Bob-stick for flattening ribs

[Redneckmech]

Hey y'all!

I just put together a quick video of using the Bob stick for flattening my end ribs. I haven't seen this technique described in any other videos however I believe it is the method Bob describes in the Bearhawk Book so I can't take any credit for development of it.

Hope this is able to help someone else since I have greatly benefited from the tips and videos everyone here has shared.

[SpruceForest]

Nice! Made mine from scrap Delrin after a couple skinny wooden ones split, but wood ones hold up well if they are made a little on the beefy side like yours.

[Redneckmech]

Well, I hadn't planned on doing a video of using the bob-stick to fully form a flange but when I was looking at my end ribs I saw opportunity to add an additional lightening hole. Since it was only for two ribs I didn't feel it was worth it to create an MDF template to form the flanges with a press so I formed them purely with the bob stick. {CAUTION: Plans deviation, do not attempt unless you are certain that it will not cause structural issues or interfere with assembly!}

[Redneckmech]

Nice! Made mine from scrap Delrin after a couple skinny wooden ones split, but wood ones hold up well if they are made a little on the beefy side like yours.

Delrin was my original plan based on your advice elsewhere in the forum. I was unable to source it locally so used a busted ax handle that I had laying around figuring if the wooden one broke I could order a piece from McMaster or other online supplier. Delrin would likely have better self-lubricating properties and make it easier to slide around the flange though.

Thanks for all of the info you have shared on the forum, it has helped immensely as I have been going through my build!!

[SpruceForest]

Delrin is likely the most expensive choice as well... sort of gold-plating what is supposed to be a perfect K.I.S.S. tool! Looks like that little chunk of hickory (?) worked just as well and kept it out of the stove this winter.

[fairchild1934]

I wonder if a home made tool could be made that would have 2 brass rollers having a .026 gap and a handle ---- and maybe a base with rollers so you
could have the base down in the hole to keep it centered and just spin it a couple turns and then raise the inclination and spin a couple more turns.
the rollers could be plastic too I guess. Hard PU like roller skate wheels.

[jaredyates]

There was a fellow in the old group named Larry Foor who created a flanging tool that worked by rotating.

larryfoor.jpg

Here are some of his words:
I have attached a breakout of the tool components. It is made of a piece of 1X1X1/8 mild steel tube for the main bar and two pieces of 1 1/4 square tube 2 inches long. The fingers were made out of a 3/16 plate of mild steel 4" X 24" and formed at the local fabrication shop. I got 2 feet to make it easier to hold in the cutoff saw. I then cut off two pieces 5/8 wide and ground them to shape and drilled and taped them fo 5/16 threads. These are then welded to the two pieces of 1 1/4 tube. The 1 1/4 tube is drilled and tapped to 1/4-20 to accept the adjusting knobs. The rollers are 1 1/4 X1/4 nylon with a 3/8 center hole( Available at your local Hardware store along with the two 3/8 X 1/4 Shoulder bolts and the two adjusting knobs. everything else is std 3/8 all thread and nuts and washers.

[Bcone1381]

A brainstorm on rib flanging. Factory ribs have a round ridges pressed around an hole instead of a flange. When we flange it we stretch and stress the inside radius of the hole. When a round ridge is formed into the rib the edge is not stretched out like that.

Does a round ridge accomplish the same goal as the flange but is easier to form? Could it have less stresses that potato chip the rib, and quicker for the EAB home builder to fabricate. Rubber pressing them might be a good fabrication method.

My wings are at the hangar. I bet someone can take a photo that shows the factory lightening hole

[DaveNelms]

Thanks for this!

[SpruceForest]

Both work, but if I recall my fracture mechanics stuff from four decades ago, unless you already have a crack in the edge of the lightening hole coming out of the forming process, any available strain energy to promote a fracture needs a stress concentration point somewhere on that edge to initiate/continue fracture/cracking... the radius of the holes in the rib are all just too large for that to happen short of a manufacturing defect. Same idea behind stop-drilling a crack... increase the radius to get below the threshold stress value at the tip of the crack needed to continue crack formation. I did NDI on my first few ribs and then careful physical inspection on the rest... while all my rib lightening holes were hydraulically formed, I doubt Bob-sticking them results in a meaningful increase in probability of crack formation. Also found that despite cautions about cold-forming the alloy, at the sheet thicknesses we use, there were no issues detected on inspection of the parts across two airplanes-worth of ribs.

Also keep in mind that the factory ribs are formed in -0 temper material, then batch-hardened, so they can get away with a lot more in the way of forming pressure without risking crack formation.

A brainstorm on rib flanging. Factory ribs have a round ridges pressed around an hole instead of a flange. When we flange it we stretch and stress the inside radius of the hole. When a round ridge is formed into the rib the edge is not stretched out like that.

Does a round ridge accomplish the same goal as the flange but is easier to form? Could it have less stresses that potato chip the rib, and quicker for the EAB home builder to fabricate. Rubber pressing them might be a good fabrication method.

My wings are at the hangar. I bet someone can take a photo that shows the factory lightening hole