Cad program to map parabola

[rickwinters]

Hi All,

Got the router mostly assembled and functional. Now I'm looking at CAD software. Years ago, in school we were trained in autocad. I haven't used any CAD packages in quite awhile.

My first project involves cutting ribs for a concentrating trough collector. The ribs must accurately follow a parabola for it's inner surface. I don't recall ever plotting an equation as part of my autocad training. Is this something that is done very often today? What packages would be best (& cheapest) for this sort of thing?

Any recommendations would be really appreciated......

[ger21]

You can't draw a parabola in AutoCAD. I looked around, and found some macros that would simulate it with a series of short, straight line segments. I'm not sure if any CAD programs would natively handle a parabola. Even if they could, then most likely you'd have to convert them to short, straight segments, because you can't define a parabola in Gcode.

[rickwinters]

Originally Posted by ger21 You can't draw a parabola in AutoCAD. I looked around, and found some macros that would simulate it with a series of short, straight line segments. I'm not sure if any CAD programs would natively handle a parabola. Even if they could, then most likely you'd have to convert them to short, straight segments, because you can't define a parabola in Gcode.

Ouch! Sounds like this might get painful! :-)

If you personally putting a parabola into CAD, what app would you use? Thanks very much for the assist.....

[JFettig]

It wouldnt actually be all that difficult to graph one in autocad, it depends on how much you would need actually
just line 0,0 to 1,1 to 2,4 to 3,9 and possibly even able to draw an acurate arc through them to make it look really good

Jon

[ger21]

Originally Posted by JFettig It wouldnt actually be all that difficult to graph one in autocad, it depends on how much you would need actually just line 0,0 to 1,1 to 2,4 to 3,9 and possibly even able to draw an acurate arc through them to make it look really good Jon

It would have to be a series of arcs, similar to an ellipse with pellipse variable set to 1.

Rick, since I use AutoCAD, and found two options to create parabola "approximations", that's what I'd use. You really won't notice that it's actually made up of many straight segments after you route it.

I don't have any familiarity with anything else to be able to recommend any options. Sorry.

[Dogsione]

I was just playing in Rhino...

It allows you to draw a parabola by Vertex or Focus. When I saved a sample as a .dxf and imported it into Sheetcam(and Mach2, for that matter), I could definitely see that it was rendered in segments - 22 to be exact. I've a feeling, though, that if I dug around I could find a way to control the number of segments. I (used to) know the commands in Acad 13...

I've attached a .dwg file for you.

[ger21]

I tried the Rhino parabola, and exported as .dxf. In Rhino's .dxf export, you can specify the polyline segment length. I created the g-code with my AutoCAD macro, and loaded into Mach2 and did a simulation. Here's a screen shot. Hopefully you can see that it looks pretty smooth, even though it's made up of short, straight segments.

[rickwinters]

Originally Posted by ger21 I tried the Rhino parabola, and exported as .dxf. In Rhino's .dxf export, you can specify the polyline segment length. I created the g-code with my AutoCAD macro, and loaded into Mach2 and did a simulation. Here's a screen shot. Hopefully you can see that it looks pretty smooth, even though it's made up of short, straight segments.

Thanks very much! Maybe this won't be so painful after all :-)

[G0G49Z0]

you actually can create a parabola in Acad thru vba as a polyline I might be able to find the routine i made to do this but here is some of the equations I did in ProE
it will take some digging around to find the acad routine as it's on my other puter
in plain english a true parabola is a cross section of a cone ,parallel to the angle of the cone...

you have to create 3 parameters name .. cone_ht,cone_rad, and offset

then you can show those under model tree and edit the values.. be sure and
enter a start value , so you don't get a divide by zero error

then this goes in the relations table
---------------------------------------------------------------------------------

ratio=cone_ht/(2*cone_rad)
cone_ang=atan(cone_ht/cone_rad)
off_base=offset/sin(cone_ang)
max_offset=(2*cone_rad)/sin(cone_ang)
trig_leg=cone_rad-off_base
para_base=sqrt((cone_rad^2)-(trig_leg^2))
para_ht=((para_base^2)/offset)*ratio
focus_dist=(para_base^2)/(4*para_ht)

-------------------------------------------------------------------
this is the datum curve equation
------------------------------------------------
x=para_base*t
y=((x^2)/offset)*ratio