Thread: joints - strength and when to use one type over another

Dovetail joint vs box joint
Dovetail vs. box joint strength

Wood joint strength testing

Bookmark that site, its full of useful stuff.

Amazon.com: Joint Book: The Complete Guide to Wood Joinery (9780785822271): Terrie Noll: Books

This is also very useful and I think you can get it from Carbatec. If not most good book shops should have it.

Regards,

Rob

This article in FWW observed that the smaller dovetails failed at a higher level than the larger ones. The conclusion was that "all of the joints we tested were stronger than the most severe load that they would ever likely be subjected to in normal use or even abuse"

This pop wood page by Rob Lang describes the use of an anvil. They do point out that there is not much point doing pseudo scientific tests in magazines. The conclusion that i drew from this article is that it is necessary to understand the relationship between what you will use the joint for , the timber you have and the glue you will use.

So I went on to the scientific literature. I started reading some of it but like most science it applies to a specific situation ; glue type etc.

So what to do?

Firstly to the " man who has abundant determination and confidence, but does not have the benefit of natural talent or a decent tutor," I am no expert. In reading both the "the complete dovetail" Ian Kirby and "the essential woodworker" Robert Wearing and looking at posts on this forum people who practice basic skills for joint making such as marking out and sawing, and give dedicated time to do woodworking seem to succeed. Recently I tried this approach, it seems to be working.

Thanks blokes, I've checked the links and will see if the local library has that book.

These are my random thoughts - after 20 years and with a few failures you tend to learn from your mistakes.

Early in my wood working career everything got a bit of PVA (poly Vinyle Acetate) White wood glue.

After one particular dining table came back - it had ridges of glue had erupted under the varnish after a few years, so I sanded it all back with the intention of re gluing it. The 'ridge' was soft to the thumbnail and seemed to be a rubber like consistency. When I tested the joints they failed meaning a bit more work than I bargained for fixing it all back up.

At the time I worked for the local forestry dept so raided their library for literature on glues for WA Hardwoods.

This research paper I read pointed out that naturally most forest soils in WA are slightly acidic on the Ph scale. As a result the sap in the timber - is likewise acidic a& over time it attacks most glues, and many joints will tend to fail.

Same thing happened before PVA Glues, when they were often made from rendered down horses hooves etc.

I those days the makers realized taht glues werent long lasting so designed a lot of physical mechanical strength into their joints (Pinned mortice and tenon for e.g. in chairs for e.g).

One of the glues this report touted was UF (Urea Formaldehyde) sold as brands like UF62 etc.

I tried this and its not bad but after 10 - 15 years I started seeing joints failing on furniture I'd made for family in my early days, where I used UF62...the glue was crystallized and broke easily with the slightest movement - the acid in the sap within the timber had done its work again - albeit this time over a much longer period.

Then I went to epoxy resins exclusively and never looked back - because they are impervious to acid attack - which is what breaks down most glues in joints over the years.

My solution was to abandon things like electric instant cured glue, cyanoacrylate (super glue), UF (urea Formaldehyde) glues, and stick with epoxy resins.

Then I also adopted mechanically strong joints like stop pinned mortice and tenon-ed combined & secret draw screw joints, combined with the use of epoxy resins and all the former problems went away!.

Mechanically strong joints, followed by glue joints with a LOT of surface area, are the go...

I'm not a fan of any glue bar Epoxy resin for Australian hardwoods myself and if you combine it with a mechanically strong joint you can't go wrong.

My 2c

Grrrr.... 3 attempt to reply to Timless Timbers post, and lost each one.

I ike the comments about acidity and glue breakdown, epoxy (I'm a big fan) and mechanically strong joints... being able to 'look back' on 20 years of work is fantastic.

I guess my issue is that I know "what to use where" by following the joint design/use from older pieces of furniture I've pulled down or studied, but I'd rather know '"why to use a particular joint" over another... e.g. rather than copy or follow, I'd rather use knowledge to inform the decision.
For example: when to use a haunched mortice and tenon over a mortice/tenon.

Surely there are some books on the subject?

Yep - tired of losing posts - waste of time typing them out!

In essence yes there's books on it - but most use the machines & tools they have at hand and joints they are familiar with and that have worked well for them in the past.

Otherwise you'd have to tool up for every conceivable type of joint - cost prohibitive & impracticable in many cases.

Very interesting comments about the strengths of various glues. I'm a big epoxy fan, but I've also used PVA on various things (sometimes on the advice of woodwork teachers). I think I will switch to Epoxy for nearly everything after reading this - except for maybe MDF and plywood gluing where epoxy will probably just be a waste. I know that PVA doesn't work well on european oak - we had some 1960's scandinavian oak dining chairs that we picked up second hand. It was obvious that the original glue had failed at some point (probably some kind of hide glue) and then repairs had been made with PVA - which doesn't work well with oak. I repaired a couple with epoxy and no problems. And then my darling SWMBO got sick of them anyway and replaced them...

In essence its not so much the strengths of the glues...its their longevity in the presence of acidic sap...or the rate at which they break down.

If you destruction test glued joints and the timber around the joint breaks, rather than the glue used to join it - then the glue is stronger than the parent timberit is joining - so by definition is a great glue - however if its effective half life is of short duration due to chemical changes occasioned by the ever present acid within the sap naturally occurring in the timber - then that's where inert glues like epoxy resins - come into their own for longevity.

I'd always wondered how these places that make solid timber cupboard doors could supply them at about 50% of my cost to hand make them...

Then one day I had to go to the city and collect a batch direct from the factory and the ownber who I knew well because I'd been buying solid timber kitchen cabinet doors from for 10 - 15 years - but never met face to face - was so chuffed that i came to see him he took me right thru his factory.

What an education in time and motion study...and the value of machines compared to mine...

It wasn't hard to see hsi time savings over my methods.

The fielding cutter for his fielded panels was the same cutter that made the joints on the end of his styles etc so the whole door just fitted together like lego - anyone could do it, (i.e. no real trade skill required - a kid from school, straight off the street could assemble the doors with about 10 minutes instruction).

Then the cramping - all done on air tables, and the glue electronically cured in seconds with electric paddles (like a heart starter machine/defibrillator at the hospital) whereas I used all the cramps I had on hand to cramp my epoxied mechanically strong sto[p pinned mortice and tenon-ed joints overnight, and if there were more doors than cramps, then 2 or 3 nights!

He was saving days on me just from his clamping and gluing setup, then the sanding a 3 drum sander - compared to me one door at a time on the stroke sander and random orbital.

No wonder I was only making $3 an hour - and he could supply doors at half my cost.

The only difference?

After 7 years his doors self destructed, while mine will last 200+ years!

But,

Most people replace their kitchens every 6 years in Oz! So who needs or is prepared to pay for 200 year doors?

So I used the quick cheap mass produced solid timber doors for kitchens and reserved my hand made solid timber doors for heirloom type furniture designed to last 200+ years where the effort and expense was warranted.

Everything these days is about time = money!

Cabinet making these days uses very little solid timber..it's all particle boards, melamines, corian tops, granit tops, post formed tops, vinyl wrapped doors (from cnc routed MDF panels).

Theres not a lot of timber work skills left in cabinet making these days IMHO.