Centering Rings - the CAD way

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vcp

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A centering ring can be a really straightforward example to introduce you to how you can use CAD in your rocketry hobby. Many of you probably already have a favorite CAD program that you use, but if not, OpenSCAD is a program that is free, works on all platforms, and has a pretty gentile learning curve. You'll find it's not your usual type of interactive drawing system, but rather sort of a scripting system. Some people like this (I do), some don't; YMMV.

This example assumes you want to make a pair of centering rings, for a cluster model that incorporates 'focused thrust' as explained in the recent Apogee 'Peak of Flight' newsletter:
https://www.apogeerockets.com/education/downloads/Newsletter363.pdf

That article well explains the reasons for using angled motors, plus an in depth instruction on laying out the dimensions of centering rings for this purpose. While I'd certainly agree that it is useful to know how to lay them out this way (for post-apocalyptic modeling, got my slide rule and dial calipers ready), I think you'll find that the small effort in reading and understanding the following OpenSCAD file will be well worth it.

The attached file "FocusThrustCRs.scad" is just a text file; you can open it with any editor and take a look. It's extensively commented, and you can probably get the idea by just taking a look at it.

To open it as a CAD file, requires the installation of OpenSCAD, which is available here: https://www.openscad.org/index.html It's available for Win/Mac/Lin, and installs very easily. Documentation and tutorials are also available at that site.

Here are some snapshots of what you can get from that file. All of the parameters are easily adjustable to get whatever result you want. All you're doing is placing two flat disks, then placing and subtracting the motor tubes from those disks.
CADCRmodel.jpgCADCRmodel2.jpg

You'll find that OpenSCAD includes a rudimentary text editor that does the job (or you can use your favorite external editor if you want). After changing anything in the file, pressing F5, will recompile the image (Menu Design/Compile). When you have something you want, pressing F6 does a compile and render (Menu Design/Compile and Render (CGAL)); and from there you can do a Menu File/Export/Export as STL... An .STL file is a common format used by all 3D printers, and should be usable by CNC routers as well.

Once you're bending things with CAD, you'll find that centering rings don't have to be a flat bit of cardboard or ply anymore. This example adds a shoulder around the center tube to keep the ring perpendicular, pockets out most of the ring for weight savings, adds ribs for additional strength, fillets everything to avoid stress points, and serrates the outer and inner edges to aid gluing.
Centering Ring.jpg

Have fun.

View attachment FocusThrustCRs.txt

Ah, I notice that RO doesn't allow files with obscure extensions. After downloading the file, change the extension to .scad so that OpenSCAD can find and read it.

Gary
 
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Finally, we have something that looks like a real rocket part (instead of a slab of plywood).

Will you be offering a 3-D printing/manufacturing service?

I do, in a limited fashion: https://www.etsy.com/shop/RocketryCNC

I've got dozens of items working up, and I'll do special requests if I can, but I'm keeping it low key at the moment as I only have the one printer - it breaks, I'm OOB. I will be rectifying that situation in the next few months.

Gary
 
...Once you're bending things with CAD, you'll find that centering rings don't have to be a flat bit of cardboard or ply anymore. This example adds a shoulder around the center tube to keep the ring perpendicular, pockets out most of the ring for weight savings, adds ribs for additional strength, fillets everything to avoid stress points, and serrates the outer and inner edges to aid gluing.
View attachment 169934

That's almost too pretty to put inside a body tube where you can't see it.
 
That looks pretty cool. You could cast those pretty easily and get much better strength than a printed version.
 
The beauty of being able to compute the dimensions of the ellipses is that a 2D drawing can be made from those dimensions, and toolpathed as 2D shapes. Cut three holes, cut the ring out, all done. The .STL file is a 3D shape file and requires a bit more horsepower ($) to do the toolpaths... for a CNC router. A 3D toolpath for a shape like that is VERY inefficent. The fancy shape would probably cut faster as a collection of 2D paths, but tool changes would be required (probably one, maybe two changes, skipping the edge notching)

The drawings for the cluster Maxx that was the inspiration behind the article were done in Rhino as a 3D model and the outline of the rings was exported as a DXF file. The 2D vectors in the DXF were converted to two/three toolpaths to cut the rings and cut time was about a minute/ring. 3D drawing and toolpaths wouldn't even allow for the ellipse holes to carry the 15 degree angle (using a 3 axis machine) as one end of each ellipse would be undercut. Need a 5 axis machine!!

Nat Kinsey
UpscaleCNC
 
Funny you should mention that. I've not done casting before, but I do have casting materials on the way.

I have some Alumilite molding stuff sitting in my workshop. Its pretty weak though. Aeropoxy makes something with a 25ksi tensile strength that would work much better than Alumilite. Do you have a project in mind for that sort of centering ring?
 
Very interesting post;
Been looking for a 3D modeling Cad program that doesn't have a huge learning curve. This one sounds like it may have promise.
Thanks for posting the link.
 
I have some Alumilite molding stuff sitting in my workshop. Its pretty weak though.

Are materials like alumilite compatible with fiberglass fibers? Could the strength be improved by mixing in a good dose of finely chopped fibers?
 
Are materials like alumilite compatible with fiberglass fibers? Could the strength be improved by mixing in a good dose of finely chopped fibers?

Adding fibers or anything that would impede the flow of resin would increase the chance of voids in the part.
 
When I designed the rocket that inspired the Peak of Flight article referenced by the OP, I actually started by using a very capable 3D modeling tool, Rhinoceros3D.

What I found was that, while it was useful for developing the basic design concepts, it was actually not terribly helpful in designing the pieces that I actually needed to *fabricate*. I did not wish to use centering rings made from the more commonly available 3d printing materials, not did I wish to pay to have something generated on a five-axis mill. Also, the airframe cutouts were fairly difficult to do in the manner I wanted. I notice that the OP doesn't address what to do if your tubes penetrate the airframe.

Sure, I could develop a bunch of manifold 3d solid topology, calculate the intersections, extract the faces I needed, unwrap the developable surfaces, move them all where I wanted, extract the various curves, draw the tangents, trim the lines, scale the inner tube cut pattern to the outer tube radius, etc. it turns out that requires more steps, takes more time, and is more error prone than running it through a quick excel spreadsheet and drawing everything where it is supposed to be. I suppose the CAD-generated stuff was useful as a check to see if I'd done the math right.

I do recommend using a CAD tool to visualize what you want, but be aware that there are quite a few issues to deal with when manufacturing a focused-thrust cluster, and you need to think it through very thoroughly before you start laying out materials.





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As a side note, the focus of the cluster should not be directly through the CG. Various sources have found that it is best to have the focus be about halfway between the CG and the CP.


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When I designed the rocket that inspired the Peak of Flight article referenced by the OP, I actually started by using a very capable 3D modeling tool, Rhinoceros3D.
What I found was that, while it was useful for developing the basic design concepts, it was actually not terribly helpful in designing the pieces that I actually needed to *fabricate*.
Interesting. I've been using Rhino for my parts, mostly because they were recommended by Shapeways and had a free download for Mac OS. I do get the feeling that a true CAD program, rather than a design-oriented tools, would be better, but so far haven't been willing to pay for SolidWorks.

Normally, the CAD (drawing), and CAM (setup for fabrication) are done with separate programs. Are you saying you found something that does both or that you found a CAD program that makes the CAM easier?
 
Are you saying you found something that does both or that you found a CAD program that makes the CAM easier?

No, sorry for the confusion there. I'm saying that the raw canted intersections were not what I was going to be building on a 3-axis CNC router, and the steps to go from a solid model to the pieces I actually needed to make were no less complicated than using the mathematics and drawing the parts.

It depends on the tools you are using. for 3D printing it would work well. For cutting penetrations for canted MMTs I didn't think the CAD approach was all that helpful other than as a visualization tool.



Sent from my iPhone using Rocketry Forum
 
The beauty of being able to compute the dimensions of the ellipses is that a 2D drawing can be made from those dimensions, and toolpathed as 2D shapes. Cut three holes, cut the ring out, all done. The .STL file is a 3D shape file and requires a bit more horsepower ($) to do the toolpaths... for a CNC router. A 3D toolpath for a shape like that is VERY inefficent. The fancy shape would probably cut faster as a collection of 2D paths, but tool changes would be required (probably one, maybe two changes, skipping the edge notching)

The drawings for the cluster Maxx that was the inspiration behind the article were done in Rhino as a 3D model and the outline of the rings was exported as a DXF file. The 2D vectors in the DXF were converted to two/three toolpaths to cut the rings and cut time was about a minute/ring. 3D drawing and toolpaths wouldn't even allow for the ellipse holes to carry the 15 degree angle (using a 3 axis machine) as one end of each ellipse would be undercut. Need a 5 axis machine!!

Nat Kinsey
UpscaleCNC

I assumed that most any CAM software could do the 2D slicing and toolpath generation off of the .stl file. What CAM do you use? Same assumption for changing the undercut to a vertical envelope cut. If that's a problem, it would be a really straightforward exercise to do 2D projections and go straight to .dxf output directly from OpenSCAD.

Gary
RocketryCNC
 
I think this falls into the "whatever works for you" category. Thinking ahead to how I was going to have to fabricate and assemble the pieces, i found a few little gotchas in the CAD approach, but whatever gets people building cool rockets and flying them is a valid method.


Sent from my iPhone using Rocketry Forum
 
When I designed the rocket that inspired the Peak of Flight article referenced by the OP, I actually started by using a very capable 3D modeling tool, Rhinoceros3D.

What I found was that, while it was useful for developing the basic design concepts, it was actually not terribly helpful in designing the pieces that I actually needed to *fabricate*. I did not wish to use centering rings made from the more commonly available 3d printing materials, not did I wish to pay to have something generated on a five-axis mill. Also, the airframe cutouts were fairly difficult to do in the manner I wanted. I notice that the OP doesn't address what to do if your tubes penetrate the airframe.

Sure, I could develop a bunch of manifold 3d solid topology, calculate the intersections, extract the faces I needed, unwrap the developable surfaces, move them all where I wanted, extract the various curves, draw the tangents, trim the lines, scale the inner tube cut pattern to the outer tube radius, etc. it turns out that requires more steps, takes more time, and is more error prone than running it through a quick excel spreadsheet and drawing everything where it is supposed to be. I suppose the CAD-generated stuff was useful as a check to see if I'd done the math right.

I do recommend using a CAD tool to visualize what you want, but be aware that there are quite a few issues to deal with when manufacturing a focused-thrust cluster, and you need to think it through very thoroughly before you start laying out materials.

Nope, I didn't address the tube cuts. Funny thing about that is that some years ago I wrote an application that generated printed templates for tube intersections; any angle, offsets, all the whistles and bells. Worked really well, but I didn't find a lot of interest in it. Wondering if I can find that code? Might have to pull out my old XP box and see if it's there.

But yeah, while constructing the tube/tube intersection in OpenSCAD would be easy, a method to 'unwrap' it to get a marking template eludes me at the moment. For myself, I'd probably just 3D print a template. A little manipulation to accommodate the undercuts, but I think not hard.

Gary
 
.. but so far haven't been willing to pay for SolidWorks.
...

I'd love to have SolidWorks too. If folks don't know, vets can get a copy of the student edition practically for free: https://files.solidworks.com/pdf/EDU_MVP_2013.pdf I'd do that, but then I'd still have a watermarked student edition, only good for a year, and then I'd still be looking at multi-$K after. I'd even consider that if the maintenance wasn't OOS too. Still ok if it generated income, but looking at the cost margin above what the free/low cost tools can do makes it seem even more $$.

Gary
 
Solidworks is definitely hard to justify at the price for rocketry. If you're making car parts, that can pay for it...


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Solidworks? Pshhh. Get quotes on NX and Mastercam. Some people like fancy cars. I like fancy CAD tools.
 
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