Mortise and Tenons -

dcottrill · Post by **dcottrill** » Sat Jul 02, 2011 6:11 pm

My wife has asked me to make some chairs for our grand children. A chair to bridge the gap between High Chair and Adult chair. I have been working on a unique design that would allow seat height to be changed to adjust it to the child's height.

The legs of the design are like a letter "L" leaning forward. I think that to create the legs I would use 2 separate pieces. A Base attached to the rising leg with a mortise and tenon. I'm not sure it the mortise should be made perpendicular to the edge of the boards surface or at an angle. Attached are some sketch up diagrams of what I'm trying to say in words.

The tenon's would be made so they go all the way through or as long as possible on the one that is cut at an angle.

Does any one know what is the convention or better way?

JPG · Post by **JPG** » Sun Jul 03, 2011 1:30 am

I think you have a catch 22! The lower one is stronger, but the upper one is easier to make!

Any reason the other end needs to be open? Putting all the weight on that joint at that angle is risky.

michaeltoc · Post by **michaeltoc** » Sun Jul 03, 2011 7:52 am

JPG40504 wrote:I think you have a catch 22! The lower one is stronger, but the upper one is easier to make!

I would say that the upper one is stronger - When weight is placed on the legs they will want to spread outward. The lower mortise offers no strength other than the glue bond. You would need to pin the tenon with a dowel. The upper mortise is on an angle, so it acts as a "hook" to resist the force wanting to pull the joint apart. (I probably would pin this joint as well).

dusty · Post by **dusty** » Sun Jul 03, 2011 9:14 am

michaeltoc wrote:I would say that the upper one is stronger - When weight is placed on the legs they will want to spread outward. The lower mortise offers no strength other than the glue bond. You would need to pin the tenon with a dowel. The upper mortise is on an angle, so it acts as a "hook" to resist the force wanting to pull the joint apart. (I probably would pin this joint as well).

The tenon itself acts to prevent downward movement. If the tenon (either of those depicted) is properly fit in the mortise and then properly glued there is no way that normal wear and tear (children's furniture) will pull it apart. The wood will break first.

The shorter tenon, set at an angle, will tend to pull out (pushing the legs apart) if downward pressure is applied. However, here again, if properly fit and glued it will never come apart under normal stress.

I don't see the hooking action that you mention doing much for this chair unless the kids are holding onto the chair legs and trying to pull them apart.

michaeltoc · Post by **michaeltoc** » Sun Jul 03, 2011 9:37 am

dusty wrote: I don't see the hooking action that you mention doing much for this chair unless the kids are holding onto the chair legs and trying to pull them apart.

A load will travel along the leg following the angle. The reaction of the floor to support the load is to push up, perpendicular to the floor surface. Since the load is at an angle, the load "splits" and the resultant forces are one straight down into the floor, and one at 90 degrees to the floor. This 90 degree resultant force is exactly as if the kids are trying to pull the legs apart!

The mortise on the top is on an angle. Since the outward pulling force is perpendicular to the floor, the upward angle of the tenon acts like a hook.

Clear as mud?

dusty · Post by **dusty** » Sun Jul 03, 2011 9:49 am

michaeltoc wrote:A load will travel along the leg following the angle. The reaction of the floor to support the load is to push up, perpendicular to the floor surface. Since the load is at an angle, the load "splits" and the resultant forces are one straight down into the floor, and one at 90 degrees to the floor. This 90 degree resultant force is exactly as if the kids are trying to pull the legs apart!

The mortise on the top is on an angle. Since the outward pulling force is perpendicular to the floor, the upward angle of the tenon acts like a hook.

Clear as mud?

No, there is no mud there. What you say is quite logical. We just seem to disagree as to which tenon is best for the intended application.

Either way, I prefer mortise and tenon for this application. Dowels inserted, in lieu of tenons, would be almost as good but are more demanding during the build process. I do not do well getting dowels to line up.

JPG · Post by **JPG** » Sun Jul 03, 2011 12:01 pm

The 'problem' with the angled joint is that the load results in torque being applied. The downward force of the 'load' will tend to twist the joint clockwise. The opposite of trying to pull it apart. It will try to pivot about the inside junction of the two parts. The 'hook' you mentioned will be in effect with the lower joint, less so with the upper joint. The vector forces you mention will be vertical down and horizontal to the right. The relative magnitude will depend upon the angle formed by the joint. A angle of less than 45° will result in a greater horizontal force than vertical.

I agree with Dusty that pinning is well advised. Also good bonding will prevent separation of the two parts, but I fear shear strength of the wood will be severely tested.

Again - why the angled open front????

dusty · Post by **dusty** » Sun Jul 03, 2011 1:26 pm

JPG40504 wrote:The 'problem' with the angled joint is that the load results in torque being applied. The downward force of the 'load' will tend to twist the joint clockwise. The opposite of trying to pull it apart. It will try to pivot about the inside junction of the two parts. The 'hook' you mentioned will be in effect with the lower joint, less so with the upper joint. The vector forces you mention will be vertical down and horizontal to the right. The relative magnitude will depend upon the angle formed by the joint. A angle of less than 45° will result in a greater horizontal force than vertical.

I agree with Dusty that pinning is well advised. Also good bonding will prevent separation of the two parts, but I fear shear strength of the wood will be severely tested.

Again - why the angled open front????

By a baby gorilla maybe.

JPG · Post by **JPG** » Sun Jul 03, 2011 4:30 pm

dusty wrote:By a baby gorilla maybe.

Agreed!

Newborn gorillas weigh only about 3-4 pounds (1.4 to 1.8 kg) at birth (about half the weight of a newborn human

dcottrill · Post by **dcottrill** » Sun Jul 03, 2011 8:26 pm

Thank you for the excellent discussion. It looks like a split.

The angle is for design, and it maybe off in the diagram as I'm still working on the exact angle (approximately 67 degrees). It is inspired by Scandinavian designs. They tend to have a different bend to children's furniture.

The seat will be stationary and the foot rest will adjust up or down depending on the child's height.

The pinning of Mortise and tenon joint is an excellent idea, one that I've been considering.

I thought that the forces on the joint would tend to cause a downward motion on the leg resulting in a "clock wise" twisting motion at the joint.

Hopefully, the child is under 100 pounds. It maybe prudent to design for an occasional use by Dad or Mom!