CNC Pietenpol Ribs & More

Discussion area for builders of Pietenpol aircraft, both beginners and experienced folks. Share ideas, ask questions and help build the Pietenpol community.
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cunni5ac
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Joined: Thu Feb 08, 2018 2:04 pm

CNC Pietenpol Ribs & More

Post by cunni5ac » Thu Feb 08, 2018 2:16 pm

Hello Everyone,

I am considering building a Pietenpol and I have read through a decent amount of material related to the build.

I was wondering, has anyone tried to utilize a CNC machine to cut out the wing ribs and other items requiring more than one nearly identical parts?

I would think this would help cut down build time considerably especially at scale (multiple guys working on a Piet at once). I have not looked at how the plans are but I have seen many use a stencil for the ribs which should be able to be easily digitized.

Would /could the wing rib be as strong or stronger?

tom kreiner
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Re: CNC Pietenpol Ribs & More

Post by tom kreiner » Fri Feb 09, 2018 2:54 pm

While you can do anything you like, both $$ and time permitting, producing NC'd ribs is much more involved than you might imagine.

First, you'll need a digitized Rib, probably taken from a Rib Drawing, but there are many issues with that, as the ribs to be routed will have nearly totally different methods of construction.

Second, the rib will most probably need to be drawn up in CAD, then saved in an NC readable format like .dxf, .dwg, or STEP. Once saved, the NC software can "read" the layout. Now you have a rib outline - perhaps with trusses designed in, with no good way to gusset your cap strips to the top & bottom. Compare with a typical rib where you'll have both upper and lower, along with internal truss braces, all held together with gussets.

An NC's rib, on the other hand, would be routed from plywood, and due to the cost of the wood, along with the requirement to have 1/2 wide cap strips top & bottom.

Finally, the cost of the plywood will be OUTRAGEOUS. Since Birch comes in 50" squares ( maybe 4 x 8 sheets, IDK), you may be looking at Okume, and cutting the ribs at a 37 degree angle to get best usage of a 60" x 10" chunk from each sheet. So, for 30 ribs, 6 sheets of 4x8 will be required, at $35/sheet, plus shipping - maybe around $250.

Will you maximize material usage by cutting head rib, middle rib, and trailing rib, and then glueing them together? And now that you have cutouts how do these attach? How do you fixture? All kinds of questions... that YOU, the experimenter will need to figure out.

That's why you don't see NC's ribs.

cunni5ac
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Re: CNC Pietenpol Ribs & More

Post by cunni5ac » Fri Feb 09, 2018 5:32 pm

Hi Tom,

I am so new to this (and building airplanes in general) I need some clarification on what you are referring to.

First, you'll need a digitized Rib, probably taken from a Rib Drawing, but there are many issues with that, as the ribs to be routed will have nearly totally different methods of construction.

Not sure what you mean when you say the ribs will have nearly totally different methods of construction. Are you saying the problems are there because the construction method is different from the plans or different from one another? I was hoping the CNC milling would reduce variation, add consistency and reduce time.

Now you have a rib outline - perhaps with trusses designed in, with no good way to gusset your cap strips to the top & bottom.

I was looking to have them cut out with the trusses designed in, the rib being one solid piece.

So, for 30 ribs, 6 sheets of 4x8 will be required, at $35/sheet, plus shipping - maybe around $250.

You bring up a good point about waste since the ribs would be a small area relative to the overall board size. However, $250 doesn't sound terrible. How much would you estimate the conventional method costing?

I appreciate your insight. I think not having my hands on plans may make it hard to see exactly what you are referencing.

Thanks

tom kreiner
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Re: CNC Pietenpol Ribs & More

Post by tom kreiner » Fri Feb 09, 2018 6:38 pm

I'll try to address your questions one by one.

Construction of a traditional wooden rib is done by first steaming & then bending the upper & lower cap strips to the shape of the airfoil. After these are made, they are connected together (glued with epoxy) using truss pieces - kinda like an old steel bridge - and flat pieces of 1/16 thick plates called gussets. When the epoxy is cured, the rib is flipped over, and gussets are added to the other side. All of the materials, including epoxy will cost around $120 to $180., depending on what you get, where you get it, and so on.

Making an NC'd rib from plywood DIFFERS radically from what I just described in that the upper & lower cap strips will need to be added. About the only way to make ribs this way would be to place 1/4 x 1/4 cap strips on each side of the plywood cutout. While this is possible, it's not normally done, and many details will be up to YOU, the experimenter.

NC tools would produce what you want, but in order to maintain the low weight of each rib, you'd necessarily need to use very thin material, and routing that - with any tool, manual or NC - can be a problem also, as the router bit might lift the material, especially towards the final few cuts where there is little support.

I'm sure you could figure out a way to make them with an NC tool, it's just going to be rather laborious to do it. In my work as a Mechanical Engineer, I create products for NC tools, along with all of the file types I mentioned, and I could re-create the ribs in CAD, but it would take more time than I think it's worth!

If you want ready made ribs, attend the Pietenpol gathering in July - details will be here on the BPA site - as each year folks with time on their hands during the WI winter show up with a few sets of ribs for sale for anywhere from $250 to $400 or so.

If there's a quick build in Piets, its in purchasing a partially complete project which you can pick up often by reading the Piet Newsletter, and or various sites.

Lastly, many guys will comment on how the rib process is one of the most enjoyable aspects of building an airplane. With a nice rib jig, you can make a complete rib in one day... so, in just one month, you can have them finished for the entire plane.

Best thing to do, however, is get the plans, and see what you're getting into. Then purchase the 4 book set on building an Experimental Aircraft written by Tony Bingelis, frequently referred to as "Uncle Tony." The books will lead you thru many steps toward building your ship. ALSO - Join an EAA Chapter as soon as you can! Guys in the Chapters have been there, done that, and they'll be very helpful. MY Chapter meeting is tomorrow!

Best of luck in your build, however you decide to do it.

EAB4
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Re: CNC Pietenpol Ribs & More

Post by EAB4 » Sat Feb 10, 2018 11:35 am

As Tom said-It's your project and you can do whatever you want with it, but maybe getting and studying the plans would be a good place to start before wanting to make changes to things.

While you certainly could CNC the ribs- for the amount of time you are going to spend digitizing, laying out and getting ready to cut the ribs, you are
going to have a pretty good start on building them with a jig.

Once you have the rib jig, which sometimes can also be found at Brodhead, you will be able to knock out a couple of ribs a day with a surprising level of consistency.

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Pat Weeden
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Re: CNC Pietenpol Ribs & More

Post by Pat Weeden » Sat Feb 10, 2018 8:06 pm

The Brodhead Pietenpol Association has two wing rib jigs available for loan, free of charge. Just pay shipping.
Pat Weeden, Site Admin
Brodhead Pietenpol Association

cunni5ac
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Re: CNC Pietenpol Ribs & More

Post by cunni5ac » Sat Feb 10, 2018 9:02 pm

Thanks everyone for the clarification and putting things into perspective. Since I am just starting out, I am still trying to get a feel for what are some of the most time-consuming tasks in building the aircraft and seeing how I could be more efficient.

Are there any activities you would do differently to have a smoother / streamlined process if you had to do it again?

cunni5ac
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Re: CNC Pietenpol Ribs & More

Post by cunni5ac » Sat Feb 10, 2018 9:02 pm

Pat Weeden wrote:
Sat Feb 10, 2018 8:06 pm
The Brodhead Pietenpol Association has two wing rib jigs available for loan, free of charge. Just pay shipping.
That's great!

rmueller23
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Re: CNC Pietenpol Ribs & More

Post by rmueller23 » Mon Feb 12, 2018 1:44 pm

Presumably the goal of identifying the most time-consuming tasks and seeing how they could be accomplished more efficiently is to reduce the amount of time it takes to build the aircraft--said another way, to complete a Pietenpol in a timely fashion.

I think if you distill down the wisdom from those who have completed and are flying their own Pietenpols, the consensus would be that the quickest way to finish building your own Pietenpol is to spend the most time that you can building the airplane, to the plans. The more time you spend analyzing processes and trying to determine alternative methods for accomplishing X, the less time you spend building. The more modifications and "improvements" that you try to incorporate, the more time you spend designing--and the less time you spend building. The more time you spend on the internet, the less time you spend building. Etc, etc...

If your goal is to build a Pietenpol, the best thing you can do to be more efficient in achieving that goal is to identify and eliminate the things in your life that are substitutes for actually being in the workshop. That will go much farther towards increasing your efficiency and arriving at a finished airplane than any fine-tuning or modification of the process or design can accomplish. Good luck!

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taildrags
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Re: CNC Pietenpol Ribs & More

Post by taildrags » Mon Feb 12, 2018 10:11 pm

rmueller23, you have the airplane-building wisdom of the ages. Well said! You have just summarized why I have a partially-completed experimental airplane project in my hangar and it looks much the same as it did when I moved it from Oregon to Texas in 2002 and back from Texas to Oregon in 2011. However, my downfall has been that if I followed your advice to "identify and eliminate the things in my life that are substitutes for actually being in the workshop" to finish that project, I wouldn't be flying my Air Camper or working a full-time job ;o) I guess I have the best of both worlds though... something to build and something to fly.

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