Things I wish someone had told me when I was using BobCAD

As I was writing out my angle on the CAD comparison post, I was reminded of
some of my initial frustrations in learning BobCAD v17. Particularly 3D
toolpaths.
So, for what its worth, here's a strategy I've used for the development of
complex 3D toolpaths from wireframe models in BobCAD. Perhaps some of this
would be
useful in Vector, as well.

OK, I also cheated at times and used ACAD for building the wire-frame
(better osnap and viewing, coordinate systems). But, this is all BobCAD
below. :)

- Keith Rumley

* Note: This is for 'I've got more time than money' types, and for speeding
up odd fillet creation. Also assumed is a working knowledge of BobCAD v17

The main concept is using the 'skin' tool to create boundary control curves
which
are offset the cutter center distance from the wireframe, then using a
cutter of
zero radius to create a toolpath. Another way of looking at it is creating
toolpath 'skin' that doesn't require any cutter offset figuring by BobCAD.
It seems that most of the problems with contours come from offsets
intersecting to form undercuts and wierd shapes.

1. Using the 'skin' tool, create a widely spaced toolpath, offset the
proper center-of-cutter to part distance. (ALT, 3, U, S in sequence)
The 'Max distance between lines' should be set to a distance greater than
that between your section segments. The resulting boundary curves
(the main toolpath curves) are created only at each path/section
intersection, and at
entity
endpoints. Use the 'ruled' (parallel to the path entities), 'zig-zag'
(parallel to the section entities), and side of surface check box to get the
proper orientation and side of wireframe.

2. Copy the boundary curves you need to the clipboard (CTRL-C), then 'undo'
(CTRL-Z) back to before the skin command. Usually the curves you want are
the beginning and ending ones of
the toolpath. (ALT, E, S, C in sequence - select chain..)

* Alternatively, If more control curves are needed, set the 'Max
distance between lines" to less than the distance between sections. By
creating more
curves (denser toolpath) it is possible to create the upper and lower bounds
at the same time, too - using the point tool and 3-point curve tool (or
whatever). snapping to the toolpath curve endpoints. (so that you have four
sides...)

3. Paste the curves back in, and modify/divide if necessary to obtain the
prober entity counts, tie in to the paths/sections, etc. These curves
become your toolpath surface boundarys. As is good practice, keep path and
section entities on different layers and colors. Make sure the pasted
entities are assigned correctly, too.

It may be necessary to run the skin tool twice to create all four sides
* Note that you can still have many more than four paths and
sections.
* BobCAD v17 allows sections to have non-matching entity counts. Useful
if you use splines for sections.

4. Run the skin tool again, this time using your offset control curves as
the paths and sections. Set the cutter to zero radius. For a 'ruled'
surface, stepover is set by the 'Max distance between lines' in the final
dialog box of the skin tool. For a 'zig-zag' toolpath, you need to catch the
first dialog box - 'Max length of division' - to set stepover.

This has the benefit of fitting exactly to your boundaries, allowing a
toolpath up over a fillet edge, etc.



Other Stuff:

Another strategy is to lay out the main model planes, and using the ruled
surface tool create quick roughing schemes. Then follow with finish, then
fillets.

Ruled surf is also good for any surface that can be 'extruded'. Rotate your
sectional curves (usually non-planar...) around to plan orientation (looking
down from Z) and 'move to Z' entities. Sometimes it's necessary to divide
arcs, put points on entity ends, and 'move to Z' the points, then recreate
the arcs using the 3-point arc tool or the spline tool. Then, with the new
'flat' profile, rotate back to desired angle, and position as needed. (May
need to leave some points suspended in Z, in order move flat up, then rotate
back....)

For non-orthogonal fillets, especially arcs in different planes...study a
book on plane and solid geometry. Basically you do the thinking, and use
BobCAD for the calculation. Much better than paper, right? I started to
do a convoluted description, but it was worse than the above stuff. Really.

So, a few tips - I'd change the view into the plane of the arc I needed to
create (ALT, V, L, 'orient', in sequence) Select arc center first, then the
two entity end points, or points on the entity. (order of the second two not
that important) This is sometimes easier than finding and writing down 8
digits of two angles.
Sometimes it's faster to find the angle using construction lines, and
rotate the view to it.
Once in the object plane, an arc of known radius and included angle from
the center point can be drawn. I found that this makes for accurate trims
and entity creation, especially if the lines coming in aren't on the same
plane as the arc needs to be for your toolpath. Trim to point is usually the
way to go here. Or, create one in an orthogonal, and rotate it around.
* BobCAD creates new entities at your viewing angle, and also reports
line angles that way. Incorrectly, if you aren't in the object's plane.

Arcs won't trim on the right side? -put endpoints on the
object-to-trim-to's intersection, and at the end of the arc on the side to
keep, divide the arc in two, put an end point on in the middle, use 3-point
arc selecting the endpoint, midpoint, and trim-to intersection point.

BobCAD's IGES import of surfaces and models give the trim curves, usually
doubled on top of each other. They need a fair amount of cleanup.

'Copy to Clipboard' and 'UNDO' is a useful process, because it cleans up all
stray construction entities (especially small spline and toolpath entities,
and those blanked out). If you missed copying something, you'll remember to
save the file before 'undo'-ing next time. :) On a 'big' file, this cuts
down on contruction layer congestion (what were those 'surfaces' for?)

BobCAD is good at crashing Win95. More stable on WinNT. (at least you can
kill the process without a re-boot)

'Divide' has the benefit of leaving arc entities as arcs, interpolate splits
arcs into lines. (Smoother 2D cuts)

I liked having my path layer named 'P' and section layer named 'S', as
BobCAD will go to the first listing of the keyboard key tapped when in the
select layer for path/section dialogs.






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