Slop in the Miter Gage

terrydowning · Post by **terrydowning** » Fri Mar 25, 2011 1:19 pm

does the incra v120 support the use of the Miter Gauge Stop Rod (Item 505629)?

JPG · Post by **JPG** » Fri Mar 25, 2011 1:26 pm

Let us step back and look at this 'situation' logically with emotion removed(the sky is NOT falling:D)!

Let's also assume the slop is 0.010.

The miter gauge bar is 16" long.

When the bar rotates in that ten thousandths slop the angle traversed is 0.0358 degrees.

A ss bar in a 3/4" slot would only produce an error of 0.0895 degrees.

I do not think we can detect that angle in any wood cutting operation!;)

However a 0.010 lateral movement would show up if the workpiece were ridgid to the face of the miter gauge. Smoothness(or rather the lack of it) is a more likely indicator.

dusty · Post by **dusty** » Fri Mar 25, 2011 2:39 pm

terrydowning wrote:does the incra v120 support the use of the Miter Gauge Stop Rod (Item 505629)?

No, it does not support the stop rod. It would be a rather simple task for you to do that though.

dusty · Post by **dusty** » Fri Mar 25, 2011 2:43 pm

[quote="JPG40504"]Let us step back and look at this 'situation' logically with emotion removed(the sky is NOT falling:D)!

Let's also assume the slop is 0.010.

The miter gauge bar is 16" long.

When the bar rotates in that ten thousandths slop the angle traversed is 0.0358 degrees.

A ss bar in a 3/4" slot would only produce an error of 0.0895 degrees.

I do not think we can detect that angle in any wood cutting operation!]A math lesson that shows how you came up with that .0358 degrees would be interesting.

michaeltoc · Post by **michaeltoc** » Fri Mar 25, 2011 3:54 pm

dusty wrote:A math lesson that shows how you came up with that .0358 degrees would be interesting.

Arctan(0.010/16)=0.0358

JPG · Post by **JPG** » Fri Mar 25, 2011 4:19 pm

dusty wrote:A math lesson that shows how you came up with that .0358 degrees would be interesting.

Hokay!

Visualize a triangle formed by one side of the miter bar(16" and tight against the slot at one end), one side the 0.01" slop(at the opposite end), and the third the line back to the starting point along the side of the slot. Thus you have a right triangle formed by the side of the bar, the 0.01" slop and the side of the miter slot. At the small angle here either long side can be considered the hypotenuse. Since sin(?) = opposite/hypotenuse, sin(?) = 0.01 / 16 = 0.000625. ? = angle whose sine is 0.000625 = asin(0.000625) = 0.03580986.

Double checking - we did make an assumption regarding selection of the hypotenuse.

With the 0.010" assumed in all cases,

? = 0.03580986 in all cases,

tan(?) = 0.000625 = opposite / adjacent = 0.010 / 16 = 0.000625

cos(?) = 0.99999980 = adjacent / hypotenuse then hypotenuse = 16 / 0.99999980 = 16.00000313.

Then using 'corrected' hypotenuse, sin(?) = 0.01 / 16.00000313 = .000625

Hey that is the same as the sin using 16.0000000. So the assumption is reasonably reliable.

Now we are doing stuff out to 7 places which is 1 in 10 million accuracy so I can safely say the very small angle formed by the slop is quite small. In other words, the 'error' introduced by the 'assumption' is out beyond the 7 place accuracy being used.

Now doing it the square root of the sum of the squares method yields the following numbers.

Adjacent(16")² + opposite(0.01)² = 256.0001000 the square root of which is 16.00000313. Look Familiar???;)

JPG · Post by **JPG** » Fri Mar 25, 2011 4:23 pm

michaeltoc wrote:Arctan(0.010/16)=0.0358

I would not even try to explain to Dusty what an Arctan is!

It is the angle whose tangent is 0.010/16.

dusty · Post by **dusty** » Fri Mar 25, 2011 5:14 pm

JPG40504 wrote:I would not even try to explain to Dusty what an Arctan is!

It is the angle whose tangent is 0.010/16.

Good, because I don't want to have dig out the math books just to prove to myself that I understand what you might say.

I understood all that stuff at one time. It was when I hit calculus that everything turned fuzzy.

To determine the impact of this slop by methods that I clearly understand, I used the table saw, miter gauge and engineer's square.

With a piece from the cut off box, I cross cut one end of a 3" by 8" 3/4" stock while forcing the miter bar to one extreme.

I then cut the other end using the V120.

On the V120 end, with the square in place I see no light. Good 90° cut.

On the other end, I see light at one edge and none at the other. Not perfectly square. In fact, I could slide a .003" feeler gauge in the small gap.

Yes, the slop in the miter bar can introduce some (though minute) error into your woodwork.

I know. There are a few out there reading this that say so what. Me too. So what. I don't really care if your cross cuts are out of whack by that wee little bit. But I do care if mine are if all I have to do to eliminate it is get rid of that wee little bit of slop.

BTW - my test does not exactly track with the math lesson but thanks. I appreciate the refresher. Engineers obviously retain that stuff a lot longer that some others.

JPG · Post by **JPG** » Fri Mar 25, 2011 5:38 pm

dusty wrote:Good, because I don't want to have dig out the math books just to prove to myself that I understand what you might say.

. . .

Engineers obviously retain that stuff a lot longer that some others.

Only if used recently! Interesting comment re calculus. I find I remember more of the geometry/trig stuff than all that stuff that followed. I relate to physical stuff I can feel/see etc. more than that abstract stuff! Maybe I shoulda been an ME!:rolleyes:

mountainbreeze · Post by **mountainbreeze** » Fri Mar 25, 2011 7:28 pm

Thanks for the responses everyone.

You can now see what happens when an engineer tries to do woodworking. I just assumed that slop in the miter gauge was a bad thing. However, I have nothing to show that indicates it is causing or will cause a problem. For now, I'll try tape in the slot to see if that holds up. If not, I'll learn to live with the 0.03580986° error.

Thanks again to all. Next time, I'll try to ask a question that really needs an answer.