How wide should a mortise be?

Re: How wide should a mortise be?

Dean J in MN

>Let's just say my 1/4 and 5/16 mortise chisels see a lot more use than my 3/8ths.

-Dean

Re: How wide should a mortise be?

Ed Mulligan, Cape Cod

>Roy -

The finished M & T should have a strong tenon and strong mortise cheeks. A 3/8" tenon would be quite sturdy but the mortise cheeks would be only 3/16" - somewhat flimsy - so not the best compromise. 1/4" or 5/16" tenon is the time-tested size for a 3/4" board. Mortise width might be adjusted depending on other considerations such as other mortises nearby. Joyce (who else?) has a good overview.

Ed

Re: How wide should a mortise be?

Wiley Horne

>Roy,

If it's actual 3/4" thick wood, I like 1/4" mortises over 3/8" as recommended by Chris, and for the reasons he gives. I also like 1/4" because most of the time a panel is involved, and often a groove which should match the mortise in width. In the examples I've done so far, I'd rather chop the mortises and plough the grooves 1/4" wide than 5/16"--just not as much excavation. I find 1/4" tenons plenty strong, but do worry about mortise wall thickness.

For full 13/16" material, I'd go 5/16" M&T. And if a full 7/8", 3/8" M&T.

Wiley

Re: How wide should a mortise be?

Pam Niedermayer - Austin, TX

>Whatever size chisel is my closest to 6mm or larger, which is a 7.5mm mortiser, which is a little smaller than 5/16". The rule I've always heard is 1/3 the item width with the advantage given to the chisel.

Pam

About 1/3 thickness (the rule of thirds)

Andrew F in Australia

>We were trained at exactly 1/3 of thickness, cut and pare to suit.

Others on the board, who were UK trained were to go the next increment up from 1/3, so they'd be cutting about an 8mm mortice in a 19mm thick board (8mm is about 5/16", 19mm is 3/4")

Basically, the rule of thirds is that you remove either 1/3 or 2/3 of the stock thickness, depending upon the joint, unless the name of the joint includes the word "half." Works for me.

Cheers,

Andrew

That should be "thickness," not "width." Sorry.

Pam Niedermayer - Austin, TX

>

Re: How wide should a mortise be?

Sgian Dubh

>Roy, I haven't seen the article you refer to, but I have seen quite a few attempted justifications for working exactly to the third rule when setting out mortise and tenons. I've never yet agreed with any of the arguments I've seen, heard or read where it's suggested that a slightly fatter tenon than the 1/3rd thickness calculated makes the cheeks of the mortice too weak. None of the justifications along those particular lines have ever made any logical sense to me.

A skilled worker mortising by hand just wouldn't break out the mortise cheeks if the mortise takes up 5/16" of the width of a 3/4" piece of wood? This still leaves cheeks 7/32" thick-- plenty of wood for support as the mortise is bashed out. A worker that has a tendency to break out cheeks this thick whilst they're working the mortice probably ought to have a look at their methods to see what they can do to prevent this happening.

A 3/8" mortice in a 3/4" thick part I'd agree is getting a little on the large side as this only leaves 3/16" of wood in each mortise cheek. I don't see any difficulty executing the joint, by hand or by machine. But I have seen the results of such errors where the too slim cheek or cheeks break out if there is significant twisting of the frame in use, e.g., tables dragged across floors, chairs put under undue strain, and doors that are perhaps slammed shut in error, or get hit somehow near the corner of the tenoned part.

All M&T�s follow the same basic principles. More specifically, where the members are the same thickness, as in a typical door, thickness is divided into three equal parts, and the mortice is usually chopped out with the nearest larger chisel. This retains a little under 2/3 of the original material thickness. For example in a 25 mm (1�) thick stile dividing into 3 at 8.33 mm (3 X @21/64�) the mortise is cut with a 10 mm (3/8�) chisel leaving 7.5mm (5/16�) for each cheek.

Two factors go to make this proportion right. 1) A 10mm (3/8�) chisel (or router bit, hollow chisel mortiser, chain mortiser, drill bit) is best suited to cut a 10mm (3/8�) wide mortise. The normal procedure is to fashion the mortice, followed by the tenon, which can readily be adjusted to any thickness and width. 2) This ensures that a little over 1/3 of the wood remains in the tenoned member. It is the weaker partner, but bulking up the tenon this way, brings the volume of timber removed from each part closer. (Half removed from each would be the ideal balance.)

Similarly, on the outer corner of the frame the width of the rail is divided into 1/3rds., and the outer 1/3� of the tenon is cut away to form a seating. This strengthens the end of the mortice reducing the chance of end grain break out at the top of the stile. Usually a small shoulder, about 3mm, (1/8�) is cut at the other end of the tenon to hide any inentional looseness of fit in the mortise length (to allow for expansion and contraction of the tenon.) Slainte.

Nice summary, Richard

Ellis Walentine

>I have always used 5/16" chisels and end mills to put frame mortises in 3/4" stock. I don't think that 1/4" is necessarily too thin, but it makes the tenon the weakest part of the puzzle, and the shoulder provides a rather good fulcrum for unexpected leverage on the rail. But there are other factors that come into play, too. One is the wood species. A very stiff and/or tough wood will be more forgiving of the engineering of the joint. Another consideration is the thickness of the parts involved. If the mortise piece is thicker, as in a table leg or a thicker breadboard end, the tenon can be thicker too.

With time and practice (and with the benefit of discussions like this one) the proportions of joinery become more-or-less intuitive and needn't be struggled with.

Ellis

Thanks Richard...

John in New Mexico

>Complete, informative, and sensible as always.

Cutting by hand I favor 5/16. Mortise sides may be thinner, but don't forget that there's the other side of the mortise also resisting torsion of the tenon (effectively making it thicker than it appears). I've yet to have one fail, even when using 3/8 mortises in 13/16 stock (hollow chisel cut).

I did once have a 3/8" by 1" (wide) tenon break in tension due to poor stock selection and weakish wood.

John

Re: How wide should a mortise be?

ThomD

>Rule of 1/3rds seems to be the trad measure. I can remember reading articles about going to the half tenon thickness somewhere or other, as though it was a novel ideal. For something like a frame and panel door one could just eyeball the proportion for what looks best.

Once this kind of analysis gets beyound a certain point it probably needs to be considered for loads. Like the first M&T joints I made where for a canoe seat back in the late 70s. That piece looked a bit like a small ladder with two rungs. When one sits on it there are a number of loads developed the most novel of which is the pressure downwards that puts the underside of the rails under tension and the top of the rails under compression. There is also the shearing force on the tenons of the "rungs". So perhaps one could make a case for some thicker mortice walls on the tension side and a thickish tenon.

Other examples would be where a post is inserted into a sill. There the compression capabiliy of the endgrain on the post is far greater than the receiving side grain on the sill, and these days there is often some panel taking a the shear forces. That kind of thing is beyond me but I do like to give it a little think on stuff like benches that are mostly structural.

Just be carefull about...

Scott Burr in Ben Lomond, CA

>Where the top of the mortice is placed on the pieces your jointing together. As stated the 1/3 rule is the way to go IMHO. What you don't want to do is blow out the top (closest to the end of the board) of the mortice.

By the way, how do you like the mortice chisel?

Re: How wide should a mortise be? 1/4"

wilbur

>"How wide do you think a mortise should be in a 3/4" board? and why?"

Absolutely, positively, without a doubt, the answer is 1/4".

Because the only mortise chisel I have is 1/4" wide. :@)