Many thin or few thick?

[nickw]

Righto, my next job but one is to make a pair of chairs with curvy legs that will have to be laminated up. When I made The Elliptical Table (Good Woodworking issues 188-190) I used bendy ply to make up the majority of the thickness, and ended up with a nice rocking motion on the table. This will be less acceptable (understatment) on the chairs, so I am planning to make the legs for them out of constructional veneer.

Question 1 : Will I get a stiffer result (no smut please ;D) by using many thin layers of veneer, or fewer thicker ones?

Question 2: Will I get a stiffer result using Extramite or will PVA/Yellow glue be just as good?

Question 3: I will laminate the legs up in blocks of up to half a dozen, depending on the with of veneer that I find, and face - rip - face etc. individual legs off the blocks. If I use Extramite how many sets of planer blades will I get through when facing the edges of the legs (2 chairs, 32 legs, 64 faces )? Come to that, how many bandsaw blades will I get through?

Question 4: If I do use Extramite, would I be well advised to invest in a thickness sander instead of using the planer for facing the edges?

Question 5: Do I have any idea about what I'm planning to do?

[paulchapman]

nickw said:

Question 5: Do I have any idea about what I'm planning to do?

No - welcome to the club ;D ;D

Cheers

Paul

[noel]

I can answer Q5......

Chairs sound interesting, look forward to the finished items.

[cbrsyd]

nickw said:

If I use Extramite how many sets of planer blades will I get through when facing the edges of the legs (2 chairs, 32 legs, 64 faces )?

16 legs per chair, is it a centipede ;D

Re the stiffness of Extramite vs PVA not sure if there is much difference but PVA can creep over time. My old college lecturer told the tale of a settee glued up with PVA. The laminated rails had the glue lines orientated vertical. Over time the cross section of the rails changed from square to parallagram shaped as the layers slowly crept under the weight of the sitters. So I have always used extramite for my laminations.

Not sure about the other questions but sure someone will be along shortly to answer your questions, or failing that take the p*ss

[Scrit]

Hi Nick

Q 1: Generally thinner veneers are used in bent work as they tend to bend better and exhibit less spring back. There is no reason other than their flexibility (and availability, of course) to use very thin layers.

Q 2: Extramite/Cascamite (both urea formaldehyde glues) will be better on laminated work than any PVA or aliphatic resin (including the cross-linking, or "exterior grade" varieties). There are several reasons for this, principally UF glues, unlike PVA/aliphatic resin types, do not suffer from glue line creep (as mentioned by Cbrsyd - although it's highly unlikely with D3 or exterior/cross-linked grades), whilst at the same time exhibiting a much stronger glue line and also having better gap-filling qualities. The glue line will also be stiffer than that of a D3, at least according to my supplier. Take care not to overcramp and thereby starve the joint of glue.

Incidentally it is possible to modify UF glues to produce a slightly more elastic glue line by adding PVA as a modifier in ratios up to 1:1, although not all PVAs and UFs are compatible so experimentation may be necessary (as well as a reduction in the amount of water to make the mix). This modification gives UF a better "bite" into fine-grained hardwoods such as beech, sycamore and maple.

Q 3: Don't know. In effect you are doing the same as machining birch plywood so wear rates should be similar

Q 4: Why not invest in a set of TCT blades for your planer? Surely that would be cheaper than a drum sander? You can also get TCT blades for band saws, although at these quantities I'd say ity was extravagant. Unless, of course, you intend to saw your own veneers

Q 5: No. But then I generally don't know what I'm doing either ;D

Regards

Scrit

[nickw]

Thanks Cbrsyd and Scrit. That confirms what I had thought on the whole. Interesting idea about TCT planer blades - I have a Hammer P/T and am not sure if they do TCT blades for it, but will check up and report back.

[nickw]

On the planer blades front Felder/Hammer only supply Chrome Steel and Cobalt Steel.

I kind of want an excuse to buy a thickness sander anyway so ..... aaaarggghhh what to do?

[jake]

Try Method Tools for TCT planer blades

[Scrit]

As Jake says. In fact any good saw doctoring service of tooling supplierr should be able to supply TCT blades for any planer - all they need to know is the sizes and whether or not there are any grooves or slots cut in it

Scrit

[Keith]

I realise you may want a reason to invest in more machinery but if you only have 16 legs to make, and you already have a planer thicknesser, then why not just plane them up and sharpen the blades afterward. Unless you are going into production I can't see that you even need carbide blades let alone a sander.

[nickw]

Gentles all,

I see that Good Sense is prevailing in your answers, damn your eyes. ;D I shall see how bad the damage is to my blades on the first few legs, and work it from there.

On the thick/thin question, I think the answers given are probably right, however I feel an experiment coming on. Watch this space.

[nickw]

In answer to the question I posed at the top of this thread, I have done some experiments, and have some results.

To test the comparative stiffness of pieces made from thin and thick laminae, and the effect, if any, of differing glues I made 5 sample beams. One was made from 4 layers of 7.5mm bendy ply, glued with Extramite. Two were made from 12 layers of 2.5mm thick tulipwood, one glued with Titebond I, the other with Extramite. The final two were made from 20 layers of 1.5mm tulipwood, again one glued with Titebond I, the other with Extramite. The number of laminae were chose to give beams of approximately the same thickness at around 30mm. All of the tulipwood laminae were prepared from the same plank.

The beams were pressed in a vacuum bag while the glue set, and left for a further two days to dry out. All 5 beams where then surface planed one one edge, and passed through a thickness planer to achieve beams of the same width.

Each beam had a section of the same length marked out, and was then placed in the test rig shown below.


One end of the beam is securely clamped to the work bench, the other is over a DTI. A standard load (the yellow diving belt weight in the top left of the picture) was then applied over the DTI, and the resulting deflection recorded.

Material	Glue	Thickness	Deflection	Normalised (x 10-4)
Bendy Ply	Extramite	30.7	0.62	0.658
Thin	PVA	29.6	0.092	0.105
Thick	PVA	29.6	0.081	0.0924
Thin	Extramite	30.1	0.101	0.111
Thick	Extramite	30	0.072	0.0800

The normalised results (Deflection/(Thickness ^ 2))take account of the differing thicknesses of the beams. As can be seen Bendy ply is much less stiff than the Tulipwood. There also seems to be a marginal gain in stiffness to be had by using fewer, thicker laminae. Any difference in stiffness due to the glues is less clear.

However as has been noted in previous posts, Yellow glue, in common with all PVAs, is prone to creep over time. Also thinner laminae bend better so a balance between the desired radius of bend, and thickness of laminae needs to be found for any particular situation.

[Telos]

That's an interesting test. That would probably make quite a good article for F&C or GoodWoodworking. With a few more thicknesses and maybe a couple of different glues that would be quite comprehensive. Thanks for sharing