Geoff's Woodwork

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Mortise and Tenon Joints

These are the most common of all conventional framing joints.  M& T’s usually are haunched for the following reasons:

1.    They reduce the width of the mortise and tenon to maintain the strength and integrity of the material.   A wide  tenon is more likely to cup, warp and consequently, cause ‘winding’.

2.    They prevent corner joints 'breaking-out' and becoming 'bridled' .

When setting out the joints always mark the original shoulder line and the reduced widths, etc. of the joint.   The shoulder lines serve to line up the frame.  Mortises are  ‘set’ or ‘cut-back’ to take into account the reduction of the width of the tenon by grooving or rebating.   This set-back is usually on the inside of the frame on the opposite side of the haunch.

The proportions of the tenon should be the 'thirds rule'.  The tenon should be two thirds with the haunch being about one third.  The thickness you will remember should be approximately one third of the thickness of the material adjusted to the nearest chisel size whether machine or hand.   The 'tongue' or depth of the haunch should be about the same as the thickness of the tenon but should not be less that 10 mm.   This tongue should either fit into the groove formed or step into the rebate.  

The size of the haunch should be, particularly in cabinet jointing be 1/3 rd of the rail width.  So the proportions should be 1/3 rd thick, and 1/3rd haunch and 2/3rd tenon of the rail width.  Notwithstanding the provision of a haunch or cut-back, it is usual to reduce the tenon on each side by about 1 to 1.5  mm's.  This tidies up the joint and will hide any 'blemishes'  inadvertently made in the construction.  In the case of mortises being made of wider or thicker stock (such as a rail to a leg) , the larger member should rule but nicely proportioned so that no member is too strong or weak.

Historical Notes:   Traditionally in wood trades and mostly joinery, M & T’s were set out as in the following:

  • Thickness of tenon = 1/3 of  (t) thickness (to nearest chisel width)

  • Width (w)  or depth (d)  of tenon = less than 5 x (t) thickness (and no more than 125 mm)

  • Depth of haunch tongue = same as thickness but,

    • no less than 10 mm,

    • corner tenons on domestic doors are set in 36 mm top and bottom of door.  

In Cabinetry:    The actual amounts of haunching in softwood was 1/3 haunch to 2/3 tenon. Hardwoods were usually 2/5 to 3/5ths.

In Construction Joinery:         

Because of problems encountered with short-grain in softwoods, etc. in modern times (especially in construction joinery) the proportions are often changed to half haunch to half tenon.   Remember in the case of double tenons, the width of the tenon is  the sum of both tongues.   On doors and other large joinery,  tenons should be no wider than 125 mm.   All widths  quoted include all the tenon thus on  double tenon each being 25 mm, the width is effectively 50 mm.

In conclusion:  When you decide on the amount to haunch you should be left with a mortise and tenon that is well balanced and nicely proportioned and fit for the job.

If the joint is to be secured with wedges the mortise should be shaped only and formed to a wedge shape at an angle of approx. in the ratio of  1:10 but no steeper than 1 : 7 .  A saw cut may be cut in the tenon to receive the wedge to prevent splitting - especially in 'fox-wedging'.    The wedge bears to no more that 75 % of the depth of the mortise from the outside edge and is marked and cut prior to final assembly.  Wedges should not be included in the marking out because it would  cause too many lines and confuse.   Wedges are inserted only when the job is cramped up flat and square.   The wedges are then driven in with a hammer from the external corners first, working inwards taking care to balance the pressure and maintain the ‘squareness’ of the frame. Glue should be offered to the shoulder area only plus the tips of the wedges dipped into the glue pot.   Fox wedging is the process of using wedges inside a stub mortise that expands  carefully prepared sawcuts.   The length and thickness of the wedges should be no more to expand the tenon.   Failure to get the proportions right will prevent the joint closing or prove ineffective. 

The terms double and twin tenon are often confused.  A double tenon is as shown above whereas a twin tenon has two tows of  tenons and are used on say the middle rail of a door to allow a lock to be fitted without destroying the joint.   You may well find that a number of text books get mixed up from time to time and this author is no exception.

An alternative to haunching is franking for slender glazing bars and this will be discussed in another paper.

text and graphics G.Malthouse ~ all rights reserved ~  15th December 2001, 27th December 2003, 2nd January 2004

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2nd January 2004