Replies

1. 
   hammer1 | Aug 27, 2009 02:17pm | #1

   It depends on how wide the frame will be. You want to avoid short grain at the miters. In other words, you might want to use an octagon with 4" wide boards rather than a 4 piece frame with 10" boards. With a circle, you are working in factors of 360 degrees. An octagon would have 22.5 degree miters where a four side would have 45 degree miters.

   I have some pictures of a layout and a bandsaw jig that may help. It's for arched window trim, only half a circle, but would be the same for a full circle.

   Beat it to fit / Paint it to match

   
2. 
   JerryPacMan | Aug 27, 2009 03:20pm | #2

   Try the Rockler polygon calculator.

   http://www.rockler.com/articles/display_article.cfm?story_id=98

    

    

   Life is what happens to you when you're making other plans.

   When your ship comes in... make sure you are not at the airport.

3. 
   BenM | Aug 27, 2009 03:33pm | #3

   What hammer1 said.  Here are the angles for 4 to 10 sides:

   4 sides - 45 degrees

   5 sides - 36 degrees

   6 sides - 60 degrees

   8 sides - 22.5 degrees

   9 sides - 20 degrees

   10 sides - 18 degrees

   The formula is: angle = 180/n

   where n is the number of sides.

4. 
   Senomozi | Aug 27, 2009 03:42pm | #4

   Your approach would certainly work. The angle of the miters is 360 deg. divided by the number of sides divided by two. Therefore if you use an octagon the miters will be 22.5 deg each. There is no set rule as for the number of pieces. The more pieces you have the closer to a circle the shape is and therefore the less waste you have. However that means more joints, more work and more precision because a small error in the cutting angle of the miters gets repeated more times. Given the large size of the mirror I would use an octagon.

   I don't know of formulas to figure out the width and length of the pieces but since I am a fan of CAD I took the liberty of drafting something for you (see attached). This geometry will allow you to have an inner diameter of 35" so you can mill a 1/2 wide rabbet on the back side to house the 36" mirror. Hope this helps. CAD really is a great tool. In this case I used Sketchup (free software)

   
1. 
   KiddervilleAcres | Aug 27, 2009 04:24pm | #5

   Any chance of posting the .SKP file?  Importing your image for some reason isn't working for me.

   Regards,Bob @ Kidderville Acres

   A Woodworkers mind should be the sharpest tool in the shop!

1. 
   Senomozi | Aug 27, 2009 06:30pm | #7

   As requested I am attaching the .skp file.

5. 
   RalphBarker | Aug 27, 2009 06:09pm | #6

   More sides to the polygon means it's closer to being a circle, so there's less waste. But, as mentioned, that also means more work at greater precision.

   A practical consideration to think about is what you'll be using to cut the angles, and what (reasonably accurate) preset stops are available on that tool. Stops at 22 1/2° are pretty common on sliding compound miter saws, for example. Still, you might want to do test cuts to confirm the accuracy of the preset stops.

6. 
   Jimurock | Aug 28, 2009 01:41am | #8

   You might find this dohickey useful for setting up tools: http://www.leevalley.com/wood/page.aspx?c=2&p=32521&cat=1,43513

   Jim

7. 
   KeithNewton | Aug 28, 2009 02:43am | #9

   Aside from the information that you have been getting relative to number of sides and angles, I will assume that you already know that stuff, since you are coming from another trade.

   One of the next most important things is to get good joints, since a piece of glass that big can get pretty heavy, especially if it is beveled.

   I like to use a finger-joint router bit to cut the ends of the miters, then glue up two half circles, before cutting the final joints on the ends of the halves. This eliminates any problems due to being off any fraction of a degree on the miters, gets solved by cutting a straight line across the ends of oversize parts used here if you plan it right.

   Also, I like to use a router trammel to make a template which I use to shape the final circle using a template bushing, or bearing over and or under bits.

   Also, when sawing out the rough parts, after the miter, and fingers are shaped, I cut a little notch near the end / toe of the miters to clamp across the ends of the joint, then a pipe clamp across the end of the half circle closes the heel of the joints.

   Keep a scrap of the parts used to set up the router to squeegee the glue into the fingers.

   With this method, the joints will be strong enough to make the frame thin if you want, and if the wood is appropriate. Then if the chosen wood is inappropriate, making the :joints larger along with fewer segments only makes the likely-hood that it will self-destruct more likely.

1. 
   KiddervilleAcres | Aug 28, 2009 04:36am | #10

   Keith,

   I'm assuming this will be a hanging mirror so am wondering about how best to hang it?  What I mean is there a way it should be hung so the weight of the mirror won't compromise the frame?

   Regards,Bob @ Kidderville Acres

   A Woodworkers mind should be the sharpest tool in the shop!

1. 
   KeithNewton | Aug 28, 2009 05:11am | #11

   Well Rob, I don't know. You will have to ask Mr. Slowtolarn that one. You know if I were mitering arc parts which attach to something for support, I would just miter the joints and nail them to the wall or carcass, but for things like this that need to stand alone, the joint gets more important. If you have never used fingers on miters, I would suggest that you add this to your bag of tricks. It is easier than you might think once you figure it out. You set the bit height where the center of the parts hits the middle of the slope between two fingers. Then set the fence to the back of the finger, with the smallest gap that you can manage. Then you run the work down the slope of the grain, both ways. When you flip the parts, the face surface align, and you lengthen the glue-line, and change it to shear, rather than butting end-grain, so you really get a strong joint. You ought to give it a test drive, and record the results for your blog. I am sure there are a lot of folks that could benefit from a little trick like this.