Intro: 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
Things you need / Materials and equipment list:
Vector Drawing Program
Mach3, EMC, or similar (http://www.machsupport.com/) (http://www.linuxcnc.org/)
CNC Router (www.100Kgarages.com)
Baltic birch plywood – ¼” 5 ply or similar
X-acto, utility knife, something sharp
Sandpaper or foam blocks
Step 2: My STUFF
(Things I need)
Dust collection system
Step 3: 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)
My program has CAM output, so the g-code was created, and ready for the homemade CNC machine.
Step 6: 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
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
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
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
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
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
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
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
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:
Mazes for more than one player
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
Negatives for molds
Thank you for considering this submission