Flexible sheet materials to make a curved-end instrument case

[<no_spam]

I need to make some instrument cases (mandolin, mandola, etcetera) and want to make curved ends - best visual analogy I can think is that the plan view shows a shape like a belt going around a small and large pulley. The problem is what to use for the curved ends. I'm expecting to have to laminate some strips of something around a former but all I can think of to use is aero ply, and that's too expensive. The tightest radius of curvature is going to be about 100mm and the width is going to be about 150mm; I need to be able to attach the curved ends to the (rigid) long sides. What material can the team suggest?

[spuorgelgoog]

Steamed hardboard.

Several layers of it well glued up should be fairly strong once it's all set.

Owain

[mike]

Have you looked at bendy ply from a timber merchant like Arnold Laver? It's not that expensive in small quantities and pretty easy to work with.

[Andy Burns]

Doesn't that make for a bend radius of 75mm, unless you mean depth?

[Tim Watts]

Bloke next door steams wood and bends it. His steamer is a box fed by a pipe from a small simple water boiler.

IIRC he leaves it for quite some time (hours at least).

[Vir Campestris]

That's the way to do it.

I've actually straightened a bent walking stick with a heat gun, but steam is far more controlled.

A wallpaper steamer might be a good steam source.

Andy

[Tim Watts]

Yes - I think it would be a good source. You don't need huge volumes if the box is reasonably small (couple of foot long) and not too leaky.

This is a wooden box BTW - bit of WBP ply would probably do a few steamings before it died. Or use wide planks screwed edge to edge and both ends blanked. Top plank unscrews to be the way into the box.

[Thomas Prufer]

Google for "rec.woodworking Steambending wood Frequently asked Questions".

And there's a "bendy wood" at the lumber merchants. AFAIK it's beech that's been steamed and dried.

Also availabel "bendy MDF", MDF with many parallel cuts, like a comb. It needs two layers glued together (with the "teeth facing", as it were) and will probably be too thick for an instrument case.

Thomas Prufer

[Bill]

The steam box is the traditional way, but I'll repeat my method that I used where we needed to steam in situ. I got some polythene layflat tubing and tied it around the rim of the place where the lid fits on an old kettle. We threaded the tubing over the 25foot long timbers of the boat and covered the tubing with old pyjamas for insulation. The kettle could be refilled as necessary via the spout, but the tube was angled to drain condensate back into the kettle.

The only problem was the heady pine vapour and the condensate which tended to froth when back in the kettle.

I also used this for parts eg the tiller, which were strips steamed and bent round a mould, then laminated with epoxy.

[Andy Dingley]

Don't steam anything.

These cases need to be light (it's horribly easy to end up overweight), so the materials will be thin. That means you can cold bend them. 1/8" birch p ly is expensive, but affordable (price/strength/weight ratio is worth it). It's cheaper if you can find a grade with only one (or no) perfect surfaces . You can use a single layer of this, curved around a former and glued. But tress the inside corner with either lots of blocks, almost touching, a stea m bent strip of hardwood (beech or slow-grown ash) or a machined groove in some solid.

To make a compound curve (which is hard), make it from glulam. This is home

-made plywood, made from single veneers laid over a former and then laminat ed. Vacuum bagging is a good way to clamp it all up. Beech or ash are again some of the cheaper and easier veneers to find. This veneered glulam techn ique, with diagonal veneers, is a strong and light way to make a single-cur vature case too. If you're making a number of identical cases and make a go od plug mould with a vacuum bag, this is a really good way to make them.

When constructed, I find Wickes Wet Rot Hardener (bit under a tenner for 50

0ml) is a great surface hardener for wood and easier than diluting epoxy.

[<no_spam]

Interesting idea - I'm happy with bending instrument ribs but hadn't thouht of hardboard - a couple of layers may be too heavy but I'll investigate.

[<no_spam]

I'd seen bendy MDF (too heavy) but not bendy ply - I'll see what price I can get locally. Also, I'll try slotting one side of some standard ply to different depths and see if it will go round a former without breaking

[<no_spam]

I've bent wet wood on a bending iron but haven't tried steaming - I wonder if that's likely to work on some cheap non-quarter sawn 4mm deal... ?

[Tim Watts]

Seem to recall guitar sides are bent on a large diameter steam pipe, ie hot dry bent.

[<no_spam]

The ribs (sides) are frequently wetted during the bending, and it can make things easier to soak them for a while beforehand. Ribs will be around 2mm thick.

A bending iron is a curved (aerofoil'ish, viewed from the top) block of aluminium with an electrical heater inside, although I've seen pictures of people bending around a pipe with a blowlamp playing on the inside of the pipe.

[Nick Odell]

That's how I started out: a blowlamp and a stainless steel tube in a vice in my mum's garage. But I've been using the aerofoilish type for the past fifty years or so. The same bending iron too: it's had a couple of replacement simmerstats and half-a-dozen cartridge heating elements in its lifetime, though. A bit like the famous yard broom.

Incidentally, I'm reading this thread with interest because I've never made cases: I suspect an entirely different skillset is probably required.

Nick

[<no_spam]

Nick, what simmerstat did you use? I've got a home-made bending iron but I haven't got-around to fitting any form of temperature control yet.

Back to case building: I'm planning to use thin ply, joined with kerfed linings (like moldless guitar making - is the board called a solara?) and reinforced at load points with another layer of ply. This will be covered with a honeycomb cardboard layer and then expanding foam to take the shape of the instrument, and finally a fur fabric covering. I'll probably stain and varnish the outside. Hopefully this will give good protection and be fairly light. Several years ago I built a flight case for a stage piano - it worked well but was as heavy as the keyboard!

[Nick Odell]

This bending iron was a commercial product and when I needed a replacement simmerstat I just took the old one into a domestic appliance repair shop and they gave me the nearest equivalent. My current one had to be fitted upside down in the base but electrically it works fine. I'll try and remember to open it up and check the spec tomorrow.

Incidentally, with the mass of aluminium acting as a heat sink it's possible to set a constant steady temperature pretty easily and using a small tin on top of it I cook up quantities of hide glue without the need to use a water bath.

Nick

[<no_spam]

It would be useful to know how the simmerstat senses the temperature (thermocouple, bourdon tube, internal bimetal strip, etcetera).

I've used a modified baby milk heater as a water bath - OK but not brilliant.

[Nick Odell]

If you mean, how does it sense the temperature of the main heating element, the short answer is that it doesn't. The internal bi-metal strip senses the temperature of the internal miniature heating element so what you have going on is an analogue of the main situation. There's more here:

Hysteresis is quite marked on a cooker hob but as I said before, with the thermal mass of the aluminium casting, you can maintain a pretty even temperature with the bending iron.

The one I'm using at the moment simply says Diamond H Controls and it was just a replacement part intended for a cooker hob.

Nick