I want to rough mill a .25" radius along the X axis of my part with a .25" ball nose EM. I have calculated (sp?) my Z and Y points using .01 steps in the Y direction, but I don't know how to allow for the .125" radius of the EM. I would assume that it would be something like the "tool nose radius comp" used on the lathe. If anyone knows the formula to make this calculation please let me know.

Thanks

You could draw it out in CAD and pull points from it!!!!!!

It's just tangent arcs

You could draw it out in CAD and pull points from it!!!!!!

It's just tangent arcs

Sorry, no cad, I used the equation of a circle and a calculator to get my Z and Y points but don't I have to allow for the radius of the ball nose EM to get it to come out right.

If I just used the points that I came up with and a straight EM I would just a series of steps around the radius of the part.

If I understan you correctly, you want to mill a profile with the END of the ball nose end mill in the XZ plane (G18). On some controls, using G41 or G42 is an option in planes other than G17. On others, no option at all. If you program your profile using G18 and cutter comp, you'd also have to touch off your tool and take the radius amount from the tool offset, driving the center of the ball and not the tip. No help?

Sorry, no cad, I used the equation of a circle and a calculator to get my Z and Y points but don't I have to allow for the radius of the ball nose EM to get it to come out right.

If I just used the points that I came up with and a straight EM I would just a series of steps around the radius of the part.

Do it just like you did but add the two radii, the one you want to cut and that of the endmill.
Then instead of programming the end of the end mill you are programming the movements of the center of the ball of the end mill. So say you wanted to put the end of the tool at Z0.0 you would program Z0.125

you have to calculate the points of the center of your ball end mill and then move down z-.125 to the tip of the cutter.

After some work this is what I came up with. I had to picture in my head a ball with a .125" radius sitting on top of a ball with a .25" radius.

r1 = .25" =.5" DIA
r2 = .125" = .25" DIA

The point where they touch we'll call point A. This would be the same as the center of the Ball End Mill. As the .25" DIA. ball slides around (not rotates) the circumference of the .5 DIA. ball point A discribes a circle with a radius that is equal to r1 + r2. This is the circle that I needed to find the points for. Take a look at the image attached.

http://www.cnczone.com/gallery/showfull.php?photo=5108

The Equation for a circle is X² + Y² = r²

Since I wanted to find points for Z I swapped Z² for X²
which gave me Z² + Y² = r²
In this case r² = (r1 + r2)²
which gives Z² + Y² = (r1 + r2)²
I wanted to solve for Z so I subtracted Y² from both sides which gives Z² = (r1 + r2)² +Y² Take the square root of both sides leaves Z = √((r1 + r2)² - Y²). The top of my part is Z 0 so I subtracted the difference of (r1 + r2) - √((r1 + r2)² + Y²) from 0. So the final equation comes out to be: Z = 0 - (r1 + r2) - √((r1 + r2)² + Y²) Now all I had to to was substitute in values for Y and solve for Z. I started with Y = r1 + r2 which is .375" and moved over in .01" steps till I reached 0. In the actual program though the first value I used (.375") is actually .125" from the side of my part. So in my program my first points were Z = -.375", Y = .125". It all sounds kind of confusing but it makes more sense when you look at the drawing.

Calculate the centre of the Ball and subtract the cutter radius off the Z as BillyStein recommended.
Y = (0.25 + 0.125) * Sin(Angle) - 0.25
Z = (0.25 + 0.125) * Cos(Angle) - (0.25 + 0.125)
Angle = 0 deg to 90deg. This will give a even cusp-height finish.
This VB program should help.

Andre'B is right. I used to use ballnose to put radii on corners, so I wouldn't have to lay part down, and do with an endmill. To get a .25 rad. you should just start at a z-.375 & x-.125, and G2 or G3 with a G18 up to Z0 X.25 R.375 or I.375. This just programs the tip of the .125 ballnosetool to make a .375 radius, which produces a .250 radius. It works on partial radius also, but you have to do a little more math to figure out the tangential endpoints. Hope that all made sense.

wow way too much math and work, just ask someone to do it on cad for you it would take about 3-4 mins, math was fun when I was younger but now not worth the time lol ;)
if you dont have it yet, pm it to me( the dimensions) and I will give you g-code for it. I should be online tomorrow

Calculate the centre of the Ball and subtract the cutter radius off the Z as BillyStein recommended.
Y = (0.25 + 0.125) * Sin(Angle) - 0.25
Z = (0.25 + 0.125) * Cos(Angle) - (0.25 + 0.125)
Angle = 0 deg to 90deg. This will give a even cusp-height finish.
This VB program should help.

I should have remembered that one. But once I got onto the circle equation my brain was locked in.

wow way too much math and work, just ask someone to do it on cad for you it would take about 3-4 mins, math was fun when I was younger but now not worth the time lol ;)
if you dont have it yet, pm it to me( the dimensions) and I will give you g-code for it. I should be online tomorrow

Thanks for that offer but I did get it all figured out. I was thinking about it like it was a puzzle to solve. I knew I could figure it out it just took a little time. Once I had the equation I just put it in my programmable calculator and got my point in just a few min.