Rubber press forming

[BobbyZ]

I've pressed quite a few panels for race cars this way with nothing more then MDF being used on the forms in quite a few cases and have always wondered why more people didnt do this vs hand forming ribs.I'm not trying to find fault with hand formed parts but this can be a lot more accurate and easily repeated.

Even if the design requires solid flanges with flutes over notches you could do the majority of the work this way and finish them by hand fairly easily.

 

[gtae07]

I've pressed quite a few panels for race cars this way with nothing more then MDF being used on the forms in quite a few cases and have always wondered why more people didnt do this vs hand forming ribs.I'm not trying to find fault with hand formed parts but this can be a lot more accurate and easily repeated.

My guess is that making the form isn't the problem, it's making the "box" that the form, part, and rubber go into before getting pressed. Welding is likely necessary unless you can bolt it together. And either way, by the time most people figured out making the box and getting the forming process right they could have probably formed all their ribs by hand.

 

[Angusnofangus]

My guess is that making the form isn't the problem, it's making the "box" that the form, part, and rubber go into before getting pressed. Welding is likely necessary unless you can bolt it together. And either way, by the time most people figured out making the box and getting the forming process right they could have probably formed all their ribs by hand.

The box can be a problem for sure. At I place where I used to work, we turned a 50 ton press brake into a rubber press. The box was made of 3/8" steel, 2' X 4', rubber 6" thick. I don't know how many times the box had to be reinforced before it no longer split. I'm sure you could drive a tank over it by the time it was all done, and probably end up damaging the tank. Mind you, the thing got a lot of use, every day.

 

[BobbyZ]

My guess is that making the form isn't the problem, it's making the "box" that the form, part, and rubber go into before getting pressed. Welding is likely necessary unless you can bolt it together. And either way, by the time most people figured out making the box and getting the forming process right they could have probably formed all their ribs by hand.

Considering that a lot of the planes in the category I'm referring to use welded steel fuselages and have multiple ribs that take a lot of time to properly hand form the amount of time to make a press box is very little.To be honest if you cant make a fairly simple press box out of mild steel then chances are you probably shouldn't be trying to weld a chromoly steel fuselage either

 

[gtae07]

Considering that a lot of the planes in the category I'm referring to use welded steel fuselages and have multiple ribs that take a lot of time to properly hand form the amount of time to make a press box is very little.To be honest if you cant make a fairly simple press box out of mild steel then chances are you probably shouldn't be trying to weld a chromoly steel fuselage either

I'm thinking of aircraft like the Sonex and other all-aluminum airplanes, which have only a relative handful of welded parts to begin with. And you can generally purchase all of them from the factory. Most builders of such aircraft don't know how to weld.

 

[Nemo]

Would any machine shop with a CNC be able to make the form block? Would I just take the rib drawing in, say I need the block made .020 smaller than the drawing, explain the lightening holes, etc and they will take care of the rest?

 

[Little Scrapper]

Would any machine shop with a CNC be able to make the form block? Would I just take the rib drawing in, say I need the block made .020 smaller than the drawing, explain the lightening holes, etc and they will take care of the rest?

Sure. It just depends on your threshold for input costs. Many shops, successful shops anyhow, won't waste time with that little part.

The difference in making rubs vs buying ribs isn't all that much relatively speaking.

You can order a router bit with a .020" off-set and run it through a router by hand in about 5 minutes.

Or build a pattern exact to a drawing and when it's done shave .020" evenly with a belt sander.

 

[Victor Bravo]

By the time you drove to the machine shop and waited for them to finish the form block, you could essentially do what Scrapper said, maybe with one little twist: Put the paper template on the wood, mark the final airfoil outline, then take a piece of .020 aluminum strip as a spacer and mark a new "final shape" line .020 inside the outer shape from the plans, then cut it out on the band saw to the outer line, and sand it to the inner line.

That said, there are very few homebuilt airplanes where a completed airplane's (otherwise smooth) airfoil being .040" thicker than what is shown on the plans would create any measurable difference in aircraft performance or behavior. BoKu's HP-24, sure, possibly. The Rutan Voyager, probably measurable over a 10,000 mile flight. The Rutan spaceships, possibly because of high speed Mach stuff. The Lockheed PV-2 "Truculent Turtle" experimental boundary layer airplane... sure, maybe.

But any of the millions of average homebuilt airplanes, RV's, Zenith (Zenae??), any of the Cubs and their millions of tube and fabric derivatives, any of the Thorps and Sonexes.... not a chance.

The curvature, smoothness, waviness, wrinkles, use of flush vs. protruding rivets, the workmanship with seams and overlaps, even the smoothness of the painted surface will have 10X more effect on performance/handling/safety than a .040 difference in the % t/c ratio of the airfoil.

Any of you high end aero guys out there can feel free to correct me.

 

[Little Scrapper]

By the time you drove to the machine shop and waited for them to finish the form block, you could essentially do what Scrapper said, maybe with one little twist: Put the paper template on the wood, mark the final airfoil outline, then take a piece of .020 aluminum strip as a spacer and mark a new "final shape" line .020 inside the outer shape from the plans, then cut it out on the band saw to the outer line, and sand it to the inner line.

That said, there are very few homebuilt airplanes where a completed airplane's (otherwise smooth) airfoil being .040" thicker than what is shown on the plans would create any measurable difference in aircraft performance or behavior. BoKu's HP-24, sure, possibly. The Rutan Voyager, probably measurable over a 10,000 mile flight. The Rutan spaceships, possibly because of high speed Mach stuff. The Lockheed PV-2 "Truculent Turtle" experimental boundary layer airplane... sure, maybe.

But any of the millions of average homebuilt airplanes, RV's, Zenith (Zenae??), any of the Cubs and their millions of tube and fabric derivatives, any of the Thorps and Sonexes.... not a chance.

The curvature, smoothness, waviness, wrinkles, use of flush vs. protruding rivets, the workmanship with seams and overlaps, even the smoothness of the painted surface will have 10X more effect on performance/handling/safety than a .040 difference in the % t/c ratio of the airfoil.

Any of you high end aero guys out there can feel free to correct me.

I would agree. Even if you were a hack and missed it by half that still puts you within. 020" witch is pretty good.

That's the thickness of a line drawing on a plan. So just eyeball it and Git-R-Done hoss.

If you take it to a CNC then your better off using the CNC from the kit company..........which means ordering ribs.

It's a paradox of sorts

 

[Marc Bourget]

This thread (which I was surprised to find I'd contributed to, previously) warrants reference to the "slip ring and die method" Many T-18 ribs were fully formed by this process, as well.

Get deeper into your rubber pad forming. Enclosing the rubber in a box is beneficial. It can raise the pressure requirements, but there are "tricks" you can employ to keep the overall tonnage down.

A big advantage with either method is better airfoil consistency and compliance. When I described the stall characteristics of John Thorp's T-18 as milder than a C-150, many T-18 drivers called me a "D" liar. I was puzzled and embarrassed (I had no reason to lie or exaggerate!) until I inspected the leading edges of their planes - then I understood!

FWIW

 

[cluttonfred]

I have pretty much zero metalworking knowledge or experience. Could you please explain the "slip ring and die method?"

 

[Marc Bourget]

Generally,

For the T-18 nose ribs, I have a 3/4" thick piece of 2024, The external dimension of the rib was milled out and the corners radiused on both sides to permit forming "lefts" and "rights" Like the beautiful die at the beginning of this thread, I have a press die made from 3/4" Micarta with the inside dimensions of the rib. The third piece is a plywood "duplicate" of the 2024 with the airfoil shape cut out, but this time a 1/4" larger. It's used to keep the sheet your forming flat while the die is pressed into the ring over the radius on the edge. When you press the die into the ring, some forming is taking place, but it's more shrinking that's taking place instead of stretching. The sheet shrinks as it's pulled around the radius - moreso at the nose than the rest of the airfoil shape. You cut the flange at the end of the radius. That leaves the formed rib blank. Next is an overlay that acts as a trim die and locates the rivet holes for the wing's leading edge. You put the blank in the overlay and punch the rivet holes with a Whitney Jr. #5.

The "slip ring" process delivers an accurate, fully formed rib, without flutes or notches.

This is part of the "matched hole" tooling process.

 

[BJC]

Generally,

For the T-18 nose ribs, I have a 3/4" thick piece of 2024, The external dimension of the rib was milled out and the corners radiused on both sides to permit forming "lefts" and "rights" Like the beautiful die at the beginning of this thread, I have a press die made from 3/4" Micarta with the inside dimensions of the rib. The third piece is a plywood "duplicate" of the 2024 with the airfoil shape cut out, but this time a 1/4" larger. It's used to keep the sheet your forming flat while the die is pressed into the ring over the radius on the edge. When you press the die into the ring, some forming is taking place, but it's more shrinking that's taking place instead of stretching. The sheet shrinks as it's pulled around the radius - moreso at the nose than the rest of the airfoil shape. You cut the flange at the end of the radius. That leaves the formed rib blank. Next is an overlay that acts as a trim die and locates the rivet holes for the wing's leading edge. You put the blank in the overlay and punch the rivet holes with a Whitney Jr. #5.

The "slip ring" process delivers an accurate, fully formed rib, without flutes or notches.

This is part of the "matched hole" tooling process.

Thorp’s matched hole process is ingenious. Every scratch built sheet aluminum airframe should be built that way.


BJC

 

[Victor Bravo]

I'd like to see a video of this process...

 

[Little Scrapper]

I'd like to see a video of this process...

I'm a visual guy, I'd like to see that too, or even photos. Sounds interesting.

 

[ScaleBirdsScott]

I think I get it. I like the method but for a tapered wing that's a lot of material to cut!

I'm more curious how the overlay is made: I'm assuming these are like metal strips that anchor to a point on the rib that you stretch over the curve as a drill guide?

 

[Marc Bourget]

Overlays were made from .090 6061. Take the material to "W" temper and form them as described in John Thorp's article on building the wing in Sport Aviation. I'm away from the computer that has the file or I'd give you a date and/or attach the .pdf.

May get things straightened out where I can dig out the components and do a video.

 

[pfarber]

e-machineshop.com is a low quantity/prototype shop that can do just about any operation.

Not cheap, but they are 10000% easier to deal with than a local shop that won't even look at you for less than 1000 parts.

 

[PTAirco]

I once had about 50 ribs pressed on a 5000 ton press at the old Miles Aircraft works in Reading, England. The rubber bed measured 10'x10'! I used a German product called "Panzerholz" for the dies; basically a super-dense compressed plywood with a very slick finish. Also known as "Jabroc" in Britain. For one or two parts, regular hardwood ply works fine. You don't need anything as fancy as the dies in those videos.

Learned a lot from it: 6061 forms nicely, 2024T3 works too but requires more attention to bend radius and polished edges to avoid tearing. If you have access to this kind of thing, it changes your whole design, for the better, but it's hard to justify it for a one-off.