Sizing curved panels

[peter-harrison]

I make a lot of curved panels in a bag press. The trickiest thing is getting them square after gluing up. You can't just cut them square as you would with a flat board, as they also have to be true to the curve.
What I usually do is put the panels back on the former, pencil along the edge of the former, and then bandsaw and hand-plane them true. This is long-winded, and if the panels spring and don't fit snugly on the former, it can be a bit hit-and-miss.
Anyone got a better way?

 

[Brandlin]

Trying to visualise your problem. I guess the issue is that you cannot rely on any of the edges of the curved panel coming off the former being co planar so you have no known reference to say a table saw bed. Logically the only reference surface you have is the inside of the curved face.

To cut the curved ends my only suggestion would be that you build a jig that holds the axes of the curved surface perpendicular to the table saw blade with the concave face upwards and allows you to slide/rotate the panel in the jig. (I'm guessing your curvature is sufficient that blade height wont allow you to do this convex face up?) This doesn't sound ideal and a risky maneuvering the panel on the saw.

The straight sides would be equally as tricky as you cannot place the panel convex side up as the edges will not be coplanar. I think you'd need a jig that not only clamps the panel on a sled concave side up but positions it in such a way that the cut is radial to the curve of the panel. At least if you have cut the curved ends first then you can use the end to reference a perpendicular cut to them.

All in all I can't think of a simpler way of doing it than you already are. I might prefer to cut hand saw the edges rather that move the panel on a bandsaw table though?

EDIT : Oh and of course you need a different jig for each panel radii. I can imagine devising a jig with three parallel 'poles' across it. The central one adjustable for height. This would allow you to position any curved panel so that its axis is parallel to the 'poles'. Mount this on a jig that allows it to rotate and you could accurately cut each end. Add a 90 degree pivot and you can rotate this to then be perpendicular to the blade. Might be an interesting academic exrcise but i can't see it being any faster or better than what you already do.

 

[custard]

Here's a few of the lamination formers I use, you can see that I screw bits of scrap to the sides as referencing aids,

Here's a male former (there's a matching female former that goes on top), as you can see there are several referencing tabs screwed on as this one is very liable for the lamination strips to try and skew around)

Even with this you probably won't get a completely clean edge, and for bag press work it's even worse. With bag press laminations I'll sometimes deliberately set one of the lamination sheets about 3mm in from the others. This inset edge will become the reference edge, it all depends though on how well you can clean off the glue squeeze out, if you can do a good job you can then use a copy router to trim to the reference edge. If you can't get all the glue off then you can cramp another flexible sheet on top of the finished curved workpiece, inset it by a mill or so, and use that as the reference edge for a copy router (with a top mounted bearing). As long as the curve isn't too extreme I find I can copy route accurately enough with a hand held router like this, you have to pay attention but it's do-able.

Next option is to run the ragged edge of the work piece over your planer machine, or use a hand plane. Once you've got a clean reference edge it's possible to push this reference edge against the fence of a table saw and careful feed the opposite edge through the blade to trim it parallel. You have to lift the workpiece to begin with and slowly lower it during the course of the cut. Again, it takes a bit of practise but it's do-able.

Incidentally, with vac pressed curved panels I find I have to confine any subsequent cuts to close to the edges, otherwise the inevitable glue pooling in the centre of the panel gives a visible glue line. Good enough for most joinery purposes I guess, but not what you'd aim for with fine furniture.

Hope that helps.

 

[davin]

On the few curved panels I have made i encountered the same problems.
I ended up using a biscuit jointer along the long length. I pinned the panel to the former placed the former and placed on a flat surface (panel saw?) Adjusted the blade to the height I needed and move the jointer along . Worked, but this was for 6 or 9mm panels, not 18mm.
Could you do the same with a grooving cutter on a spindle moulder or router table, make a big false bed for the former?

Davin

 

[doctor Bob]

I always mark the centre line of the former and transfer on to the panel and work from that, can't go wrong.

 

[dzj]

I'd use a saw blade on a spindle moulder, a jig/ work holder to hold the curved piece and a guard to limit the amount of protrusion of the blade. The sawn-off part would fall between the table and blade and thus eliminate possibility of kickback.

 

[peter-harrison]

Thanks for your advice- it seems that a pencil and a bit of hand planing may be the simplest!
Cheers!