Rubber press forming

Discussion in 'Workshop Tips and Secrets / Tools' started by 12notes, Feb 20, 2018.

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  1. Feb 20, 2018 #1

    12notes

    12notes

    12notes

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    I built a couple of rubber press formers for my wing ribs, and thought I'd relate my experience and advice concerning them.

    When I first priced out the metal and rubber for building one, it was far too expensive. Then I found a 6'x3' roll of 1/4" thick rubber at the hackerspace, and the drop rack at Metal Supermarkets near me had 2 pieces of 3/16" C channel the correct length and a 3/16 angle the correct length to fit inside for 65 cents a pound. So I built two forming boxes, one for the nose ribs, and one for the rear ribs. I dusted off my decade-old stick welding skills using some old, moisture laden 6010 rods and made some ugly looking but strong welds. Total cost was somewhere around $50-$75, I think, but I was buying a bunch of metal at the time.

    I cut my ribs out on the Shopbot CNC, and also made forms from 3/4" MDF with flanged angles for the lightening holes. I also put a stiffening bead groove on the forms.

    I cut and layered 6 pieces of the rubber to fill the smaller nose rib box, and went to press it, only to find the hackerspace's Harbor Freight 12 ton press' jack was leaking fluid badly. Bought a new jack, got everything in place and started pressing, only to find the rubber wasn't compressing the flanges nearly enough. I modified 3 layers of the rubber to match the shape of the form, then pressed again, only to hear a loud bang as the press ram bent and the tip shoved up back into the tube - it is a press no more. It formed most of the rib including the flutes pretty well, the back lightening hole flange wasn't complete and it did not even start the stiffening bead.

    Score: .020" aluminum 1, press 0.

    I thought I'd try the rear ribs on a 20 ton press. Made the same type of rubber setup, press slowly until my weight was no longer enough to move the jack handle, then my more gravitationally abundant friend compressed it a few more pulls. The side flanges bent some, but not enough to form any flutes, the bend from the fluting pliers was the only one evident. It pressed the middle two flanges on the lightening holes, but nothing on the other two. No stiffening beads were evident.

    Score .020" aluminum 2, presses 0.

    So I decided to form the rear ribs by hand and the front ribs in the 20 ton press. Forming 24 rear ribs the "slow way" instead of pressing them took all of 2 hours, excluding the stiffening beads and lightening hole flanges. I did put the flanges and stiffening beads on one rib by putting a piece of the rubber over the rib on the form and hammering in the approximate locations, it took about 1 minute, worked fine.

    My personal opinion is that rubber press forming makes no sense unless you are going into production. You'll spend far, far, far more time building the boxes than it takes to press the ribs. If you already had the boxes, I'm not sure forming a rib with the rubber press would be any faster than forming one with a hammer. Also, it's only good for nose ribs unless you have a greater than 20 ton press, and I'm not even positive about that as I haven't gotten back to the 20 ton press with the nose ribs yet.

    I decided to do a few things with this project that I knew would be slower, as I wanted to learn some new techniques (CAD, CNC, etc). This was one of those things, and, although I'm glad I went through the process, it doesn't seem to be worth the time and expense (~$300, if using new steel and rubber) for a home builder.
     
    Last edited: Feb 20, 2018
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  2. Feb 20, 2018 #2

    BBerson

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    Well, your 14" thick rubber is likely a typo? (1/4")
    Probably need 2-3" thick rubber block and a 500 ton press or more. Figure the force to push 1 square inch a distance of 1/2" into the rubber block. And multiply that times the size of the rib.

    5-10 tons per square inch, I think.http://www.knrubber.com/pdf/loadbearingpads.pdf shows 15,000 yield strength.
     
  3. Feb 20, 2018 #3

    12notes

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    Yep, it was 1/4", I corrected the post.

    500 tons is way more than needed. The front rib was 90% formed on the 12 ton press before the press broke. I can't remember where I found the calculation, but 20 tons should have been on the edge of working for the rear rib (it's also what they use in the EAA video), and more than enough for the front rib. I'll try to find it again.
     
  4. Feb 20, 2018 #4

    BBerson

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    If you notch the ribs like EAA, then won't need much.
    I thought you were trying do real Guerin type forming.
     
  5. Feb 21, 2018 #5

    12notes

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    I'm not sure what you mean by "real" Guerin type forming, but unless I'm missing something, Guerin type forming is exactly what I was doing - bending down the flanges on the edges of the ribs into a fluted form while also pressing the flanges and stiffening beads on the face using a rubber pad. The ribs were not notched.
     
  6. Feb 21, 2018 #6

    BBerson

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    The EAA video I saw was notched, I think. So I had no further interest.
    If you can press without notches and get the rubber to flow up the side and fully form at 90°, then that would be "real", in my opinion. I think you might need solid rubber, not 1/4" laminations.
    Is the metal soft? What alloy?
    Got any photos?

    You said the process makes no sense...
     
    Last edited: Feb 21, 2018
  7. Feb 21, 2018 #7

    BBerson

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  8. Feb 21, 2018 #8

    bobm4360

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    The "Me-262 Project" used a HF 50-ton press and a 1"thick steel box 12" x 12" x 6" deep and a soft neoprene pad backed by a slightly harder neoprene pad. and would handle anything that fit within the box up to .040 steel (notched), and, IIRC .063 aluminum (drawn, not notched).
     
  9. Feb 21, 2018 #9

    lakeracer69

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    Just any "rubber' won't get it done. You need some that you KNOW is around 65 durometer. You also need it to be around an 3/4 to 1 inch thick ( per piece).
    I have had no problems using: A 20 ton press, two pieces of the correct rubber ( durometer and thickness), and a form made out of corian ( MDF got crushed too much). .025 6061-T6 formed fine and I just had to just do a final straightening on the edges.

    In your case, a bunch of thin rubber of unknown hardness didn't do the job. If you follow how others have done it, and don't substitute your own ideas and materials, it will work. I have done it a bunch so I know it works.
     
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  10. Feb 21, 2018 #10

    12notes

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    0.020" and 0.025" 2024-T3, the nose ribs had formed about 80 degrees on the edges with flutes formed and most of the flanges on the face formed when the 12-ton press broke. I'm fairly sure the nose rib will fully form (no undercut on the form, so not quite 90 degrees) with the 20 ton press, but I don't think it will be any faster or better than hammer forming it, I'll press one later this week.

    No, I did not say that. You cannot remove every qualifier in a statement and imply it is what was said. What I said was "My personal opinion is that rubber press forming makes no sense unless you are going into production." And I stand by that statement. It is my opinion that the process of making the box takes much, much longer than any time you might save making the ribs, and adds unnecessary expense. Once the box it built, it is only marginally faster - a few minutes saved per rib at best. My experience was no time saved per rib - it took about 5 minutes to form a rib either way. A 20-ton press is about the largest commonly found in a garage or home workshop, and I'm building a small (38" chord) set of wings, for those that have larger wings, this process is only good for nose ribs unless you have a larger press. In my opinion, it's an expensive, time consuming solution way to solve very minor problems if you're just building one plane. Hammer forming is a much better method for low numbers.
     
  11. Feb 21, 2018 #11

    Chris In Marshfield

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    If you check out my Expedition project in my sig, you’ll see how I used a 20-ton press in my project. I used it only to flange lightening holes, and it was great. That was the part I really needed it for. Not enough oomph to bend edges, although I did try on some small parts. It flowed the hole flanges beautifully. But wasn’t very useful for edge flanges. And it went *fast*. .025” and .032” 2024.
     
  12. Feb 21, 2018 #12

    Marc Bourget

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    Lots of good points in this thread, but there is a much larger "world" out there than set out above. Used book websites will give you good resources if you subject search on "Forming Metal." Alcoa's reference book, "Forming Alcoa Aluminum" will raise your personal forming bar significantly.

    I agree with the relative speed of hand forming versus press forming, but I served my apprenticeship in John Thorp's Burbank Shop. Check out John's Articles in Sport Aviation on Building the T-18, particularly No. 3, printed in the July 1962 issue. View attachment No 3.pdf
     
  13. Feb 21, 2018 #13

    Marc Bourget

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    Sorry double post
     
  14. Feb 22, 2018 #14

    Victor Bravo

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    Drool...jealousy...drool...jealousy...

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  15. Feb 22, 2018 #15

    Marc Bourget

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    Victor Bravo,

    No reason to feel bad, I'm more than happy to share my knowledge. This Summer, if you're ever in the Stockton area and have some time, drop me a PM.
     
  16. Feb 22, 2018 #16

    Victor Bravo

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    Thank you, but I'm still wanted as Public Enemy #1 in Stockton, over something that happened almost 25 years ago. To this day I'm puckered up toothpick-tight just driving past there on the interstate.

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  17. Feb 23, 2018 #17

    12notes

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    I do not have a Shore A gauge, but I did have samples of the same thickness of 50A and 75A durometers. The rubber I used was stiffer than the 50, but not as stiff as the 75. It's in the neighborhood of 60, but I don't know the exact durometer.

    I did follow how others have done it. I used the guide from the EAA, which said several pieces of thinner rubber would suffice:
    https://electricmotorglider.com/wp-content/uploads/2016/06/Hydro-Forming-Web.pdf

    It should work for nose ribs, but rear ribs, even the small ones I'm making, require a box that is near the limit of a 20 ton press. The pressure in the box I built calculates to be about 275psi, and the box barely fits the rear rib on a 13.5% thick 38" chord wing. That's less pressure than recommended, and that's for a very small rear rib (21"x5"). Larger ribs would require a bigger box and have less pressure with the same press.

    I never said it doesn't work, but that it's not worth it unless you're building more than one plane. Building the box takes longer than forming all the ribs with a hammer.
     
  18. Feb 23, 2018 #18

    lakeracer69

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    Did you use lube up the parts with spray on furniture wax before forming? That was something I found was a must for it to work properly. Also the box is not "required" for actually forming the parts. It is more to just contain things if they want to give out under pressure. I made mine from some scrap wood. My rubber pads are leftover scraps of mats from horse stalls.

    YMMV
     
  19. Feb 23, 2018 #19

    Marc Bourget

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    VB, I talked to her, she's forgiven you and actually would like your Ph# !!! (hahahahahahahaha!)
     
  20. Feb 23, 2018 #20

    Marc Bourget

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    12notes,

    Actually, If you have a good understanding of the process, all sorts of materials can be employed to help press and drop hammer forming.

    For those 20T press owners:

    Consider adding a "clamp plate" while rubber forming. You'll only have to "rubber" the perimeter, increasing the psi to the areas that need it.

    FWIW
     

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