Housing joints in ply

[Anonymous]

Hi again Andy

This scenario has got me thinking its an interesting project.

If you had a large sheet of the birch ply (at least 32 inches by 40 inches), and set out a "7" with the point to the right hand top corner, then a mirror image of the "7" set out with its point in the top left corner, so that when you look at the layed out sheet you have more or less a equilatteral triangle with its point toward you, then if you machined your housings across them together in your desired position's, then they would line up for assembly the same way they were machined. BUT thered be slightly more wastage, and youd need to use one of those straight edges to work along? I'd still be inclined to use a jig with a step locating device-at least then the off cut's would be free of housings!! Birch ply always comes in useful for odd jigs and stuff anyway..... :wink: If I set them out that way I'd use 2 complete one piece "7's" make about 1/8 th of an inch oversize, for each "hand". Like you say you can get best side out both sides if you do this way. Locate together with a few dowels or biscuit's to stop them slipping, glue up, then trim using a nice template finished to a high quality (attach the template with screws into a spot that will later be housed out?) and then machine the housings. No need for a M&T in that scenario 8) Hand cut the mortices (or use a router jig-you seem to have more experience than I do with the routers). Plane up the cross strut's etc, you've already got the backrest lamination sorted. Go for it! I could see this being a real nice job. The challenge wood be to get the same precision and crisp finish. My boys tripp trap gets hammered 3 times a day for about 3 or 4 years now and it still looks great (well when its free of pasta curry and peas etc :wink: )

What sort of colour/finish would you be considering?

Cheers Mr S

 

[Anonymous]

Sorry shultzy I wasnt being insulting, What I meant to describe was the likelihood of acumulated error, same scenario as cutting and fitting the little piece's between stair spindles, you have to constantly check every few fixes for plumb, so that you havent wandered too far, and thats using identical spindles and filler pieces. Plus I think it would be a bit more fiddly and messy to keep every thing in line and keep all your fabricated housings parallel to the each other.

Cheers Mr S

 

[Shultzy]

I wasn't insulted at all, this forum is used for exchanging ideas and long may it do so.

When I look at a project I like to try and think "outside the box", if that's the right term, and look for easier and more accurate ways to do a job (I think that's what jigs were invented for).

Remember this project revolves around two identical handed pieces which start from the same baseline. If you cut spacer pieces (or the piece between the housing) together they will have the same hight, cut in half they still have the same hight. Because they start from the same baseline on each leg (the floor), any error will be mirrored on the other leg. If you cut the spacers, and the pieces between the housings, as two long pieces and then cut to approx lengths no inaccuracies.

After the glue has dried cut the legs to size, this will trim all the pieces.

 

[dedee]

Mr S,
you flatter me on two counts or I've misled you :lol:

1)

or use a router jig-you seem to have more experience than I do with the routers).

Er I doubt it. I have a Trend T3 and have never made a jig

and

2)

What sort of colour/finish would you be considering?

As you can see I've not worked out how to make it yet so finishing decisions have not even made it to the back burner :lol: - Probably Oil maybe Patina. Might try and stain in to match my oak dining table & chairs - more trial and error.

I do like to pick projects that stretch the old grey matter. Until I layout my templates and until I make a jig for use on an offcut I cannot decide whether to go for the router, spacer, or two piece 7 route.

Ideally (as I was rudely reminded last night) I would like to make two chairs from the one 8x4 sheet, now being as need to laminate the 7s to get the desired thickness, and finished face, this may have a bearing on the construction method.

Thanks all for the input.

Andy

 

[wrightclan]

Hi again Andy

This scenario has got me thinking its an interesting project.

If you had a large sheet of the birch ply (at least 32 inches by 40 inches), and set out a "7" with the point to the right hand top corner, then a mirror image of the "7" set out with its point in the top left corner, so that when you look at the layed out sheet you have more or less a equilatteral triangle with its point toward you, then if you machined your housings across them together in your desired position's, then they would line up for assembly the same way they were machined. BUT thered be slightly more wastage, and youd need to use one of those straight edges to work along? I'd still be inclined to use a jig with a step locating device-at least then the off cut's would be free of housings!! Birch ply always comes in useful for odd jigs and stuff anyway..... :wink: If I set them out that way I'd use 2 complete one piece "7's" make about 1/8 th of an inch oversize, for each "hand". Like you say you can get best side out both sides if you do this way. Locate together with a few dowels or biscuit's to stop them slipping, glue up, then trim using a nice template finished to a high quality (attach the template with screws into a spot that will later be housed out?) and then machine the housings. No need for a M&T in that scenario 8) Hand cut the mortices (or use a router jig-you seem to have more experience than I do with the routers). Plane up the cross strut's etc, you've already got the backrest lamination sorted. Go for it! I could see this being a real nice job. The challenge wood be to get the same precision and crisp finish. My boys tripp trap gets hammered 3 times a day for about 3 or 4 years now and it still looks great (well when its free of pasta curry and peas etc :wink: )

What sort of colour/finish would you be considering?

Cheers Mr S

Mr. S's idea is basically what I was thinking, except I was thinking of taking the housings right through the width of the panel. Hence my earlier comment about a lot of wastage.

Brad

 

[Anonymous]

One good thing about the 7 shape is they stack against each other with no wastage on cutting out of the one sheet.
Alternatively you could dispense with a "7" form and use a sort of slanted T square shape instead, with the tongue at a 60 deg angle instead of 90, and the "stock" part with one short and one long end-then you could use the through wedged M&T, you'd have maybe 2 inches or so of relish (on the foot end that faces towards the table) to give you additional strength? I think Mr George Nakashima used something like this on his chairs. Or even laminate a alvar Aalto style bent shape from birch venner's with the foot and angled upright in one solid piece?

Whatever you finally decide to do, I hope it all goes well 8)

 

[dedee]

I've been thinking about angled mortice and tenons to make the 7 from two pieces which method would be stronger not forgetting that this is being made in 1" ply and the stresses involved.

Andy

 

[Lord Nibbo]

I've been thinking about angled mortice and tenons to make the 7 from two pieces which method would be stronger not forgetting that this is being made in 1" ply and the stresses involved.

Andy

It's got to be the bottom one, the top one wont work as the tennon is higher (sticks out) more than the board is thick. Good ideas though

 

[dedee]

Is the only problem with the top one the length of the tenon - badly drawn sorry

Andy

 

[Jake]

Or use three layers of ply, and have the middle layer as a through tenon, full size as it were.

 

[j]

I recently asked on this forum about making a similar joint, although mine would not have held any weight, but the general opinion was that making an angled tenon is much easier than making an angled mortice.

I think the first one looks stronger.

As for the seat and foot rest, what stops them from sliding right through the housing? These chairs look very interesting, i've got some 1inch ply in the garage that needs a good project. As it's 8foot by 1foot boards, i'd need to make the 7s in two pieces.

Maybe i'll try and find a chair in mothercare or somewhere, see how theirs is made. Do they make the 7s in one piece or two?

Thanks
J

 

[dedee]

J,
the two sides are held together by the two black rods in the picture, the back rests and the piece that joins the two feet together. A bolt goes through the sides into the steel rods and knock type t-bolts are used for the back rests and the foot piece.

See my earlier thread on the various ways of making those pieces.

In the original the two 7s (sides) are made in two pieces and in the one I have are of solid wood. The seat, footrest and backrests are made from 1/2" ply.

Andy
edit: I have seen them in larger Mothercare stores and John Lewis.

 

[Anonymous]

When you look at the trip trapp its really an excellent bit of timber engineering. Theres no evidence of any metal fixings (unles they put a steel reinforcmet inside at the corner, laminated inside somehow??) Its not even a through tennon with a peg or wedge, so all that stress is coming onto a very small glue area :shock: I'd be very intrigued as to what sort of glue technology they used. I mean I always thought that ZED chair looked flimsey but it has a full width glue line at each joint (with plenty of room for dowels or biscuits etc) The trip trapp has VERY small joints at the main area of stress. Full credit to Stokke for a successful design

I know its slightly different using ply instead of regular staraight grain stock, but tennons should always be parallell with the grain, if they go off at angles you get short/cross grain which instantly reduces its strength under stress. EG On timber frames (proper beefy ones, not 4x2 nailed stud frames) you need to have brace's entering posts/plates etc at 45 degree angle. But only one side of the mortice is chopped to 45 deg, the other is chopped with a 90 degree side so as to avoid undercutting and weakening the mortice unecessarily so that it doesnt end up bigger than it need be.
I have somewhere a series of design principles off of the TFG (Jack Sobon/Ed Levin) for good joint technology with M&T's etc, I'll dig it out If I remember on

To answer your question Losos the seat and foot rest are clamped by simple friction as the 2 metal cross pieces and the seat back's and base cross piece are tightened with little screws, like a clamping action.

 

[dedee]

When you look at the trip trapp its really an excellent bit of timber engineering. Theres no evidence of any metal fixings (unles they put a steel reinforcmet inside at the corner, laminated inside somehow??) Its not even a through tennon with a peg or wedge, so all that stress is coming onto a very small glue area :shock: I'd be very intrigued as to what sort of glue technology they used. I mean I always thought that ZED chair looked flimsey but it has a full width glue line at each joint (with plenty of room for dowels or biscuits etc) The trip trapp has VERY small joints at the main area of stress. Full credit to Stokke for a successful design.

Exactly the reason why I thought making the sides from a single piece of ply would be such a good idea.
I was hoping David C might have chipped in with how he made his. I'll send him a PM.

Andy

 

[promhandicam]

Hi, Dedee.

I can't help thinking that you are making things more complicated than they need to be with regards to making a join at the base. As you are intending to make the sides out of two pieces of 12mm ply why not use a half lap joint - hopefully the picture will explain if my terminology is wrong. You can always put a couple of dowels through if you don't trust the glue or even whack a couple of screws in.

You can then easily mark out where the slots go with the two side pieces back to back and then with a jig like the one below cut the slots with the router.

The jig is the yellow/green/purple bit - with the angle between the yellow and purple sections 60deg or whatever angle you decide on. The distance from the edge of the green board to the purple batten is determined by your router and the cutter you choose - so that as with the circular saw guide jig the edge of the slot when it is cut is in line with the edge of the board.You can then clamp the jig to the work and all you have to do is line the jig up with your marks. The light brown shows the slot. I hope that makes sense!

You'd never believe that I originally worked as a design draughtsman - I just can't get the hang of 'drawing' on a computer. Give me a pencil and paper any day! BTW, if you take this advise, the use of the exact colours that I have shown is critical to the success of the chair - if there are any problems at the end then its because you've used the wrong coloured paint and nothing to do with the actual design!

All the best,

Steve

 

[engineer one]

deedee, i wonder since you are laminating two boards together, have you thought about doing this???

why not make your angled pieces, then for your laminates, cut small sections which fit between the seat positions, a simple spacer would hold them in the right position, then you could glue and pin the laminates.

the laminates could all be cut on the table saw using the same settings.

paul :wink:

 

[Shultzy]

Hey - engineer one, I thought of that first :lol: :lol:

 

[engineer one]

your right mate, but i am sure i made it clearer :roll: :?

paul :wink:

 

[dedee]

Hi All,
Steve, your router jig for the housings is exactly what I had in mind, much better drawn than I could have achieved - thanks.

Paul & Shultzy, every night I go to bed convinced on how I am going to make the housings (and the main joint) and every morning I wake up with a different approach.

This morning I'm convinced that the strongest joint will be not to make one at all ie cut the sides in one go. As for the housings I will trail and error both the router with jig method and spacers. :idea: I even dreamt up another method this morning.
I'll try to explain..... Make the two sides and screw them together. Make an angled table for the morticer and wack in a 1/2" chisel. Turn the sides on edge and drill out 1/2" square holes centered on the join between the two sides. Separate the two sides & I should have housings 1/2" wide by 1/4" deep that must be directly opposite each other.

Andy

 

[Wanlock Dod]

Andy,

I'd go with Steve's suggestion on this. I wonder if making the sides in 1 piece will be as strong as in two pieces. Since you are using ply wood one of the "arms" of the "7" will presumably not have any long grain (although whether or not this means that it won't be strong enough is another question).

Cheers,

Dod