Laminating/Cold forming

[dedee]

I'm slowly but surely making progress with my Tripp Trapp chairs. The sides have been laminated and the seats and foot rests cut to size.

My attention is fixed now on making the curved back rests. I have cut the 1.5mm ply to size and I will be using 7 layers.

If I were to use an existing back rest as a former how close to the original shape should I expect my laminated piece to be?

Andy

 

[Chris Knight]

Any,
You could expect it to be very close with minimal springback - as long as it it is all clamped up well during the glue-up. Also, leave the clamps on for at least 24 hours or even 36 hours, there will be a lot of wet glue to dry out.

Depending how wide the back rest is, the challenge will be to get sufficient clamping pressure in the central area. I would plan on making up the first back as a caul to be used in subsequent glue-ups.

 

[dedee]

Chris,
thanks. The backs are 2 3/4" high and in fact there are two backs on the chair, see here, so I have a ready made caul even for the first one.

I think I've cracked the fixing method for the backs and the steel ties but I dare not go public in case it does not work.

Andy

 

[Chris Knight]

Andy,

You might well mark the existing backs with pressure from the clamps so don't plan on reusing them. Also leave enough extra length at each end to allow a nice square cut off of your laminated pieces.

There will be a tendency for your ply laminations to try and straighten your former and caul (ie the two backs) also, because you will be using a back as a caul, the radius (ie for its convex side) will be slightly more than you theoretically need (to match the concave side of your laminations). Thus your made up backs will likely differ slightly from the existing backs - probably not enough to worry about I suspect.

 

[dedee]

Chris, this bit worries me

You might well mark the existing backs with pressure from the clamps so don't plan on reusing them.

I had thought of protecting the backs with clingfilm to protect from glue squeeze out and using rubber or compressed form as protection from the clamps bruising the wood. The 7 sheets of ply are very bendy so I hope that excessive pressure will not be required.

I guess the first one at least will be a trial and error experiment.

The alternative I guess is to cut out a former & caul from a piece of 3x3, this might be achievable on the scroll saw but the length will be a problem - would have to cut from either end.


Andy

 

[Chris Knight]

Andy,
By all means come and cut it on my bandsaw!

 

[woodbloke]

Andy - you shouldn't have too many problems but I advise to use the old 'Cascamite' or 'Extramite' as it's now called. PVA adhesives always have a certain degree of flexibility (desirable in chair joints for example) but not needed in a laminated structure as this will almost certainly guarantee a small element of 'springback'. Extramite on the other hand sets glass hard (if mixed correctly) with no movement whatsoever and is always recommended for laminating - Rob

 

[Scrit]

Andy

There is a rather basic formula for calculating spring back which can be used as a starting point, it is:

s = r / p**2

Where:

s = spring back at edge of curve
r = rise of form in centre, e.g. the distance from the centre of the form to the highest point of the curve
p = number of plies

This will at least allow you to gauge the amount of springback you'll experience for a given form curvature.

Scrit

 

[dedee]

Chris, thanks for the offer. I'll have a play at the weekend and so how I get on.

Rob, thanks for the info. I did laminate the handles on my chisel cabinet last year with PVA but the much larger curve probably deserves a different approach.
I have not found mach technical info on Extramite. Does it have a shelf life?

Scrit, thanks for that, using your formula I calculated a spring back of about .28", which I hope will be insignificant.

Andy

 

[Shultzy]

dedee, you could make the formers out of multiple layers of chipboard or mdf and use sash cramps to apply the pressure.

 

[dedee]

I got hold of some Extramite and was quite surprised how far it went. Which means I made up more than twice what I actually needed

The original chair back was well wrapped. 7 pieces of 1.5mm ply were still quite flexible so clamping was straight forward

Hardly any spring back at all

I'm not quite sure what happened at the left end but after trimming I do not think it will be a problem.

The back will be set into shallow mortices on either side and bolted and as I hope this shows the "not quite flat end" (now on the right) should not cause a problem.

Andy

 

[Chris Knight]

Andy,
That is a good result. I guess the larger springback at the LHS might be a result of those quick Grip type clamps not giving as strong a grip as the G cramp on the right. As you say however, after trimming you shouldn't really have a problem.

 

[dedee]

This is how all 4 back rests turned out

I am quite pleased with the uniformity of the curves. Once the ends have been trimmed they should all fit very neatly into the sides.
I can see no discernible difference between the three that were stuck with Extramite (powdered resin) and the one that was stuck with Titebond Original (aliphatic resin). Time will be the true judge..

Andy