Tuesday, 21 February 2012

Continuous Arm Bending Strap

This morning I received a request from Tim in the U.S. regarding the bending strap I use to bend the crest or bow for the continuous arm windsor. This new and improved strap is result of using the original, which was essentially knocked together to get us through the first class we taught on the chair and then went on to serve us faithfully for another four full classes, bending countless dozens of crests.

That strap, a bit of packing strap welded to some square tube cut in half and with a few screws here and there to add grip was the result of a good friend Pete Mc Curly giving some good advice and suggesting the strap to control the breakages and delaminations that were occurring.

At that point I had hand shaved 23 crest rails to shape in one day and steamed them, only to lose all 23 to some form of breakage. Demoralising to say the least. With the new strap we had immediate success and went on have a 95% success rate.

What was also immediately apparent from that day was that "we weren't in Kansas any more Toto" or Tennessee, where we had steam bent white oak crest rails with Curtis Buchanan with spectacular ease, not even the slightest hint of failure and certainly no strap.

So while we don't have the beautiful ring porous hardwoods of the Americas' ideally suited for bending, we do have beautiful timber in it's own right and we just have to learn how to tame it.

As is widely known the strap essentially changes the point of neutrality in the piece from the centre, where there is expansion of the timber on the external side and compression of the timber in the internal side, to the very back of the external face, forcing the entire piece to compress instead. Now while this is nothing new, the issue with the continuous arm crest is bending the piece in two differing planes whilst maintaining that compression. This must also occur in good time so as not to allow the piece to cool.

So this is what we came up with. A 2 piece strap that contains the primary bend of the back and then allows the secondary bend to occur whilst maintaining the overall compression of the piece. To achieve this the shaped piece or blank is fitted into the main body of the strap.

The secondary straps are then placed into the clamping portion of the strap at the point where the taper occurs from the thicker back to the thin hand hold. The bolts are then tightened into locating holes, clamping the secondary straps into that curve. The serrations on the underside of the clamping pad grip the piece and limit slippage ( See below ). This has now taken care of the compression of the first bend of the back.

To contain the expansion of the handholds the secondary straps have stop-ends attached which simply house on the end of the blank. Pressure is maintained on the strap when the secondary bend is being performed to ensure that the stop-end remains in place and contains the expansion.

The reason for stainless steel? The strap is fitted to the piece before steam bending and is put into the steam box. There is simply not enough time to fit the strap to the hot piece after steaming and it would cool too quickly. The stainless has eliminated the staining from the original packing strap metal, leaving a blank which requires less cleanup afterwards.

Having said all that, there is still no substitute for clean riven or sawn, green, straight grained stock. The bending strap is a great aid, but it is not a cure-all. Unsuitable species, grain run out, branch inclusions or other defects will almost certainly result in a failure of some sort, strap or not.

As for dimensions, the strap should be fitted to the bending form you make. Mine is setup for an overall crest rail blank length of 1490mm ( a fraction over 58.5 inches ). The over all length of the main strap is 860mm ( 33.85" ). The secondary straps should be custom fitted to the length of your handhold. The bolt down clamps should just be large enough to accept the blank and the clamping pad.

My bending form is sitting at the bottom of a large pile of stuff while I set up my new shop, but when I get to it, I'll post the dimensions for those of you who'd like to make one.Any other questions or dimensions, please ask! Hope this is of some help. Here's Pete Galbert's thoughts on the strap.

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