How to trace a piece of wood with curved edges

Question

I have a metal bench (like a park bench) with wood around the edges, but the wood is extremely worn and I would like to replace it. Because the top and bottom of the bench is curved, I am going to try to trace the existing wood pieces and use my bandsaw to match.

Of course, the existing wood doesn't have square edges which makes it difficult to trace a line that would perfectly match the width without letting the pencil creep under the curve. As I thought to myself how I might ensure a perfect trace (using a regular #2 pencil), I thought I might angle the pencil such that the angled tip is flush against the side. Hopefully that makes sense... but if not don't worry because while in theory this should work if I am extremely careful, I'm not that careful.

The above is my thought process so far, but I'm trying to find a better way to match the existing piece upfront because experience tells me if I go forward with this first idea off the top of my head I'm just going to end up with something that requires way too much extra effort to correct afterward. (And maybe I'm wrong about that; maybe this is a common solution, which is why I'm asking the question to find out).

I did see a Stumpy Nubs video just now about using a router table to create positive/negative templates and realized that if I had a router table I ought to be able to match the curve on another piece of wood pretty easily by stacking them. However, I don't have a router; much less a router table. I do plan to buy a hand held router, but a table is out of the question at this time. Not sure if I could easily accomplish the same with a handheld router?

So the question is a) Do I try the pencil trace method and use a bandsaw to cut?, or b) would a handheld router give me decidedly better results?, or c) is there another superior method that hasn't yet entered my mind?

Answer 1

I've seen a couple/few complex hacks over the years for doing this that, and while some are very interesting TBH they're just not worth the effort.

So, great tip for this I've seen used by some when tracing the complex shape of saw handles: sand the end of your #2 pencil in half.

Doesn't actually have to be in half exactly, just enough to expose the lead. This way the edge of the piece to be traced and the edge of the pencil are in alignment, so no offset error as normal with a pencil. Simples.

Answer 2

Take the piece outside on a sunny day. Aim the face of the piece right at the sun, put a piece of cardboard behind it, and then trace its shadow. The sun is so far away that light rays from it are effectively parallel, so the shadow of the piece wil be the same size as the piece itself.

Answer 3

You don't clearly say whether you can remove the existing board intact, and whether it is flat in its long dimension. I assume the answer to both is yes in my answers.

Your fundamental problem is to get a tool to reliably follow the flat plane of the pattern board, while a bearing surface perpendicular to that plane follows the edge of the pattern board. The bearing surface can guide either a pencil, a router bit, or a bandsaw blade.

a) (How to trace reliably) This is easily done by making a holder for the pencil. Cut a flat board, say 2" X 4" from a thick piece of lumber. 1 1/2" dimensional lumber is good for this, if it's smooth and flat. Now drill a hole that a pencil fits snugly through all the way through the 1 1/2" dimension near one end. Put the pencil in the hole, projecting the thickness of the pattern board through on the sharpened end of the pencil. You can now lay your pattern board on top of your replacement lumber, lay the pencil holder flat on the surface of the pattern board, and hold follow the edge of the pattern board with the pencil held vertical.

b) (Is a router better) You can do it with a handheld router, But it'd be tough to do well, and safely. To reliably duplicate a pattern with a router, the foot plate of the router must sit firmly on the plane of the pattern, and the edge of the pattern that the bit follows should be perpendicular to that plane. Your pattern edge is rounded. This can still work if you have a wide enough bearing on the bit to follow the rounded edge of the pattern board, but your going to end up with a lot of router bit sticking out of the router, and probably need to make the cut in several passes. It would not be easy to do this well and safely with a handheld machine, unless you're well practiced with handheld routers.

c) You can make a pattern follower for your bandsaw. Jonathan Katz-Moses shows how this is done in this video. He builds it with a router table, which you don't have, but once you see the idea, it's not hard to see how you can make the template follower using your bandsaw and hand tools.

Were this my problem to solve, I'd use method a. c will work just fine. I'd avoid b.

Answer 4

A handy tool to have in the shop for occasions such as this is a flexible curve. It is basically a long piece of (usually) lead* encased in flexible plastic. All you need to do is press it in place along the face of the curved piece you're trying to copy, then transfer the line to your replacement piece of wood. They are generally inexpensive - the ones I linked to currently cost between $8 and $13 (USD), plus shipping. They're available from many sources.

Depending on how long the pieces of wood you want to copy are, a contour gauge (also known as a profile copier or duplicator) may be an option. They tend to be more expensive than flexible curves, and the largest I found with a brief search was only 20" long, but with a decent one you won't have any slop or flexing like you might get when moving a flexible curve. They are also very handy tools to have if you do any wood turning, as they're great for copying profiles on turned spindles and balusters.

^* Because of the plastic casing, there's no contact with the metal.