Introduction: CNC Wing Ribs

Picture of CNC Wing Ribs

At our monthly meeting of the Experimental Aircraft Association, I was telling the members that I had access to a CNC router. Quickly, one member asked if I could cut out wooden wing ribs. The traditional approach is to make a jig, take hundreds of small sticks and glue and staple them together. Considering his project is a bi-wing aircraft, and has 32 wing ribs, that’s a lot of tedious hours. I said I’d give a go. I have a background in fabricating Museum, Zoo and Nature Center exhibits. For 25 years I was asked to build the most amazing things. This project would be a fine experiment.

Step 1: Your STUFF

Picture of Your STUFF

Things you need / Materials and equipment list:
Large scanner
Computer
Vector Drawing Program
CAM generator
Mach3, EMC, or similar (http://www.machsupport.com/) (http://www.linuxcnc.org/)
CNC Router   (www.100Kgarages.com)
Tooling Bits
Baltic birch plywood – ¼” 5 ply or similar
Staple gun
X-acto, utility knife, something sharp
Sandpaper or foam blocks

Step 2: My STUFF

Picture of My STUFF

(Things I need)
Patience
Dust collection system
Writing skills
Photography Skills
More tools
Bigger tools
Faster tools

Step 3: Scan Original Blueprints

Picture of Scan Original Blueprints

I was given the sheet of blueprints (actually white zeroxed copies) to work from.  I taped the relevant portions together and traced the image to be cut on white butcher paper. Then took it to a local architectural copy shop (FedX-Kinkos would do) and had them make a digital file. They also sized it a bit to match the dimension given on the plans. I was enlarged 1.6%.

Step 4: Drawing Programs

(CAD – Computer Aided Design)
There are so many design programs out there (http://crunkish.com/top-ten-cad-software/) (http://www.freecad.com/). Some are free, some are not. Try the free ones, or the trial downloads,  and spend some time assessing their capabilities. Make sure you look at what they can import and export. This will be important in moving your drawing files to the CAM generator (more about that later). The higher end ones have a steep learning curve and take a while to be comfortable with.  The lower end and free are getting better all the time.  Check out Sketchup (free) with the ruby script that allows output in .stl and .dxf formats. Sweet! (http://www.guitar-list.com/download-software/convert-sketchup-skp-files-dxf-or-stl)
I loaded the file into my CAD program and with node editing, smoothed the exterior curves, or as known in boat making, fairing the lines. Using created circles at each intersection within the internal trussing, I smoothed and cleaned up the intersections, thus replacing gussets that would have been present if built the traditional way.

Step 5: Output to CAM

(Computer Aided Manufacturing)
Some drawing (CAD) programs have no way to get g-code from them. G-code is a language defining where the router must go to perform the task you have in mind and is generated by the CAM. Some CADs have CAM with them but they tend to be the pay-for types.  For those of you who are using a CAD program that does not have this function, you’re going to have to find a separate one to do this. Here is some links. (http://www.rhino3d.com/resources/display.asp?language=en&listing=545)
(http://www.freecad.com/CAM_Programs/))  For those of you who want to learn about g-code there are sites that will show you what it all means. (http://www.cncezpro.com/gcodes.cfm)
(http://www.cnccookbook.com/CCCNCGCodeCourse.htm)
My program has CAM output, so the g-code was created, and ready for the homemade CNC machine.

Step 6: Selecting the Bit

Picture of Selecting the Bit

On our homemade CNC machine, is a Porter Cable Trim Router  As I had selected Baltic birch plywood, I chose a ¼”shank 1/8” end mill. This means it is strong (thicker shank), and will cut 1/8” wide with a square bottom.
Our router only goes one speed in rotation and maxes out at 25 linear inches per minute, so the only control we have is how deep it cuts per pass, and the inches per minute speed it travels thru the material.  Before any machinery is turned on, install the bit into the router you have. Be VERY careful, they are quite sharp. I wouldn’t know that, but, there are a wide range of band aids and compresses readily available in our shop, just in case.

Step 7: Load the Material

Picture of Load the Material

I had some cardboard to use for the test cut. Make sure that the piece you load on the machine is bigger than the project. Seems obvious, doesn’t it?  Also that it is oriented correctly in the X and Y plane. I never make that mistake.
Stabilize the cardboard (later the plywood). Many treacherous opportunities lie here. Cut the cardboard/plywood  larger than the project.  You want extra room for clamping down the outer edges with screws, cam locking devices, or even double-sided tape. I used a staple gun with ¾” staples around the outer edges of the cardboard and the baltic birch, out of the way of where the router bit would travel. Regardless of the method, one wouldn’t want the highly rotating router bit or the router itself crashing into something hard and unyielding. Some very interesting results WILL occur and you really don’t want that experience. The least is you may lose some steps in the g-code and get the remainder of the cut all cockety-wampuss, and the worst is flying bits of high speed steel flying around. Which brings us to the all-time favorite subject of OSHA,   da-da… SAFETY.

Step 8: SAFETY

Picture of SAFETY

Here we go, ignore this at your own peril. Ok, the condensed version:
---Ears - CNC machines are loud. I like to put on noise cancelling earphones with my favorite music playing in the background. Not        too loud as you want to lightly hear the machine as it runs, giving you a heads-up to any problems.
---Eyes and face – Flying objects do sometimes come from the machine, best to protect them. I use a face shield.
---Breathing – All materials have some dust created during the cut. Use at least a filter mask for non-toxic woods and cardboard something better for MDF and plastics.
---(If at all possible make or buy a dust collection system to keep the fumes and dust to a minimum. That’s the next project for us after this contest. Wish I had done it earlier.) (http://solsylva.com/cnc/vacuum.shtml)

Step 9: Power the Machine and Computer

Picture of Power the Machine and Computer

At last the fun began. I powered the machine and computer and opened up Mach3. This is the interfaces that actually translates the g-code into movement by the router. Mach3 works with windows. EMC is the open-source Linux version, Next I loaded the g-code using a flash drive. Use that or wireless or whatever you move files with, from your cozy office to the cold and manly shop. I then  clicked the Reset button. Now I could move the non-running router around to position the 3D coordinates(X, Y and Z) of the bit by using the number pad on the keyboard or an separate number pad.

Step 10: Zero, Zero, Zero

Picture of Zero, Zero, Zero

You really want to fire that thing up. You have been at it for a while. You are impatient (well that, at least, describes me at times). But we have to get off on the right foot, or inch, or thousandths of an inch. X, Y and Z must be zeroed. Most of the time, this point will be the top-lower-left corner of the drawing.  Be slow and careful to NOT run the bit into the material or hold-down clamps. The tooling bits, I’ve found, do break, and are pricey. Once over the zero, lower the bit ever so gently down to just above the top of the piece. Then I place a feeler gauge below the bit and, extremely slowly, move it down to just touch the gauge. Using .002 gets the bit located in space just half-a-hair above the material. You do remember NOT to have the router on during this step. Right?

Step 11: The Magic

Picture of The Magic

It’s finally time to turn the router on and start the sequence of cutting from Mach3.  Just like your mother used to say “don’t forget your mittens”, remember the safety stuff.  Turn the router on first.  Then I clicked the green button on the screen labeled Cycle Start. Off and cutting. Yeah!!!  I prepared to stop the cycle if it ran off in some bizarre direction. Remember the adage that computers, (along with dangerous machines attached to them) only do what you tell them to do, not what you want them to do. If all is going well, just watch and enjoy the moment. To me, this is amazing stuff, I used to hand cut all of these projects. It’s a wonderful world!
When the cycle ended, and the router came back to zero, I turned it off, then removed all the staples. I remove the waste sections, and vacuumed or blew off the dust so the machine is ready for the next experiment.
It almost worked perfectly. Several of the interior waste pieces moved around in an uncomfortable way, and two of the trusses bent out (got too thin) as the router bit pushed them sideways.  Lastly, if the project is cut completely from the waste material, I have a template that will prove useful. So, I went back the CAD and placed a number of tabs around each part. (Tabs are small bits of the wood that hold all the various parts together. They are less thick than the plywood, short in length, and are easily cut with an x-acto knife.)
For this run I had the cardboard template to trace on a piece of Birch plywood. I could place staples more accurately to secure the material safely. Plan B was run thru successfully.

Step 12: Examine

Picture of Examine

Now is the time to really look at the product.  Test it, measure it, look it over closely. If it needs improving, return to Step 4, and go for Plan-C, or Plan-D, etc.  Most of the time, it takes several tests and iterations to get it right.

Step 13: Finishing

Picture of Finishing

I used foam sanding pads, and smoothed all the surfaces, then cleaned with.
I then met with the aircraft builder for his approval.
Now, only 31 more.

Step 14: Why We Want to Win

Picture of Why We Want to Win

Last summer Mike and I began getting together at least once a week, or more, for what we called guy time. We would work on some idea, hack, project, or repair of some item with what we had in the shop. This resulted in some clever solutions using rather odd materials. The interplay between us was so fun we began to invite others to our “club”.  Thus a Maker Space has taken over my shop. I love it. About this time Mike introduced me to Instructables.com and I was hooked. He also brought over his homemade CNC Router.  We got it up and running and have been creating a number of interesting projects that are being submitted for this contest.
Our club of makers would continue to explore the vast world of CNC routing. We have in mind a large backlog of ideas that would be furthered by winning the PRS Standard 96-48-6 ShopBot .  Our homemade CNC can cut 31” x 21” and cuts at a maximum of 25”/ min.  The 4’ x 8’ table and 300”/min will vastly improve our ability to produce prototypes, develop and improve designs, create larger projects and test out 2.5D, then move to full 3D sculpting.
Here is a small selection of my personal list of projects:
2D
                    Mazes for more than one player
                    Multi-level mazes
                    Giant mazes on a half sphere to be played by young children
                    Interchangeable mazes of increasing difficulty
                    Picture frame mazes
                    Spirographs – small 8.5 x 11
                    Giant Spirographs using chalk for outside
                    Patterns for Cabinet doors
                    Wood Blocks for printing from ancient lithographs
2.5D
                    Bas-reliefs
                    Cameos
                    Negatives for molds
                    Negative busts
3D
                    Sculpture
                    Full Busts
                    Art prototypes

Thank you for considering this submission
Rick Shore

Comments

ultralajt (author)2012-09-26

I suggest that a comparative load test should be done between that CNC-ed rib and ordinarry build up rib from spruce sticks. Ordinary rib will be superior as grain orientation and strength of the spruce (or douglas fir..) is way higher than of the plywood. The backdraw of the plywood is also the grain orientation, and it can not be used to its best strenght.
Pure plywood ribs should be designed for that lower strength of the material. Caps and diagonals should be much wider. And this will resulted in heaviest ribs, than odf classic design.

Othervise it is clever idea to simplify ribs manufacturing and to speed up the process. The newest design from team AeroMax use many CNC done wooden plywood parts in the aeroplane structure.

dlayton (author)ultralajt2013-11-29

I see the concern but ribs on most airplanes are not heavily loaded and even aerobatic airplanes are using routed ribs now.

I've noticed that many times we get buried in conventional thinking that is based on a theory nobody fully understands in aviation. Many theories are just rumors even. Sometimes it's better to just see if you can get something to work since everything is a great compromise in design anyway. Most great advances come from trying something new.

dlayton (author)2013-11-29

What airplane is it for? I've done some ribs for people too. Kitfoxes have routed wood ribs but the factory does them by hand. Sad huh? haha.

wally6800 (author)2013-03-09

I would like to know more about how you faired the rib profile before you created the "G" code. I have never been able to make a rib that I could use right off the CNC table without doing a lot of hand work to get the profile fair. By that time I have lost the exact shape and the rib is worthless.

I hope you can help me. I have two Marske Flying wings that I want to build. 70% of the work is in making the ribs and making them to the correct size.

Thanks.

Wally

dlayton (author)wally68002013-11-29

I'm building a flying wing right now too. I use Rhino 3D which has a feature in the view menu to place a background bitmap, jpeg also works. I then use the control point curve to trace the outline and then scale it to the right size in the "transform 2D" function.

I've owned a shopbot for 15 years and after you have the machine it's all about the CAD skills isn't it :) Have fun, I'll post my project "floater wing" soon.

rickharris (author)2012-03-01

Just a CNC tip - We often for complicated cuts used to print out the drawing - full sized - and the lightly tack glue to the material. that way we could see a) that the material is big enough - b) where to put the hold down clamps that is going to be out of the way.

We always used a piece of sacrificial MDF under the material so we could cut all the way through. this also allowed us to pin the material - usually wood - down with panel pins. Much better than double sided tape and easier than various clamping systems.

CameronSS (author)2012-02-27

I'd be curious to run a stress test to failure on this and a traditional built-up rib. In standard rib building (as I understand it) the wood grain runs parallel to each member, whereas with plywood the grain is going every which way. Then again, plywood would have increased strength in other directions, and wouldn't have failure-prone joints. Have you looked into this?

Please don't take this as a disparaging comment, it's a great use for a CNC router, especially if it gets another airplane in the air. Well done!

Toxictom (author)2012-02-27

Having dealt with wooden wing ribs I can see where this will save the builder lots of time and money. Good job.

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