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Forum topic by Robert posted 08-04-2020 06:50 PM 266 views 0 times favorited 8 replies Add to Favorites Watch
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Robert

3950 posts in 2334 days


08-04-2020 06:50 PM

I’ve been building bow front tables lately using a strip of white oak for a drawing bow.

I’ve also used long aluminum rulers (better results).

But I’m not getting the shape I want. there is a nice arc, but the outside is too straight.

I’m thinking of trying a 1/8” thick strip of Azek.

I really don’t want to pay 50 bucks for a drawing bow.

What are you guys using?

-- Everything is a prototype thats why its one of a kind!!


8 replies so far

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sras

5566 posts in 3982 days


#1 posted 08-04-2020 06:58 PM

By definition a bent strip is going to be straight at the contact points – unless you put some twist at each end (not sure how one would control that).

Options:

Use a large trammel and strike an arc

Set the contact points outside the width of the desired bow

-- Steve - Impatience is Expensive

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Jerry

3484 posts in 2501 days


#2 posted 08-04-2020 07:13 PM

If you take a look at the pictures on my kitchen island build here you can see where I taped a string to the floor and put a pencil on the other end of it to get my arc for the bottom stretcher.

-- There are good ships and there are wood ships, the ships that sail the sea, but the best ships are friendships and may they always be. http://www.geraldlhunsucker.com/

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SMP

2476 posts in 758 days


#3 posted 08-04-2020 07:51 PM

I have used graph paper and math to draw out various arcs(parabolic curves). You can fine tune the arcs by changing a variable in drawing the lines point to point.

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AndyJ1s

419 posts in 608 days


#4 posted 08-04-2020 08:45 PM

You can taper the end(s) of a bow to decrease the radius of curvature at the end(s) of the curve.

Like a fishing rod, a uniform taper creates a uniform progression of curvature with tension on the end.

If you draw the curve in two, symmetrical halves, you need only taper one end of the bow, and don’t have to achieve the same taper/flexibility at both ends of the bow.

With such a single-tapered bow, you can achieve various combinations of curvatures depending on which region of the bow you use, but you need to use the same section of the bow for both halves of a symmetrical curve.

You can also use an ellipse jig to draw (or cut, with a router or jigsaw) variable radius curves.

-- Andy - Arlington TX

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MPython

298 posts in 665 days


#5 posted 08-04-2020 08:59 PM

Don’t try to draw the complete arc. Use a French curve or some other device to draw a quarter arc on a folded piece of paper. When it looks right cut it with scissors and unfold it. You should have two quarter arcs that are mirror images and meet at the crease in the paper. It may need a little tweaking, but again, do the tweaking with the paper folded in half. You should be able to come up with a pleasing half circle.

As a side note, unless you’re bent on perfection, it is not necessary to have a perfect half circle. As long as the curve is fair and symmetrical end-to-end, it will probably be pleasing to the eye. If you want perfection, sras has the answer. Make a simple trammel out of a board a nail and a pencil. it will draw you a perfect circle – or any arc thereof.

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Robert

3950 posts in 2334 days


#6 posted 08-05-2020 12:03 AM

@Jerry, yes! Geez I forgot I’ve done that type thing but used a solid piece of wood instead of string.

And you can can make parallel arcs for edge banding, aprons etc.

-- Everything is a prototype thats why its one of a kind!!

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theart

225 posts in 1407 days


#7 posted 08-05-2020 01:28 PM



By definition a bent strip is going to be straight at the contact points – unless you put some twist at each end (not sure how one would control that).

Options:

Use a large trammel and strike an arc

Set the contact points outside the width of the desired bow

- sras

The bow should be parabolic (“should be”, but the math kind of breaks down at large deflection), so the ends aren’t exactly straight. There’s also no part of the span that has constant curvature. The only reliable way to get a constant radius arc is to use a big trammel and calculate the radius based on your desired chord and sagitta.
https://www.mathopenref.com/sagitta.html

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theart

225 posts in 1407 days


#8 posted 08-05-2020 01:36 PM



You can taper the end(s) of a bow to decrease the radius of curvature at the end(s) of the curve.

Like a fishing rod, a uniform taper creates a uniform progression of curvature with tension on the end.

If you draw the curve in two, symmetrical halves, you need only taper one end of the bow, and don t have to achieve the same taper/flexibility at both ends of the bow.

With such a single-tapered bow, you can achieve various combinations of curvatures depending on which region of the bow you use, but you need to use the same section of the bow for both halves of a symmetrical curve.

You can also use an ellipse jig to draw (or cut, with a router or jigsaw) variable radius curves.

- AndyJ1s

Designing a tapered bow that will flex into a constant radius is not trivial, and it will only work for one radius.

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