[alex781]

Does a larger ballnose bit give a better surface finish than a smaller one, in general for 3d contouring? According to estimated run time I think it is at least a little faster.

[Joe Crumley]

Excellent post.

I also wonder if the step over could be reduced and speed ut the cutting time?

All 3D files don't need to take a long time to route. I've also been experimenting with different techniques like this. The stepover was extreme and the cut time was short.

[andy55]

you want to match the curvature of the cutter to the curvature of your part. If you only have vertical walls and 90-degree corners a cylindical endmill is most efficient. For finish-cutting general 3D shapes a ballnose is most versatile.
a Bull-nose or filleted endmill is somewhere in between a flat endmill and a ball-nose and is much more efficient for roughing, and if your 3D part has curvature only in one direction then the fillet of the bull-nose provides a nice small-R fillet for finishing and is much more rigid than a similar radius ball-cutter. If however your 3D part has curvature in both directions on a tiny scale then the only way to do it is with a small ball-nose cutter and the machining time will go up.

[sinha_nsit]

If a larger diameter is chosen, you can increase the step-over distance for the same surface finish (unevenness). This would reduce the size of the program file. So, unless you have to machine a tight corner, always select a higher diameter.

[Joe Crumley]

Thanks fellows.

I'm in the experimental stages of 3D work and have been for the last five years. At my shop we have five or six jobs a month and each takes different machine techniques. So far we've not engaged in small deliclate work.

Unlike most CNC shops I'm always in an experimental stage and find a sharp chisel improves most every carving we do. Sometimes I'll use a mop for finishing. It can be a real time saver by not having a high stepover rate. When I first started out I'd spend too much time expecting the CNC to render detail. We all find our way.

Joe Crumley
www.normansignco.com

[TMGSF]

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[Joe Crumley]

Here's another example of increased step over to achive special effects. The color was achieved by using Sculpt Nouveau.

For us, there are three carving materials. Trupan, Extira, and HDU. I've had to move away from all 15lb HDU products except for Duna. All other brands require 18 or 20lb.

What are others using? Does anyone have examples to post?

Joe Crumley
www.normsignco.com

[alex781]

Wow, those designs look fantastic and very substantial size! I don't know much about working on that scale, with my little X2 .

[Cmailco]

Short answer: Yes, if you did nothing but change tools from say, a 1/4" to 5/16" ball, you would achieve a better surface finish with the same step-over values due to the fact that you've reduced the cusp height.

[Joe Crumley]

Very interesting. What do you mean by "better surface finish"?

Joe
Norman Sign Company, Norman, Oklahoma, SandBlasted Signs, Sign Painter, Redwood Signs

[Cmailco]

Joe, surface finish is a function of the maximum deviation from the surface. If you were to look at a surface under a microscope, machined with say, a 1/4" ball mill stepping over at .005" each pass; you'd see a little area between each pass that peaks at "x" height. Generally speaking, the lower the peak, the lower the deviation and the greater the reflectivity of the surface. You can lower the peak in two ways; use a larger tool or reduce the step-over. One is certainly more expensive than the other but the basic rule is: use the largest tool permissible with regard to the part geometry.

Very general but there's a lot to this subject and I'm feeling a bit lazy at the moment. Other factors that affect surface finish are radial tangency to the workpiece, tool run-out and toolpath... just to name a few.

Regards,
Chuck

[Joe Crumley]

Excellent point Chuck.

I've not been to concerned with those factors since my work is so crude and the materials I use doesn't carry much detail. I've been choosing my tools by the curvature effect. For example: When I'm doing 12" letters and want a ribb look I down size the bit. For really deliclate work I'll go down as small as 1/8". with 98% stepover. On large letters, like those I posted, a 1/2" ballnose would have worked out fine. I used a 1/4" bit on those.

One of the main advantages to this technique is the added reflection when gold or gold mica is applied. It gives lots of pockets so the gold will flash.

Once again let me state, I'm not much of a machinest. My background as a painter leaves me very short on router technique. Everything I do is one up.