Straightening Thin Dowels
A woodworker struggles with a crooked dowel problem in a mass production setting. October 1, 2010
Question
Anybody have ideas on how to straighten small diameter birch dowels? They range in size from .100" to .156" and are 16" long. I need to figure out a way to get them relatively straight as quickly as possible. I have to process at least 10,000 dowels a week, so I'm trying to find something that is quick and easy. I've tried bundling them together with rubber bands and even hose clamps, wetting them and letting them dry before releasing the bundle. It works somewhat, but the bundles need to be small, otherwise they take forever to dry out. This makes the process fairly time consuming. I'm thinking that steaming might be a better way to go, but I haven't tried it yet. Anybody deal with this problem before?
Forum Responses
(Furniture Making Forum)
From contributor G:
Ten thousand a week? That rules out rolling them between your hands. How badly off "relatively straight" are they now? Are you making them or do you know how they are manufactured? And what are they going to be used for?
From the original questioner:
I've tried rolling them between my hands, pinching them between my fingers, and putting them between two boards, leaning all my weight on the top board and then rolling it back and forth against the bottom board.
The degree of straightness varies from almost perfectly straight to bowed by 3/8" or more. The majority of them are bowed around 1/8-1/4 inch over their length. Some of them are straight for 12" or 13" and then take a sharp turn. Some are "S" shaped, some are wavy... In short, they are all over the place.
I get them from a dowel mill in Maine. I've been told they are produced on a molder. Wide boards fed through two opposing knives with a series of 1/2 circle cut outs. The manufacturer can pack them in sealed plastic bags (right now they just come loose in a box) and I'm thinking about installing some climate control in a room in my shop (right now I have nothing). Limiting the dowels' exposure to moisture might help keep them somewhat straight, but I don't see that fully solving my problem.
On my end, the dowels are fed into a machine that bundles a pre-set number into a hexagon shape. If the dowels aren't straight, the machine doesn't work very well.
From contributor G:
Plastic packaging can't hurt. Does the dowel mill use KD birch with consistent MC? I think wood that thin would be quickly affected by RH changes, so your climate controlled room should approximate the RH at the mill.
Will your hexagon machine accept pre-bundled dowels (or can it be adapted to)? I'm thinking of some sort of jig, perhaps a funnel, where you could wrap a loose handful of dowels with tape, say at the ends and in the middle, before feeding into the machine.
Does the dowel mill have any further suggestions? They must know that when a customer orders straight dowels, S-shaped won't do.
From contributor O:
For arrows you can heat them and bend by hand over heat source, but that quantity seems unlikely without a more automated method.
From the original questioner:
Funny you mention taping a bundle together first, since that is what this machine is actually supposed to be doing for me. It is designed to count out a pre-set number of dowels, form a hexagon shape, and then put a piece of tape on each end. When the dowels aren't really straight, the machine can't form the hex shape properly. Unfortunately the dowel mill hasn't been of much help. The response has basically been it is impossible to keep dowels that thin straight. Thanks for the ideas anyway.
From contributor G:
Maybe something other than birch would stay straighter.
From Professor Gene Wengert, forum technical advisor:
There are two issues: First, the dowels may be warped because there are stresses in the wood - drying stresses (rarely) and growth stresses from the tree (more common). These would show up right when the dowel is manufactured. Second are stresses that show up after manufacturing - a day or longer - because the MC is changing after manufacture. You need to determine the cause before a cure can be tried.
Wrapping the dowels in plastic will not always work, as the problem might be that one or two dowels are too wet and they will dry out in the bag and maybe increase the MC of the drier pieces.
Bending the dowels by hand is temporary, but such bending can straighten a piece so you might be able to run it in your machine. But for 10,000 pieces, it does not seem practical. For a drum stick manufacturing operation, crooked sticks are hard to avoid... Yields of good pieces might be 80%.
From contributor J:
You can bend hardwood up to 1/4" thick with dry heat and it will stay in that shape when it cools. The system I use has a 2" diameter steel pipe about 8" long that is mounted horizontally. I blow a propane torch into one end and rub the wood on the other end until it gets soft and bends. You could heat batches of 100 or so in an oven until they are soft and then roll them flat as they cool.
From the original questioner:
Gene, thanks. The material I get is kiln dried. Obviously I can't control the environment in which the dowels are manufactured. For that matter, controlling the environment in my shop really isn't a practical solution either. The dowels come from Maine, and I'm in Vermont, so I figure any change in environmental conditions should be relatively minor. That's why I think I need to find a way to simply take what I get from the mill and straighten them somehow right before they are loaded into the hopper on my machine.
Contributor J, thanks for the advice on dry heat. That was actually the next thing on my list to try. I was going to take a bunch of dowels, bundle them together in as straight and tight a bunch as possible, and stick them in the oven for a while. Hopefully when they cool they will be relatively straight. Any idea what temperature and for how long? I was thinking something low, like 200-250 degrees, for not more than 10-15 minutes, but I suppose like everything else in life it will take some trial and error.
From contributor J:
I don't have a temperature setting on my rig so I can't help you there. It is below 250 though. The wood softens rather quickly and starts to bend in less than 2 minutes, but the heat is localized in a small area.
If you bundle them, the interior ones will take a long time to heat. I would suggest lying them loose on a tray in the oven until they are soft and then taking a bunch of them and bundling them around a steel rod and tying them with tie wire or zip strips until they cool. Trial and error is what is going to work for you.
From contributor R:
I like the idea of looking at your machine and modifying it so that it can handle the bent dowels. I'm thinking of a sheet metal cowl that starts round and funnels down to the proper hexagon shape. Even if you had to hand load the machine at the funnel, that's no more (and likely less) man hours than trying to straighten 10,000 dowels a week, then feed them into a hopper. I'd like to see pictures of the machine and how the dowels don't feed properly. I really think the answer is in the hopper-to-machine interface.
From contributor W:
That's a lot of dowels. You sound like a good customer. Can the dowel mill bundle them for you? If not, there are others to try. If the pieces are going to be straight at any time, it will be when they are freshly milled.
