weird results with a sine bar.

[wheeltapper]

Hi
some time ago I showed a sine bar I'd made.
http://madmodder.net/index.php/topic,8735.msg94914.html#msg94914

I've recently been trying to master basic trig and have been stacking various height objects under the bar and working out the angle, then checking it and I'm getting some weird results.

It seems the higher I go the more the error increases.
It's a 5" bar, the rolls are exactly the same diameter, the distance between centres is 5" to a tenth of a thou and the distance between the base of the roll to the top of the bar is identical at both ends.

should be all right, not so.

with a stack of 407.5 thou.( I miked various pieces to hand so odd sizes )
theoretical angle is 4.6 degrees, actual is 4.7.
at 763 thou theoretical angle is 8.7, actual is 8.8. so far so good.
1509 thou theoretical 16.8, actual 17.6. oops.
1950 thou theoretical 21, actual 22.4.
2738 thou theoretical 28.7, actual 33.2. big oops.
at 3" should be 30.9, actual 36.6.
and at 4" should be 38.6, actual a whopping  53.2.
What the hell is going on!!!

If I'm doing something stupid please tell me because this is driving me nuts.

cheers
Roy.

[BillTodd]

Is that 5"between rollers or 5" between the centres of the rollers? ( as it should be)

Bill

[wheeltapper]

Its centres, I did say that.
on reflection I think I may be using the wrong formula and confusing myself.

I never was any good at maths.

Roy.

[Fergus OMore]

Having been brought up on a diet of Euclid, I would guess that you have a crap mike!

What you should be using is measurement tools of a known reliability- ie Jo Blocks.

I might be wrong  but  ALL the details necessary are yet to be provided.

Regards

N

[wheeltapper]

I just read this  on a different website,

"Limitations of Sine bars:

Sine principle is fairly reliable at angles less than 15o, but becomes inaccurate as the
angle increases."

so maybe its not me?

[Jasonb]

I think its your maths

Working it out for the 0.407" stack I get 4.669 which is near as dam it your 4.7actual

0.407 / 5 = 0.0814.
sin 0.0814 = 4.669

For the 1.509 stack I get 17.566 again closer to your 17.6 actual than your 16.9 theory.
1.509 / 5 = 0.3018
sin 0.3018 = 17.56574

What are you using to "check" the angle

Just twigged you are using tan not sine there is a clue in the name of the bar

J

[mklotz]

I think its your maths

...
sin 0.0814 = 4.669

...
sin 0.3018 = 17.56574

Don't add to the confusion.  You should have written:

arcsin (0.0814) = 4.669

arcsin (0.3018) = 17.56574

[wheeltapper]

I thought I was using the wrong formula.
I said I was no good at maths.

Oh well , back to the drawing board, I'll just get my coat.

cheers
Roy.

Edit
I just figured my mistake, thanks guys I know were I went wrong now.

[mklotz]

I thought I was using the wrong formula.
I said I was no good at maths.

Here are the pertinent formulae:

Let:

h = stack height
L = sine bar length (distance between roll centers)
A = angle

then:

sin(A) = h/L

h = L * sin(A)

A = arcsin(h/L)

[wheeltapper]

cheers for that.
Roy.