Help with joinery - best options for hand tools only

[Robert Trotter]

Here are some pics of what I want to make - White ash

White Ash Bench Seat exploded view.jpgWhite Ash Bench Seat 1.jpgWhite Ash Bench Seat alternative support B.jpgWhite Ash Bench Seat support A.jpgWhite Ash Bench Seat Front Elevation.jpg

(CORRECTION: Leg assembly rail is 40x60mm)

First, is the design OK against racking with the mortice and tennons and 60x30mm? (Basically a chair isn't it?)

Seat support - which option would be better? just screw the seat boards on or put them in grooves (?) ?

Mortice and tennons -
What size tennons and shoulders would be appropriate?
Do I need to use double tennons anywhere?

I have 8/4 and 6/4 lumber. Member sizes could be changed. Just using 30mm a lot as it would mean minimal removal and waste of wood. But I can go thinner - it just means I get to use my new scrub plane more.

Thanks for your help.

Rob

[Zach Dillinger]

Rob, I would be concerned about the width of the rail to leg tenons. The weak part of that joint is the mortise cheek, and there isn't much "meat" to make up that cheek. I would definitely make the tenon thinner. The rule of thumb is somewhere around 1/3 to less than 1/2 of the overall width of the board. And ash splits very easily, so you could blow out the cheek. Make it thicker by making the tenon slightly thinner.

Other than that, the only issue I see is to be very careful with the stock selection on your seat supports. With the angle cut into the end, you have the possibility for some very short grain at a place where you need lots of strength. If you have good straight grained wood, you should be fine, but if it runs out you could easily split off the end. And I would definitely think about the additional seat support. No reason not to and I think it would add some strength to the overall design.

[Robert Trotter]

Zach,
Thanks for the response. I made a correction on my original post - I made a mistake on the drawing and the leg assembly rail should be 40x60. That would make the tennon 1/2 of the width and give 10mm shoulders. OK?? or still make thinner?

The tennons all generally have 10mm shoulders all round.

Seat support - you mean I should think about 3 support members? or just 2 and use the cut outs like in a bridle joint. In my drawing it is the "optional seat support". For this I meant to still have only 2 supports but could use the design without the cutouts or the flat topped design. Would three be necessary?

I see your point about careful grain selection for the ends of the seat support members. Maybe it would be better to go with the chamfered look rather than the to-a-point look.

Rob

[Jim Koepke]

The look is appealing, but my concern is if my 240 pounds were to be plopped down off center would it hold?

jtk

[Robert Trotter]

Jim thanls for the reply as it got me thinking...
What areas of the design are you wondering about?

[Robert Trotter]

Jim's comments got me wondering where a weak point might be and I realized that the single tennons of the stretchers into the leg assembley rails(?) would be mainly end grain to end grain. So I tought about the following options - adding twin tennon. There are two options, which would be better? "A" is just cutting a notch in the tennon I had, but I am thinking if option B just in case Jim came over to Japan. There would be more long grain glue surface.

Or...Do I need to go further and use wedged twin tennons? I would prefer not to have through tennons but if strength is required then OK. I would use a dark contrasting wedge.

Ash Bench seat stretcher tennon options.jpg

Also, with cutting the tennons and mortices is there anything to take care of when working with white ash?

Thanks

Rob

[Prashun Patel]

My concern for a bench like this is that the load is not transfered directly to the legs. It puts a lot of stress on the joint.

I think you will improve the stability and longevity by making the seat supports over the side rails - or at least adding an 'invisible' support to transfer some of the load more directly there.

I would also add a center support. I appreciate the design on this; the seat does appear to float. But I'd accomplish that by moving the support 'wings' inward; the seat can cantilever over them a couple inches.

Last, I would consider a lower, centered stretcher supported by a lower rail between the legs. You can optimize the mortise and tenon joint only so much. In the end, there's a lot of torque down on the bottom of those legs. It's most efficiently stymied by a lower stretcher, IMHO.

Screwing or notching the supports won't make a lot of difference, IMHO. If this is an outdoor bench, then personally I prefer screws, since it'll enable you to take the top off every couple years for easier refinishing if necessary.

[Jim Neeley]

Rob,

Perhaps I missed it but I didn't catch where you described the purpose of this table/bench. You ask about racking but the racking load on a dining room table is very low (while existent) but if this is intended for a work bench for hand planing the force is very high.

What kind of racking load must it resist?

Jim

[Jim Koepke]

My thought is someone of my weight sitting on one end of the bench will have a leveraging influence on the far joint.

If sitting on the edge of the bench it could also put pulling forces on joints designed for compression.

My tendencies may be over cautious. I am sometimes hesitant to put arm rests on chairs for fear of people trying to sit on them and having them fail.

jtk

[Zach Dillinger]

Jim, that's a good design principle actually. Fewer parts means less things to fail / break / go wrong. If the doesn't need arm rests, it shouldn't have them.

[Robert Trotter]

Jim, the bench is for the entrance to our house where it will mainly be used for putting things as we come and go from the house. And as we live in Japan it would be used for people to sit and take off their shoes if need be. But I don't anticipate it being a heavily used item for sitting on. However it has to be able to be sat on as it will be used like that occasionally.



I may increase the main stretchers to 80mm at the leg assembly and put a curve to reduce the visual depth at the centre.Bench narrow curved 1.jpgBench narrow curved 2.jpg

And I might introduce a middle support but it gets a busy at the ends when looking through the seat planks.
Bench narrow curved 3.jpg
I prefer the look of the two supports when looking from the top
Bench narrow curved top.jpg

I was thinking that the main stretchers would not deflect much so the extra moment forces on the stretcher to leg frams joint wouldn't be too great. It would be maily shear stresses (vertical) stesses. ONly if someone heavy sat on it and was rocking side to side would there be big stresses there. Not likely of often happen.

Ideas and suggestions.
Rob