centre joint questions

[jowl]

I am just wondering if it is always necessary (1) to glue size before gluing a centre joint , & (2) to do a rub joint on a centre joint?

[fiddlecollector]

Ive never sized a centre joint whether using clamps or just  rubbed joint. I just quickly apply glue to both surfaces and lightly rub together to get excess glue out and clamp or often dont bother clamping with violin /viola wood. Sizing is really only necessary with end grain.

[David Burgess]

I always pre-size centerjoints these days, after many years of experimenting with trying to break the joint on cutoffs. Like just about every fiddle I've ever made. If it makes a clean break at the glueline, I'll probably saw it apart and give it another go..

 

It's probably not so much of an issue on tops, but curly maple backs can present significant "end grain" at the glue joint..

[actonern]

I find that on the maple, where the glue can really suck in and starve the joint on highly figured wood, if you heat the glue surfaces with a clothes iron (not high heat) till the wood is quite warm, the glue that you paint on will remain fluid quite a bit longer, giving you time to re-mop the glue surface a few times before rubbing them together.  This extra time seems to allow the wood enough time to suck glue into itself, and the resulting joint is more reliably strong.

 

E

[fiddlecollector]

I always pre-size centerjoints these days, after many years of experimenting with trying to break the joint on cutoffs. Like just about every fiddle I've ever made. If it makes a clean break at the glueline, I'll probably saw it apart and give it another go..

 

It's probably not so much of an issue on tops, but curly maple backs can present significant "end grain" at the glue joint..

Never had a problem unless the joint was bad on occasional troublesome cello backs. I think ive sized one or two oppio violin backs in the past that had pretty wild figure that showed some slight end grain at either end.

Though i dont know if a broken centre joint  would be that bad if it was clean ,certainly better than breaking somewhere else where the break wouldnt be straight.

[Melvin Goldsmith]

I really feel that if someone has a tested method that works for them it is a good one and more than one method certainly  exists. I am also a serial break tester of off-cuts and like David would re do a join where an off-cut broke at the join. My method is quite a bit different to David's and focuses on getting a consistency for the glue in relation to the wood I am gluing to hit the join as soon as it is finalized from the plane. I will use off cuts from the wood I am joining to test how the glue behaves on the wood before arriving at an optimal viscosity for the glue and make sure the glue is Hot & ready to hand as soon as the joint is perfect. After all the prep work, planing a center join normally only takes me a couple of minutes. Before this I might spend much longer adjusting the viscosity of the glue for the job. Definitely the glue viscosity I choose will differ for every piece of wood. I always do the joint as a rub joint but I clamp it too because clamping is a fast and easy insurance.

[nathan slobodkin]

I don't size the center joints and as far as I know have never had a failure. I have no compunction about cleating center joints though on the theory that insurance never hurts. I also tend to break the cut offs to check the joint and while I don't usually get a clean break I often don't get as much wood failure as I'd like to see. So David, do you replane the joint after sizing? I would think that would be awfully hard on my precious jointer plane. Likewise What strength glue do you size with?

[Marty Kasprzyk]

I really feel that if someone has a tested method that works for them it is a good one and more than one method certainly  exists. I am also a serial break tester of off-cuts and like David would re do a join where an off-cut broke at the join. My method is quite a bit different to David's and focuses on getting a consistency for the glue in relation to the wood I am gluing to hit the join as soon as it is finalized from the plane. I will use off cuts from the wood I am joining to test how the glue behaves on the wood before arriving at an optimal viscosity for the glue and make sure the glue is Hot & ready to hand as soon as the joint is perfect. After all the prep work, planing a center join normally only takes me a couple of minutes. Before this I might spend much longer adjusting the viscosity of the glue for the job. Definitely the glue viscosity I choose will differ for every piece of wood. I always do the joint as a rub joint but I clamp it too because clamping is a fast and easy insurance.

I use Gorilla glue.  It never breaks.

[Michael_Molnar]

Let me back this thread up a bit. To me, the most important issue is the flatness of the joint seam. I inspect that very carefully before deciding to apply glue. I have tried clamping, but found that I get a better seam with a rubbed join. One other tip is that I rub the glued surfaces with my finger to work the glue into the wood.

[Michael_Molnar]

I use Gorilla glue.  It never breaks.

That explains those inch-wide seams. 

[DarylG]

I use Gorilla glue.  It never breaks.

 

Not sure if you're joking or not, but I recall that Fine Woodworking magazine did a series of tests a while back and Gorilla glue didn't live up to the hype.

 

http://www.oldbrownglue.com/pdf/HowStrongisYourGlue_FWW.pdf

[DarylG]

I always pre-size centerjoints these days, after many years of experimenting with trying to break the joint on cutoffs. Like just about every fiddle I've ever made. If it makes a clean break at the glueline, I'll probably saw it apart and give it another go..

 

It's probably not so much of an issue on tops, but curly maple backs can present significant "end grain" at the glue joint..

 

Hi David. Do you sharpen your plane blade flat or with a slight camber for center joints?

[Marty Kasprzyk]

Not sure if you're joking or not, but I recall that Fine Woodworking magazine did a series of tests a while back and Gorilla glue didn't live up to the hype.

 

http://www.oldbrownglue.com/pdf/HowStrongisYourGlue_FWW.pdf

I do use hide glue where I might later want to take the joints apart--- plates to ribs, nuts etc.

[NewNewbie]

I am just wondering if it is always necessary (1) to glue size before gluing a centre joint , & (2) to do a rub joint on a centre joint?

Glue is cheap. 

A starved joint is expensive.

Rubbing is good exercise.

Failure is not an option.

[Bill Yacey]

I've used a rubbed joint to good success with the top join, but for the back I feel better clamping.

 

I size the surfaces really well and use a hot air gun to work the glue into the wood fiber. Working quickly while everything is still warm, I apply the final glue and join the pieces. Sometimes I even go over the join with the hot air gun after it's clamped if I feel the glue might have been jelling slightly while  tightening up the clamps.

[jowl]

thank you everyone for your useful comments

[Marty Kasprzyk]

thank you everyone for your useful comments

Heat lamps help too.

[Craig Tucker]

I'm curious about something that comes up in this conversation.

Since I never pre-size center joint edges when joining.

 

I do make absolutely sure of the flatness of the edges of both pieces first, before I join and them, and then I will apply liberal (always) fresh made hot hide glue to one joining edge of the plate that's face (edge) up in the vice, and then I'll apply liberal hot hide glue to the other face of the second piece of the plate's edge, that will be rubbed, face (edge) down, against the piece that's face up in the vice...

They are rubbed in order to remove the "excess" glue, and allow the plates to "seat" properly, with the joining piece firmly against the other adjoining piece - the seam always works out perfectly, to be both invisible and strong when the glue becomes dry.

 

No clamps necessary, although I did use clamps at one time.

 

I have become accustomed to this joining method - and it has worked well for me, for years.

 

I find it difficult to envision how two plates can fit perfectly, if they are pre-sized with glue first?

Wouldn't that ruin the perfect flat surface at the join line that planing the raw wood produces? I don't doubt that a method of doing this is practical - I just don't readily see why it would be done that way? Are the plate edges re-flattened again, after the glue has been applied?

 

Is it's basic purpose, a method to insure no glue starvation? 

 

Just curious.

[Jacob]

I imagine that sizing will be followed by slight planing....?

[Craig Tucker]

I imagine that sizing will be followed by slight planing....?

 

That's what I'm thinking also.

[Bill Yacey]

I don't allow the glue to dry. I apply the sizing, work it in with the hot air gun until I see the glue isn't soaking into the wood any further. Then I apply the final glue and join the pieces.

[Craig Tucker]

I don't allow the glue to dry. I apply the sizing, work it in with the hot air gun until I see the glue isn't soaking into the wood any further. Then I apply the final glue and join the pieces.

 

Interesting, what kind of wood are you joining?

 

Sounds workable enough...

 

Just curious - are you clamping?

[Bill Yacey]

Hi Craig,

Good to see you around!

 

The wood is just standard issue spruce and maple, red maple as of late.. I use a rub joint for the spruce, but I still prefer to clamp the maple.

[jowl]

@ctviolin

You are right that you need to plane some more after glue sizing, if you allowed the glue to dry. This was the reason I asked the question. I was taught to glue size, dry, and then revisit the surfaces before final gluing. Usually this can be achieved with just a few passes. Recently there was so much glue left on after sizing I had to take it all off before I could proceed! I think the glue I used for sizing was too thick ...

The 2 questions are related. When gluing the centre joint, I was taught to do a rub joint until it bites, break the joint, apply more glue and then rub again. I guess the first rub sizes the surfaces some more. I don't really know the rationale behind the rub joint and how it differs fundamentally from a clamped joint - can someone please explain?

[David Burgess]

Hi David. Do you sharpen your plane blade flat or with a slight camber for center joints?

The blade is shaped to make the joint ever-so-slightly concave (transverse), so that when moisture (glue) is applied and swells the wood, it becomes flat.

 

On glue sizing:

The glue is applied very thin, wiped off thoroughly before it gels, and allowed to dry. Sometimes the joint needs to be touched up with a plane after the sizing has dried, but usually not. I try to glue the joint soon after sizing (overnight at the longest). If you let it go too long, the wood can change shape enough that the joint no longer fits.

 

But whatever works for you. However, if you haven't tested the strength of your joint by breaking your cutoffs (which I see Melvin also does), you may not know how well your method is working.