Math has never been my strong point, I've looked up the formula in the machinist handbook but can't seem to figure it out. Anyway I built a special ID bar that has a adjustable cutting angle for making spiral grooves in different sized bronze bearings but I need to know how to preset it so it won't rub, can somebody explain in plain English how to figure this angle? I included a simple drawing of the tool I made and I need the angle listed, say for a 2.2" bore and a 6.5 in. lead. I made this spiral groove yesterday and just eyeballed it to about 46 deg. and it worked great.

A Helix is strange. The angle at the top of the groove is different than the bottom, for example.
That being said, the helix angle at the surface can be calculated by taking the circumference, and the lead, and solving a right triangle. Think of it as wrapping the helix around a barber pole, The helix groove is the Hypotenuse, the circumference is the base, and the lead is the height of the triangle.

That is the simple way to understand it.
Now ten people will all have a different formula, so I won't cloud the waters with one more. As long as you can picture it on a knapkin, you can work it out pretty easy, using the S=O/H, C=O/H, T=O/A permutations.

S sine, C Cosine, H Hypotenuse, O Opposite side, A Adjacent side, T Tangent.

Cuda
What type of machine and control are you doing these on.
We have had variable results doing long lead grooves on our lathes.
Mostly not good.

Using a 1989 Mori Seiki SL25 with Fanuc 15T control, I've never had a problem with spiral grooves using either G92 or G76 cycles, but you must use a M24 chamfer off code or the tool pulls out way before the stop point and pulling the groove tool works much better than pushing. And thinks for the replies above but it's still kind of greek to me, can someone go through it step by step please without using a scientific calculator? Let figure the one I started with above, 2.2 Dia. 6.5 lead.

Cuda
The only way to to this without a calculator would be to lay it out on graph paper.
Draw a line 6.911" long along the bottom ( this is the bore or adjacent side)
Draw a line 6.5" long along the right side ( this is the full circle pitch or opposite side )
Draw a line connecting the two ends ( hypotenuse )
Measure the angle formed by adjacent & opposite sides with a plactic protractor from geometry class. This method should be close enough to set your tool.
Do you cross feed in X at the same time you feed in Z using an I word?
I would be curious to see your code. Can you post a piece of it here.

Cuda
The only way to to this without a calculator would be to lay it out on graph paper.

Draw a line 6.911" long along the bottom ( this is the bore or adjacent side)
Draw a line 6.5" long along the right side ( this is the full circle pitch or opposite side )
Draw a line connecting the two ends ( hypotenuse )
Measure the angle formed by adjacent & opposite sides with a plactic protractor from geometry class. This method should be close enough to set your tool.

I meant a calculator without all the trig functions. But drawing it on paper would be a good idea too if it would work good enough.

Originally Posted by jwk42

Do you cross feed in X at the same time you feed in Z using an I word?
I would be curious to see your code. Can you post a piece of it here.

I just use a simple thread G code, I don't use the I word at all as the groove is straight. The code for the above Left Hand spiral looked something like this, X started at 2.200 and ended at 2.29 for a .045 deep groove...
G97 S035 M3;
G0 X2.100 Z2.0;
Z-6.75;
M24;
G76 X2.290 Z1.0 I0 K.045 D001 F6.5;

Cuda
Boy did I mess that up. I was looking at the hypotenuse and typed Opposite!!
I realized what I had done after I left town for the weekend so I couldn't correct it until now. I like MRaineys Excel method better than mine.

"Measure the angle formed by adjacent & opposite sides with a plactic protractor from geometry class. This method should be close enough to set your tool."

It should read --

Measure the angle formed by adjacent side and hypotenuse with a plactic protractor from geometry class. This method should be close enough to set your tool.

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