how do I convert distance offset to angular value?

[Jesse Tutterrow]

I received an e-mail from Woodpeckers offering a limited edition square (http://www.woodpeck.com/1pccarpentersquare.html?et_mid=381130&rid=29489241).
They state that they use a feeler gauge to verify that each surface if straight within 0.0015 inch.

This started me wondering, if the 90 degree is off at the top by 0.0015 inch, what is the real angle?

Or better yet, how do you convert between the two?

[Bruce Page]

Simple trig, 89.98567606° Close enough for government work.

[Brian Tymchak]

I used to know the trig to do this, but I just use an online calculator (http://www.1728.com/trig4.htm)now. Assuming the .0015" error is measured at the end of the leg of the square, you can think of the problem as an isosceles triangle with 2 sides the length of the square and the third side of length .0015". So, knowing 3 sides, you can compute any angle of the triangle.

On the 3 3/4" square, that would equate to a 0.022918* angle opposite the .0015" side.

On the 6" square, that would equate to a 0.014324* angle opposite the .0015" side.

So, 90* - these angles.

[Brian Tymchak]

I see Bruce beat me to it...

[Jesse Tutterrow]

Wow, Fast Response.

Thanks Guys!

[Bruce Page]

I should've said that my numbers were for the 6" square. :rolleyes:

[Tom Walz]

We have an angle chart online.

http://blog.carbideprocessors.com/wp...gles-Chart.pdf (http://blog.carbideprocessors.com/wp-content/uploads/2011/04/Saw-Tip-Angles-Chart.pdf)

You can download it or you can call and we will send you a laminated 11" x 17" version free. Ask for Emily or Renee'

Tom Walz

[Chip Lindley]

And once we have quantified the deviance from square of .0015" in 6" (89.98567606°) what can that information be used for?

It would be more useful in woodworking to find the deviance from square for 3/4" and 1-1/2" lumber as the amount of gap produced if butted against true 90° --.00019" and .00038" respectively. Not much! Plenty good for woodworking.

I set the blade of my TS to "90°" with a cheap plastic drafting square; a penlight laid on the table behind for backlight. Amazingly, the surface of the blade plate undulates along it's width a minute amount. Thus, the "90°" setting must be visually interpolated with the least amount of "daylight" between blade and square at any one time. Mine are premium Freud blades, not some cheapies.

[Andy Mail]

Just to finish the play, you take the arctangent of (the blade length divided by the deviation), so angle = atan(.0015/6) = atan(4000) = 89.98567606°

[Mitch Barker]

The way I read that ad, the 0.0015" refers to the straightness of each edge. For example they would place a side of the triangle on a super flat reference surface, and feel for any gaps between the triangle and the reference surface. If the feeler slips through, it needs to be reground.

The ad doesn't mention the accuracy of the right angle. But if they did it would be given in degrees, minutes and seconds.

[David Thompson 27577]

.....................They state that they use a feeler gauge to verify that each surface if straight within 0.0015 inch.


Measuring for straight, is not the same as measuring the angle at the corner. While I agree that IF the surfaces are completely flat/straight, then the trig thing works. But if one leg of the square has an edge that is simply not straight, that has nothing to do with the angle.

[Bruce Page]

Measuring for straight, is not the same as measuring the angle at the corner. While I agree that IF the surfaces are completely flat/straight, then the trig thing works. But if one leg of the square has an edge that is simply not straight, that has nothing to do with the angle.
Very good point! People assume because a part was machined on a CNC that it is perfect. That is not always the case.