|Some Theoretical Issues|
|Copyright (c) 2002-15, Brent Beach|
Checking over the jig, it became clear that over time the hole in the dowel for the file tang has gotten a little lose. In fact, because the tang is 6-sided, it is not easily gripped. The tip is much easier to grip - it is three sided and a little larger.
Perhaps recalling the way Woodnut4 holds his files (see Other jigs), I decided to move the file holding dowel from the tang to the file tip. The result is Version 3, which in use is actually better than earlier versions.
As well as switch ends, I also increased the lengths of both braces by 1". This gives a little more room between the file and the back dowel during use.
This is the far end of the jig, the end on the other side of the saw.
The far brace (it is on the other side of the saw during use) is 5-1/4 inches long. Holes for the two dowels are drilled at 45 degrees. I use Forstner bits to drill these holes. Most other types of bits do not work very well when you try to drill a hole at an angle. Forstner bits work fairly well on angled holes.
The file holding dowel is 3/4" in diameter, with a 10/64" diameter hole in the middle of one end. The triangular tip of the file fits into this hole. Tap the dowel on a hard surface to seat the file.
The back dowel is 3/8" in diameter and about 5" longer than your longest file. This one is 14" long, but could be longer. Any extra length extends beyond the far end of the jig, where it does not interfere with anything. (Extra length is a problem only when mailing jigs!)
The near brace is 4-1/4" long. The hole for the back dowel is at 45 degrees.
The other end is mitred at 45 degrees. A 7/64" diameter hole to hold the tang tip is drilled into this mitred face. You should not drill this hole until you have done a test assembly of the jig and determined where the tip of the file should be so that the file is parallel to the back dowel.
Since the file tapers the file cannot be parallel to the back dowel over its entire length, of course!
My limited experience with this version of my jig (sharpened 4 saws) in this configuration confirms its superior file holding ability. However, it is a little tiring on the right hand. I am starting a modification to the near brace to solve this problem. Once built and tested, the new variation will be shown here.
This picture shows the simple mechanism that allows for both adjustment of the brace along the back dowel, and adjustment of the file holding dowel, but also allows locking of both dowels once the desired adjustment has been made.
There is a fairly wide saw kerf from the edge to the dowel hole (I use my widest kerfed rip saw for this).
A T-nut in one side, a machine screw from the other side. The end of the machine screw is visible in the "far brace" in first picture. I normally counter sink the t-nut to make the braces easier to hold.
Tightening the screw once the jig has been adjusted quickly locks the position.
The file holding dowel is a 1-1/2" piece of 3/4" hardwood dowel.
The file tip (in version 3, tang in version 2) fits into a 7/64" diameter hole tightly enough that the file cannot rotate.
The adjustment for rake is done by loosening the machine screw and rotating the file holding dowel. Notice the plier teeth marks on the dowel. Make sure you rotate the dowel rather than the file, since turning the file will put unnecessary wear on this hole.
The slider is a small wooden block that slides easily along the back dowel.
The hole is at an angle to the length and determines the bevel angle. I use bevel angles from 5 degrees for Rip saws to 25 degrees for crosscut saws. I have many sliders with different dowel hole angles for special purpose saw filing. The hole is always square to the width and should be in the middle of the face.
The hole in this slider is angled at 5 degrees. This hole should be just a little bigger than the back dowel to allow free movement. You can enlarge the hole slightly with a round file if it binds. You can also oil or wax the dowel to reduce friction (thanks to Ken for suggesting this), or even use a metal rod and a metal bushing (thanks to Walt for this idea).
Notice that the ends are not square to the edges. One end is at 5 degrees, the other at 15 degrees. These angles are used when setting the rake angle. Setting the rake angle is discussed in the saw filing step by step description.
There is a saw kerf down the middle of each side. The slider rests on the saw with the saw teeth in this saw kerf. Usually the slider is in contact with the saw vice, sliding on it rather than having the saw teeth in contact with the bottom of the saw kerf.
In order that the jig slide along the saw from tooth to tooth, this kerf must be wider than the kerf on the saw you are sharpening. I initially worried that continued use would slowly saw through the slider. This has not happened yet and I have done many saws.
This view shows the two saw kerfs, ideally perpendicular to the face.
If you want sloping gullets, you slope these kerfs.
When filing, I almost always hold the near brace in my right hand and the slider in my left hand. So while my right hand does the actual filing, my left hand positions the jig relative to the gullet.
When I want to file one side of the gullet more than the other, I exert a little force with my left hand on the slider, pushing it, and hence the entire jig, toward the tooth that needs more filing. For exotic filings, where the front and back bevels are not equal, this extra control over the jig position is essential.
The file can be on either the left of right of the back dowel -- here it is on the right. In this position, the third or fourth finger press down on the file. This can be a little awkward. I normally work with the file on the left of the back dowel, pressing down with my index finger.
After some use, I think yet another modification of the jig would make it more comfortable. More soon.
Here the slider is not between the file and the near end of the saw so you can file teeth right up to the end.
It involves just flipping the jig over, then setting the rake again.
The file moving in the gullet locks the jig in place along the teeth. As you push along the line of the file, the back dowel moves back and forth in the hole through the slider. While there is not a lot of friction here, if you move your hand so that the dowel is no longer aligned along the hole, the dowel can bind. It locks in the hole and does not move at all.
The problem is not friction, it is a catch. If the back dowel is not aligned with the slide hole, the harder you push the more it binds.
Walt Jaap has mode a simple modification to the system that may reduce the chances of binding by converting from a wood-wood boundary here to a metal-metal boundary. He uses a metal back dowel and a metal bushing in the slider. He epoxied the bronze bushing into the slider - with careful drilling a friction fit or a glue that works on wood and metal may be enough. If you carefully size the rod and the bushing, the chances of binding due to slight misalignment will be reduced.
A metal rod can be much rounder than a wooden dowel. A metal rod will not change size with changes in humidity. Because wood swells differently along the grain than it does across the grain, as wood swells it becomes more oval. The quality of the fit changes each time you use it. The change is small and I have few problems. If you want even fewer problems, using a metal rod here is a good option to try.
You might even be able to find a Teflon bushing which is slipperier again. If you cannot start to bind, you cannot get a bad catch.
Notice that none of the metal bits come in contact with the saw teeth!
Ken says: "It's not as pretty as yours, but it's been well used. I used spray oil on the dowel which helped a lot, and that's some left over chalk on the file."
Looks pretty enough to me. Anyone using a jig like this or like mine will soon realize how important it is to have soft corners on the parts of the jig you hold. A few minutes with a file to soften all the corners and you will be a lot happier when filing.
The idea of putting oil on the dowel to improve sliding is one of those things that knock you over when someone points them out. Why did this never occur to me?
It's a simple wood block with a hole drilled in it a little smaller than the end of the file you are going to use. Draw a line of what rake angle you want pretty close to tangent to the hole. Push the file in lining up one side of the file with that line. As long as you keep the top of the block fairly parallel to the top of the saw edge you'll hold the angle pretty close.