Morticing of curved stair stringer
I’m thinking of building a curved stairway and would like information on cutting the mortises for the treads and risers. I have built stairs with straight flights but this will be my first attempt in the round.
I have all of the good texts on the subject by they all lack detailed description in this area with respect to using modern equipment. I would like to see photos/sketches of your setups. Thanks to all.
UBC
Replies
If you haven't already done so, post over in Breaktime, as there are a couple of stair builders there that do great ones.
UBC,
While I have not built a curved staricase, I have studied them a fair bit and have pondered the same curved stringer mortise processing issue that you ask about, and here are some of the thoughts I've had.
It seems to me since the stringer surface will be following a pre-set curvature, and probably a fairly broad curve at that, it would be logical to use a template to guide a router to produce the mortise. That is, the template would have an opening for the router bit, sized at either actual size, for using a bearing guided bit, or at a size with the use of a collet guide factored in.
One could produce a mortise with a flat bottom using a template that was flat on the router side and curved on the other side to fit against the stringer. You may however prefer the mortise to follow the curve of the stringer. I think I would - it would be a much better connection. If that were the case, I'd approach the jig thoughtfully. One would need to make up a cradle that guided a router along a path of the same curvature as the stringer. You could shape two pieces of MDF with the stringer's curve and construct a saddle-shaped jig, using the curved templates as opposing 'walls', and connecting them, perfectly aligned and squarely, with an MDF 'floor' and appropriate bracing. Then you would make a platform (which becomes the 'ceiling' in relation to the other parts of the jig) of MDF to carry the router. The floor and ceiling of the jig are in plane with the stringer, and the 'wall' of jig, curved along their top, are perpendicular to the stringer. The walls would need to be separated widely enough to allow the stringer to follow its line of descent.
The router platform would need wheels at all four corners - these wheels would run in a guided fashion along top of the curved MDF edges.
Once you had the path of travel for the router sorted out, then a means to control extent of travel would need to be done, using some sort of limiting stops of some sort. Also, there would need to be a means of fine tuning the position of the router so as to cut the mortise in precisely the right place(s).
The tread tenon(s) would be simple to process with an MDF pattern-guided router approach.
It should work for a stringer that is curved as a section of a circle - one jig would be needed for the outer stringer, and another for the inner. However, if the stringers were in logarithmic or other non-cicular curves, then a different approach might be best, since each location of tread would have differing curvature. I'd have to think some more about that.
Edited 11/5/2006 4:17 pm ET by sparrow
Edited 11/5/2006 4:18 pm ET by sparrow
Edited 11/5/2006 4:22 pm ET by sparrow
Edited 11/5/2006 4:22 pm ET by sparrow
A couple of dowels on the bottom of the router base will allow it to follow the curve. You can then set up a fence that you can clamp to the skirt and rout away.
J.P.
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