Steam bending yet again

[Arron]

Hi,

Well I have started on a new suite for the spare bedroom - in brushbox. First up is a chair, so again a slab of timber needed a gentle bend. This time it was a single piece of brushbox 650 x 210 x 22. Brush box is rated as 'unsuitable for bending', so knowing I had little chance of bending it in the proper technique I went for the 'just crush it between male and female forms with a trolley jack method'. After 55 minutes steaming, out it came and into the form, couple of cranks on the trolley jack, some awful noises ensuing, then left for a few hours to cool down. Results weren't too bad at all. The springback was around 30% as expected giving me exactly the required bend, and there is no visible damage in the sense of stress fractures or overcompressed fibres. Probably bending in this manner is a pretty awful thing to do to wood but the piece is non-structural and cleaned up afterwards it will look just fine.

The odd thing tho', is that some cracks right across the middle which were almost unnoticeable before have opened right up. They are lengthwise to the bend so I think their appearance owes to the heating/cooling, not to the bending stress.

Probably this job would have been better suited to laminating. However I was attracted to the idea of getting it over and done with in one hour and being able to fit the finished component the same afternoon.

Also, someone asked previously about getting information on the suitability for bending of Australian timbers. I picked up a brochure from State Forests NSW on bending which has a rating for 40 or so species. My scanner is not working so I have done a digital photo of the list which I will post in the next two replies. Apologies for quality, but its readable if you try hard enough.

Arron

[Arron]

First half of the list:

[Arron]

And the not so easy ones:

[Rocker]

Arron,

It still seems to me that glued bent laminations are the way to go for curved chair parts, such as back slats or rockers, rather than steam bending; there is minimal spring-back with with glued laminations, so the resultant shape is much more uniform and predictable. Curved back rails do not need to be bent anyway - the curve can simply be band-sawn from thick stock.

Rocker

[Arron]

You are probably right, Rocker, laminating would have given a more predictable and marginally more attractive curve. However, like most weekend warriors I have to make the best use of the gear I own. I do own a kettle barbecue but dont own a bandsaw (to cut from single piece) or a thicknesser (to make the thin strips for laminations), so I dont have much choice.

I suppose I could have paid someone else to cut the laminations for me, but it all adds to the 'frigging around factor'. To cut from a single block would have needed a piece 550 x 200 x 70mm - even if you could find that in brush box I think I would need a second mortgage to pay for it.

Still, I would like to try laminating next time. Am I overlooking something ? Is there somewhere I can buy thin strips of less common timbers like brush box , or is there some easy way of making thin strips I am overlooking ? I know from experience that the strips in brush box would have to be about 6mm thick to allow a nice bend without cracking.

Arron

[Rocker]

Arron,

I think you are missing something. It is easy enough to rip off thin strips with the table saw, so long as you have a zero-clearance insert. Although it means that you have to keep re-setting the fence, it is safer to to cut the strips with the strip on the opposite side of the blade to the fence. You can can plane away any saw marks with a hand-plane. Of course, this method works best for strips narrower than 150 mm, since, with a 250 mm blade, you can cut strips that wide in two passes by flipping the workpiece. It is still possible to cut wider strips on the table saw, if you use a hand panel-saw to cut away the waste between the two passes. But, generally speaking, the bent laminations you usually need in chair-making are only 30-40 mm wide anyway.

For some years, I was in the same position as you in lacking a band-saw. During that time I used a router to hollow out the concave side of chair rails. Sandor Nagyszalanczy, on P. 69 of his book Woodshop Jigs and Fixtures, describes a jig where a sled-mounted router rides on two curved rails to cut a concave curve in the workpiece. The workpiece is clamped in a different position after each pass. It helps if you have a compass plane to clean up the concave surface, but at a pinch you can do it by sanding.

The convex side of the rail can easily be shaped with an ordinary bench plane.

Rocker