I think what was meant in the data you have is that it is difficult
to cut a 3D object with precision. The greater the desired
precision, the more passes you will have to make. Here's a simple
Imagine cutting half of a sphere out. You are using a standard 1/4"
end-mill. You would sneak up on the shape by cutting circles at
varying depths, such that the bottom edge of the cutter just touched
the desired finished hemisphere, with extra passes to hog out the
other material. Just how close that you get to the shape of the
hemisphere depends on the number of passes that you make. At any
rate you will end up with a staircase effect. You can smooth this
out considerably by choosing a ball end-mill for your last few
passes. If, for example you wanted half of a one-inch sphere,
accurate to within .01" you would need to describe about 35 circles
over its surface for the finishing pass alone. You won't cut this in
an hour or two on a Sherline (unless you make some special tooling -
specifically for this job). Now, just suppose that you had switched
to a 1/4" ball end-mill. What happens at the bottom of your cut?
You need to go to smaller and smaller ball end-mills or switch to a
standard end-mill to finish off the last 1/8" of the project to get
the accuracy you desire.
As the shape gets more complex, and the detail finer, you must use
smaller tools. This dramatically increases your chances of breakage,
and the time that it will take to do the project.
There are software packages (add meshcam to your list) that will
generate the tool paths for you, but have lots of coffee on hand to
do the job.
If you are thinking along the lines of production quantities some
things you may consider is using the mill to make a 3D mold for an
item. Roughing on the mill, and hand finishing is another option.
Cutting shapes in 2 1/2D, then heating and forming into a 3D object
is yet another. A final suggestion would be using the mill to detail
a moulded blank.
With this in mind, I am in agreement that a mill can't totally
manufacture a true 3D object. At some point in the process you'll
have to unleash the artist inside.
--- In SherlineCNC@yahoogroups.com
, "Andrew Werby" <a.werby@c...>
> Message: 2
> Date: Mon, 01 Mar 2004 14:50:08 -0000
> From: "Bryan Hassing" <bryanh@l...>
> Subject: CNC milling limitations?
> In a previous post, someone invoked the somewhat humorous "face in a
> plaque" project as something CNC milling cannot provide. What
> limitation of CNC milling does this refer to? I have seen how CNC
> milling can yield complex shapes with milling in the X and Y axes
> only. Is the inclusion of varying Z-axis elevations a significant
> limiting factor with current CNC programs? For example, could one
> machine half of an egg-shape using current CNC programs? Thanks.
> [That's not a limitation for most of the CAM programs I've used,
> which I sell through my site. CNC programs are different; they just
> machines, and don't write programs. For that, you use a CAM
> Assisted Machining) program to convert a model to a toolpath
> feed the resulting G-code file to the CNC program. A face in a
plaque is a
> common sort of project, and half an egg (or a whole one, for that
> no big deal. If you've got a CNC mill with live x, y, and z axes,
> you need is a model in STL format and a CAM program like DeskProto.
> don't have a model, download Rhino from www.rhino3d.com and produce
> the program will work until you've saved 15 times. (The egg is
> face might take some work.) Once you've got that, save it as an STL
> download DeskProto from www.deskproto.com . Its free demo will write
> toolpaths for a month so you can evaluate it on your machine. To do
> whole egg, if you don't have a 4th axis, try DeskProto's "2-side
> which will support the shape in a block and give you instructions
on how to
> set it up and flip it over so the back side registers with the
> milling does have some limitations- for instance, sharp inside
> undercuts are problematic- but this sort of thing is what it's good
> Andrew Werby