Monday 9 July 2018
Most of the timber for my spindle turning work is cut from commercially available boards, mostly American and European hardwoods, all of which are readily available from timber merchants. All of these timbers are easily obtainable in sizes up to around 75mm (3in) thick, but there are associated problems with thicker timber, oak (Quercus robur) especially tends to have deep cracks in the centre of thick boards and stability can also be an issue. So what happens if I get a job which requires something with a greater diameter than 75mm (3in)?
The simple answer is that it is quite acceptable and indeed traditional to glue up two or more thinner boards to make up a larger blank, but it is not quite as simple as slapping the glue on and clamping it up, there are a few things to be taken into consideration first. In this article I will look at the best way to approach this sort of glue up to achieve perfect results by showing you the stages of making up a pair of 115mm (4 1/2in) diameter finials.
The first thing to consider is the best way to align the boards. Take a look at the end grain of the board and notice the annual rings. To ensure the blank stays glued and doesn't try to come apart at the edges you should put the boards with the rings 'back to back.' Additional boards should be added so that, should they try to cup, the edges stay tight. Once you have selected the timber and decided which faces are to be glued together you need to prepare the timber. I usually cut it oversize just to be on the safe side. I have industrial machines such as a table saw and planer/thicknessner which make light work of cutting and flattening timber
Your timber should now be prepared and marked ready for gluing. Lay out all of your cramps first and 'dry fit' to ensure that you don't get too stressed during glue up. Spread an even layer of good quality PVA or resin-based glue on one side of the joint, position in the cramps and tighten well by hand. The joint will pull up and there should be an even amount of glue squeezed out of the joint. I use sash cramps for this but you could use G or F cramps. The key is to use as many as possible to achieve a good, even clamping pressure
Once your blank is dry and ready for turning you need to consider how you should mount it on the lathe. My first choice for this type of work is the between centres method, although you could choose to hold it in a chuck in compression mode. If working between centres with a blank that has a central join, it is important to use some sort of ring or Steb centres at both ends as the traditional prong drives could weaken the joint or at worst, drive the joint apart, which you do not want
With the glue dry and your blank mounted securely, you can now turn the blank as normal. I used a copy template to replicate the design. Bear in mind that you are now turning a large lump of timber so begin at a slower lathe speed and build up. To reassure you if you are nervous of this kind of work, modern glues give a joint that is stronger than the actual timber and with jobs like this there is a large gluing area so it is extremely unlikely that the joint would fail, either during working or indeed in the future