Bending 3 mm plywood

[h.g.]

I am looking for curved panels with a radius of 150 mm and the stiffness of flat 18 mm plywood sheets. 1.5 mm 3 ply aircraft plywood looks good but is too expensive. 3 mm 3 ply marine plywood is just about affordable but is the radius too small? Table 1-2 on page 1-25 [of an old aeroplane manual I do not have permission to link to] suggests "thoroughly soaked in hot water" would work whereas 10% moisture would not. So the radius looks to be close to the limit. Does anyone have good or bad experiences of soaking plywood to make tight radii?

 

[PAC1]

Yes I have steamed ply to form a bullnose step for stairs. It does need to be well steamed I also did it over a number of curves to bring it down to the required curve otherwise it could split

 

[h.g.]

Yes I have steamed ply to form a bullnose step for stairs. It does need to be well steamed I also did it over a number of curves to bring it down to the required curve otherwise it could split

Thanks that is encouraging. I presume it was a tighter radius than 150mm? and what thickness was the plywood? It was normal plywood and not the flexi-ply with the soft core?

 

[PAC1]

Yes ordinary ply and about 100mm radius

 

[7sheds]

I have seen some very attractive curved panels for boat interiors made from 9 or 12mm ply by making saw-cuts from the reverse using the same technique coffin-makers use for side panels.. They can be made very strong by filling the grooves with a thickened epoxy as they are bent. For added strength one can coat the inner face with a layer of veneer using a vacuum bag (Polythene, parcel tape & vacuum cleaner) or if it is not visible, a layer of glass fibre cloth.

 

[worn thumbs]

It depends on the ply to a large extent.If you buy the stuff with paper thin face veneers and 85% of its thickness in the thick core veneer,prepare to be disappointed.If you buy 3mm gaboon with all veneers of similar thickness maybe it will work.A quick way to find out is to take a small piece and pour boiling water straight from a kettle onto it and then bend it.If your ply isn't WBP grade it will probably fall apart.

If you follow the advice about using a vacuum cleaner,prepare for a spell in the doghouse as you will probably burn it out without a flow of air to cool the motor.A proper vacuum pump can normally be had from ebay for around £45 and may be a good investment for domestic harmony.

 

[h.g.]

I have seen some very attractive curved panels for boat interiors made from 9 or 12mm ply by making saw-cuts from the reverse using the same technique coffin-makers use for side panels.. They can be made very strong by filling the grooves with a thickened epoxy as they are bent. For added strength one can coat the inner face with a layer of veneer using a vacuum bag (Polythene, parcel tape & vacuum cleaner) or if it is not visible, a layer of glass fibre cloth.

Thanks for the suggestion but the curved panel needs to be as stiff as 18 mm flat plywood. The requirement comes from the need to keep the lowest resonant frequency of the structure above a particular frequency where it will not get driven by vibration. I don't think I have much wiggle room.

 

[ColeyS1]

Slightly off top but you can get flexi ply

Not sure if it would be rigid enough after gluing to make 18 mm though .......

Coley

Sent from my SM-G900F using Tapatalk

 

[h.g.]

It depends on the ply to a large extent.If you buy the stuff with paper thin face veneers and 85% of its thickness in the thick core veneer,prepare to be disappointed.If you buy 3mm gaboon with all veneers of similar thickness maybe it will work.A quick way to find out is to take a small piece and pour boiling water straight from a kettle onto it and then bend it.If your ply isn't WBP grade it will probably fall apart.

Thanks for the warning. I was tempted by the 3mm(ish) £10 sheets at the local builders merchant but eventually accepted I would have to pay more like £30+ a sheet for marine ply to get the required stiffness.

 

[h.g.]

Slightly off top but you can get flexi ply
[...]
Not sure if it would be rigid enough after gluing to make 18 mm though .......

I am fairly sure it would not be stiff enough on it's own but bonded to an inner and outer layer of something stiff it might work.

 

[ColeyS1]

Don't think you mentioned yet, but what are you using it for ? I'm curious

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[h.g.]

Don't think you mentioned yet, but what are you using it for ? I'm curious

It is the low frequency section of a prototype (quick and cheap!) loudspeaker. If the lowest frequency resonances can be pushed to frequencies above the highest frequency handled by the driver they will not get excited and the cabinet will remain quiet. Unfortunately, a near miss is a big fail because the lowest resonance will then sit at a frequency where music has the most energy, the ear is fairly sensitive to cabinet noise and in a cabinet with almost no damping. One generally needs to sacrifice stiffness to get significant amounts of damping (e.g. constrained layer damping) so one tends to go all in for one or the other and in this case it is stiffness.

 

[Droogs]

Could you not make a mould and build it up yourself from thin strips/boards of solid?

 

[h.g.]

Could you not make a mould and build it up yourself from thin strips/boards of solid?

Solid wood is usually avoided with speakers because it moves too much. Although stiff in the direction of the strips I suspect such a construction would be insufficiently stiff in the circumferential direction because of the need for something soft to accommodate movement between the strips?

PS Or have I misunderstood your suggestion?

 

[Droogs]

i menat make your own ply in the shape of the mould and use UF glue

 

[h.g.]

i menat make your own ply in the shape of the mould and use UF glue

Yes that is the intention. The structure has an internal framework to make it stiff and so there will be something to bend the ply around without the need to construct something separate to do the job. Had I been able to afford very thin ply things would have been straightforward but I am looking to use 6 or 7 sheets of cheaper thicker ply that requires heat and water to bend to the radius.

Having not done it before I have some concern that by bending the ply to the desired radius and not beyond it is going to spring back too much when drying. In which case I guess I will have to construct something to bend around. Should I be concerned?

 

[PAC1]

If the dimension is critical then hot water bending is not going to work without permanent formwork or good non creep glue.

 

[Beau]

Kerf some 18mm ply. Have done this for a friends horn loaded speakers. Don't think the radius was as tight as 150mm but there was no problem with rigidity as the sides were glued to the sides of the cabinet.

Not mine but gives you the idea http://p10hifi.net/FH/images/dallas-2-k ... jpg?105,84

Google "kerf ply speaker building" and you will get lots of different builds some of which are very tight radius's.

 

[h.g.]

If the dimension is critical then hot water bending is not going to work without permanent formwork or good non creep glue.

Thanks. There is a permanent internal framework that the bent plywood will be glued to but the outside will be free. What type of glue would you recommend?

 

[h.g.]

Kerf some 18mm ply. Have done this for a friends horn loaded speakers. Don't think the radius was as tight as 150mm but there was no problem with rigidity as the sides were glued to the sides of the cabinet.

Not mine but gives you the idea http://p10hifi.net/FH/images/dallas-2-k ... jpg?105,84

Google "kerf ply speaker building" and you will get lots of different builds some of which are very tight radius's.

Thanks for the suggestion but I doubt it will be stiff enough in the circumferential direction (although I am not stating this with high confidence given the wide range of glues). The cabinet is designed to work in a particular way which requires a higher than normal stiffness.