good evening,
it,s very nice to be able and put my question on to you. in my next project, one of the challenges are two bowed frames in which a also bowed glass wil be fitted. forunately I’ve made an agreement with the glass company to hand over the frame to them so that they will size the glass to the frame. my question is about how to build the bowed frame. I’ve plannned the thickness of the frame about 20-22 mm, the frame width is 75 cm and the radius is about 21 degrees, whereas the bowed “height” is only about 23 cm. the styles and the rails have to be made in the 21 degree radius. could you give me a clue how to handle. if you like I can send a photo of the project. thanks for replying.
Replies
Hi,
From what I can tell from your query, and if I understand it correctly, it appears that you will have a radius of 42cm for the bow. That is if the case measures 75cm across and the height of the bow is 23cm. The arc of the curve may be that 21 degrees you mention.
The most reliable method for making this frame is to glue up a bent lamination. For a frame only 20mm thick, your laminates will need to be fairly thin. It is the number of laminates used that will determine springback. The formula for springback is 100/ n squared -1, where n is the number of laminates. Thus:
2 laminates 100/4-1 = 100/3 = 33% springback
3 laminates 100/9-1 = 100/8 = 12.5%
4 laminates 100/16-1 = 100/15 = 6.67%
5 laminates 100/25-1 = 100/24 = 4.16%
6 laminates 100/36-1 = 100/35 = 2.86%
So, making up a two part gluing form, male and female, with two different radii for the inside and outside of the curve, you can glue up your laminates to create the bowed rails. I don’t believe you’ll need to curve the stiles. I use MDF to make the forms, first creating templates of the curves required. Then after roughing out that shape on the band saw, I template rout the first layer of MDF. From there you can use the previous layer as the template for the next. Rough out
each layer on the band saw and then rout to shape to create your gluing forms.
With these forms prepared, mill up a piece of suitable stock, and mark it with a triangle on its edge so it glues back together in the same order. This will ensure a more consistent looking lamination. Resaw the laminates off on the band saw and then plane or sand them to a consistent thickness. Get your clamps and glue ready. For larger glue-ups, I would use plastic resin glue to glue up the laminates so that you have greater working time and less creep in the joints over time. Good luck.
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