Creating complementary curves

[Scott Cardais]

I have a question about a technique for creating complementary curves. This technique seems easier than those. Have read about or videos I've watched on this subject. I'm wondering if anyone has experience with the method I'm considering.

The ultimate goal is to create a panel consisting of two contrasting types of wood - joined with a perfectly fitting complementary curve. ( think Yin / Yan shape).

Here's the process I'm considering.

Mill two panels of contrasting woods ( e.g walnut , maple ) to approx 1/8" thick.
create template of the curve I want using bandsaw.
stack both panels on top of each other.
place template on top of stack.
use router with 1/8" straight cutting bit and bearing on top.
cut through both panels at the same time with bearing riding on template.

In "theory," this should create 4 pieces with perfectly matching curves. Has anyone tried this method?

Thanks.

Scott C.

 

[ehpoole]

You could also cut out your design using a very thin blade on the scroll saw (just stack the two pieces of wood) and you will automagically have your complementary curves without any fancy templates. Keep in mind that a bandsaw has a much larger kerf than a scroll saw.

I'm sure some of our dedicated scroll sawyers could offer some expert advice on the subject.

 

[CarvedTones]

There is a hole in your theory. I have cut puzzles on the scroll saw lots of times and the tiny kerf creates enough of a gap for the pieces to easily be removed and assembled but even at that size you can tell the arcs are affected. In a curved section, the piece on the inside will have an arc with a radius one kerf width less than the piece outside. In your example, that would be 1/8" and that will be enough so that they don't fit exactly.

 

[Henry W]

Scott: My first take is that the two curves will not match entirely perfectrly. Take the limit as you approach small radius curvatures. On the inside piece the radius could be 3/8", and on the outside piece the radius of the same curve would be 1/2". These curves won't fit together, without a 1/8" strip of metarial between them.
Fine woodWorking had a great video on a technique, but I have long since lost that - access to their archives might reveal it. that convinced me it would work. I believe it involved making a smooth cut with a bandsaw to create two templates, and the using these temples to create complementary pieces. Of course my argument about radii above still holds... but I can't recall what they did to offset that. Smooth large radii curves were needed as I recall.

Henry

 

[Mike Davis]

You should try this with paper first, with some curves when you remove a 1/4 inch strip between the two parts you create a gap that does not reflect the opposite sides. In other words the parts will not fit.

 

[ehpoole]

There is a hole in your theory. I have cut puzzles on the scroll saw lots of times and the tiny kerf creates enough of a gap for the pieces to easily be removed and assembled but even at that size you can tell the arcs are affected. In a curved section, the piece on the inside will have an arc with a radius one kerf width less than the piece outside. In your example, that would be 1/8" and that will be enough so that they don't fit exactly.

A scrollsaw blade with a 0.01" kerf will only create a 0.01" kerf (1/8" is closer to a wide bandsaw blade or tablesaw blade). A kerf of a couple hundredths of an inch will close up if a water based glue is used. Perhaps you were using some of the very heavy scrollsaw blades -- although I don't think even they create much more than a 1/32" (~0.03") kerf?

I know the method I suggested above is used quite a lot in intarsia work.

I'm not trying to argue, but rather seek clarification because I have seen the method used quite extensively and, even as an occasional scrollsaw user, I have some extremely fine blades that would create a barely visible kerf.

 

[CarvedTones]

It's one of those "in theory versus in practice" things. They make a puzzle blade that is really small, but I usually use a #5 or even a #7 if it is a puzzle for kids and a little more gap is a good thing. Anyway, puzzles have some ridiculously tight curves and unless you have some mad skills some of the cuts will be slightly sloppy. By that I mean more than a normal kerf width. Anytime you get a little side pressure on the blade, it increases the kerf width. The teeth do the cutting, but the side can wallow out a little more when it is hitting.

 

[Gotcha6]

Cut 2 templates (yin and yang) and be sure they match each other when one is rotated 180 degrees. Then proceed as mentioned using both templates set on the workpieces with enough clearance for the pattern bit. They should fit.

 

[Scott Cardais]

All

Thank you for your prompt replies. You saved me. Indeed - there was / is a hole in my theory. I didn't consider the effect of the change in circumference caused by the waste from the 1/8th bit.

I watched the FW video mentioned by HenryW. I'm going to use that technique as I've seems to work.

Thanks again to all.
ScottC

 

[CarvedTones]

A scrollsaw blade with a 0.01" kerf will only create a 0.01" kerf (1/8" is closer to a wide bandsaw blade or tablesaw blade). A kerf of a couple hundredths of an inch will close up if a water based glue is used. Perhaps you were using some of the very heavy scrollsaw blades -- although I don't think even they create much more than a 1/32" (~0.03") kerf?

BTW, this is a perfect opportunity for my tangential "why haven't we gone metric?" rant? The back and forth between fractions and decimals and approximations. The blades I commonly use are between .3 mm and .5 mm straight line kerfs.

Also forgot to mention that I have dome some intarsia work and there is some sandpaper involved in a perfect fit.

 

[Henry W]

Scott: Do you have a free access link to that technique? I'd like to see it again.

Henry

 

[Scott Cardais]

Henry:

Here you go.

http://www.finewoodworking.com/subscription/workshop/workshoparticle.aspx?id=5312

I think this requires membership.

Thanks for your help.

Scott

 

[Flootsie]

Cut 2 templates (yin and yang) and be sure they match each other when one is rotated 180 degrees. Then proceed as mentioned using both templates set on the workpieces with enough clearance for the pattern bit. They should fit.

I think this method from Dennis would work. When you cut for the clearance, would you need to cut to the same "side" of the clearance gap?

 

[Henry W]

Yes - requires membership....

THanks Scott

 

[Scott Cardais]

FW also published a companion article "Cut Matching Curves". The author is Carol Koebbeman.

Scott

 

[Jeff]

Sorry. I'm not following this discussion and I'm thoroughly confused, but interested in what Scott is trying to accomplish. A sketch/diagram of some sort with dimensions would be helpful for understanding.

Scott said a "panel with contrasting woods", but he didn't say anything about making puzzle pieces, just complementary curves that match. I'm missing the concept here, but here is an offering in cherry and maple for discussion.

 

[Scott Cardais]

Jeff:

Your drawing captures the concept.

FYI ... I was interested in this idea to create a key cabinet similar to the one shown on the attached photo. I was toying around with the idea of creating a large panel with two complementary and contrasting woods (e.g. walnut / maple, cherry and maple or similar.) The cabinet in the photo is made from Brazilian Tulipwood and maple.

I would make the drawers and raised panel from the panel. This would give the entire front of the cabinet a flow.

I've studied the article referenced earlier in this thread and I think it will work. I'm going to try it anyway! A friend of mine told me half of sewing is ripping out. Her comment certainly helped my attitude about woodworking!

Scott C.

 

[FredP]

you can do this [i think] with a template and router fitted with a collar. 1/2" collar and 1/2" bit for one side and 1/4" bit with the same 1/2" collar for the other side. same template for both. pay attention the the rotation of the bit so as to be pushing the collar against the template. I have not tried this but the theory makes sense to me. This is the method used for inlay work. I have a special collar with a removable spacer for inlays. woodcraft and klingspore both sell these collars. it works on the same principle. ymmv

 

[Jeff]

I think that I'm beginning to understand the concept finally, a real duhhh? on my part but we're all learning and understanding.

So this 1/8" thick panel would be incorporated within a rail & stile frame for the key case. Correct? The Sketch Up drawing is 3 pieces with complementary curves. Of course, executing that perfect fit in the shop is another matter.

An interesting exercise and I'm curious about how to approach such an undertaking.

 

[Scott Cardais]

Using two complementary woods, i would glue up a panel approx 24" x 12” x 5/8". From this glue up, i would cut drawer faces, rails, stiles and raised panel for door. I'd arrange these in such a way that the pattern flowed from bottom ( the drawers) to top.

thanks for your comments and interest.

Scott C.