Steam Bending Jig

Steam Bending Jig

I will add some pictures asap
Inspired by failed bends and James Mursell’s book Rob Exton builds a steam bending Jig.

I have been involved with many bendings over the past few years and have experienced my fair share of failures. It takes a long time preparing a blank and steaming for the requisite time, only for the bend to fail. This is one of the most frustrating elements of green woodwork and one which got me motivated to lessen the risk.
It goes without saying that the first requirement of a good bend is a good blank. With Ash, a knot free, straight and evenly grown length is desirable which has been prepared to tight tolerances in both dimensions.
Steaming time is another critical factor with many opinions regarding the correct amount of time and how long you have to complete the bend once removed from the steamer. Such is the variety of opinion I pass no comment as this article concentrates on the mechanics of the bend.
My observations regarding failure are that one or more errors have crept into the process which has caused the blank to twist and/or lift. These errors are exacerbated when 2 people are pushing/pulling on each bending arm as their differing strength creates unequal forces. The errors that creep in are:-
The arm lifting off the bench creating an upwards twist.
Wedges working loose as other are hammered in.
Unequal pressure with one side being completed while the other is only half way there.
A weaker person tugging at the arm with a jerking motion.
Using a poor strap. (I have used heavy metal ones and nylon webbing, neither of which are ideal)
The end block being narrower than the blank being bent.
All of the above are difficult to control if you are working alone.
Tim Gatfield and I had lengthy discussions about the reasons for failure and how we could overcome them. We came up with some Heath Robinson ideas with pulleys and levers looking more like a piece of torture equipment than a simple woodland device and they would have taken considerable engineering to build. What we needed was something simple, but effective.

Two Pieces of the puzzle.
Two of the problems were easily tackled with an updated bending strap. Tim ordered several straps of varying width all 60 inches long made from 1mm stainless steel for £8.50 each from an engineering workshop equipped with a suitable guillotine. These straps are ideal as they don’t stretch but are still flexible.
I made end blocks from hardwood, with cut out sections to ensure that they stay flat on the bench, which were bolted to the stainless strap. It is important that these blocks are wider than the wood being bent to ensure that the correct compression is achieved on the inside of the curve. If the blocks are short the corner of them will dig into the blank and it will not compress the edge.

The Eureka moment
No it didn’t come in the bath, rather on the drive home when I recollected reading James Mursell’s book “Windsor Chairmaking”  sometime earlier and was pretty sure he would have some words of wisdom.
Sure enough on page 54 he describes in words and pictures his jig which for want of a better description is akin to a giant cross bow. Via email James kindly answered some questions I had with his final comment being “the most important part of the rig are the ‘fairleads’ available for chandlers” I did not appreciate the full meaning of this until later.

Making The Cross Bow
The engine for this device is you, with your power being transferred at a ratio by a winch. I got my winch from Machine Mart   (TW600 Hand Operated Winch) for £22 and appears to be weather resistant and substantial enough for this work. 
Any wood will suffice for the frame with tanalised 4×2 being the easiest available to me. You need three lengths:- The central shaft at approx. 60”, a cross piece to support the end at the bench at approx. 30” and a piece of sufficient length to act as a leg which I attached with a T hinge to aid transportability and storage.
With pole lathe cord pulling from the end blocks it is clear that the unit must be mounted to the bending table, level and at such a height as to render the cord dead centre of the blank to ensure a smooth, flat bend with the end blocks flat on the bench at all times. The rest of the design/construction is really a matter of personal preference and common sense. The winch being mounted some 48” back from the bending former.
I had used 2 rows of steel, eye hooks, screwed into the timber as guides for the cord going down the central spine before the cord diverts to the ends of the cross. At the very first testing these were ripped out of the timber due to the very high pressure exerted by the winch pulling against the ash blank. This is the point of maximum stress and is the point that James made to me about using fairleads here as they are designed to take huge lateral stresses without pulling out of the timber.
Apart from this small mishap the jig performed wonderfully, we completed 3 bends in an hour with each being calm, controlled, smooth and a perfect shape. Very little effort is required in turning the winch and the bend is completed in only a minute or so.
Such was the excitement that I neglected to take pictures at the time and hence the re-enactment pictures with a Hazel rod and an already bent and dry chair back which I trust explain these jottings clearly enough.

Cam Wedges.
Another of my areas for concern is that of wedges which are inserted at various points around the bend to keep the bend tightly edged against the former. This helps to keep the shape but also acts to hold the wood flat against the table and helps to prevent twist. I have noticed that with hammering of wedges and general rocking and rolling exertions of the bending, that previously inserted wedges can work loose. This delays the whole operation and can contribute to twist getting started.
After much reading and development I cut some cam shaped wood the same thickness as the bending strip and inserted a 19mm dowel right through so that it can be used in any direction. The 19mm dowel is a great fit for the holes in the bench and holds as tight as any wedge. As the bend is taking place it is a simple and gentle matter of inserting the dowel and turning the cam until it locks in place very securely and exerting good pressure.
After completing all of these developments and alterations my steam bending set up is much more professional, less frustrating and can be undertaken on your own.