Housing joints in ply

[Shultzy]

This is a very interesting post - like the idea of using the mortiser, but not with plywood as I don't think the finish will be clean enough. Remember that you will have to make sure the two pieces are clamped together at the position where you cut the mortise.

Ply is only thin pieces of wood glued together, so gluing another piece to the first won't produce a weak joint. I'm sure that the weakest part of a joint is very rarely the glue line.

If you mortise, is the 1/4" slot adequate support for the seat?

Here's another idea - tape over the first piece in the slot positions, glue on the second piece, cut through second piece to create slots.

 

[Anonymous]

The chancy part of using the morticer is you would have to find some way to acurately (and firmly with all the pressure off the lever to prevent distortions) hold it at the exact angle and also with a 100 % perfect plumbline where the tempory join between the 2 halves ocurrs. If it went off plumb one way or the other youd get a deeper effective housing one side, and a shallower one the other (theyd be wedge shaped rather than parallele) And as Shutlzy says it wouldnt be the best of finishes, the morticer leaves a rough surface any way, which by its nature was never intended for viewing.

PS I think you would easily get at least 8 solid 7's from one 8x4 sheet to do 2 chairs, and still have usable bits for cross bars etc :wink:

PS why not cut some 7's from a piece of scrap and simply test it to see how much weight it takes to break it??

 

[dedee]

Shultzy,

1/4" housings are what's used on the original- admittedly in hardwood though.

Jonathan,
Although doable I never really thought I'd go ahead with the morticer idea for all the reasons both of you mentioned.

To get the 8 solid 7s the grain directions would be all over the place and I do not think I can get the seats and foot rests.

Having mulled this over again all day I am now erring on the half-lap solution for the main joint. This will reduce waste & allow consistency of grain direction on inside and out of both sides. And just leaves me the problem of getting the housings accurate - but how hard can that be :lol:.

I need to get on with this as all this thinking is doing my head in :lol:

Seriously though I'd be a darn sight worse off without this forum to bounce ideas off - Thanks again

Andy

 

[Shultzy]

Why don't you have make a test version as mr spanton says. You could then try the various methods that have been suggested and see which one works best.

 

[Jake]

Half lap takes you down to one glue surface, though.

I can't remember if you have the ply already or not, but if not, why not think about the laminated mortise and tenon approach with 1/2" ply?

 

[promhandicam]

Half lap takes you down to one glue surface, though.

But that is what ply is isn't it :?: so if clamped up well stucturally it will be no different.

Dedee, If you were worried about the strength of the half lap joint, you could always modify the design slightly to increase the surface area of the joint by flaring out the ends of the upright and base.

Steve

 

[Jake]

Half-lap has very little mechanical strength in this application, though, unless I'm misunderstanding.

You could pin it, but then the pins would be under twisting stress. M&T balances the forces.

I don't think it is right to equate the glue joints we can create in a joint with the ones achieved under heat and pressure in a plywood press.

 

[promhandicam]

Various posts on the UKW forum come to the conclusion that in a well prepared glued joint it is invariably the wood that fails before the joint. I did say in a previous response that if Dedee was worried he could put a couple of dowels through the joint. At the end of the day it is only a kids high chair, although I suspect that it should be made to withstand mum or dad using it as a ladder. :shock:

Kids Chairs that I've built out of low quality 3/4 ply were able to withstand 80kg without any problem and the construction method was very basic as that was all I was really capable of doing.

Cheers,

Steve

 

[engineer one]

if you are going to cut the housings and want them accurate, then you should make a routing board.

make it square to the angles, and then cut another angled piece to go on top, this will be parallel to the floor/baseline., then you slide everything up and down, and route the slots. no halflaps, just full rebates.

can't draw it . but think like this your angled piece is the same width all the way up and down. so make a clamped spacer that fits and is restrained on this, then add the extra piece to make it parallel to the floor.

paul :wink:

 

[Jake]

Well, the more glue, the better then?

Kids Chairs that I've built out of low quality 3/4 ply were able to withstand 80kg without any problem

But from what I recollect of your fine chairs , there isn't anything like the concentrated stress on a single joint that there is in this design.

 

[promhandicam]

But from what I recollect of your fine chairs , there isn't anything like the concentrated stress on a single joint that there is in this design.

True - but the larger chair which I managed to squeeze in was similar to the one that dedee wants to copy except that the area in questions 'hinged' so that the angle of the chair could be adjusted. the upright were on the inside with the base on the outside. the adjustment was done by having two 12mm holes drilled through the side pieces, lining up with holes in the upright and a piece of pipe put into the hole and held in place by a short 8mm coach bolt with a penny washer on the inside and a wing nut. The adjustment was achieved by having a series of holes in an arc on the upright at the for the rear set of holes. One issue we did have was that the physio wanted to have a large foot rest at the front that stayed parallel to the seat cusion, but such that a child could climb on to the foot rest to get into the chair. We then had to redesign the base to bring it further forwards as there was a tendancy for it to tip up with a child standing on the footrest unless they were supporting themselves on the chair with their hands. With the design that Dedee is copying I think that there would be a danger of the chair overbalancing if a child tried to climb up it from the front - although I guess that the designers took this into consideration.

Bon nuit, Steve

 

[j]

I think an M&T with some dowels trough would be strong enough.

If it's really can't take the strain, you could add a brace, to make some kind of triangle. although that would compramise a really cool design. Maybe ill do a sketchup of the triangle one to see what it looks like.

Thanks
J

 

[Anonymous]

If you did do a half lap you could use some tensile steel peg's fitted into blind hole's to "dowel" the joint, they would not show on the outside face's as a drawbored peg would, but would still provide mechanickal reinforcement and extra glue surface area where the 2 halves met? They might need some special epoxy resin glue or something? And an acurate jig to get the holes perfectly aligned

 

[Scrit]

Having manufactured batches of children's furniture in the past I'd say that a dowelled joint or a half lapped joint on it's own would soon fail under racking or pivoting, especially in plywood which (counter-intuitively) can be less strong because of the short cross grain on every other layer of veneer. Pin the lapped joint and it will last longer, but still fail in time. The mortise and tenon might work, especially if double pinned, however I'd rather not risk it on a joint which is subject to both racking and pivoting stresses. A better choice could be something like steamed beech with a deep finger joint and through pinned. My preferred method of manufacture, however, would be to cut the foot and leg in one piece from plywood thus sidestepping a potential strength issue.

As far as glueing together different layers of plywood, I'd only do this if an appropriate strength glue (such as Urea Formaldehyde, e.g. Extramite) were available, a cross-grain layer of construction veneer or a strengthening material such as fibre glass/carbon fibre mat were glued into the middle of the sandwich, and sufficient clamps were available to ensure that constant pressure could be applied during curing. Under no circumstances would I entertain glueing parallel strips to form the grooves - my suspicion would be that in so many glue joints there might be too much of a risk of failure unless supplementary support dowels or screws were used (not a bad idea when one considers the loadings that a No. 8 steel screw can withstand)

Scrit