Sunday, October 21, 2012

Using a Trephine to remove broken screws in Wood

If you do any furniture repair you have probalby come cross a screw or nail that has broken off flush with the wood surface.
Digging it out with prys or pliers generally damages the surrounding wood severely and can compensate the repair.

Enter the Trephine.

Borrowed from the medical and dental field this hollow steel tube sharpened with saw teeth can accurately core out a small section of the surrounding wood leaving a concentric hole that is easily repaired with a dowel plug.
Trephines currently come in 3 sizes and will handle most screws or nails found in common furniture products.
Here's an example of how they are used.
This picture shows a screw broken off flush with the end of a rung. If you have trouble centering the trephine cut a small guide hole in plywood scrap with the trephine and mount that with double sided tape to the top of the rung.
When the surface is cleaned to allow the drill to sit flush against the rung the trephine is gently pressed into the surface with short drill bursts and allowed to clear the debris before continuing.
Once the screw has been tunneled along it's entire length you can use a sharp instrument to pry away the wood between the screw and the kerf until the screw frees up.
Here the screw is out and the hole debrided you can fit a length of doweling in the hole or custom
 turn a matched piece if the repair will show.
A narrow chisel will take care of the remaining splinters by twisting it in the channel. If you have a suitable drill bit you can try this as well.

Once the screw is out and the hole debrided you can fit a length of doweling in the hole or custom
turn a matched piece if the repair will show.
Here's the finished repair ready for a new screw or nail.
I purchased mine from Highland woodworking . You'll find them under screw extractors.

Sunday, May 20, 2012

Taming tear out on finger joints - Baltic Birch plywood

I’ve become accustomed to using quarter inch plywood with finger joints to build storage boxes for some of my tools.The reason I chose this method is that by using finger joints I don’t require any additional support for the box and given the rather thin quarter inch exterior is not a lot of room for additional support for dadoes or splines.

Most recently, I purchased a new sheet of quarter inch Baltic Birch. The stamp on the material tells me it came from Eastern Europe and quite possibly Russia.

I ran into a problem with the sheet in that there was  unreasonable amount of tearout while cutting and milling the product. Repeated attempts to cut a joint that at least, could serve as a storage box in the shop led to disappointment. The wood splintered, it chipped front and back, the internal layers between fractured and fell out.
In frustration, I gathered up a couple of samples and tossed the rest of the sheet into the truck and returned to my supplier. They explained to me that they "had not had any complaints except for mine" regarding this product and suggested that I tape both sides of the joint with painters tape and or use sacrificial wood strips on either side of the joint.

I returned home rather disappointed and patched up enough wood for the project to put it bed for the time being.
 Today I had another opportunity to go over my work and take a very close look at the bits I was using to cut the joints.
I had tried a single blade quarter inch a double bladed blade quarter inch on 1/4 inch shaft as well as a double blade quarter inch on a half-inch shaft. In addition I used a 1/4 inch solid carbide spiral up cut carbide all without reasonable improvement.

I began to think that there must be something in the process of making the plywood that made it different from other batches I had used over the years. There were the same number of plies as before and the thickness of the Baltic Birch itself was dead on one quarter of an inch so I ruled that out. I tried to see if the material used inside the plywood was different from what I’d use previously but was unable to tell from such a small sample. The internal layers where breaking off as I milled the finger joints so they were either made from a very brittle wood or the glue used to binding the layers was more brittle than I was used to.

I decided to try longshot, I got a small spray bottle, put some water in it and lightly sprayed both sides of a test sample.
There was just enough moisture there to be visible to the naked eye and I used paper towel to remove the excess.I let the wood set for about five minutes in order to have the moisture penetrate and proceeded to mill several joints.
Surprisingly, this method worked and I was able to mill about 10 joints without a single joint exploding against the bit.
I think at this point, I could recommend this method to help overcome brittle wood when cutting dovetails keys or finger joints. I also found tear out along the table saw cuts as well and if you’re having trouble there you might try this method as well.

I have concluded that the wood used is not as good as it  once was and that the filler layers may be substandard for this purpose. I also wonder if the wood had been subject to undue heat and pressure in order to speed up the manufacturing process and increase profits. At any rate the material cuts like glass and needs every precuation to prevent tearout. For the time being, this seems to work for me.