Bending a Circular Stair Handrail
Bending a circular stair handrail, or indeed producing any curve with lengths of wood, can be a difficult job to do - if you don’t know how to properly carry out dry or wet bending techniques. These techniques allow you to produce elaborate curvilinear shapes that have structural integrity, making them suitable for use as stair handrails. Dry and wet timber bending techniques produce curves with the timber grain running with the curve, rather than across it, to give the bent wood its strength.
Dry Bending
The technique of dry bending wood is more correctly called - kerfing. Kerfs are equally spaced grooves made part way into the wood with a saw. The width of the saw blade used to make the kerfs and the spacing between the kerfs has a bearing on the radius that you can bend the wood through. Whilst you can cut the kerfs by hand with a tenon saw, you will find using a power saw gives you greater accuracy in terms of the depth you cut.
If the bend is not quite tight enough gently use a triangular file to ease open the kerfs, it is important here that you take the same amount out of each kerf to maintain the bend you require. Use a web clamp to pull the curve or bend into shape, applying glue into the kerfs to maintain the curve.
For extra strength you can glue a laminate to the inside face. If the inside of the bend is to be exposed you can produce a smooth bend by laminating together two kerfed timbers back-to-back.
Steam Bending Basics
It has to be said that wet or steam bending will usually produce a better curve. However, this does require either a very large workshop to do it in or very good weather so that it can be done outdoors. Steamed wood can actually be bent into quite tight radii as the steam softens the wood fibers allowing them to bend and compress.
You will need to make or buy a former, a support strap and steam chest; and be prepared to experiment before tackling your main handrail project.
Selecting and preparing the timber is important before attempting to bend it. Newer air-seasoned wood is always easier to work with; however, if you have some well seasoned wood simply soak it for a few hours before starting to work with it. Also, the wood should be straight grained and without any major knot holes or obvious flaws in it and be smooth to the touch. The strap is simply a frame that can hold a piece of metal the exact length of the wood you want to bend, whilst the former is the shape you want to bend the wood into.
The steam chest is the piece of apparatus in which the timber to be bent will be steamed. Commercial ones can be purchased from DIY stores or you could make your own by constructing a plywood box with a steam inlet hole at one end and a plug at the other.
Then rig up a steam generator, running a hose from it to the inlet hole of the steam chest. As a rule of thumb - leave the timber to steam one hour for every 25mm (1 inch) of timber thickness, don’t over-steam it as the wood fibers can start to break down.
Bending Steamed Timber
Once steamed you’ll only have a few minutes to bend the timber into shape, but you can help the process by having the former and strap pre-heated to help retain heat in the handrail. Fix it into the strap then clamp the strap to the former at the spring point or center.
Bend the wood around the former and clamp it into position as required according to the length of the timber, a second pair of hands is invaluable here. For the best effect have a dry rig to clamp the bent wood to after about fifteen minutes and leave to dry thoroughly for anything up to a week.