I'm trying to reattach a mirror to an old dresser. The dresser has two upright supports and the mirror is supported between these two supports by wood threaded dowels that pass through the supports and screw into the mirror frame. Unfortunately, both dowels are broken.
My first idea is to replace the threaded dowels. To do this, I need to thread a 1/2" dowel with 12 threads/inch (tpi), but the only threaders I can find on the web for 1/2" wood are 8 tpi. A cabinetmaker in town suggested looking for a metal threader/die that cuts "coarse" metal threads at 12 tpi, but I can't seem to find any item like that either.
Appreciate any thoughts about how/where to find such an item and/or alternate suggestions on making this repair without aesthetically altering the dresser.
The closest that any "metal" threads come to that is 1/2-13. As far as I know, anyway. Are you sure that the holes you have aren't 1/2-13? If you really want to make this a project, you can thread a couple of dowels by hand. Cut a right triangle that has the proper pitch out of paper. I think the pitch would be: 1/12 of an inch rise for every pi D of the OD of the dowel. The length of the paper (on the long side, not the hypotenuse) would have to be: one times pi D per thread. -----I think! If you had a piece of paper 12 inchs long, then you'd make a right triangle with 1.57" (about 5/8") as its short side, going to zero at the other end. This would give you about 12 / 1.57 or about 7 1/2 threads.
Wind and tape the triangle onto a dowel carefully and you will see that the hypotenuse gives you the outline of the correct pitch thread. With a fine bladed saw, cut through the paper into the dowel to mark it. Think about this when you tape the paper in place so it doesn't fall off when cutting. Then remove the paper and file in the threads with a
3 cornered file. I didn't say this would be easy. I saw Roy Underhill do this on TV once.
If you do it, I'll bet you'll be the only guy on your block who Has!
One last thought: One could open the holes up slightly (17/32" drill)and tap them 5/8-11. (you might get away by only opening the holes to 1/2"). Then thread new 5/8" diameter dowels 5/8-11. Both the tap and the die are available readily and they are pretty coarse.
As usual, there's no free lunch. One way is cheap and time consuming, the other way is fast, but costs more. I would be the cheapest tap and die I could find if you go that way.
By the way, I do use metal cutting taps and dies in wood. Mainly for
Do they really need to be threaded? I'd make tapered dowels and jam fit them into the threaded area... I'd probably also glue them in, but I'm guessing that you may wont to remove the mirror some day and don't want to heat and beat the dowels out.. ;-]
For threads in metal, 1/2 x 13 is now the US standard coarse thread.
1/2 x 12 used to be fairly common, but you'd probably not have much luck finding a die.
However, 1/2 x 12 is the Whitworth or British Standard coarse thread. You might be able to get a die of that size from an English supplier. The Whitworth thread form is somewhat different from ours, with 55 degree sides and rounded crests, but for a wooden thread it would be just fine.
A couple of cautions, though.
One, that would be a pretty fine thread for wood and would be fairly easy to strip.
Two, a die for metal threads cuts in a different way than one for wooden threads. The wooden die uses a single cutter sharpened to a knife edge, which cuts into the wood and removes a chip. The die for metal cuts using multiple teeth with more of a scraping action. On a wooden piece, you might find it ripping off the threads and tearing the wood.
There are a couple of other options.
Someone with a metal-turning lathe could make them for you. While cutting threads on a metal lathe utilizes the same type of scraping action that a die does, you do it one thread at a time and in multiple passes. Less likely to tear the wood.
If you have a wood lathe, it is possible to cut threads with a hand chasing tool. It looks like a scraper with saw teeth on the end. You'd need one cut at 12 tpi. It's up to you to feed it along the work at the same rate as the thread pitch. It's actually easier than it sounds, but takes some practice. The chasers are available new, but I don't recall who offers them.
Whatever method you choose, using a fine grained wood that cuts cleanly will help. Hard maple is fair, but boxwood is ideal. So are the rosewoods, lignum vitae, and many other very hard woods.
So, the original dowels had 100% failure and you want to redo it exactly the same? Bold move!
Hand-threading can be done by winding a string around the dowel and then unwinding the thread, pencil-marking its path as you go. Then carve a groove where the pencil mark is. Finetune the pitch with varied string size, or by thick-string/thin-string double thread wrapping.
Remember the thread is a short-grain member, it won't be strong, and 12 threads/inch seems rather fine (which hurts the strength).
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