# How to make a smooth curve from a tracing?



## steambc (Jan 16, 2012)

Hey Guys,

New kid here. I'm restoring a vintage amusement park ride and the only thing I need to completely rebuild from scratch is the seat.

The shape of the seat bottom is roughly that of a slight 2-D elipse cut in half, so that the straight part is under the knees of the riders while the curved part conforms to the curved backrest of the ride. To be specific, it is a Mangels Whip car from the 1920s.

With great difficulty I removed the ancient upholstery from one side of the extremely rotted wood to expose the shape, and traced it out with a pencil onto a large piece of foam board. So now I have the rough tracing of one half (one side) of the seat bottom.

My question is, how can I transform this rough, jiggly outline into a perfectly smooth outline on the foam board, so I can cut it out and transfer to the new wood?

I was thinking of hand-drawing the rough outline with a black magic marker, taking a photograph, importing that into a CAD program, creating the perfect curve, and then somehow getting that curve blown up and printed.

That seems to be quite an involved method, so I wanted to run it by you guys to see if you are aware of simpler methods. If I haven't explained my challenge properly please let me know and I'll try to clarify.

Thanks,
Brian


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## woodnthings (Jan 24, 2009)

*any drafting experience?*

Ever hear of a French curve? It has an accelerating curve from a small radius to a large radius.... http://images.search.yahoo.com/search/images?_adv_prop=image&fr=ytff1-tyc7&va=french+curve

There are other drawing curves that have a progressive curve to them. You can "clean up" a curve using pan lids, barrel tops and other radius's. 
You can make a bent thin strip that will conform to a rough curve by placing finishing nails along the curve at key points. You can then trace the curve on a paper, fold it, cut it out and you have 2 identical but opposite curves about a centerline.  bill


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## cabinetman (Jul 5, 2007)

steambc said:


> My question is, how can I transform this rough, jiggly outline into a perfectly smooth outline on the foam board, so I can cut it out and transfer to the new wood?


 





 
If you have the rough edge cut, make it a bit narrower enough to account for the thickness of a strip of 1/4" plywood and place it on the edge. It will hit the major edges, lay smooth, and on the other side you can draw your line.










 







.


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## bob talk (Jan 17, 2012)

Hi. A good trick that may or may not work here is to get some sort of flexible material but not completely flaccid (not string for example). A thin piece of wood might even do the trick. You can then form this wood to the curve and tack in down every few inches or so (if you have a pneumatic brad nailer it helps). It's a balance act between keeping the existing curve and letting this material form the more fair curve. You can then draw your new 'more fair curve' 
edit: basically the same thing as what cabinet man said...

-t


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## firemedic (Dec 26, 2010)

Sounds like you are really over thinking this. Cut the foam out while your mind and eyes smooth out the line as you go.

~tom. ...GEAUX TIGERS!... ...GEAUX SAINTS!......


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## GeorgeC (Jul 30, 2008)

firemedic said:


> Sounds like you are really over thinking this. Cut the foam out while your mind and eyes smooth out the line as you go.
> 
> ~tom. ...GEAUX TIGERS!... ...GEAUX SAINTS!......


I think that overthinking is correct.

You are probably going to cut the final shape on a bandsaw. Your eyes/brain will enable you to turn the rough line into a less rough saw cut.

After you do the cut your sander of choice will turn the rough cut into a smooth cut.

George


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## jjrbus (Dec 6, 2009)

I usually keep a piece of sheet metal around, maybe 1 or 1 1/2 in wide, 3 to 4 feet long. Comes in very handy for curves


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## steambc (Jan 16, 2012)

Thanks guys, for all the tips.

I actually ordered a "profile gauge", a 37" piece of metal with a series of levers and locking knobs along the length in the back. It allows you to conform the band to the piece, lock down the knobs, and then trace that contour to a new piece of wood.

However, I was indeed overthinking it. I decided to try to hand-draw the rough tracing with a magic marker, and was suprised to find that I was able to accurately smooth it out. Then I further smoothed it as I cut the template shape out of the foam board. I then had a nice, smooth template which I used to place the outline on the new wood. All I have to do now is to cut it out with my jigsaw.

I'm taking delivery on my contour gauge tomorrow, even though the job is accomplished. I didn't cancel the order because I can see that this device will come in very handy for other restoration projects I'm working on.

This was a case of not having faith in my own abilities. Thanks again to all!


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## jjrbus (Dec 6, 2009)

Many times it is just someone pointing out the obvious!  I love these forums :thumbsup: JIm 0311


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## bob talk (Jan 17, 2012)

If your transfering curves from one form to another that is wood, it can be helpful to use a flush cutting router bit. pattern bits. Basically you could take that strip of plywood cabinet man recomendded using and referencing the bearing of the bit off that and its great to reproduce many of the same shape ove and over on the router...


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