5 cut method?

[Tony Shea]

I've heard people refer to the 5 cut method around here a couple times. What is this and how is it done. I saw a real cut-up version of it but still is a little off to me. Do you have to start with a square peice or does it matter? Some decent explaination would be great so I could really see how accurate my sleds and miter guage really is.

[Steve Rowe]

The method is to check for square on a sliding outrigger fence. I suppose it could be done with a miter gage but it seems iffy at best unless you have a long fence. You could also use it for a crosscut sled.

The process starts with about a 2' x 2' piece of plywood approximately square. Place one edge against the crosscut fence and cut a sliver off. The second step is to rotate the piece with the edge you just cut agains the fence and repeat the cut 3 more times. For the fifth cut, you cut off a piece where your offcut is large enough to measure (say 1/2" or so). Then measure the width of the offcut with calipers, micrometer or whatever. If the width of the offcut is the same at the front as it is the back, your fence is square.

[Rod Sheridan]

Steve has done a great job of describing the 5 cut process.

It's used because the error is magnified by using the cut surface as a reference for the next cut multiple times.

This makes it possible to measure the error with common woodworking measuring tools ( or even better machinist type tools as Steve indicated.)

Regards, Rod.

[Frank Drew]

I'm probably not thinking of something, but starting with the same roughly 2' x 2' piece, wouldn't two cuts tell you if you were crosscutting at 90º: Make one crosscut then flip your board over so that the same edge is still against the crosscut fence and cut the opposite edge, then measure? If the cut edges aren't parallel, wouldn't the amount they're off by be twice your cutting error?

[scott kinninger]

I'm probably not thinking of something, but starting with the same roughly 2' x 2' piece, wouldn't two cuts tell you if you were crosscutting at 90º: Make one crosscut then flip your board over so that the same edge is still against the crosscut fence and cut the opposite edge, then measure? If the cut edges aren't parallel, wouldn't the amount they're off by be twice your cutting error?


Frank, it would be impossible to measure a 2 foot distance with a typical caliper. Thats why you only cut off a 1/2" sliver or less on the fifth cut. When I used this method to square my sled I could measure to 4 decimal places with a caliper, much more accurate than rounding with a tape measure. Your thought process is probably correct but no way with common tools to measure accurately.

[David DeCristoforo]

Only thing I would add is forget the micrometer. Just snap the cutoff in half, flip one half and butt the two pieces end to end. They should be the same width.

[Myk Rian]

I'm probably not thinking of something, but starting with the same roughly 2' x 2' piece, wouldn't two cuts tell you if you were crosscutting at 90º: Make one crosscut then flip your board over so that the same edge is still against the crosscut fence and cut the opposite edge, then measure? If the cut edges aren't parallel, wouldn't the amount they're off by be twice your cutting error?
This is the method I use. You can do it with a 6" piece.
As long as the front and back edges are the same length, you're good to go.

[Alan Schwabacher]

Advantages of the 5-cut method over the crosscut and flip are that you don't need starting parallel edges, and that you can test squareness to extreme levels of precision. But it's possible to go overboard and think you are tweaking the fence to no error when in fact changes you see are mostly caused by random errors. That's why it can be worthwhile to repeat the process to see if you get the same result, and to keep in mind that the first cut only establishes a straight edge, so you only need four more cuts for each repetition.

The advantage of measuring with calipers is that the measurement can tell you how to shim your fence to get closer to square.

[Christopher Morgan]

Here is a repeat of the points made above with some basic diagrams:

http://www.thewoodshop.20m.com/five_cut_method_swf.htm

[Mike Cruz]

Christopher, thanks for the diagram. While I understood the description, having it diagram form made it that much easier to visualize.

[Robert Chapman]

And the diagram tells you what to do if your edges aren't the same length.

[Tony Shea]

Makes sense. I really didn't understand the 4 times the amount your off but is clear now.


I suppose it could be done with a miter gage but it seems iffy at best unless you have a long fence.

I don't just mean the miter guage by itself. It has a sacrificial fence mounted to it that is about 2' long. I would assume that this is plenty of fence to use the 5 cut method on.

[Gary Kvasnicka]

Only thing I would add is forget the micrometer. Just snap the cutoff in half, flip one half and butt the two pieces end to end. They should be the same width.

David,

That is the exact same method I always used to adjust the slider where I used to work. Additionally, I marked the leading edge of the 5th cut so I would know which way to adjust.

[Steve Rowe]

I don't just mean the miter guage by itself. It has a sacrificial fence mounted to it that is about 2' long. I would assume that this is plenty of fence to use the 5 cut method on.

IMHO, with the standard miter gage, it is best just to use a square to check it against the blade and rock on.

The 5 cut technique is very precise because of the aforementioned error magnification. Based on most cabinet saw miter gages that I have used, I think you would be chasing square all over the place using this method. The slop in the miter gage slot as well as the coarse adjustments of the miter gage makes using this method for checking and adjusting square tenuous at best. Unless you just have an outstanding miter gage that holds its settings and is repeatable when you move it back to the 90 degree position, the 5 cut method will waste a lot of time.

I should point out that the only time I use this method is during initial setup of the crosscut fence on a slider. I haven't had to repeat this yet nor has my setup even given cause to suspect the settings.

[tyler mckee]

So i just tried this out on my RAS and i ended up with about 1/16" difference front to back, so in reality im only off about a 64th? Not bad for the old RAS, at that point i don't think i want to bother trying to dial it in.

[Tom Veatch]

Depending on the size of the original square. If the original block were about 1 foot on the side, and you took off about a half inch with each cut, and were off 1/16" on the last cut, that would be an angular error of around 0.08 degrees or about 5 minutes of angle. If you started with a smaller block and had 1/16" difference, the angular error would be greater.

The difference in the 5th cutoff divided by the cut length of the last four cuts is a good approximation of the tangent of the angular error. I just modeled the process in Sketchup, and made minimum width cuts on a 1 foot square with an intentional error of 0.02 degrees (1.2 minutes of angle) and got a linear difference of about 1/64" on the last cut. (1/64")/48" = tan(~0.019°)