Mortise & Tenon proportions

[ptrott]

I want to make an "old style" wooden framed flyscreen door.
I imagine that the strongest joints would be M & T. The wood is DAR Jarrah
at 19mm X 80mm. What I would like to know is what proportions should the joints be? It seems that because the wood is only 19mm wide the mortises & tenons will have to be mighty thin. Is it a practical joint in wood that thin?
Any suggestions appreciated.:)
Phill.

[johnc]

I usually aim at keeping the tongue about 1/3rd the timber thickness, around 7mm in this case. The glue provides the strength when it bonds the tongue into the mortise. I'll now wait to be corrected.

JohnC

[Island Nomad]

I wouldn't M&T them.

I'd dowel them with 2x8mm dowels (just cut the heads off a couple of galvanised bolts) in each joint, and glue (dowels only) with a good exopy. Might be better to prime joints prior to assembly.

[Harry72]

My front screen door is made from 19mm hard wood using M&Ts and its over 50yrs old still going strong.
JohnC is correct general rule of thumb(GROT)is 1/3 for the M&T joint... but being 19mm you'd need 6.3333333333mm, you wouldnt want to make the tennon any thinner than that so you'd go up to say 8mm(common router bit size)... my door uses 3/8"s.
Dont use dowels they will not last like a M&T.

[Island Nomad]

I stand corrected.

It's just that modern dowelling techniques have come a long way with the epoxy glues now available.

[Different]

My front screen door is made from 19mm hard wood using M&Ts and its over 50yrs old still going strong.
JohnC is correct general rule of thumb(GROT)is 1/3 for the M&T joint... but being 19mm you'd need 6.3333333333mm, you wouldnt want to make the tennon any thinner than that so you'd go up to say 8mm(common router bit size)... my door uses 3/8"s.
Dont use dowels they will not last like a M&T.
Im with Harry on this one I make a lot of M/T joints in 19mm stock and usually use a 9mm router bit or drop back to 6mm if stock is thinner.
You can try variations on the theme of the attached pic to gain strength in the joint if you wish

[Wood Borer]

I made a similar door about 20 years ago. I used through Mortise and Tenon joints and glued with 308 glue. The timber was Tassie Oak and the door faces South which means it cops all the rain.

It is still in one piece. The timber was 19mm and the mortises were 1/4 inch wide. I routed from both sides of the rails and chiselled the middle where the router wouldn't reach.

It meant a cheap door and some fun making it. It has lasted beyond my expectations.

[silentC]

Maybe you could consider a bridle joint (or a mitred bridle - Ross would love to see that ;) ). You could pin it as well and it will last forever.

[TassieKiwi]

I did this with Cedar, as the local door blokes wanted about $1800 for 3 flyscreen doors:eek: .

I used 90x32mm cedar - nice and light. Routered 12mmx60mm M&T joints, which went really well. I made a couple of errors:

Didn't leave enough 'meat' on the cheeks of the tenons for trimming, so they weren't 100%

Didn't have the doors restrained whilst the finish (cabots - never use this again) was drying, so one door has a 5mm twist

Materials were about $350 all up. The doors work fine.


I would be wary about making them from heavy Jarrah, though at 19mm this won't be too bad. The design should allow for bracing - maybe a third rail in the centre?

[Different]

Maybe you could consider a bridle joint (or a mitred bridle - Ross would love to see that ;) ). You could pin it as well and it will last forever.
Hey Silent why dont ya stick in a couple of rivits to be sure!

Ross

[Wood Borer]

No, I think a few of those wonderful bow tie type dovetails (Ross does them so well) across the ends of the mortise and tenons would help hold it also. The problem is what timber could you use to make them? :rolleyes: :rolleyes: :rolleyes:

[Island Nomad]

I reckon you should use 31mm at least for what you are doing.

[Woodlice]

Mitred with buscuits??

[ptrott]

Correct me if I am wrong, but isnt the strenght of the biscuit the strength of the joint? It sure would be a lot quicker to mitre and biscuit it, but I need strength and reliability. This door is the main one into the house so will get a LOT of use. As for using bigger wood, it is governed by the frame size (thickness) which is already there.

[julianx]

what about haunched M&T with wedged through tenons like a traditional panel door

[journeyman Mick]

Correct me if I am wrong, but isnt the strenght of the biscuit the strength of the joint? It sure would be a lot quicker to mitre and biscuit it, but I need strength and reliability. This door is the main one into the house so will get a LOT of use. As for using bigger wood, it is governed by the frame size (thickness) which is already there.

I read an interesting article in a book which was a collection of articles from an American wood working magazine. In it, they constucted a wide range of joints, out of identical stock species and sizes and glued them up using identical glues. They then took them to a university engineering lab and tested them to destruction. The joint with biscuits had the highest ultimate strength, (ie took most pressure to break) however it broke without any warning. The M & T joint was the next strongest but it came loose well before it failed entirely. In practice you would pick up the loose joint and repair it before it failed.

Mick

[ptrott]

Interesting Mick. I wonder what direction they applied the stresses for this exercise. In a door the joint would be flexing in a twisting motion every time the door is opened unless the person opening it got it dead centre every time. If they tested it by pulling straight against the joint it could well be stronger, but what if you applied a twising motion. The M&T has a much thicker "biscuit" than the biscuit itself. What are your thoughts on that?

[journeyman Mick]

I must say I was suprised at the results. It's been a few years since I read it, so I can't remember the exact details. I wouldn't use biscuits for any full size doors or for chairs and tables but have used them for cabinet doors in the past. Used with a good glue, they're stronger than most people give them credit for. I've never had one fail and I've been through quite a few bulk boxes of biscuits.

Mick

[Woodlice]

If every you start recycling wood from busted furniture left on verges for council collections, you will come to understand this as I did. Salvaging wood from other joints is more often than not a lot simpler, and straight forward, when you dont want to resort to a table saw or similar.

[Tarty]

I'd go with thicker timber say 25mm and M&T joints dowel pinned. That is you drill right through the morticed section about 10 mm from the joint line, mark the position of the hole on the tenon when it's inserted, then drill through the tenon offsetting the hole about a quarter of the dowel diameter closer to the shoulder. When reinserted the hole through the tenon won't quite match the hole through the mortice. You then chamfer the end off the piece of dowel and drive it through the offset holes. This pulls the joint tightly together. The dowel is then trimmed off flush with the face of the timber. Of course glue is applied to the joint before pinning. This makes a very strong joint which is very unlikely to let go or come loose. Good Luck!

[Vasco]

Phill,
I made and sold several wooden flyscreen doors going back 20yrs or so. There were some Pine and some Jarrah. All were 19mm x 60mm all had a centre brace and some had decorative corner braces. I used wooden dowels 2 in each joint using only white PVA glue. I would have used M/T had I known how to make them back then with the limited tools I had. The thing is I drive past the houses that I installed them into and they are still there and looking good. Never did make one for myself much to SWMBO disapointment :o

Hope this helps give you some confidence.

Kind regards Vasco

[ptrott]

Thanks Vasco. It makes me wonder if it is worth all the effort to go M&T if a biscuit joint (which should be stronger than dowelled joint) will suffice. If I had loads of time on my hands I would go M&T just for the experience, but with several hundreds of hours of reno's in front of me I am not prepared to "overkill" at the expense of time right now.

Phill.