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Sharpening, Continued


Sharpening the Teeth

This section is on re-sharpening existing teeth on a saw.  If you would like to re-tooth a saw, there is a section in the back saw project on cutting new teeth into the blade that might interest you - but as that isn't the norm, I'm not going to include it here.. 

One note about sharpening fine teeth - even your stance at the vise can make a difference in how you apply pressure to the file - and if you don't move as you progress down the saw, it will make a difference in how the saw is being sharpened.  Stop every so many teeth and adjust yourself accordingly!  Besides, you need to make sure you can see what you are doing.  Now, either because you have re-shaped the teeth in the section above or because they weren't that bad to begin with, you can count your strokes if it helps you.  The real trick is to keep a good eye on the tips of the teeth, looking for the flat from the previous step to just barely disappear.  

Another quick tip - learn to lift the file on the backstroke.  Doing so will significantly lengthen the life of your files, as dragging the file backwards across the steel only serves to dull it.

Filing Rip

For sharpening rip saws - file across every other tooth from one side, then flip the saw and run the file from the other direction on the remaining teeth with at least one end of the file jammed into a block of wood like shown below.  Filing from each side is done so the filing is consistent from each direction, and any inconsistencies that are created by your technique are repeated in an opposite and equal manner.  The angle should be the same one chosen previously (the lower is the less aggressive pitch):

It often helps to use something to darken the teeth so you can see which teeth you've filed and which you haven't.  You can use chalk, layout dye, or "Sight Black", which an anti-reflective aerosol spray used by gun-owners for reducing glare when sighting through their scopes.  It wipes off easily, leaving nothing behind.  Another great method is to use the soot from a burning candle or alcohol lamp.

As shown in the above diagram, file the teeth at 90 degrees to the blade, using the same angle used in shaping the teeth above.  Here's the real-world shot:

Usually, only a light stroke or two of the file is needed, just enough to remove the "flat" part left behind in the step above.  The flat will help you as a reference, filing the very last of it away should leave all of the points of the teeth in the same plane along the length of the saw.  Use the set of the saw to help guide you - for example, keep the tooth that is bent away from you to the right side of the file.  When you flip the saw, the orientation will remain the same - and if you darkened the teeth as suggested above, it should be easy to see where you've filed and where you haven't.

Filing Crosscut

The process for crosscut saws is much the same as for rip saws above, but with two very important differences - the first being the angle the teeth were cut at - which will be the same angle you chose when shaping the teeth previously, similar to this (the 12 degree angle is the minimum, most aggressive angle I would use on a crosscut saw - more rake angle will give you less aggressive, easier starting, but slower cutting performance from your saw):

The other difference is the fleam angle.  For our purposes, fleam angle is just the angle that you file the teeth at, which basically means 20 to 25 degrees off of perpendicular, as shown below, whereas a rip saw is file perpendicular.  File every other tooth, then flip and file the remaining.  Always file pointing in the same direction relative to the saw - if you do one side angling the file towards the toe of the saw, make sure that you also file towards the toe of the saw when you flip and file the other side.

Or, using a real-world photo, something like this:

Just be careful, and watch that you don't remove too much that it makes it difficult to file the teeth from the other side.  If you have to, just file a little, flip the saw file the other side, then repeat until you are finished.  It's easier if you remove too little than if you remove too much.

It is important that you file the proper teeth for crosscut, and how you file them is determined by the set of the teeth.  I tend to pick a side (i.e. the tooth bent away from me will always be on the right), and then file towards the toe of the saw.  It seems silly, but look at how the teeth are forming in this diagram, with the cutting points of the teeth on the outside edges of the blade:

I know this might seem trivial, but I've caught myself filing the teeth backwards!  So watch yourself!  To help clarify the angles a bit more, here's an overhead view of how the files should be held:

For back saws. I use about a 20 to 25 degree angle.  A steeper fleam angle results in a sharper saw, but also one that dulls quicker (on panel saws I use a 15 to 20 degree angle, because I don't like sharpening them as often).  You should end up with evenly shaped teeth, as below:

It can be hard to tell (and even harder to photograph), as because of the sharpening every other tooth might look small - so look at it against a light colored background and look at the entire tooth.


Crosscut Teeth - Graphic

I thought it might be a good idea to show a couple of graphics describing crosscut teeth.

  Here, you can see the bevel of every other tooth, as well as the back of the alternating teeth in this side elevation of a set of crosscut teeth:

When viewed from on end, just above the plane of the teeth, the above teeth look like this:

You can see the point of every other tooth is located on the opposite side of the blade - To show this a little better, here's a perspective view of the same teeth:

Quite a difference from the 90 degree angle of a rip saw's teeth:

I hope this helps illustrate what crosscut teeth are compared to rip...


Broken Teeth

What do you do about broken teeth?  If it's just a couple, ignore them and keep sharpening the saw as if they were there. They will eventually "grow" back, more and more with each subsequent sharpening. If it's a bunch, then the blade is probably too hard to be set, and may have been a metal cutting saw, or poor steel, or someone has tried to set the teeth the opposite way that they were originally set.  Hopefully it's the last rather than the steel...

If it's a lot of teeth, there's may be more wrong with the steel of the saw than is worth fixing. You can try joint the teeth down a ways and re-shape the teeth - it's an ambitious project, but not too hard to do, just a bit tedious. Take a look at the back saw project where I added teeth to blank steel to see a bit more on it.

I've only had teeth break on one saw I owned - and on that one, about 1/3 of the teeth were too hard to take a set. Thing is, about another 1/3 of the teeth were too soft to hold an edge.... Just poor steel, and not worth repairing in the end. Shame, too - it looked like a cool old saw, but was basically unusable.

Adding Set to the Teeth

Now is a good time to "set" the teeth.  "Setting" the teeth simply refers to bending every other tooth over slightly one way (the remaining are bent the same amount the other way) to increase the kerf size cut by the blade.  You can see the end effect in the rip and crosscut profiles diagrammed above.  The reason for this is to keep the blade from binding against the wood its cutting.  Softwoods need more set, as they "spring back" more.  Wet woods need the most set.  Hardwoods need less, and for cabinet grade hardwoods, it's best to try and get away with as little kerf as possible.  It's been my experience that you need at least some.

Setting the saw is done with a tool called, surprisingly, a saw set.  Stanleys are among the most common with the 42x being my preferred - but others can be just as serviceable.  For teeth finer than 14 PPI, it might be necessary to file down the hammer of one of these so it fits properly over the saw tooth.  The ultimate goal is to end up with a set where about 1/2 of the tooth is bent over.  Subsequent sharpenings reduce the amount of set simply by filing it away as you file down the tooth, therefore requiring you re-set the teeth at least every third sharpening, and maybe more often with finer teeth.

Using a saw set - notice the mark I've made on every other tooth with a marker:

Start by setting every other tooth on one side, then flip the saw around and set the remaining teeth the other direction.  Like I said, I don't always set the teeth during every sharpening, as sometimes I don't file enough off to significantly affect the set.  But when I do, I will always set it *just* a little more than I think is necessary.  You'll see why in the tuning section.

A word of warning - if you set a tooth one way, then decide you need to set it the other way, you could break the tooth off.  It is generally good practice to avoid doing it, so heed the following advice.  One problem I always seem to have when setting fine teeth is that I lose my place while setting every other tooth.  To overcome this, I often take a marker such as a Sharpie and mark every other tooth for reference, as I mentioned above.  This helps me avoid problems like mentioned above where you accidentally set the  teeth the wrong way.  If you do - don't fret, it's not the end of the world.  Subsequent filings will essentially remove the set, and at some point in the future you can start over.  Till then, it won't make a huge difference in how the saw performs so long as you are consistent in the set from side to side.  But do try to avoid it.

Once I've set the teeth, sometimes I will go over the saws once again with a file to sharpen them one last time - very lightly - to account for the new set.  Now the saw is sharp... but I'm not done yet.  Now saw is ready for final tuning.


Graduated Teeth  (Added 05-05-05)

A friend (Jonathan Skipsey) and I were having conversations about sharpening saws (yes, I know - what fun!) and the subject of spacing graduated teeth came up.  Graduated teeth are where the teeth at the toe of the saw  might be spaced at 8 teeth per inch, and at the heel the spacing is 5 TPI.  This is done to make starting a saw easier, but as the cut progress, the saw cuts more aggressively and as a result - cuts faster.  Jonathan had seen several saws where the spacing was masterfully done, and we wondered how the sharpener could spaced the teeth evenly, when Jonathan thought up the answer:

Text written on board above reads  "Edge of saw" - "Fronts and tops of teeth marked out - need square or sliding bevel etc." and "Line from biggest to smallest gullet depth".

Here's his commentary on it:

"Obviously I exaggerated the slope on this example but you can see the basic idea. I reckon it will be critical to get the edge filed and stoned absolutely straight before starting; Paste plain smooth paper along then set it out with a sharp 2h. The depth of the gullet for the biggest and smallest TPI are the 2 important dimensions. I'd calculate them separately on paper.
You could use a Swanson square or sliding bevel, or even 2 little wood or brass templates, one cut to each of the 2 angles. "

If you have a CAD program, this would be quite easy to do, too.

Thanks, Jonathan!  If anyone has another idea, I would like to hear from you!


Tuning a Saw for Optimum Performance

Yes, a saw can be tuned for optimum performance, and there isn't that much involved that I haven't gone through already in this article - but to put it all in one place...

Wax the blade of the saw using a paste wax such as Johnson's Paste Wax.  Avoid waxes that contain silicone, as this can contaminate some finishes... simple paste wax is best and it is amazing how much it can improve performance.

Lightly stone both sides of the teeth to even them out.  Errant teeth both reduce efficiency and leave the surface rougher than need be.  Stoning the sides evens the set of the individual teeth.

When sharpening, you can adjust the rake of the teeth to provide either a more aggressive angle that cuts faster, or a less aggressive angle that provides more control.  I've had back saws sharpened so aggressively that they can reach their full depth with a single stroke, albeit by leaving a very  rough surface behind.  

Crosscut saws can be sharpened with more or less fleam... using 30 degrees of fleam provides a very, very, sharp saw - though the edge retention is less than desirable, and you would need to re-sharpen such a saw quite often.  On the other hand, a saw with less than 15 degrees of fleam needs sharpening less often, but isn't as sharp.  Generally I try for a 20 to 25 degree fleam on my back saws, and a 15 to 20 degree fleam on my hand saws - but it is an opportunity for optimizing performance that you can utilize.

One thing I've heard of, but never tried - I guess there are now triangular shaped ceramic stones available now that you could hone the cutting edge of the saw teeth with.  Seems like quite a bit of work to me, but interests me enough that I might try it one day.

Once you get used to sharpening and tuning your own saws, you will develop a feel for what types you need, and how to tune each for it's intended purpose.  I've tried laying out a few simple guidelines, but so much of it comes from using it - well, you'll see!  Once you've made those first cuts with a sharp saw...