Squaring my Incra miter guage - a different way

[Lee Schierer]

I didn't see this thread the first time around and I have a small problem with a few of the comments regarding the blade not being parallel to the miter slot still giving perfectly square cuts. While that might be true for the angle of the cut being perpendicular to the direction of travel of the miter gauge, the cut is no longer vertical if you measure up from the table surface. While a few thousandth of and inch out of parallel might not be very noticeable the saw blade is a circle so as get out of exactly parallel with the miter slot you will begin to get a curved cut of the cut surface perpendicular to the saw table top.

You can see what I mean by getting down to table surface level with your eye looking straight down the blade so you only see the thickness of the blade. Now move to the left or right and you will start to see an ellipse instead of a flat blade.

[Nick Decker]

That’s a great method, thanks for posting (and bringing back an old thread). I hate the 5 cut method!

I'd like to echo that, Johnny. I vaguely remember someone posting that link a while back, but it really didn't sink in then. The five-cut is a PITA. Laying a square along the blade and fence is fine for measuring a certain point in space, but it doesn't measure the path traveled along the miter slot.

[Grant Wilkinson]

I started to think that I was dense. I didn't understand how a cut could be square if the blade is not square to the miter slot. However, now that I've read the comments on Dan's video, I see that I was not missing anything after all. He says, "No matter what method you use to square your miter gauge, your blade should always be parallel to the miter track. Everything should be aligned to the miter track." That's the way I always understood things.

I would appreciate if those here who have said that the blade does not need to be parallel to the slot could explain how you can end up with a square cut. I'm not trying to be argumentative. I'm simply trying to learn how, if the miter gauge is square to the miter slot, but the blade is skewed to the slot/gauge, you can still get a square cut.

[Ted Derryberry]

Felder forgot the miter slot when they built my saw.

[Glen Gunderson]

I started to think that I was dense. I didn't understand how a cut could be square if the blade is not square to the miter slot. However, now that I've read the comments on Dan's video, I see that I was not missing anything after all. He says, "No matter what method you use to square your miter gauge, your blade should always be parallel to the miter track. Everything should be aligned to the miter track." That's the way I always understood things.

I would appreciate if those here who have said that the blade does not need to be parallel to the slot could explain how you can end up with a square cut. I'm not trying to be argumentative. I'm simply trying to learn how, if the miter gauge is square to the miter slot, but the blade is skewed to the slot/gauge, you can still get a square cut.

The final cut occurs at a single point on the blade's travel. So the part of the blade nearest the miter slot is going to produce that cut on the entire width of the piece you're crosscutting. So as long as the miter gauge's fence is perpendicular to its path, you'll get a 90º cut. In fact, sliding table saws are often deliberately set to toe away from the blade at the back, but they still produce perfectly square cuts because the crosscut fence is set exactly 90º to the path of the table.

Now if it's way off, you're going to have problems because the kerf will run into the body of the blade which will want to pull the wood one way or the other, but for small inconsistencies it doesn't matter. The miter gauge being 90º to the slot what's important.

[Nick Decker]

Grant, I'll try: Visualize your saw from above. There's a straight line, running from front to back. That's the miter slot, which guides your stock from front to back in a straight line past the blade. OK, there's another straight line which is your saw blade. Ideally, it's parallel to the other line, which will cause to wood to pass by it with the least resistance. But, if that line that is your blade is skewed out of parallel, since it's a saw blade it's gonna cut everything that passes by. It'll cut a little wider swathe, but if the wood is secure it'll still cut a straight line.

So, the job of the fence on your miter gauge (or sled) is to hold your stock perpendicular to the line that the miter gauge follows. Blade gonna do what blade gonna do, regardless.

[Grant Wilkinson]

Nick and Glen: I appreciate your efforts to explain this to me, but I'm still not there.

Here is how I am thinking and you can point out where I am wrong. Let's say that everything on the saw is perfect. The gauge fence is perpendicular to the slot. The blade is parallel to the slot and, therefore, perpendicular to the gauge fence. Therefore, any cut must be perpendicular (90°) to the edge of the board against the fence. I think we can all agree on that. (I hope. :-) )

OK, now we change the fence setting from 90° to 89°. All else stays the same. The slot is still perp to the blade, but now the the fence is not perp to the slot. Nor is it perp to the saw blade. So, we get a cut that is no longer 90° to the edge of the board against the fence. It is 89°. I assume that I'm OK so far, as this is how we cut miters on a table saw, by changing the angle of the miter fence with reference to the blade/miter slot.

OK, now lets change the fence back to 90°. Nothing else changes and all is well.

Finally, we change the angle of the blade to the slot. Depending on whether the trunnions are table mounted or cabinet mounted, we loosen something and bump the table until the slot is no longer parallel to the blade. It is, in fact 1° out. Now, the miter fence is still perp to the slot, but it is no longer perp to the blade. it is out by 1°

If I understand what you both are saying, the cut will still be perp to the edge of the board against the fence.

This is where I am lost. In both of my examples, the fence of the gauge has been changed by 1° with reference to the saw blade. In my first example, it was changed by adjusting the miter fence by one degree. In the second example, it was done by moving the miter slot by that same one degree. (We just did this by changing the relationship between the slot and the blade.)

If you agree that, changing the miter gauge to 89° will result in a non-square cut, how does changing the miter slot by that same one degree not result in the same non-square cut?

[Nick Decker]

The miter slot never changes. Re-read your last sentence.

The only things that change are the position of the BLADE, relative to the miter slot, or the fence, relative to the miter slot.

If the blade is skewed off of parallel (with the slot) then it will basically "plow" through the board rather than slicing cleanly through it. The resulting cut will still create a straight line, just a wider straight line.

[Grant Wilkinson]

But, Nick, the slot does change - relative to the blade. We can say it any way we want. The blade angle changes relative to the slot, or the slot angle changes relative to the blade. The result is the same. The slot and the blade are not parallel. That seems to jibe with your second sentence.

The issue to me is not the angle of the slot to the blade. It is the angle of the board we are cutting to the blade. If the fence is 90° to the blade, the cut will be 90° to the edge of the board that is against that fence. If we change the angle of the fence to the blade by loosening the miter gauge and changing the fence from 90° to the blade to 89° to the blade, we get a cut that is no longer 90° to the edge of the board that is against the fence.

If we change the angle of the fence to the blade by changing the angle of the miter slot to the blade, we have, in effect, changed the angle of the fence to the blade, since the angle of the fence to the blade is dictated by the angle of the slot to the blade. (I know this is starting to get convoluted.) So, why is the cut not 89°?

Maybe if I use a sliding miter saw as an example, it will help explain what I am trying to say. Using a sliding miter saw, we hold the board against a fixed fence and angle the blade to the wood. If the blade is set to 90°, we get a cut that is 90° to the edge of the wood that is against the fence. If we change the angle of the blade to the fence by 1°, we get a cut that is 89° to that edge. We can get the same result by putting a 1° wedge against the fence and putting the board against the wedge. With the blade at 90° to the fence, the cut will be 89° because we have skewed the board using a wedge.

So, I keep coming back to not understanding why, if the blade of a table saw is skewed from 90° to the board, it can still make a 90° cut.

[Nick Decker]

Your miter saw example might be instructive, although it doesn't apply. With the miter saw, the saw blade is traveling, not fixed in place like a table saw, but the miter saw blade is still traveling (hopefully) in a straight line. Try imagining that the miter saw blade is traveling in a srraight line, but that the blade is slightly out of skew with that straight line. What happens? A straight cut is made but the blade plows a wider swathe through the board.

(I once had a miter saw whose blade didn't travel in a straight line, so I got rid of it.)

[Glen Gunderson]

But, Nick, the slot does change - relative to the blade. We can say it any way we want. The blade angle changes relative to the slot, or the slot angle changes relative to the blade. The result is the same. The slot and the blade are not parallel. That seems to jibe with your second sentence.

The issue to me is not the angle of the slot to the blade. It is the angle of the board we are cutting to the blade. If the fence is 90° to the blade, the cut will be 90° to the edge of the board that is against that fence. If we change the angle of the fence to the blade by loosening the miter gauge and changing the fence from 90° to the blade to 89° to the blade, we get a cut that is no longer 90° to the edge of the board that is against the fence.

If we change the angle of the fence to the blade by changing the angle of the miter slot to the blade, we have, in effect, changed the angle of the fence to the blade, since the angle of the fence to the blade is dictated by the angle of the slot to the blade. (I know this is starting to get convoluted.) So, why is the cut not 89°?

Maybe if I use a sliding miter saw as an example, it will help explain what I am trying to say. Using a sliding miter saw, we hold the board against a fixed fence and angle the blade to the wood. If the blade is set to 90°, we get a cut that is 90° to the edge of the wood that is against the fence. If we change the angle of the blade to the fence by 1°, we get a cut that is 89° to that edge. We can get the same result by putting a 1° wedge against the fence and putting the board against the wedge. With the blade at 90° to the fence, the cut will be 89° because we have skewed the board using a wedge.

So, I keep coming back to not understanding why, if the blade of a table saw is skewed from 90° to the board, it can still make a 90° cut.

For your sliding miter saw example, the same applies. What's important is the travel of the slide mechanism, not the angle of the blade. If the blade was slightly misaligned with the slider the angle of cut would still be dictated by the slider's relationship to the fence, not the blade's angle.

Let's use an extreme example for a table saw. Imagine your blade is skewed 45º to your miter slot while your miter gauge is set at 90º. Now imagine you're cutting 1/32nd off the end of a board by using your miter gauge normally. What do you think would happen? It certainly wouldn't be a 45º cut. Essentially you'd end up with a poor cut, there'd be a cove on the end of the piece because of the blade's skew, but ultimately the end of the board would be a 90º cut because it's the miter gauge's fence's relation to the path of travel that dictates the angle of the cut.

[Grant Wilkinson]

I want to thank both of you again. I'm really not trying to be stubborn here. It just comes across that way.

I'm sitting here with pieces of paper representing saw blades, miter slots, miter gauges and chunks of wood. At some point, the light bulb will come on and I'll get it. The damn thing is that I do quite a bit of flat work. It simply has never occurred to me that having the saw blade anything but parallel to the miter slot is no big deal, at least when it comes to the accuracy of the angle of the cut.

Interestingly, the author of the video that started all this discussion is mistaken. He says that his method of dialing in a miter gauge depends on the miter slot and the blade being parallel.

Take care, and thanks for the discussion.

[johnny means]

Wait, explain why you wouldn't just square the miter gauge to the square?

[Chris Parks]

Felder forgot the miter slot when they built my saw.

I use this method to square the cross cut fences on my slider because it is so quick and accurate. Woodpeckers are offering a new one time tool that is ideal for the job, a phenolic 18" square. The thickness of it will make it easy to use the DI which with thin bladed squares can be a bit painful.

[Lee Schierer]

You can't get a straight cut in the plane perpendicular to your table top if the blade is not parallel to the miter slot. The line of the cut along the table surface will be straight and you will get a wider cut and probably some teeth marks and burning. Since the blade is a rotating circle perpendicular to the table cut is not a straight line. Look at the two sketches below:
straight saw.JPG This is what your wood sees in a cross cut with a blade aligned with the miter slot.

misaligned saw.JPG This is what you piece of wood sees when the blade is not aligned with the miter slot. Note that the cut right at the table surface will be wider than the cut near the top of the blade.



exaggerated.JPG To take it to an extreme, would you cut wood on this saw?