How Do You Cut a Lathe circle in Half and Keep It Straight?

[Technodandy]

When I cut a circle, I created with lathe, in half it does not keep it symmetry about the center. It bows into a parabola instead of keeping it's half moon shape. I then have to staighten it out slowly by hand, one Control point at a time.

 

Anyone know a solution?

[Stuart Rogers]

There are two solutions: add more CPs to define the curve, or play with the gamma and magnitude settings of the existing CPs (particularly the end ones).

 

OK, so there's a third option... Create your half circle, lathe it to make a sphere, and then start disconnecting (but not deleting!) CPs in the section of the sphere you don't want.

[MMZ_TimeLord]

I usually use the last method that Stuart mentioned.

 

I lathe my circlular object then I remove part of the side I don't want and break the splines so there is one branch of the circle remaining on each side of the half I cut. That way the curve remains unchanged.

 

Edit: Here's a screen shot of a quick 8 lathe sphere.

 

NOTE: on this particular 8 lathe sphere, I could delete the 'tails' that remain without distorting the remaining half. On a 4 copy lathe it will distort unless you leave the 'tails'. Most times if the number of lathe divisions is 8, 12, 18, etc. somthing that allows it to have a seam at the axis you want to cut it on, you should get very little distortion. At least the way I make spheres.

 

The technique I use is to lathe a 2 point spline with 8, 12, 18, etc. and delete the half that is off the axis, rotate it 90 degrees and delete half of it. Move the two points on the axis that remain just one 'nudge' with the arrow keys to the side your half circle is on. THEN lathe this spline... you should get a nearly perfect sphere (minus the ugly pinch points in the north and south poles). This is the type of sphere I have cut in half here.

 

I've attached another screen shot compilation showing how I build the 8 lathe sphere...

 

WARNING: The compilation is nearly 900k in size!

[heyvern]

Holy crud...

 

I never would have thought of that...

 

I am too obsessive! I would never even consider leaving those hanging splines! I always just use more points.

 

a low patch half sphere... nice.

 

Vernon "!" Zehr

[MMZ_TimeLord]

Another item of note... I believe I found what way Technodandy was cutting his sphere.

 

If you cut across the 'Y' axis, then you will have a parobolic dish as he said, unless you leave the 'tails' like this.

 

If you cut along the 'Y' axis, as I did in my first example above, you should see no distortion. You are left also with two sides that are not perfect (i.e. - pinch spots). Where in his cut, you will have only the one pinch spot to deal with.

 

There are many ways to get a near perfect half sphere. There are many ways to get a near perfect sphere too.

 

The way I do a near perfect sphere is this. I make a sphere, and cut off the poles. then copy and paste it (with no offset) and rotate it 90 degrees on the 'Z' axis.

 

This results in the appearance of a perfect sphere. The downside is if you have to decal it, you may have to do two 'stamps', one for each partial sphere.

 

I have also attached a wireframe and shaded wireframe of this type of sphere for your info. I made each partial sphere a different color to show what is happening.

[Kamikaze]

Damn, you mean there is no key you can hold down to maintain curvature while deleting splines ...........Smile...............

 

Michael

[Technodandy]

Damn, you mean there is no key you can hold down to maintain curvature while deleting splines ...........Smile...............

 

Michael

 

 

Exactly the solution I was hoping for Michael but the other solutions will work to eventhough they are not as straight forward as I would like.

 

Thanks everybody, but if anyone knows if there is some key for maintaining curvature please let me know.

[mtpeak2]

I'm not sure I understand what you are trying to do. I think you are missing a step when deleting half the circle/sphere, there's no need for dangling splines, unless I'm missing something here. After lathing your circle/sphere and deleting half of it, you need to break the spline that AM keeps to continue the loop (it depends on which half you are deleting, side or top/bottom). You can also select half the circle/sphere and copy/paste it. If you are trying to close the flat side, you need a 4 pt spline, using the 2 middle cp to close the loop and deleting the ends after you attached the cps.

[Technodandy]

I'm not sure I understand what you are trying to do. I think you are missing a step when deleting half the circle/sphere, there's no need for dangling splines, unless I'm missing something here. After lathing your circle/sphere and deleting half of it, you need to break the spline that AM keeps to continue the loop (it depends on which half you are deleting, side or top/bottom). You can also select half the circle/sphere and copy/paste it. If you are trying to close the flat side, you need a 4 pt spline, using the 2 middle cp to close the loop and deleting the ends after you attached the cps.

 

 

Hi Mark,

 

That was it. I new it had to be something simple. I was just one step away from doing it right.

 

Thanks,

Technodandy

[mtpeak2]

Glad I could help.

[Kamikaze]

Cool...mtpeak2................

 

Michael