Shop Tips & Tricks #13: Duplicating Parts in Wood and Smoothing Curves

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Blog entry by GnarlyErik posted 04-17-2013 09:14 PM 3979 reads 1 time favorited 2 comments Add to Favorites Watch
« Part 12: Making Long, Round Things in Wood - with the Norwegian Dowel Cutter Part 13 of Shop Tips & Tricks series Part 14: “Repeat After Me”: Making identical parts in wood »

Let’s say you need to made two, three or even four parts in wood, all precisely the same size and shape. It is straightforward and fairly easy to make your parts if they are all rectangular and can be done on the table saw. But, what if they are curved or irregular in shape, and you only need a few? The following technique may seem elementary, but it is surprising how many people aren’t familiar with this idea. Simply put, this is as easy as making two or more parts at once, in one setup, then separating them apart into component pieces.

Here’s how it is done:

1. Mill out enough stock for your several pieces to the proper thickness, making sure there is enough stock of proper width and length to make all pieces;

2. Rough cut your stock to width and length. From here there are two approaches;

a. Using a pattern made for your stock, rough cut each piece to shape about 1/8” over all around (easier, takes a little longer);

b. Proceed without rough cutting to size (faster, easier to keep squared, so long as your saw blade is perfectly square to the table);

3. Glue all your parts together in a stack as follows:

a. Use a glue thinned down about 1:1 with glue and thinner. If using white glue, water is a thinner. I use something called ‘School Glue’ which is made for school art projects and is cheap and easy to get;

b. Apply a coating of thinned glue to one part, then a layer of newspaper, more glue, the next part, and so on. Make sure there is a layer of newspaper or other thin paper between each part, and each part is accurately aligned with its neighbor. Clamp all together tightly and allow the glue to set.

3. When the glue has set, all the parts may be worked and shaped ① as one piece, making sure to use your square to keep all edges squared to the side face of the work. The thinned glue will hold everything together nicely until you are satisfied;

4. When the shaping is done, gently separate each piece from its neighbor with a chisel or screwdriver. I usually leave a little extra material at the ends for this purpose, which is trimmed off later. The pieces will separate within the paper, leaving a paper/glue residue on the matching surfaces. This will easily sand off, leaving you with perfectly matched parts!

Another way sometimes used for making two pieces only is not quite as easy, although it is the same idea. This is to use stock sufficient in thickness to allow for the finished thickness of both parts, plus an allowance for finish planing. After shaping, this is then resawn through the middle and each piece planed back to the desired finish thickness. This is often done in boatbuilding when planking a wooden boat. A plank made for one side should exactly match one in the same place on the opposite side as closely as possible, so they are sometimes made this way, then beveled after they are sawn apart and planed to the finish thickness.

If you are in production work making many identical parts, you likely are already familiar with making a pattern for each part for a router/shaper to follow.

① A note here about making your curves smooth. The eye is a curious and easily fooled organ, and can easily mislead your senses. Color or texture changes in your material can make you think there is a bump, or that a curve is smooth when it is not.

One sure way to check the smoothness of a curve is with a bendy straightedge or piece of wood made for the purpose. Boatbuilders call the tool they use to check curves a ‘Fairing Batten’. This is made from a straight-grained wood like pine, and usually square or rectangular in section, and bendy enough to make the curve without breaking. But, any bendy straightedge or other material works just as well for short curves so long as it has no ‘hard’ spots.

This batten or straightedge is laid on the curve in question and tacked or clamped in position. Any deviation in the smoothness of the curvature will immediately show up as a high or low spot, and then can be worked down to get your nice, smooth curve. The pictures below explain things pretty well I think.

The particular pieces in these photographs with the fairing batten are intended as rockers for a rocking chair, so I am being extra careful to make the curves nice and smooth. Bumpy rockers on a rocking chair can be disconcerting and irritating for anyone except maybe a six-year-old to ten-year-old child!

-- "Never let your dogma get run over by your karma!"

2 comments so far

View Robb's profile


660 posts in 3506 days

#1 posted 04-18-2013 08:10 AM

I like the fairing batten idea especially. Good thoughts, thanks for sharing them.

-- Robb

View Mauricio's profile


7115 posts in 2724 days

#2 posted 04-21-2013 08:04 PM

Another good one, thanks for sharing.

-- Mauricio - Woodstock, GA - "Confusion is the Womb of Learning, with utter conviction being it's Tomb" Prof. T.O. Nitsch

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