Depth of mortise?

[Ray Turney]

Hi, can somebody give me a rule of thumb on the depth of a mortise. I'm building a bed for my grandson out of oak and am going to mortise the legs for the head and footboard. I am using 3/4 stock. Thanks

[harry strasil]

tenon length + 1/8 inch

[harry strasil]

I bet what you really wanted was the tenon length!

[Paul Ryan]

Ray, my rule is half the thickness. Like I do with rabits and dados. Now that is adjustable. If I am putting a mortise into a 3" thick leg I dont make a 1 1/2" deep tendon/mortise. But when I use 3/4 stock the mortise is 1/2" deep for a 3/8 tendon. With of the tendon is half the thickness and tendon depth is 1/8 deeper than the tendon. I made a mission style bed this summer, the tendons for the slats were 3/4 deep by 3/8 wide. The slats were 3/4 stock going into 5/4 stock. I feel a 3/4" deep tendon with today's glues is plenty strong.

[Frank Drew]

Longer tenon = better, IMO; more mechanical strength plus more glue surface. For example, if the job was a table with rails mortised into legs, I'd make the tenons as long as possible without fouling the tenon coming in from around the corner, and might miter the ends of the tenons to give me just a bit more gluing surface.

Just in terms of strength, I don't think you can make a door tenon too long; a wedged through tenon is the ideal joint for doors. Again, IMO.

[Neal Clayton]

yeah, for doors i do 2/3 the stile width. maybe 1/2 on a door with many rails (more than 4).

[Don Wurscher]

If your Grandson is like most, I would seriously consider pining the mortises.
The pins would greatly add to the strength of the joints, as well as add some
style if contrasting with the rest of the wood.

I pinned mine when I built a Captain's Bed for my Grandson, and the joints are still sound after all the jumping etc. that they are not suppose to do on a bed.

Use will only get rougher when they are teenagers and have their buddies
over. Of course roughhousing is not allowed;)

[Mike Cutler]

Rule of thumb for tenons. 3-5-3

The tenon is 1/3 the thickness of the stock. The tenon width is 5 times the thickness, and the length is 2/3 the width if the mortised member.

This is a rule of thumb only, so it's meant to be broken, or whacked with a hammer.:D

[John Thompson]

You didn't mention how deep you can go into the foot-board or head-board Ray. You only mentioned you were using 3/4" oak stock. It that just the rails or is 3/4" the thickness you are morticing into on the HB and FB also?

I think 3/4" oak will be fine for a young child if.. if.. you have laminated or thicker stock to tie into on the HB and FB. If so.. I would go as deep as I could with the mortise and accomodate the tenon accordingly as Frank Drew mentioned. That gives you more glue surface and pinning the tenon would also help in the case of a bed for a child. They do have a tendency to plop.. tussle.. wrestle with friend if we are talking beyond 4-5 years old.. etc. That relates to forces that cause movement to and fro and diagonal force that can rack a joint and in this case the joint is the only thing holding the rails.. HB.. FB together.

If this were a "big-boy-big girl" bed.. I would use 1" rails and at least 1 1/2" thickness to tie into the FB-HB, then pin it and add a center (front to rear) rail to help support the box spring-mattress-two adult weights in the center to aid the slats. Or.. I simply use male-female hook-latch hardware I mortise flush and attach with the longest screws I can use to hold them. And that includes a 3" deep mortise into the end grain of the rail.. then glueing a plug of oak into that with the long grain showing. Screws don't bite well into end grain and the reason for the end grain plug.

Sorry to be so detailish but.. depends on what your other dimentions are.. the age.. the weight of the matress-box and of the child intended to sum it up... All things must be considered IMO.

Good luck...

[Chris Friesen]

Here are a few different "rules" on sizing mortise and tenon joints as suggested by Chris Schwarz.

http://blog.woodworking-magazine.com/blog/Tenons+Rule+So+Here+Are+The+Rules+On+Tenons.aspx

Generally you want to balance the strength of each part of the joint. You generally want some shoulders to provide bearing surface and to hide any joinery flaws. The length of the tenon (aka depth of mortise) can be as big as you want, but a common rule is 5x the thickness.

[Frank Drew]

In Chris' list of M&T dos and don'ts, he early in the article notes, "Traditional texts say the tenon's thickness should be one-third the thickness of the stock being mortised (an important distinction)."

Unless this is just a typo, I'm not sure what the thinking is there; making the tenon one-third the thickness of the tenoned piece, sure, but how is the thickness of the mortised piece the determining factor when sizing the thickness of the tenon? E.g. consider mortising a 1'' thick rail into a 3" table leg; obviously you'll have a tenon much thinner than 1".

Of course, and this applies particularly to table joints, you might not place the tenon in the middle of the thickness of your rail (i.e. the tenon shoulders might be uneven) in order to give you a thicker outer cheek to the mortise.

[Chris Friesen]

In Chris' list of M&T dos and don'ts, he early in the article notes, "Traditional texts say the tenon's thickness should be one-third the thickness of the stock being mortised (an important distinction)."

Unless this is just a typo, I'm not sure what the thinking is there; making the tenon one-third the thickness of the tenoned piece, sure, but how is the thickness of the mortised piece the determining factor when sizing the thickness of the tenon? E.g. consider mortising a 1'' thick rail into a 3" table leg; obviously you'll have a tenon much thinner than 1".

The point of that rule is to make sure that the strength of the two pieces is roughly equal and that the mortise sidewalls don't blow out. With machine-cut mortises it can be as much as half the thickness of the mortised piece.

In your example, the tenon could actually be the full 1" thickness. However, having a bit of a shoulder gives the joint some mechanical resistance to racking so it would likely be a better joint with a tenon thickness anywhere from 1/2" to 3/4".

[Tony Bilello]

How thick are the corner posts going to be?
The reason I ask is that a M&T joint might not be the best solution. Bed frames are under a lot of stress. There is a lot of leverage that can be working against the joint.
There is also the option of standard bed frame hardware.

[Faust M. Ruggiero]

I may have missed this in your thread but do you intend to install bed bolts? Good bed frames were made using a mortise and tenon that was only dry fit. A long hole is drilled through the leg and tenon and into the bed rail where a captured bolt is fit into a mortise made from inside the bed rail. The bed bolt has a large shoulder and is recessed into the leg. A decorative cover is installed over the head. Then you can disassemble and reassemble the bed as needed. Your mortise will not have to be very deep since it is held tight with the bolt. Also, the bolt can be tightened as the wood contracts.
Bed bolts and covers are available through all the normal catalogs.
fmr

[Frank Drew]

If they're not too big to make it upstairs when assembled, the head- and foot-board assemblies can be glued up, but the side rails will have to be inserted in place so using some sort of hardware (bed bolts, specialized draw-tight hardware, whatever) for the side rail connections is the best technique, IMO. And, of course, using hardware (particularly bed bolts) the side rail tenons can be much shorter than you'd use for a glued joint.


The point of that rule is to make sure that the strength of the two pieces is roughly equal and that the mortise sidewalls don't blow out.

I agree, but what surprised me was that it wasn't stated that way; as written, it's rather nonsensical.