Create a non-circular helix 2

[CADCAMMIKE]

Greetings all,

I am having a problem creating some geometry! The profile of the part is basically a ellipse! This ellipse needs a helix in it with consistent spaces between!

The basic design is an oval part with a helical groove to wrap a wire around! Once drawn, I need to program a cnc mill to cut, but for now I'll just worry about drawing it!

Thanx to all who reply

 

[RiBeneke]

You will need to create a 3-d model :
Draw your ellipse and the major and minor axes in a plane.
Use one of these axis lines to trim away half the ellipse.
Then switch to a coordinate system centred on one end of the half-ellipse and with the end of the ellipse line perpendicular to the xy plane.
Draw your groove shape and a small part of the internal solid cross-section as a closed polyline.
Use the extrude command on the polyline and specify the nearest end of the ellipse as a Path for extrusion.
(Extruded polylines cannot self-intersect, therefore your polyline cannot extend to the centre of the ellipse).
Mirror to get the other half.
Draw an irregular polyline in the plane of the ellipse to cover the central hole, extrude it upwards, and union it with the earlier solid to close the central hole.

 

[RiBeneke]

Rereading, I don't think I have given the answer you are looking for.
If your shape is like a screw thread squeezed into an elliptical rather than cylindrical outline, then my method will not help you.

 

[FRED70]

hi cadcammike
if you need a 3D helix i think you'd better use script files:
write a program (for example in quickbasic)to find ellips' points(x/a)^2+(y/b)^2=1(you'll find x,y and z(which is your optional increaments in z direction ))then save your points in a file with "scr" suffix.then in autocad call the points. it automatically draws your 3D helix,i don't know if you have ever used script files before? if not you might need some help to do it.but it works.

 

[CADCAMMIKE]

The problem is that the profile is not perfectly elliptical, it is about 2.075" wide, 5.44" long and 2.875" high, but it has 1.5" flats on the length. When the part was originally designed, we just took a piece of sheet metal with twenty 0.025" deep grooves in it @ 0.125" spacing between and wrapped it around the profile and matched the first groove up with the second to create a continuous helical path!
Now we would like to make it out of one solid piece and mill the helical path into the profile!

 

[RiBeneke]

HAve you tried to create a rough approximation by generating a 3d polyline path and then using an extrusion method similar to what I suggested earlier.

You can use FRED70's method to create the 3d polyline.

You can also create an approximation by taking some bits of circular curve (say 10 deg at a time) and tilting them to the correct slope in 3d (based on end-to-end positions). Position them end-to-end in 3d , join into a 3d-polyline, and trim back to 180 deg of plan curve, add a piece of straight, and stack these together. It is a bit tedious, and the accuracy is better if the pieces are smaller.

You could also use a helix lisp to create the curve part.

 

[FH]

Dear CADCAMMIKE,
Please send me a copy of the 3D model and the exact specification of the helix you need. I have created some complicated helix shapes for my job up to now.
email address: heydarif@yahoo.com

Farzad

 

[CADCAMMIKE]

Thanks for the offer FH, but the helical ellipse prooved too difficult to program on our mill. The ellipse had to be interpolated into many small 3 axis moves, that overflowed the controls memory before the entire part could be completed (8k max). Discussion with the customer allowed me to redesign the profile to simple 1" rads. Using an incremental sub simplified the code to just 4 lines. Thanks to all who posted, I'll let you know if I have any more problems!

 

[rocheey]

hmm, Im still having a hard time picturing what you're doing... something like milling a thread on a hard-boiled egg, that has been dropped a few times?

When I milled cams into helixes, I used a 4th axis, and yes, I used 'chatter milling' (zillions of little G01 blocks)

A lot of CNC controls can be duped into drip-feeding huge programs by setting the 'tape reader' to the RS232 port, check the control docs for this.

If your elliptical/parabaloid-thingamagingy profile can be represented as a 2d cross section, dice your closed profile into angular segments, and increment your X axis moves to correspond with each angular increment: ie, if you are milling a 2.00" pitch, and you dice your profile up into 5 degree sections (72 steps) each angular movement would correspond to an X axis increment of .083 (2.00/72)

Be aware that using an end-mill to mill helixes may generate unexpected dimensions/tool tangencies; your tool will not be cutting on its side, but slightly in front-of-center. This can be calculated by multplying the sine of the helix angle * the tool radius.

 

[CADCAMMIKE]

Just finished the job today, thanx for all the help

Ended up using a small 0.015 slit saw, offsetting the cad profile by the rad of the tool, calculating the perimeter, and incrementally programming the x and y while moving the z axis proportionately to the pitch per rotation!

Worked great, 4 lines of code and 20 sub calls!