Re: Stretcher to Post Joinery
Posted: Wed Feb 03, 2016 11:06 am
In the preceding examples, we looked at the half dovetail connection, done blind. As noted there, a longer dovetail and deeper mortise mean greater surface area for interlock and thus the longer example should be the stronger example.
Taking this idea a step further, we could bring the half dovetail right through the receiving member:
Note that the wedge hasn't been detailed in the above drawing, and there is no mortise provided for the wedge, so the above is simply for sake of example.
To maximize the dovetail, it is extended to the exit face of the receiving member. It's hardly against the law or anything like that, but there is no reason to configure the connection so that the dovetail ramp surface sticks out any further. With a portion of the stretcher protruding beyond the receiving piece, it is more desirable to present the stretcher as a clean pair of horizontal lines passing through, and not have the joint mechanism be any more obvious than it already is. remember, this form of connection is generally employed on framing that is not visible to view, and so the aesthetics are less important in that context. If you choose to adapt it for furniture work, there is absolutely nothing wrong with that, however there are plenty of alternatives. The loading on the parts should guide the joinery decisions ahead of aesthetics in most cases, and there may be several options which meet performance goals and yet offer quite different aesthetics.
With the connection passing fully through the receiving member, we have options to wedge the connection as before, from the inside face:
Or, we can drive the wedge in from the outside face:
Notice that in either case, if we keep the wedge taper the same, at 1:8, the lengthening of the wedge can result in one end of the wedge becoming quite large. This may be objected to on aesthetic grounds in cases where the connection is exposed and not buried in a wall, but also there is the issue that, as the wedge section becomes larger, it becomes prone to greater overall seasonal movement. We could make the wedge ramp angle shallower, say 1:10 or less, to bring the dimension of the fat end down a bit, however the trade off there is that the friction between the wedge and abutment goes up, making it a little harder to fully seat the wedge and definitely more difficult to remove the wedge should the need arise. Also, if the wedge requires more force to drive in, then the potential for damaging the end of the wedge by insertion increases. These are not huge issues, and are less a factor with softwoods than hardwoods.
Another issue concerning aesthetics relates to the end of the wedge relative to the end of the stretcher. Should they be flush? Probably not, as you would likely want to chamfer the arrises around the end of each piece. Having the wedge project beyond the end of the stretcher doesn't make much sense, but if it is to be held back from the stretcher end, then by how much? There is no set answer to that. Generally, the wedge looks best, I think, if it is proud of the post the same amount on entry and exit face, so one might think simply in terms of having the stretcher extend a bit further than the end of the wedge, and that is all.
Otherwise, whether one drives the wedge in from the outside face or inside face is largely a matter of which face provides more convenient access for the hammering.
Another issue arises with driving a wedge though this sort of connection. Here is a video which shows the problem clearly:
https://www.youtube.com/watch?v=rybMmT10rhw#t=52.819174
Notice how the wedge starts to blow out a piece of wood on the exit face? In the next post we will look at that situation a little more and how it might be dealt with, and then discuss other wedging options.
Taking this idea a step further, we could bring the half dovetail right through the receiving member:
Note that the wedge hasn't been detailed in the above drawing, and there is no mortise provided for the wedge, so the above is simply for sake of example.
To maximize the dovetail, it is extended to the exit face of the receiving member. It's hardly against the law or anything like that, but there is no reason to configure the connection so that the dovetail ramp surface sticks out any further. With a portion of the stretcher protruding beyond the receiving piece, it is more desirable to present the stretcher as a clean pair of horizontal lines passing through, and not have the joint mechanism be any more obvious than it already is. remember, this form of connection is generally employed on framing that is not visible to view, and so the aesthetics are less important in that context. If you choose to adapt it for furniture work, there is absolutely nothing wrong with that, however there are plenty of alternatives. The loading on the parts should guide the joinery decisions ahead of aesthetics in most cases, and there may be several options which meet performance goals and yet offer quite different aesthetics.
With the connection passing fully through the receiving member, we have options to wedge the connection as before, from the inside face:
Or, we can drive the wedge in from the outside face:
Notice that in either case, if we keep the wedge taper the same, at 1:8, the lengthening of the wedge can result in one end of the wedge becoming quite large. This may be objected to on aesthetic grounds in cases where the connection is exposed and not buried in a wall, but also there is the issue that, as the wedge section becomes larger, it becomes prone to greater overall seasonal movement. We could make the wedge ramp angle shallower, say 1:10 or less, to bring the dimension of the fat end down a bit, however the trade off there is that the friction between the wedge and abutment goes up, making it a little harder to fully seat the wedge and definitely more difficult to remove the wedge should the need arise. Also, if the wedge requires more force to drive in, then the potential for damaging the end of the wedge by insertion increases. These are not huge issues, and are less a factor with softwoods than hardwoods.
Another issue concerning aesthetics relates to the end of the wedge relative to the end of the stretcher. Should they be flush? Probably not, as you would likely want to chamfer the arrises around the end of each piece. Having the wedge project beyond the end of the stretcher doesn't make much sense, but if it is to be held back from the stretcher end, then by how much? There is no set answer to that. Generally, the wedge looks best, I think, if it is proud of the post the same amount on entry and exit face, so one might think simply in terms of having the stretcher extend a bit further than the end of the wedge, and that is all.
Otherwise, whether one drives the wedge in from the outside face or inside face is largely a matter of which face provides more convenient access for the hammering.
Another issue arises with driving a wedge though this sort of connection. Here is a video which shows the problem clearly:
https://www.youtube.com/watch?v=rybMmT10rhw#t=52.819174
Notice how the wedge starts to blow out a piece of wood on the exit face? In the next post we will look at that situation a little more and how it might be dealt with, and then discuss other wedging options.