mitre joint

[Didi]

G'day,
I would like to get some advise on how to join the mitre as shown in the attached drawing. I will be laminating two pieces of Jarrah 30x175x700 for the legs on either side and two 30x175x1200 for the horizontal section.
Should I use biscuits (horizontally or vertically?), dowells, steel brackets etc.? Thank you so much for your input.
Best regards,
Didi

[Harry72]

There are several ways, what tools do you have?
Biscuits would work, but you will need to gang them up as 1 or 2 will not be strong enough IMHO, if you add a bottom piece between the legs it will be a fair bit stronger.
Dowels will be stronger, 175x30mm should give enough area for 6~8(6x32mm) dowels, you may need to cut the outer side dowels down as the outer cross section of wood is smaller in a mitre joint.
In your design, whats the top going to be glass?

[rrobor]

My choice would be dowels. I would drill for dowels before you cut the mitre. that way you are sure of the depth and angle of the dowel. Or if the top is wood, glue it up without then dowel from the top.

[Didi]

Hi guys,

Thanks for your info. Dowels seem to be the go as both of you reckon that they will be stronger. The glass table top will rest on two stainless steel square hollow sections spaning the two legs and weighs about 110kg.
rrobor: Thanks for the tip about drilling.
harry: Haven't got any specialised tools yet. Is there one to drill the holes at exactly the same position on either piece or do I just mark them exactly and drill them on the drillpress?

[rrobor]

You can buy centres in Bunnings. What you do is drill one side then put the centres in the drilled hole and bring the other piece to where it should be. Give it a tap and you got perfect centres for your drill on the other piece. They come in packs of 4 and all different hole sizes.

[Didi]

excellent. thanks. i'll go and have a look

[Ian Smith]

Didi,
Dowells are very unforgiving in so far as if you get their location even slightly wrong you won't get a flush fit on adjacent faces. ( edit...and if you have a few of them, and they all don't match up exactly you won't even get the pieces to meet)

If you have some sort of step between the legs and the horizontals it's probably not going to matter so much but if you want as flush as possible match between the two make sure you use a flat surface, such as a table top. Lay the faces that matter on top and then bring the two pieces together when marking the centres.

You'll still have to be careful to prevent the drill bit wandering so use those specialised dowelling ones such as Haron make with the centre spike which are less likely to cause a problem.

Unless the Foul-up Fairy is taking the day off I'll bet you will still need to do a bit of sanding

For mine I'd be using multiple biscuits


Ian

[Didi]

Ian, I can see what you mean with the placing of the dowells. If I was going to do the buiscuits, would I need to place them horizontally as the pressure on the joint is from the top? How many would I need per joint? What sizes? Would it be strong enough?

[Ian Smith]

Didi,
The weight of opinion seems to be that biscuits add little to the strength of a joint - they are primarily a locating device, and its the glue that does all the work, so it probably doesn't matter what direction they face. Having said that, they are not all that fragile so I'd reckon they must contribute something.

In any event, given the weight of your project an epoxy glue would probably be a good choice

Why not try a floating tenon? If you are laminating a couple of pieces you could work on creating the mortice on the inside of one piece of the laminate. Might be a bit easier than carving it out after they are assembled, and cut the tenon from a bit of your scrap jarrah.

Before you ask, I guess you should make the tenon as large as practical


Ian

[Didi]

Had a practice run with dowels and I really can see the problem that they are not going to match perfectly thus creating gaps in the joint.

I will have a try with the floating tenon and mortise. Another option I was thinking of is routing a channel on the internal side of the laminated timber (30mm wide, 50mm long in the horizontal and vertical part) and inserting a steel angle. I assume that would solve all issues with strenght..........or could this cause a problem down the track?

[chowcini]

i would glue the join using epoxy glue without any joinery at this stage, once dry i would bore two holes through the mitre and insert dowels that run through the two peices of timber and strengthen the timber you could also use a contrasting timber or metal dowel like brass stainless etc ....... thank me later :2tsup:

[carpoid]

I don't understand drawing 'A' , what am i looking at there?

[Didi]

chowcini, that is indeed a good idea! No mucking around with biscuits, dowels and steel angles. I think it tops the list so far. I would go for stainless dowells eg rods.
Thank you!

carpoid, picture A is the leg of the table and also marks the area for glueing. Instead of having plain square legs I'm going for legs with beveled edges.

[Harry72]

To dowel a join like that you'd make up a one off jig, couple bits of scrap/offcut 175mm wide with one having a 45° bevel the other needs to be thick hardwood(30mm+) these would be joined by nails/glue to from a 45° "V" at each end of the v glue on some sides, this made correctly should slip over your mitre cut 175x30 jarrah with no movement/wobble on the fit.
Mark out your dowel placings so they are reversible, you need to be pretty accurate here(within .5mm), then if possible using a drill press drill the holes square to the jig.
Now you have a jig capable of doing the job quickly and accurately, the guide for your drill is the thick hardwood so it should last this job.

[Didi]

Thanks Harry. It took me a while to understand your instructions because, you might have guessed, I'm new to this kind of work. I quite like your idea and will definately have a go at it on a practive piece.
Didi

[Harry72]

Half the work in WW is making jigs for repeatable accuracy and speed

[Ian Smith]

Half the work in WW is making jigs for repeatable accuracy and speed
ahh yes ..for jobs you'll never do again :D

[Rocker]

Why not use floating tenons in mortices routed with a jig and a spiral-upcut solid carbide bit? As pointed out above, multiple dowels are very unforgiving. If you have a thicknesser, a router table and a roundover bit, it is easy to make floating tenon stock.

Rocker

[Sir Stinkalot]

Didi ..... it sounds like you are facing the same dilemma that I did a while ago on a similar project.

http://www.woodworkforums.com/f40/blackwood-glass-steel-dining-table-20678/#post189522

I went for the biscuit joint B in the post below.
http://www.woodworkforums.com/f11/biscuit-joining-options-7868/

After a few years it hasn't had any problem. There is a little creep of the pva glue along the joint line but it isn't anything that you can notice and hasn't altered the strength .... you can just feel it if you run your hand along the joint.

Hope this helps

Stinky.