How to avoid shrinkage problems on wide miters?

[ian maybury]

I'm playing with a design for a carved/contoured coffee table that would require three way miters at the corners in 3 - 4in square section hardwood - probably oak, walnut or similar.

I guess i'm wondering if anybody out there has any smart techniques to prevent the miters opening up as a result of cross grain movement?

One option I can think of is to place loose tenons or ply tongues near the centre of the joint - so that the pieces are fixed at their centre lines. That way movement would at least be reduced to that occurring to each side of the centre line - in 1/2 the width of the pieces.

The other issue is how best to approach properly drying the 4x4in solid hardwood stock without causing splits etc. - laminating thinner stock isn't all that attractive because it would leave joint lines. I'm thinking maybe air drying, followed by careful finishing off in a kiln.

The Chinese and the Japanese seem to be able to have managed it in years gone by, and in climates with very high levels of humidity at certain times of the year too.

Thoughts anybody?

[Troy John]

If time allows, I recomend sizing your miter parts within a 1/2 inch and let them sit in the climate they will stay in. Pre miter oversize and let them climate to their new home. Most material sits outside at the wholesaler, then gets moved inside to a different climate. Ive found if you rough plane it and rough cut it you remove the excess material and let the part settle to the new climate, then finish cut everything and pin it how you choose. It may take 2-3 weeks to adjust?

[ian maybury]

Thank you Troy, that sounds like good advice.

Taking a closer look at what i gather is the layout of the traditional Chinese 3 way mitre http://mercury.lcs.mit.edu/~jnc/jpg/joint70_50.jpg (http://mercury.lcs.mit.edu/%7Ejnc/jpg/joint70_50.jpg) it seems to incorporate all sorts of subtleties in the one joint:

- there's lots of lapped area which should give a good glued joint
- the square dowels prevent the pieces working their way apart (and can be extended to protrude as a decorative feature if required)
- the relatively narrow mating faces of the mitres (being relatively thin) can more easily be fine tuned with a chisel or a shoulder plane to give a perfect joint line.
- the square dowels provide a strong mechanical interlock when the joint is assembled
- The square dowels also locate each mitre on its centre line, and so as above result in movement being limited to whatever comes from half the width of the leg/rail

Quite some joint - it combines the strengths of both a mitre (mostly the clean look), and the strength of a mortice and tenon - but gains a little extra along the way.

ian

[Jamie Buxton]

Yueng Chan, in his book Classic Joints with Power Tools, shows a way to make that three-way miter joint, utilizing loose tenons.

[Rick Christopherson]

I explain how to make a locking parsons joint in the Festool Domino manual (http://www.waterfront-woods.com/festool/Domino_DF_500.pdf). Because the tenons are in opposing directions, it will lock together even without glue.

http://www.waterfront-woods.com/festool/graphics/DF500-22-small.jpg

[ian maybury]

Thank you very much for that guys.

I've ordered a copy of the book Jamie, and printed the Domino manual Rick.

That supplementary manual is a lovely job, and very useful. I don't know why, but somehow the English speaking/European academic tradition (i'm Irish/in Ireland by the way) has resulted in an opaque style of writing manuals and product documentation (and in the broader sense of text books) that's more designed to make the writer seem smart by having the reader struggle to understand the topic than anything else.

US written manuals and texts frequently tend to do what they should - that is seek to help the reader. It reminds me of when i was doing my engineering degree back in the 70s, it was always a good idea to seek out US maths texts because they normally included complete explanations, and all the steps. The English variety liked to jump about four lines in derivations...

As it happens i have a Domino, so it sounds like there's some choices to be made between the new and the old versions of the joint. :)

A complicating factor is that i'd like to use 3D curved outer faces, and to blend the rails and the legs, but care will be needed to figure out the effect on the joint.

The next step for me once the shop is ready is to see about sourcing some 4x4 in air dried oak to set drying, then to get stuck into trying some test pieces...

ian

[Rick Christopherson]

Thanks for the comments Ian. I agree about the common technical writing style and it is a pet peeve of mine, but I don't think it is a U.S. vs E.U. thing. I see it here as much as anywhere else. Many writers think the "technical" in technical writing means they have to write with precision, to the point of sounding like "legalese". I used to be a magazine editor (and engineer and woodworker) so I write my manuals with a little bit of a magazine style presentation and heavy on graphics. I also try to make the manuals educational, not just about the tool. You should check out the Kapex manual too. It's my favorite.

As for your parson's joint, I am a little concerned about the air-dried 4x4 and the possibility of the corners opening up. Normally I advise people to start with really dry wood because projects are more survivable with expansion than with contraction. However, for mitered joints, they will probably be more survivable with a slight contraction versus expansion, because the resulting gap will be in the inside corner instead of the outside corner.

Even considering that, I think you are going to want to dry these with a slightly elevated temperature (like a closed garage in the summer) to better dry the interior, and then let them rest for a bit at a normal indoor temperature and moisture level.

[ian maybury]

Getting 4in square oak or whatever to properly dry without splitting or the joints opening up is probably the major factor regarding the feasibility of the approach I've been (idly) playing with Rick.

Combination air drying and final indoor conditioning is as you say where the thought process may be going. It's not something i've seen done - I'm not even sure it's feasible. (it might be best to take the hint! :))

Another complicating factor (which might even play as an advantage with the right technique) is that the 4x4 in is really only for the blanks, the finished legs and rails would be heavily contoured from within that volume (it's the curves that require the thick section) down to much thinner (say max 2in x up to 4in thick) sections.

Meaning that maybe the plan could be to roughly pre-shape the pieces to greatly reduce the sections (think of the junction of two rails and a leg on a small table - joined by the a way miter) and let them move/equilibriate before making the joint and doing the final (hand) shaping.

One issue with pre-shaping would be the need to leave tabs, or to provide for fixturing to generate reference surfaces from which to work to cut the joints.

Looking more closely at the joint options and thinking aloud it looks like the traditional version of the 3 way miter is probably quite a lot more robust than the Domino version - the latter depends almost entirely on the tenons and the grip they get. There isn't really any other effective gluing area in the joint, as the rest of it is more or less all end grain to end grain. (or have i got it wrong?)

ian

[ian maybury]

Just to say that I've been reading 'Classic Joints with Power Tools for a while now Jamie as you recommended, it's a really good reference book.

It's basically a great balance of thorough explanation and concise writing, and is illustrated right through with colour photos which must have taken so much time to set up.

It's aimed towards those using a table saw, bandsaw, router and table and drill press. It covers all conceivable joint types (100+), and their cutting methods - including photos and details of required jigs and fixtures. Plus materials, adhesives, principles and so on.

It's not like many of these photo books confined to the old reliables either - for example as well as the basics it covers some beautifully thought out joints like some of those traditional to China. (e.g. the 3 way mitre that was the topic of this thread)

Thanks

ian

[scott vroom]

I'll admit to minimal furniture making experience and zero experience with 3 way miters. Wouldn't a good wood glue combined with dried wood keep the miters tight? Not sure how splines would help except with alignment.

[Chris Friesen]

I'll admit to minimal furniture making experience and zero experience with 3 way miters. Wouldn't a good wood glue combined with dried wood keep the miters tight? Not sure how splines would help except with alignment.

Gluing end grain isn't as strong as gluing long grain. Miters are a combination of end and edge and typically they don't hold particularly will without some help...thus the invention of keyed miters, splined miters, mitered half-laps, mitered bridle joints, etc.

Also, furniture can see high stresses. I sit on my coffee table frequently...wouldn't trust a pure miter joint.

[scott vroom]

Thanks....I learned something new!

[ian maybury]

To Chris' point Scott. Take a look at the more traditional version of the joint shown in the link from my post no. 3 above. It's complicated, but one of the reasons to go to that much trouble is that it makes available lots of long grain surface for gluing.

If I was to be a little cautious about the Domino version of the joint it'd probably be because it relies pretty much 100% on the relatively limited area and section of the Dominos to transfer the load between the pieces.....

ian