Re: Measuring accuracy

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Gregory Kamysz
<smsupply@a...> wrote:

> Doug, that's what I thought, but which software under 1000 USD

actually

> does this? I've tried several and running a G02 is much more

accurate

> for cutting a circle than any of the CAM software I tried.

Greg,

You are confusing two things here.

1) The accuracy of CAM software, as it is applied to roughing
surfaces with radiused cutters. Vector, StlWork, and DeskCNC will
produce 3D code as accurate as the beginning model. They all allow
you to back off from this utopian position and make a more realistic
calculation for gross roughing of material with finishing stock left
behind. They also will let you process the surface data at a finer
resolution than the original data in the case of .stl files which are
triangle approximations. In the case of Vector, it will evaluate a
nurbs mathmatical surface with whatever interpolation accuracy you
desire. The model is defined numerically and there is by definition,
no measurable granularity in either the original design, or in the
resulting toolpaths. All our commercial software will produce
accurate programs, I cannot speak for other programs. What program
are you using that has this problem?

2) The other thing you need to consider is realistic accuracy
achievable with the cutting tool available. A ball nosed end mill or
router bit is a spinning lever with the spindle bearings being the
fulcrum, and the force applied at the cutter tip. Ideally the
spinning cutter will shear material with sufficient ease, that the
deflection from the lever will be within expected tolerances on your
machine. In reality, the cutter has a "dead zone" right in the
center where you have programmed it to do all it's cutting. This
dead zone is caused by the decreasing surface speed as you approach
the center, as opposed to the programmed speed out by the full sizeed
edges. This slower cutting area will cause the tool to scrape and
plow material away, instead of the nice planned shearing. The
resulting surface is rougher than the programmed accuracy, simply
from the leveraging of the slow moving cutter through the material.
A longer bit will produce even worse results. If instead of using a
ball nosed cutter, you use a radiused corner (bull nosed) or a flat
ended end mill or bit, there will be no attempt to cut with the "dead
zone". (unless you plunge vertically along the Z axis). This will
make for much better finishes and for more accuracy on the sides and
bottoms of the cut. You will however, not be able to cut concavities
smaller that your cutter end flat, and a flat end mill is not
productive for smooth curved surface. The flat end tools however are
much better at roughing as they cut full depth at a higher cutting
speed than a ball nosed cutter.


I suspect your perception of inaccuracy is operator error in creating
your code, and that when you get around to making parts, you will
find that mechanical cutting has a much greater liklihood of causing
inaccuracy, than the worst CAM program.

Fred Smith - IMService
THE source for low cost Cad-Cam
$75 sale, still on, http://www.cadcamcadcam.com/hobby