Re: Measuring accuracy

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

> Ok, I was a bit upset when I saw what was happening before.
>
>
> Fred, since you answered I'll pose these questions about Desk CNC.

Why

> is it that with my parts I need to leave 3mm of stock on roughing

to

> avoid gouging the part?

You need to leave stock if you use coarse surface evaluation steps.
That's the only reason. If you use a fine evaluation, there is no
chance for gouging. It's a tradeoff, one that you are allowed to
make in DeskCNC and StlWork.

>Why does it seem that the software will not cut
> a vertical wall?

This depends on your tool, model and the stepover distance along the
direction of cut. An angled tool will not cut a vertical wall. If
you evaluate your model every .001 and the model has a .001 surface
resolution, your result will be within this tolerance. You, the user
make the tradeoffs in cut time and resolution. If you want a
vertical wall, cut with the side of the cutter instead of the end.
(different cam software tool, same design tools, still included in
DeskCNC)

> I actually cut a part with code DeskCNC generated. I
> had a lot of variation in wall thickness that wasn't nearly as

obvious

> when cut with another software. I had set .25mm z steps and .25mm

step

> over for finish.

Variation in wall thickness? We are talking about cutting surfaces
here. That has nothing to do with wall thickness. Either the
surface is accurate or not.

DeskCNC does not have a single stepover for finish. It has 2, X
direction and Y directino, one is step over and one is step along,
you pick which. A .25 Z step means that you are cutting the entire
region at .25mm depth planes. No wonder you thought the program took
a long time. Your Z step should be = to you total model depth, thus
taking ONE pass instead of thousands.

As regards pocketing, Vector can project your geometry onto a flat
plane in the blink of an eye and the Vector Cad-only version is about
$60 with the group discounts. You need a flat, closed contour for
pocketing with flat ended cutters.

>To say the least progress was extremely slow.

3D surfacing is a slow process, if you try to run on a machine that
does not support continuous contouring. It involves cutting many
thousands of tiny linear segments. These segments are turned into
smooth curves by the lookahead of your control and the
mechanics of your machine. Your settings made it even slower by
several magnitudes.

> Will DeskCNC write a G02 or G03 command if I'm
> working from an STl file?
>

Why would it? There is nothing in an stl file that even approximates
an arc. It is composed of triangular faces, period. No matter how
you slice them they produce lines. Vector can extract waterline
contours and arcify them, but unless you are trying
to run on an old machine with limited controller memory, you should
not need to do this.

> Anyone who wants to see what I'm making, an iges file is available

here.

> www.edfinfo.com/temp/test.igs
>

I looked at your model before and I belive I suggested that you need
to invest in a rotary 4th axis capable system to cut your parts. Not
one based on .stl. Vector would be a good choice, but you are going
to have to get over the concept that you can use surfaces to machine
arcs, on a hobby budget. See

http://www.metacut.com/Products/MCFinishNutshell.htm

What would you have to pay someone to
commercially produce that part? What do you think they paid for
their software and training in its use?

> I understand the accuracy of the machine does not have anything to

do

> with the accuracy of the files, but why bother with CNC if the code

is

> wrong to begin with?

Fix your mistakes, ask for help.

StlWork is only $75 bucks right now. Have you even tried it? How
about Vector? The Stl processing in DeskCNC is equivalent to
StlWork "LITE". That is no waterlines, no tangency selections, and
no rect-zone selection. I can't remember if it even has "no flats".

You need to use a combination of techniques to make your part, and
you need to use the right tools.

Right tool #1 Use a plain old end mill for the open areas, define
these contours with a closed shape and POCKET the massive amounts of
material. Correct software tools would be Vector or DeskCNC

Right Tool #2 Use the same cutting tool to rough out the vanes,
quickly
Correct software tools would be Vector DeskCNC StlWork

Right Tool #3 Finally use a fine interpolated toolpath for just the
finish pass
Correct software tool, any of the three, just look at the picture and
make sure that you are only taking one finish cut.

Vector Cam can do all this, including designing the part, and working
with an iges or Rhino 3dm surface file. And will also machine along
the design curves of the model, or any other pattern you design.

StlWork can do the roughing and finishing of the blades, but will not
as efficiently pocket the through areas.

DeskCNC WILL pocket the open areas and WILL rough and finish the
blades with some help from your cad system. It won't let you use a
cutting pattern for finishing other than parallel zig-zag.

Fred Smith - IMService
Group discounts and specials are at:
http://www.cadcamcadcam.com/hobby