Sharp transitions on curved surfaces

[dauntless]

We are struggling with how to get sharp transitions from compound curved wing plug surfaces to the flange, and I am hoping you guys can help us out. This photo illustrates the challenge:

http://www.dauntlessracing.com/forums/Forums/Dauntless/Rear%20Wing%20Test%20Section.jpg

The flattish section at the top of the piece is the mold flange (with an alignment bump at right). A shallow trench separates the flange from the compound curved surface of the wing itself. What we are trying to do is get a sharp, crisp corner directly in the plug material without resorting to a trench that we later fill in and sculpt to the desired sharpness.

Suggestions for tooling strategies, software, or 'tricks of the trade' are invited. Thanks guys!

Stan

[dray]

I have an incredibly hard time with stuff like that.

Its the 2d or 2.5D to 3d transition.

There are allot of good 3d guys on the forum here that can draw it up in 3d then you can 3d mill it.

[bleeth]

Stan: Assuming that you are cutting this with a small ball nose if you do your part without the trench in the design and have the flange and plug surface borders defined by a vector in 2-d you can run that ball nose on the vector after carving to clean up the line. At that point your transition would be a small radius. If the radius is too large you can rerun that section only. Define the area with a vector box and machine within the vector with a smaller radius ball nose or clean it up with a 60 degree v-bit and a small stepover. At that point you should have only your normal sanding to do.

[robtown]

Unless I'm missing something, you could just machine this without the trench and get your inside corner clean and sharp with a sanding block couldn't you?

[beacon14]

What about cleaning up the perimeter with a straight bit? Without the trench you'll still get a slight ridge where the ballnose cutter can't machine all the way down to the surface but it should be easier to remove by hand than the trench.

Another option is to machine the raised portion separately, then insert it into a pocketed area in a flat background.

[dauntless]

Thanks for the replies, guys!

Dave, yes, I am using a small ball nose for the finishing cut. I don't see how a 2-d vector can solve for this, though, as the intersection is varying continuously in both y and z as x increases.

Rob, we are hand finishing the transition now, but what I am looking for is to do it on the 'bot. ;)

David, we thought about doing it in two pieces and post bonding them, but getting the alignment right appears very difficult. Your suggestion to use a straight bit sounds interesting, and raises a new question...

All the tool path programs we've looked at raster in a straight line (or spiral). Are there any that will follow a curve? This project is almost like terracing a hillside. If the tool could follow the 'contour line' instead of running up and down x, it would produce a much nicer finish.

Thanks again for the suggestions!

Stan

[dauntless]

I couldn't edit my initial post, so am uploading the image here.


8210

[stevem]

Stan, for the "terrace machining" you need to use CAM software that allows horizontal finishing, as opposed the parallel finishing pass that you're doing now. RhinoCAM is the one I use, but there are several others.

As for alignment of two pieces, just place locating pin holes in the bottom of the plug and the top of the flange for glue-up alignment.

Locating pins will also work much better for aligning the finished moulds than the dimples you're using now.

Feel free to call me if you need help.

[beacon14]

So the "background" area is not flat but varies in Z height? If so I missed that from the preview.

I think it should still be possible to machine the raised area with a vertical "cliff" at the perimeter (the shape defined by the transition between the raised shape and the background when viewed in plan). Then machine a pocket into the background. The bottom of the pocket and the bottom of the raised model would both be flat in the Z axis. The alignment is provided by the fitting of the model into the pocket. If the models are accurate the transition should be crisp even though it will vary in Z height.

[dauntless]

Thanks guys!

Yes, David, the z height varies with x. Here is a photo I took of the actual piece. The y is also varying with x. The photo is taken from near x=0, and is looking along the x-axis.


8211

Steve, I'll look into Rhino...thanks!

Stan

[dauntless]

Steve, do you mind listing some other programs that do horizontal finishing? Thanks! Stan