How to build a stool with plywood?

[michaelmkchu]

I am building a stool out of 18mm plywood and would like ideas on how to make it more structurally stable. 

I own a jigsaw, drill, random orbital sander, clamps, wood chisel, japanese wood plane, but no table saw or jig for super straight edges and complicated joins.

I have read that I should carve a shallow dado on the underside of the seat, but I am not confident. I am considering adding a corner block. Any advice here? I am a beginner.picture of chairUnderwise from the back. not there are small gaps due to me not cutting 100% straightholding a potential corner block for reinforcing the joint

[EricL]

Hello @michaelmkchu 

That is truly a very creative way to use plywood and turn it into a seat. Your idea of using a corner block should make the seat stable and secure. Will you be using screws to keep the pieces together? If you are, I would also like to suggest using Gorilla 236ml Wood Glue to secure the wood together. 

Let me call on our experienced members @Dave-1, @Nailbag and @JoeAzza for their recommendations.

If you need further assistance, please let us know.  

Eric

[AlanM52]

Hi @michaelmkchu,

I agree with @EricL's suggection.

Cheers

[Dave-1]

Good Morning @michaelmkchu 

Now thats an interesting way of slotting a seat together and having it lock itself together

I am thinking of adding another block/panel to your project. Located half way between the underside and the front of the foot It spans the width of the underside iof the seat.

So it continues in your slot type building style. You have the two verical slots (In black, thro probarlly only half as long as I have drawn them so it slots)

The blue slash section is the new plywood panel, It spans just shorter then the width of the seat part itself. 

Note the "slots" will be cut on an angle, just a little to match the way the feet of the chair are splayed.

You could mark out the angle on the underside by drawing the straight line where I have suggested it up to the feet sides. Then dismantling and extending it through to either side. 

Dave

[Nailbag]

Hi @michaelmkchu 

What an awesome project. Following along @EricL suggestion, running a good wood glue along the top of the legs/body is step one. Next I would insert from the top of the seat 4 recessed screws filled and sanded down from the top of the seat. This would be ample without the need for additional blocks.

To me, the potential weakest part of the design is the left/right movement in particular at the lower legs, which I don't think those under seat blocks would help here. Two options would be to either to run a single board across under the seat similar to what @Dave-1 has mentioned. Or if it didn't take too much away from the aesthetics would be to insert an aluminium rod between the legs via recessed holes either side.

Nailbag

[michaelmkchu]

DO you think I should use screws in the corner block? The corner block itself is around 25mm, so I guess since it's 18mm plywood, I should aim for perhaps 35mm length screws? 

Also should I glue, then screw right after to help with clamping, or wait until the glue sets?

[michaelmkchu]

Thanks Dave! I like the idea but my concern is that I'm not really good at cutting and it's been already a big struggle to get my pieces to fit (you can see a few gaps in the picture already and they were meant to be 90 degree / parallel cuts), so cutting the slots at an angle may be a bit too difficult for me as I have a jigsaw, but no other saws.

What if I add a cross beam of 25mm x 25mm wood like the diagram below? Would that be enough, or do you think I need to really "cut through" the plywood legs to make it work?

 

[JacobZ]

Hi @michaelmkchu,

Congratulations on the stool so far, it looks great. Now lets make it a bit more stable.

The main issue, like @Nailbag mentioned, is that you would have side-to-side movement, and this is because there is nothing really holding the legs together. Without some attachment, these legs are essentially separate pieces with a timber piece just sitting on top. 

Adding blocks, as you've mentioned, will help support the top, but it does nothing to stop this side-to-side movement. For this, you will need to brace them together by running a piece between them. In construction, this is called lateral bracing.

The option @Dave-1 proposed would be a good start, and if you cut rebates into that timber, then slot it over the top of the legs, it could be done in a way without using screws.

Having the lateral bracing lower down, as Nailbag has mentioned, would, however, be better as it will stop the side-to-side movement lower down on the legs, where it will be the most pronounced. Running a timber piece or a metal rod from one leg to the other, as he has mentioned, would greatly strengthen the structure as a whole. Having a lateral brace at the front and back of the stool would also be worthwhile to brace it all over.

If you need to use screws, then 30mm timber screws would be the way to go.

Hopefully, this helps to clarify things a bit. Please don't hesitate to reach out if you have further questions.

Jacob

[Dave-1]

Good Afternoon @michaelmkchu 

I was trying to keep away from screws and staying with the slotted way you have built the chair

Since that is off the list, I would then go for a bar between the legs much like @Nailbag has suggested.

If you use Hobson M12 x 3000mm Hot Dip Galvanised Threaded Rod to span between the legs and use Macsim 50 x 50 x 3mm M12 Galvanised Square Washer - Each either side of each leg plus nuts it would lock things together nicely.

I would install the rod at least half way down the leg if not 2/3rds of the way.

Dave