Thread: Fixing a deteriorating joint in dining table

Originally Posted by elanjacobs

It'd be interesting to see the long term performance of new finger joints with PU glue, from what I've read it doesn't break down by any known means

elan, any well-made finger joint with any 'good' glue should be as strong as (or stronger than) any traditional joint when freshly made, I accept that unreservedly. I don't think the chemical composition of the glue is necessarily all that relevant to the longevity of a joint, unless you intend exposing it to extreme conditions. Hide glue, which is a biologically degradable material (but also 'stronger' than the wood it joins under ideal conditions) has hung-on in many pieces of furniture for hundreds of years, while I've seen plenty of 'permanent' synthetic glues fail in a fraction of that time. Besides faulty technique, things happen at the wood/glue interface that we don't necessarily understand. Of course, being an imperfect world, not all finger joints will be born equal - cutters wear & tear and wood is an inconsistent medium, so there will inevitably be some variation in any run of joints made. However, you have a huge area of glueing surface, so even allowing for some segments that aren't perfect, you should still have a very potent joint 99.9% of the time.

I long ago accepted that despite all due care and attention to detail, any glued joint still stands a chance of failing, eventually. I gather you do plenty of repairs, so you will be well aware of this. But unless we switch to using all glue-less joinery, we are stuck with glued joints in traditional 'western' furniture. My point is, why tempt fate by putting them where we don't need to? What you'd save by using the cheaper finger-jointed wood in a spot like a table apron just isn't worth the risk, imo, however slight it may be - it can happen, as demonstrated above. Then there's the appearance factor - if the joined pieces are well-matched, the join may be hard to spot from a few feet away, but they aren't, often enough, and they do seem to become more evident over time.

To each his/her own, but it will be a rather cool day in Hades before I'll use finger-jointed material in any piece of furniture I expect to last more than a week or two......

Cheers,

Originally Posted by Kuffy

finger jointed material is very easy to break over your knee and the timber around the joint just tears away "if" the glue actually holds...

Kuffy, I think you need to specify what finger-jointed material you are talking about? If it's soft pine mouldings, then I can't argue, but sometimes, at least, the joins seem to be as "good as the wood"! About 10 years ago, we had an extension put on our back veranda, and the builder used finger-jointed Merbau railings. I wasn't too damn pleased about it when I saw them (already in place), but he assured me they were fine & would hold up, and met all the specifications of the building code, yadda yadda.... Me being the doubting type, I didn't believe a word of it, so I took some off-cuts and put them in the vise & tried to break them. I couldn't break the short pieces of full railing, so I ripped them into narrower bits which I managed to break, but they all broke with at least 80% and mostly closer to 100% wood failure. I was somewhat mollified, and even ended up using a couple of lengths of the leftovers to make a pair of handrails inside the house, but I was careful to place the couple of joins I couldn't eliminate where they were likely to receive little or no stress.

I made sure the outside rails were well-painted immediately, and maintain them carefully. So far, no problems at all, but I would still have preferred solid wood!

Cheers,

Originally Posted by IanW

Kuffy, I think you need to specify what finger-jointed material you are talking about? If it's soft pine mouldings, then I can't argue

in Kuffy's "experiment", he is breaking the finger joint by stressing it across the joint. Most scarf joints would fail if stressed this way.

Originally Posted by IanW

... sometimes, at least, the joins seem to be as "good as the wood"! About 10 years ago, we had an extension put on our back veranda, and the builder used finger-jointed Merbau railings. ... he assured me they were fine & would hold up, and met all the specifications of the building code, yadda yadda....
Me being the doubting type, I didn't believe a word of it, so I took some off-cuts and put them in the vise & tried to break them. I couldn't break the short pieces of full railing, so I ripped them into narrower bits which I managed to break, but they all broke with at least 80% and mostly closer to 100% wood failure. I was somewhat mollified, and even ended up using a couple of lengths of the leftovers to make a pair of handrails inside the house, but I was careful to place the couple of joins I couldn't eliminate where they were likely to receive little or no stress.

I made sure the outside rails were well-painted immediately, and maintain them carefully. So far, no problems at all, but I would still have preferred solid wood!

my point. As strong as solid wood.

But IN NORMAL USE a table rail should not be stressed beyond about 15 to 20% of the material strength -- if even that much -- so I do wonder what abuse the OP's table suffered in the house move.

How does one make such a large finger joint in a small workshop? It has me curious.

Small ones are easy, one would just use a router bit: 1-3/8'' Finger Joint Router Bit | Rockler Woodworking and Hardware .... but big ones! So cool!

It would be an interesting effect to incorporate into a chopping block or boxes and lids.

I like ElanJacobs method of routing out a bridge - fast and attractive. You could make it trendy by making a Nakajima butterfly

finger joint bridge.jpg

edit: I was thinking I could use one on the lower half of my boxes to make splines that are small, but run up the corner ... only part way in (i.e less than 1/2" the thickness of the timber) and 1 1/2" tall from the bottom

The soft pine moulding finger joints are hopeless. If you drop a piece of 90x42 moulded handrail on concrete, the impact is often enough to break one of the joints. The structural 4mm long joints are just as bad. It is too much of a end grain joint for my liking. the longer 10-15mm fingers are much better. But any gap in the joint, which is evident by gaps at the peak/valley of the finger is a sure sign of a very weak joint that is easily broken.

You can make longer finger joints using bigger machinery such as a spindle moulder with tenoning attachment, or a square end tenoner with different cutter head etc. making the joint is fairly straight forward, pressing it together well is difficult.