Stepover and ball/end mill

[Sanghera]

Hi,
I was wondering what is the stepover percentage to produce a smooth surface with a ball mill and an end mill. What is recommended for smooth surfaces, a ball mill or an end mill? I have heard people say that a ball mill is better for this. The ball mill only leaves the surface which was cut at the very end of the bit, so more passes are probably needed, but in the end does it make any difference if a ball mill is used or if an end mill is used to make a smooth 3d surface?
Thanks.

[dertsap]

step over percentage is dependant on many factors , finish , tolerance , tool dia , the overall profile , to name a few , ballnose is most prefered for 3d surfaces due to better surface finish ,less chance of gouging ,etc
most cases a surface will be roughed out with an EM and finish with a ballnose
and once again this will also depend upon the overall profile

[Sanghera]

okay, can you give me some examples? for the type of finish or whatever?
Thanks.

[tobyaxis]

A 1.0" daimeter Ball Mill with a stepover of .035 will give between a 125 and 200 micro finish. This is pretty broad, but it's fairly accurate. This part would need polishing.

I skipped this part earlier.
There again it all depends on what you are making and the tolerance you have to hold. Don't forget that the tool has to fit in the area being machined. If you machining a clevis that is .5 wide your not going to use a 1.0d Ball Mill, but use the 1 inch in the areas it will fit, and then go back with a smaller end mill to finish the inside of the clevis/blends.

Like Dertsap mentions in his post the tighter the stepover the better the finish will be.

example part below

[dertsap]

FINISH ON WHAT ?
picture using a v bit which comes to a near perfect point , now think about using that on a surface , pocket or what have you, .1" step over , the finish is bad , .05" its better,.025" better yet , and so on and so on
same deal goes with any tool , including a ballnose , depending on the step over with a ballnose you will feel the finish even though it looks good ,
.005 step over with a .25 bnose on an aluminum mold for example will produce a near perfectly smooth finish ,.05" step over you can expect to do a lot of sanding to accomplish the same finish
when you have a surface that has radius's tapered walls tapered pockets or islands etc, more or less a part that isn t dealing with straight floor and walls it is general practice to surface with a ballnose and the step over depends upon surface finish requirements and your amount patience , as this style of machining is a long process

[lakeside]

step over refeer to cusp hight. How much of the radius from tool did not cut to Z 0 So for every .05 step there would be a rise from ball on tool of .00125 and if your looking for a smooth surface with this step over you will be hand sanding
***** Cusp Height / Cutter Stepover Calculation *****

Tool Radius = 0.25
Cutter Stepover = 0.05
Cusp Height = 0.00125

[Sanghera]

Well if you used an endmill instead of a ball nose wouldn't the surface end up being smoother because there is more area cut by the flat of the bit which is going to be the surface. And you wouldn't have to take as many passes, plus you wouldn't have as many cusps because the end is flat. Is this right?

Also, what do you mean by tolerances?
Thanks.

[ger21]

If the surface is flat, than an endmill is better. If it has vertical curves, than an endmill will only be cutting with the corner, leaving a stairstep effect. For a smooth finish on curved parts, you almost have to use ballnose tools.

Stepover size will vary with tool diameter. The larger the tool, generally the better the finish with bigger stepovers.

[Sanghera]

Thanks, but all bits probably have the same step over percentage for a given surface finish.
Does anybody have a good example for wood.
Say I want to cut a 3d profile with lot of curves with a 1/4 inch ball nose cutter in wood. What would produce a near perfect finish, what would make a good finish which doesn't need sanding, what would make a finish which would have to be sanded.
Thank you.

[ger21]

Whether or not it needs to be sanded may be a matter of personal opinion. And a few minutes of sanding might save you an hour of machine time. And a perfect finish may depend on the type of wood.

It's all going to take a bit of trial and error. Start with a small, simple part, choose an arbitrary stepover, say .03, and see how it comes out. A few simple tests should answer all your questions.