Replies

1. 
   sapwood | Oct 03, 2008 05:10am | #1

   For this sort of arc, I usually make a template out of 1/4" mdf. I find it much easier to fair the thin soft material rather than my lumber. Draw the arc, cut to the line with bandsaw or jigsaw, then plane to a fair line with a block plane. With the template, use a router with a bearing bit to make the final cut. You will want to use both a top bearing and a bottom bearing bit depending on the grain direction. Keep the bit rotating the direction of the grain as the arc crosses it.

1. 
   wilkesland | Oct 03, 2008 03:25pm | #2

   Thanks for your response. I thought about a template, but what is throwing me a little bit is the cut is longer than 8'. I guess for thin MDF you could cobble a splice plate to get the length you need?

   I guess what I really needed was to reassure myself there is no miracle trick to set up this cut. Thanks again for your response.

1. 
   SARGEgrinder47 | Oct 03, 2008 03:58pm | #3

   Ditto sapwood on his approach. I will add that if the arc is over 8' and the arc is one continuos flow.. you can lay two pieces out on your work-bench and draw it off. Let's say your arc is 10'. Use a full 8' piece and a 3'-4' piece-mill butted up to draw it. Once drawn you are assured of accuracy.

   But... you don't have to mod the entire MDF piece down. You measure to exactly half way on the 8' piece and then take it down to 1 mm with your files.. rasp.. etc. to get it exact. If you mess up.. MDF is cheap and you just bite the bullet and start again.

   Once down to precise and cut exactly in half.. transfer that to the actual piece and draw one half.. then turn it over and attack from the other side to meet the half you just drew. Cut it with your jig-saw to about 1 mm of the actual line on the real piece. Then replace the half template and I personally counter-sink two sheet metal screws through the MDF into the back-side of the real piece to attach.

   Load an over-head bearing bit in your router and take that last 1 mm off. Then remove the template.. turn it over and attach to the un-cut end. Rout that side to where they meet. You are done with a baby-butt smooth actual piece as long as you smoothed your template properly.

   By using the half template on that large of a piece.. you save much time cutting and smoothing your template to precise. Not a bad idea to save templates as you never know when you might duplicate a piece and that saves a world of time also.

   Or.. just bring it by my shop and I'll cut it for you. ha.. ha... ha..ha..ha..

   Good luck...

   Sarge..

2. 
   oldusty | Oct 03, 2008 04:56pm | #4

         Another simple way to create arches is make only half of a template of the desired arch , mess with it until it is near perfect , then lay it down and draw the first half , then flipping the template draw the second half , the template only needs to be perhaps slightly more then half as wide as the arch .

         Cut with a band saw or a jig / sabre saw slowly , leave the line thick and sand to the line , I use an edge sander but a belt sander can also work as well as a drum sander mounted on a drill press is also possible .

       If you have a router any of the methods already mentioned will also work .

                     good  luck  dusty

3. 
   Jamie_Buxton | Oct 03, 2008 06:01pm | #5

   Must your arc be a piece of a circle?   That's implied by your mention of a trammel.  However, there are many other arc shapes which are easy to produce in a large "radius".  For instance, use a technique from boat-building.   Start with a long stick, narrow enough to bend, but not so narrow that it is floppy.   Secure it at each end of the arc.  Push the middle to your desired bulge.   There's your arc.   You can use it just to mark a curve.   Or, if you put it on the off-cut side, you can screw it to the off-cut, and use it directly as a guide for a cutting tool.

4. 
   PhillipB | Oct 03, 2008 11:44pm | #6

   Here's a trammel that was about 50-inches. I used to for making templates so I wouldn't have to use the trammel over and over again. Sometimes you just have to bit the bullet and make these awkward rigs to git 'er done.

   Best of luck,
   Phillip Anthony

   
1. 
   MikeHennessy | Oct 03, 2008 11:55pm | #7

   Uh, 50 inches is a pretty far cry from 40 feet! He could maybe set up in the shooting range while he was biting the bullet. ;-)

   Mike HennessyPittsburgh, PA

5. 
   basilg45 | Oct 04, 2008 12:24am | #8

   G'day

   This can be done by making a jig for the router.

   The jig consists of two arms of an equilateral triangle, open at the bottom and fixed at the top with the router fixed on the top. The distance from the base to the inside of the router cutter is the rise of the curve ( in your case 12 1/2 minus 7 1/4 equal 5 1/4 ) which must also coincide with the apex of the triangle the distance across the base will be the distance across your opening. When cutting the arc the arms of the jig run on a pin at either end of the opening ( these could be dowels, nails or screws) the arms of the jig therefore need to be a bit longer than the width of the opening. to use you simply slide the router around holding the arms against the two pins and it will cut the arc you are seeking either on the workpiece or a template. Correctly made the router cut will just kiss the two pins as it traverse the arc ( it may be preferable to use dowels or stop short of the pins if nails are used)

   Regards

1. 
   wilkesland | Oct 04, 2008 02:33am | #9

   Thanks everyone for your replies, they have been very helpful.

   Sarge - while Wilkes county, NC is a long way from your part of Georgia (you are still there -right?), I plan to visit my father in law in two weeks as he has a joiner he wants to give me and he lives in Clarksville. Methinks that is not so far from your shop....... careful what you offer.... :^)

    

   Basilg45 - your jig intrigues me, but I have read your post about a dozen times now and my synapses are still not firing completely. I get the equilateral triangle part and the necessity of the router to be at the apex. Where I am getting lost is the legs of the triangle. When you speak of the opening, are you referring to the length of the cut, which in my case would be 101 inches?

   Regards

   Barry

1. 
   SARGEgrinder47 | Oct 04, 2008 02:44am | #10

   Clarkesville is about 70 miles north-east of me in Lawrenceville. It's on 441 before Clayton and the N. C. line that passes into Murphy I believe. Frankly my metro county which "was" rural 40 years ago is not rurul anymore and I would love to live around Clarkesville.

   But.. drop by anytime as I was not kidding about what I said. I started a coffee talbe Wed. but.. I don't have deadlines.... anymore anyway. :>)

   Sarge..

    

1. 
   wilkesland | Oct 04, 2008 04:58pm | #13

   Thanks for your kindness - I couldn't resist teasing a bit. However, I think I need to learn to make this cut. As you say, never know when another might come along.

   My FIL's place is very nice. They have a nice view of the mountains. When he retired and built a house up there, he built his wife a house on top of his woodworking shop, if you know what I mean. Now, he is beginning to pair back on his woodworking :^(  hence his offer of his joiner to me.

   Regards

   Barry

2. 
   QCInspector | Oct 04, 2008 04:58am | #11

   http://ca.geocities.com/web_sketches/circle_calculators/circle_radii/circle_radii.html"Basilg45 - your jig intrigues me, but I have read your post about a dozen times now and my synapses are still not firing completely. I get the equilateral triangle part and the necessity of the router to be at the apex. Where I am getting lost is the legs of the triangle. When you speak of the opening, are you referring to the length of the cut, which in my case would be 101 inches?"If you look at the math drawing linked, your router would be fixed at the apex of the triangle and the arms touch the pins at the ends of the base (your 101"). The arms need to be long enough that as the router goes from one side to the other they make contact with the pins throughout the cut.

3. 
   basilg45 | Oct 04, 2008 08:57am | #12

   BarryYes, you are correct it would be 101 inches along the base of the triangle. Sorry if I confused you a drawing would have simplified things, but I have not figured out how to include a drawing in responses. Good luck with the project.regards

1. 
   wilkesland | Oct 04, 2008 05:04pm | #14

   basilg45,QCInspector - thanks for your replies, I've got it now. Given the length of this cut, I think I will wait till I have a need for a shorter arc and a much smaller jig. I will definitely keep the jig you suggested in mind.

   Thanks again for your responses.

   Regards

   Barry

1. 
   basilg45 | Oct 04, 2008 11:54pm | #15

   You're welcome have fun with your project