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How Square Is Square?


Timbo

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I need a spot of help with a math problem. My brain does not do measurements, I just cannot get the bits that stroked out to work properly.

I'm currently building a couple of table saw sleds. One is a crosscut sled, and the other is a mitre cut sled (for making boxes). I'm trying to align the fence that the lumber to be cut will register against. To do this I'm using the FIVE CUT METHOD. I've used the result to adjust my fence and retested using the five cuts again. So far so good and I have my results but cannot understand them and I'm hoping someone with more technical ability can help?

I've managed to discern that my fence is -0.04mm out of square or 0.001574803 inch over the length of my table saw top. Is this square enough? 

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I do it by eye.

If the "work" starts to pinch between the blade and the fence, then the fence is turned towards the blade but if you have to maintain the work against the fence, by hand pressure, when running it through, it is turned away from the blade.

I have often found that this is more a matter of making sure that the blade is running square to the table, than the actual alignment of the fence to the table.

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Err....so I have to divide by 5? I'm struggling here, to give you a clue, Ellie has to check my change at the shop because I can't add it up...right, let's see...

I've made five cuts on a board that is 219mm long.
On the fifth cut I've taken a wider strip that at the front measures 11.65mm wide and at the back (closest to the fence) measures 11.60 mm wide.
So I subtract one from the other which gives me 0.05 mm difference.
Divide this by 5, and I now have 0.01 mm 
If my 'summing' is right :35_thinking:

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Weeeeelllll, I'm on the level...

Let's put it this way I just cross cut and put a square up against it and it was square, but how square is my square? I couldn't see daylight...but then I was blinded by that shining from my ear holes! :facepalm:

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34 minutes ago, CambridgeCabby said:

Reminds me of a joke I was told years ago concerning two engineers watching a blacksmith at work and commenting to him that in their work they have to be accurate to within a thousandth of an inch , prompting the old blacksmith to state that he prefers his work to be exact  

How does another story go:

measured with a micrometer

marked with a pencil

and cut with an axe

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Just now, KaptinKev said:

If the measurement had to be close, it was always referred to as "being within a gnat's cock".

Hopefully I haven't broken any rules with this comment. 

 

on a model railway forum I belong to, the automatic net nanny would have changed that to "being within a gnat's large chicken:default_biggrin:".

 

This is awkward when describing a locomotive on the LNER with it's official name being

Large Chicken :default_biggrin: O' the North

https://en.wikipedia.org/wiki/LNER_Class_P2

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Hi,

so far I know, after having done the five cuts and did your calculation, it has to be devided by 4, the 5th cut is just to get your peace to measure the alignmend.

Than it depends how long your cut is actual to get the missalignment for 1 meter ( approx 3 feet  ). 

that means 0.0125 * 1000/219 = 0.058 mm per meter.

If you cut a peace which is 33 feet in lenght you will be missaligned 0,0023 inches. For a space station may be not ok  but i think its harder to get it better. I think think you are really square

 

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So here is the first finished sled...I used a design by a chap from Germany (as his English is much better than, well, most peoples) as he seems to have an almost identical inventory of tools and manufacturers to mine. 

DSC_0269.JPG

The test cuts I've made on projects so far seem to indicate I'm getting much better results using the sledge than the mitre gauge. It also feels much safer to use.

My test cuts were made on plywood and MDF as I assumed a man-made board would be much flatter and uniform but perhaps not.

This week is turning out to be a 'jig building' week as I have the mitre sledge to build next (with the blade at 45 degrees). A separate sledge for mitres means I have a 'zero clearance' slot for the blade in the bed. Makes a better job of cutting those mitres.

I also have to build a planer sledge as my jointer is not wide enough to take some of the boards I want to flatten. I do have hand planes but...I still have problems getting them flat. It must be the 'one-handed' thing!

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