How smoothe ?

[tusses]

for glueing

when glueing something smoothe you are advised to rough it up to get a key.

what about wood ? what makes the strongest joint ? a good saw blade ? a planer /plane ? sand paper / what grit ? is it different for different woods ?

;D

[davyowen]

I've never had a problem with a glue bond on anything from the crappiest sawn 'DIY shed' timber to freshly planed and perfectly smooth timber. The reason I don't think wood requires an extra key (and this might well be bullshit : is because wood is porous and even when freshly planed there is still enough of a key to get a good bond, unlike metal or plastic for example.

One thing I was told was that by scuffing up a planed edge you raise wood fibres that are normally pushed flat, and this helps to hide the glue line - nothing to do with adhesion though.

Davy

[paulchapman]

From all the stuff I've read by people who should know, they reckon that the best glueing surface is that left by a hand plane. Probably right given that few adhesives have any gap-filling properties and therefore need close contact between the two surfaces. Mind you, it would depend on how good your planing was ;D

But unless you've had a joint fail, why worry about it

Cheers

Paul

[dunbarhamlin]

Yep, I always go for a vacuum fit straight off of a straight bladed plane.
From what I've read, the glue/wood bond is strong, so the ideal would be a single molecule thick layer of glue, with each glue molecule bonding to both pieces of wood, as then surface tension as well as chemical bonding helps hold the stuff together (or was that for yellow glue?)
Having said that, I've tried breaking (HHG) joints I've been uncomfortable with in vulnerable joints, and invariably worn myself out before breaking the wood, not the (scarf) joint.
Cheers
Steve

[tusses]

I guess, because I am self taught (sp) I just wanted to know if there is a definitive answer. I am starting to make things for other people - and want it to be the best it can be - not just 'good enough'

[engineerone]

it actually all depends on the glue.

if you are doing what we mainly do, which is for instance make
table tops from a number of boards, then you have to remember that dc suggests you have a slightly bowed joint which the cramping will spring together. however this also gives glue a better chance.

there is of course the problem that with too tight joints is that you could push the glue out and starve the joint, so it is kind of a fine line(soory ) but it is why you are advised to rub the glue along the joint with the two pieces of wood together.

so far i personally have had little problem with pva's except if you
drop the joint on a concrete floor at a strange angle ;D


paul

[Alf]

Jul 10, 2008 11:18:45 GMT tusses said:

I guess, because I am self taught (sp) I just wanted to know if there is a definitive answer.

Important lesson right here then; there's seldom a definitive answer in woodworking.

[tusses]

Jul 10, 2008 12:37:29 GMT Alf said:

Jul 10, 2008 11:18:45 GMT tusses said:

I guess, because I am self taught (sp) I just wanted to know if there is a definitive answer.

Important lesson right here then; there's seldom a definitive answer in woodworking.

ok - 'optimum'

I'm sure the glue manufacturers would like an 'optimum' smoothness for a glue joint ?

[jay]

I think I got this from some bloke off a woodworking forum, so it's probably rubbish; but my understanding of these things is that you're looking for brightly cut wood fibres. Fresh coarse sand paper will do it for you as will a suitably sharp plane blade.

[nickw]

This probably won't help. As won't this. Nor this.

[tusses]

nope ... not signed up

Thanks anyway

[wizer]

You probably should Tusses, it's cheap and you can access any FWW article going back to the beginning. It's a great resource, I go there first when I want to learn about something. Odds on they have covered it 3 or 4 times over the years.

[paulchapman]

Another approach you could try if you want to ensure that joints won't fail, is to always use some sort of mechanical device in your joints - like loose tongues or biscuits. One of the best pieces I have ever read on the subject is in Alan Peters' book - 'Cabinet Making - The professional Approach'.

I can't quote from it because I don't have the book, but basically what he said was that he always uses PVA because of its flexibility (wood keeps on moving) together with a mechanical device, such as a loose tongue. I've always done this and with the limited tests I've done (trying to break the joints with a hammer) they have always broken somewhere other than the joint.

Cheers

Paul

[Scrit]

Jul 10, 2008 10:15:43 GMT davyowen said:

One thing I was told was that by scuffing up a planed edge you raise wood fibres that are normally pushed flat, and this helps to hide the glue line - nothing to do with adhesion though.

My understanding about using a toothing blade to scuff the surface is so that you provide a mechanical key as well as additional surface area to your glue joint. This was possibly necessary inthe days of hide glues, but modern glues don't really need it, so as smooth as you can, I say. The main thing is that the surfaces need to be freshly planed as the fibres are supposedly better able to absorb moisture from the glue and haven't oxidised (hence the old advice to cut and glue-up in the same session/day?)

One thing I certainly wouldn't do is sand to get a flat surface - mainly because of the risk of dubbing over the edges

Jul 10, 2008 21:11:08 GMT paulchapman said:

Another approach you could try if you want to ensure that joints won't fail, is to always use some sort of mechanical device in your joints - like loose tongues or biscuits.

A well executed rubbed joint is as strong as anything supported by dowels or biscuits. Half of the reason for using such devices is to get accurate/positive location during assembly rather than strengthening the joints. It is true that in man-made boards such additions to joints can add considerable mechanical strength to the joint (as well as mechanically locking the components together in the case of a biscuit joint) but that's mainly because the base material is so lacking in structural strength

Scrit