Re: Re: Compare & Contrast/Vector & Bobcad

Thank you Fred... that was incredibly informative and straight forward to understand.

One more question. If I created the part in Velum Solids as a surfaced part and then saved it as a IGES file to you think either of these programs could read the result in and be able to process it??

GaryPasson
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imserv@... wrote:

> From: imserv@...
>
> > Date: Mon, 06 Dec 1999 07:39:19 -0800
> > From: Gary Passon <gary@...
> >Subject: Re: Compare & Contrast/Vector & Bobcad
> >
> >Fred, I'm also interested in doing a propellor blade and was pondering
> >how. Please give us a quick tutorial on Vector's capability in this area.
> >
> >GaryP
>
> Gary, Both Vector and Bobcad have a function called the "Skin" It is also available in some other Cad-Cam software, where it is sometimes known as a Coon's Patch surface.
>
> The surface is defined by a series of generally left to right and top to bottom
> cross sections. I call this pattern a quilt square effect. Like a quilt it can have an infinitely variable 3D shape, but the quilt squares ALWAYS retain boundaries on four sides, and always retain a perfect intersection at each of the 4 corners. The skin function generates a back and forth,3D (X-Y-Z coordinates for each move) tool path, properly compensated in 3d for the radius of a Ball or Bull nose cutter.
>
> In the case of a propellor (or airplane wing, turbine blade), you would define
> the airfoil shape that you wanted to cut, every so often along the length of the propellor, then with Bobcad or standard Vector, you would separate the shapes into a top and bottom half, depending on where you could reach with an endmill or router cutter (zero degrees). By generating a spline along this seperation line, you now have defined both the cross sections, as well as the "paths", which are the other direction for the skin.
>
> Setting up the skin for Bobcad or Vector involves selecting the paths and cross sections in a logical order, with a common starting or reference corner. In Vector once you have selected a path, it is retained in memory, and you have lots of opportunities to correctly select the cross sections going in the opposite direction. In Bobcad to aid selection, you can select to the end of a chain, restricted to a single layer, in Vector you can blank the unneeded geometry while you select it. If you generate a skin in Bobcad, and the step over is not correct, or you need to change from "inside" to "outside" the selection process must be started from the beginnning, in Vector, undo-repeat commands will take you immediately back to the skin dialog, where you can enter your new parameters and immediately repeat the skin generation. You are allowed to change your viewing angle & zoom location and scale at any time during the selection process with Vector, Bobcad is VERY restrictive in!
> this respect.
>
> Bobcad has an option on the skin dialogue that will render a wire frame as a shaded surface. This is a nice feature, as it shows what the surface should look like when cut, but before generation of G-code. It also is nice for making pretty pictures to show customers.
>
> Both program have an NC-verify/simulation capability, that will allow you to specify a tool size & shape & observe the cutting action as displayed in 3D. They are restricted to rectangular stock shapes, Square end and radiused (bull or ball) end mills. In Vector this is a separate stand alone program, in Bobcad it is contained within the main program. Both simulators work, but require a serious amount of memory and a very fast processor. I do not consider either a professional tool for this purpose, but they will do a fair job of identifying gouging and rapids through stock material.
>
> I have not yet had a chance to really get serious about a shape like a propellor with the Vector rotary axis option, but I know that the Vector skin function will wrap around in 3D, and it should be possible to make a propellor with a rotary setup (single workholding setup), without having to generate a split line like that needed with Bobcad or standard Vector. It should be possible to define the geometry as a series of cross sections in an external program like excel, paste a compensated representation in to Vector, and directly cut it with the rotary option. This could be done as a series of rings or crosssections.
>
> Remember that Vector can only handle 32000 entities. If you are making model airplane propellors, you will not have any trouble with Vector. If you are making full sized airplane propellors, and want to have a smooth feeling surface after completion, you may need to use Bobcad, just for the higher entity count requirements.
>
> My rule of thumb on this is that I cannot see less than about 1/64 (.015). A scallop height of .015 is usually acceptable in wood, as it will yield a nice surface, with just a little sanding. In Plastic and metal, a scallop height of .002-.005 is about the maximum that I use, as the finishing is much more difficult. For Mold surfaces, often .0005 or .0001 scallops are desirable, to get a mirror finish, with minimal sanding or polishing. Remember that a larger cutter radius will reduce scallop height MUCH more than making more thousands of passes with a tiny endmill. Also the cutting time with a large radius is MUCH less that with a tiny radius. Always use the largest radius your machine can handle, which you can afford, and which will cut the internal contours you require (none for a propellor).
>
> Hope this helps a little, sorry about the length, but it's a complex subject.
>
> Best Regards,
>
> Fred Smith
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