1. Member
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## Compound angle question

I over heard someone talking about getting a fine feed using the compound. I want to set my compound to get a 10 to 1 ratio so what degree do I need to set it at?
Tim

2. dp
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In which direction - cross feed or in line with the ways?

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Cross feed is what I think I need. If I move the compound .1 indicated I will get .01 actual movement.

4. HSS
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I believe if you set your compound at 30º you will get 10:1, if I remember correctly.

5. dp
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You want an actual infeed of 1/10 of the dial on the compound. Set the compound 5.71º off parallel with the ways. Call it 6º. It is a tangent function and the ratio is 1 in 10, or 0.1. The arc tangent of 0.1 is 5.71 degrees.

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It is cosine (or sine if you measure from the axis of spindle rotation), not tangent but the answer is almost the same.

0 degrees from the X axis (compound travel) gives 1:1. 84.26083 degrees from the X axis gives 10:1 and will also result in a shift in Z of 0.9998744" per 1" of motion of the compound.

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...or, to put it slightly differently, if X is the ratio you need and A is the angle of the compound, then

A= arccos (1/X)

For X=10, A=arccos 0.1 = 84.26 degrees

As you see, to achieve this kind of ratio you'll have to put the compound almost parallel to the ways.

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Thats what I did was the tangent function. I made a triangle of a=10 b=1 and came up with the angle of 5.71, just didn't look right.
So thanks for the help after many pages of triangles my head started to spin and I had to make it stop...
Tim

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## Slippery slope

Originally Posted by whitis
It is cosine (or sine if you measure from the axis of spindle rotation), not tangent but the answer is almost the same.

0 degrees from the X axis (compound travel) gives 1:1. 84.26083 degrees from the X axis gives 10:1 and will also result in a shift in Z of 0.9998744" per 1" of motion of the compound.
Good points whitis.

The values of Tan theta and Sine theta are very similar but at about 5 degrees but they diverge rapidly after that. I limit the use of similarity to 3 degrees.

Cosine function as well as cosine error were neatly covered to.

Some might like to review the assumed consistent accuracy of a sine bar for similar reasons.

0.001" "lift" at zero elevation on a 10" sine bar = asin 0.001/10 = 1 : 10,000 = 0.0001 = 0.00573 degrees (20.63 seconds) where-as a 0.001" over or under the theoretical 7.0711" at 45 degrees = asin 7.0721/10 = atan 0.70721 = 45.0084 degrees = a difference of 0.0084 degrees = 30.11 seconds.

The reason I said that is as whitis says, is that there can be considerable differences between the "travel along the slope" (top/compound slide) and "travel along the flat" (lathe carriage). Both are only the same when the top/compound slide is parallel to the lathe bed (and the spindle) axis.

I do think that the OP and dp (Dennis) were referring to the classic "rise (1 vertical) over run (10 horizontal) which is as dp says is a tangent function.
Last edited by oldtiffie; 09-26-2010 at 01:15 AM.

10. dp
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Originally Posted by whitis
It is cosine (or sine if you measure from the axis of spindle rotation), not tangent but the answer is almost the same.

0 degrees from the X axis (compound travel) gives 1:1. 84.26083 degrees from the X axis gives 10:1 and will also result in a shift in Z of 0.9998744" per 1" of motion of the compound.
Oh - you're right, and I should have gotten more sleep last night! The compound represents the hypotenuse and not the side adjacent. But what a happy coincidence (which doesn't scale). On Asian lathes that have zero indicated with the compound parallel to the ways, sine works exactly with the compound angle indicator to scale the compound feed dial. For other machines where the compound reads 90º when perpendicular to the ways, cosine is preferred.

But then I've been up all night and now all day, too

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