand or contract because the door must fit within 3mm all round between the door and the jamb. If it’s more then it’s prone to draughts (pre-rubberised draught excluders). If it’s less the door will swell in seasons and stick.
      So, in general a single tenon is used when the rail overall width is 6″ or less. That’s because the tenon, allowing for a 1/2″ panel groove and a 1″ haunch will be narrow enough at 4 1/2″ not to be affected by shrinkage and expansion. When you move past the 6″ rail we consider increasing the length of the haunch so if you have an 8″ bottom rail, you can make the haunch 1 1/2″ long so that with the 1/2″ groove your tenon is then 6″. You can make the haunch up to 2″ and still have enough support and strength to keep the shoulder tight against the rail. In general, a double tenon becomes more practical at 10″ and up. Regardless of the thickness of the tenon. Proportionally, the tenon width increases the strength of the tenon so wider is better, but the problem of weakness is then transferred to the door stile if you remove the whole tenon width or make the tenon too wide with no retaining grain between the inner and outer walls of the mortise and that’s why we use stop gaps intermittently to create double or triple inline tenons. A 10″ wide bottom rail is quite common as a single tenon with a wide haunch of 2″. I am not saying this is the best. It’s quicker and more practical for mass makers though. I like my tenons to be a minimum of 2″ wide, better to be 3 but it depends on that rail width. So on an 10″ rail I use a 1 1/2″ haunch, a 1/2″ panel groove all of which leaves me with 8″ of wood. This I divide into two 3″ tenons and 2″ gap. As the rail increases to 12″ I add extra to the gap and the tenons. 1″ to the gap and 1/2″ to each tenon.
      As I think I said in my previous blog, we don’t use wide rails so much these days so these answers are becoming increasingly redundant, but, hopefully, for the few that are made, this information will be out there forever somewhere.

      • Thanks Paul for the additional details. How does it work with thicker material? Do you maintain your maxim of 3/4 inch think tenons? I am making a bunk beds with a stair case. The uprights are 2 3/4 in square and the cross rails are 2 1/4 inch thick and about 4 inches wide. I had previously used a 3/4 inch tenon but your write up got me thinking. Would a twin tenon be better this second time around?

  2. This is what I learnt at college doing carpentry and joinery city and guilds.
    Quite simple rules to follow.
    A tenon should be 1/3 thickness of timber. Remember in carpentry you may be jointing bigger timbers together so good rule to follow. So a 4″x 4″ would have about a 1 1/8″ thick tenon.
    A tenon should be no greater than 5 x thickness of timber. When greater than 5 x’s thickness a haunched tenon used.
    In softwood a haunch should be 1/2 width of tenon and in hardwood haunch should be 1/3 of tenon or 2/5 haunch and 3/5 tenon for either. I normally stick to a 1/3 on most things I make in either types of wood.
    Most external doors 1 3/4″ so 1/2″ > 5/8″ perfectly acceptable width of tenon.
    As for double tenons Paul has given great explanation above 😉 like Paul says bigger rails not so common now.
    So for furniture double tenon would be quite rare but you never know. 😉
    Doors have 8″ and even 6″ bottom rails are common for cheaper construction doors but even then most do not have mortise and tenons any more use of dowels on mass production . ;(

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