Matching Curves

[Narayan Nayar]

Hi everyone.

I've done plenty of template routing, but I'm working on a step stool for my son which has an arched "cut out " handle. I'm having a bear of a time getting the arches to match perfectly.

There are a couple of things which trip me up:

The curve is not a section of a circle; it's more elliptical. So I can't use a circle cutting jig.
If I'm thinking through this correctly, I need a convex template for the top of the stool and I also need a concave curve for the inside edge (and the rest) of the handle. It's these two templates which need to match exactly. The length of the handle (about 10") and the thinness of the handle (about 1" will really show any deviation between the two shapes.


I've made the hardboard template for the top arch, sanded it down on an OSS, and have traced that shape onto another piece of hardboard. It's difficult to sand both pieces exactly.

Any thoughts? Just keep making templates until I get it right? :confused:

[Jamie Buxton]

Why does the handle need to have that uniform 1" thickness? Consider a handle which is thinner in the middle than at the ends. There's nothing wrong with that, is there? It looks pretty natural in my mind's eye. After you're relieved of the burden of making the curves perzactly parallel, you have a little more freedom in shaping the templates.

[Tim Sproul]

If I understand correctly, you want the handle to be uniform thickness between the arch of the cutout and the arch defining the outside of the side.
One easy way is to change the bearing on the bit. You might have a hard time locating a bearing that is 2 inches larger than on your pattern or template bit.
Another alternative is to make a bent lamination as the part that defines the arch. Make the lamination 1" thick and bend it to the shape you want. Attach/fasten to other wood to define the sides of the work.

I thought you were needing to make a negative of a certain curve, when I saw the post title.

[Doug Shepard]

Making a template for the inner arch would be good for penceling starting and stopping marks on the stock but for doing the actual cutting I dont think I'd use a template. If you want the inner one to exactly follow the outer one a router edge guide would do the trick. You'd want the type with 2 small lobes for following curves rather than a single straight fence.

[Jamie Buxton]

Would you consider using a CNC to cut the template? The Sawdust Shop (http://www.thesawdustshop.com/) has a Shop-Bot which you could use.

[david kramer]

When creating a template, how about:

- Create the upper curve however you like
- Set a compass to 1", run the point along the upper curve while the pencil scribes the inner curve.
- Rough cut the inner curve to within 1/8 of the line with a jigsaw
- Finish on an oscillating spindle sander or a rasp

The corners will need to be chiseled out, shouldn't be too bad

David

[Ed Bamba]

Lonnie Bird has a tip in one of his books about mathing curves between the top and bottom of a goose neck molding. This will make the distance between the curves uniform. He suggest using a round, smooth disk, preferably made on a lathe, that matches the diameter of the curve's offset. A marking hole needs to be at the exact center of the disk. To draw the offset line, place a pencil in the hole and draw the second (offset) line while rolling the edge of the disk against the template.

If my interpretation doesn't make any sense, I can relay his exact verbage directly from the book.

Ed

[Dan Lee]

Here is a commercial version of what I believe Ed described. I would shape the top curve first then double sided tape a thin piece of wood to the curve that the wheel can bear against.


http://www.leevalley.com/wood/page.aspx?c=2&p=50272&cat=1,42936,50298&ap=1

[Thomas Williams]

I would make the cut out for the handle in the template. To establish the arch on the handle I would use a scribe set at the 1 inch you want. Cut the template and sand to a smooth and uniform arch. Once the template is made, you can then mark the stock and rough out the inside of the handle with a jig saw. Holding the finished stock inside face to inside face, mount the template and route both pieces at the same time.

[Peter Quinn]

Have you tried an ellipse jig for the router? I won't describe its construction here but there are plenty of good explanations on the web, probably a few here on the creek. Should work if it is a true ellipse.

If it is a free form curve that is neither a segment of a curve or a correct ellipse, I'd go with an edge guide with two wheels so that the second arch can be cut directly from the first to make a perfect template.

[Ed Bamba]

Here is a commercial version of what I believe Ed described. I would shape the top curve first then double sided tape a thin piece of wood to the curve that the wheel can bear against.


http://www.leevalley.com/wood/page.aspx?c=2&p=50272&cat=1,42936,50298&ap=1

Thanks for clearing that up Dan, that's exactly the type of disk that Lonnie has pictured in his book. Too bad their largest size is just one inch. I guess one can simply position the template over the first line and draw to the required offset dimension.

If I only knew about this handy tool when I was drawing an "S" shaped template for the top rail for my daughter's day bed.

[Narayan Nayar]

Hi.

Thanks for the suggestions, everyone. I thought I'd followup with the results.

The issue was never layout. Drawing the curves was not a problem--the problem was getting two curves to be cut identically. I definitely suffered from some analysis paralysis here--spent way too long trying to find a perfect solution.

First, I double-stick-taped (in the past tense, is it double-stuck-taped? :)) both sides of the stool together and roughed out the handle tops (the outer curve) on the bandsaw. I then used the template and my router table to finish that outer curve. Nothing special here.

http://www.narayannayar.com/images/curve_template.jpg

Next--a jigsaw to rough out the inside shape.

http://www.narayannayar.com/images/curve_rough.jpg

I then used some scraps and double stick tape to rout flush the straight parts of the handle. I used floats to square up the corners.

http://www.narayannayar.com/images/curve_corners.jpg

Back to the point of this followup--the inside curve. In the end, Doug Shepard's comment probably pushed me in the right direction. I looked for the legendary guide of which he spoke--the one with "2 small lobes for following curves rather than a single straight fence". And I ended up making my own...from about 1" of 3/4" oak dowel cut in half, double-stick taped to my router's edge guide.

http://www.narayannayar.com/images/curve0.jpg

When the bit was placed inside the curve, I could take multiple passes easily, pressing the guide/dowels against the outer curve, using the edge guide's microadjust to sneak up to the line with each pass. Here's a photo from underneath.

http://www.narayannayar.com/images/curve2.jpg

The guide bearing isn't riding against anything--it's lower than the bottom of the two pieces. But I did use the bit with the bearing so that I'd know where to stop (I left the straight pieces taped to the stool sides).

It all went rather swimmingly and I couldn't be happier with the results.

http://www.narayannayar.com/images/curve4.jpg

This project has taken me a long time, mostly because I've been swamped at work and time in the shop has been limited. I finished with the woodworking and surface prep before my last business trip; today I'll prefinish all the parts and glue it up. Here's the dry fit:

http://www.narayannayar.com/images/stool.jpg

This is the first project in a long time which I "documented" in photos. If anyone's interested I'll try to put together a small writeup. It was a fun project (I don't generally work on anything this small).

Thanks to everyone who chimed in!

[William OConnell]

That is very nice work there Narayan. Thank you for the pictorial it was well done also:):):):):):):)

[David DeCristoforo]

"Too bad their largest size is just one inch..."

FWIW, these are very easy to make in whatever size you need. 1/4" MDF, ply or plastic will work fine.....

[Doug Shepard]

...
Back to the point of this followup--the inside curve. In the end, Doug Shepard's comment probably pushed me in the right direction. I looked for the legendary guide of which he spoke--the one with "2 small lobes for following curves rather than a single straight fence". And I ended up making my own...from about 1" of 3/4" oak dowel cut in half, double-stick taped to my router's edge guide.
...


Well OK - the guide I have is legendary:D but their's didn't do it much differently from your dowel trick http://microfence.com/mf/PDFs/EG-Fig2.pdf
I have no idea where it came from or what it fit but I had one kicking around for years that had the lobes cast right into the body. I didn't fit any router I ever owned and was rather cheap so I pitched it finally. The project turned out well and you get extra points for using yer noggin:D