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Rib forming and bob stick issues

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  • Rib forming and bob stick issues

    I cut my first batch of aluminum a few weeks ago, and in between too much work travel over the past month, finally got around to forming my first ribs. I started with the small ones - the aileron ribs - and let’s just say I need a lot more practice. I don’t think I want these on my airplane. Pictures attached.

    When hammering the flanges, I noted that the trailing edge seemed to bow out, despite a lot of pressure in between the formblocks (2nd photo). The web should be flat, not bowed. I tried clamping the formblocks in addition to screwing in the tooling pins, but I think this was formed in the “finishing” stage outside of the formblock - hammering the flanges to 90 degrees (since the springback of the metal in the formblock only gets to 80 degrees or so).

    Also, it looks like I need to rethink my flange shapes, since the trailing edge is mis-shaped (3rd photo). Not sure if I can salvage my existing template or if I have to make a new one.

    Finally, my bob stick doesn’t seem to hold up at all. The 1/16 slot just tears and opens up after a few pulls. I used hardwood (oak) and waxed the edges of the lightening holes, but still got pretty ugly results (1st photo). Should I be trying maple perhaps?

    At this point I see myself getting back into the MDF game again for a while... new templates to do the whole press method for lightening hole flanges, and perhaps new router templates as well.

    Thoughts appreciated on what is going wrong... I have my ideas...

    ... and after uploading I see I’m in quick build, and meant to post in plans build. Sigh. Better quit trying anything else today...
    Attached Files
    Last edited by nborer; 04-20-2019, 06:20 PM.
    4-Place Model 'B' Serial 1529B (with many years to go...)

  • #2
    I made my from blocks out of hard maple. The backup block were 3/4 plywood. In addition, when I formed a rib I had the form block and the backup block clamped down to a bench or clamped in a vice for the small ribs. Just looking at your rib, It does not look like you had the form block and backup block clamped tight enough. I did not use MFD for form blocks because I don't think it is dense enough to make a good form block.

    To make my from block, I had the velum drawing printed full size and I glued that drawing to the hard maple board. The board was 1" thick. Having the wing master drawing on the form block was really handy. The edge of the master form block is to the left in the picture.

    The thing iI did not like about using a Bob stick was that I found it nearly impossible to end up with the rib as flat as I wanted. So I made upset dies out 1/8 inch aluminum. In cross section the flange is a step 1/8 high. The advantage of this system is that the process of pressing the flanges for the holes flattens the rib. In the picture the bottom block is the pressing block. It is MFD and is cut to fit inside the rib. There are reliefs in the edges for the flutes. The second piece up is the female form, then the male forms and at the top is the finished rib.

    The rib is cut in a single step using a router and a MFD form for the rib. I found that I could cut 3 ribs at a time with the process. The next step was to flange the rib and the final step was to press the flanges for the lightening holes. The most time consuming process is forming the rib flanges and fluting them to get the rib flat.

    The aileron rib was the toughest to get flat. But from what I see in your picture is an issue with the form blocks when you are flanging the ribs.


    • #3
      For what its worth, here are my thoughts on these issues:

      I used only 3/4 inch MDF for all my form blocks. The forms were each treated with PC-Petrifier Wood Hardener which made them bullet-proof! I had no problem with reliabily the 3/4" MDF used through the entire process.

      The paired sets were used to router cut all the blanks. Before trimming and modifying them to be used as bending forms for the outer flange, I used a 45-degree router bit to cut the lightening hole flange recess; one LEFT and one RIGHT (inboard vs outboard orientation). I then used the rubber press method to bend the lightening hole flanges. This method worked fantastic!!! Quick, easy, and no time spent making male and female dies. The press was $140 from Harbor Freight and I got a stack of neoprene pads from Ebay.

      After the lightening holes were flanged, I then trimmed the paired forms down to the actual rib profile form using the original master and the router table. The edges were all radiused for both the lightening holes and outer flange edge (first with the use of files and sand paper, then found a router bit that worked great!). The exposed edges were again treated with a couple of additional coats of the wood hardener. I also routered out MDF backing boards 1/8-inch less than the bending forms and match drilled the jig pin holes.

      I first started the outer flanging by clamping the sandwich to the edge of my workbench and using dead-blow hammers. It needs to be very thoroughly clamped down very tight!!!! I used 2 hammers; placing one on the edge of the aluminum, uptight against the backing board edge, then hitting that hammer with the other one. Its extremely important to set the radius for the bend!

      After doing a few ribs with the hammers, I decided to try the flow forming method with an air hammer or rivet gun (see John Snapps' N3UW YouTube video). When I priced flow forming tools I thought the cost was too high, so I bought an air hammer and steel hammer bit from Harbor Freight, then went to Home Depot and got a box of rubber chair leg feet that fit tight over the hammer (ridges in the bottom of the feet were sanded flat). I also decided to switch from clamping on the table top to using a vise, in my case I used a Rockwell Jawhorse. I was very happy with my DIY flow forming tool and it worked great! Unique to the nose ribs, because of the aggressive curvature, I added flutting valleys to the form block, and used fluting pliers early on in the bending to start the flutes and ensure perpendicularity before the potato chipping started. Once the flange was bent enough, the flutes that I started laid down in the valley, where I used a rod and hammer to set them deeper. Be Careful when pounding the fluting rod into the valley that you down intersect the bend radius of the flange.

      Once out of the form blocks, I did not try hammering the flanges to bring them the rest of the way to 90-degrees. For the smaller ribs I used a pair of pliers and/or a hand seamer (protected with tape). When using the hand seamer, particularly near the trailing end of the flap and aileron ribs, you need to clamp it down tight and push hard against the table while bending it over past 90. For the larger ribs I built and used the Vans/RV style over-bending device (does this thing have an actual name???).

      The big ribs had to go back to the bench-top clamp method using all the form sections pieced together. For these, clamping alone was not enough, so I climbed on top of the bench and stood on the sandwich while hammering!

      I'm not sure if this will work, but here is a Google Photos link with a number pictures of these processes:
      Bearhawk Bravo #1508B - Scratch Build (wings)
      Northern Idaho


      • #4
        Thanks all. Sorry for the delayed reply, was out of the country all week with little opportunity for "fun time" on the web.

        It seems like lots of folks use different techniques but get some pretty fine results, so I'm going to head scratch on this a bit For reference, I currently have 3/4" MDF formblocks, and I put two coats of wood hardener along the edges. I used 1/4" tubing for drill bushings in all of the tooling holes. I keep the blocks (and associated router templates) together with 10-24 machine screws (3/16") that fit just inside the drill bushings, along with some big washers and a 10-24 nut. I've seen some folks use 3/16" bolts; perhaps that gives a little more torque capability to hold things together, but I can get it all pretty tight and get it to stay that way during hammering.

        I radiused the master formblock to a little more than 1/16" - I fashioned a sanding block with a 1/8" hole that I used with 80- and 120-grit paper to set my radius. I checked it with a gauge I made from aluminum with a 90-degree cutout radiused to 1/16" and see that the radius is generous, but not excessive. I'd rather have slightly too much bend radius than not enough, since these will be used on .032 as well as .025 ribs. That said, I DON'T want the amount of radius shown in my initial pictures.

        I may try one more batch of ribs from the blanks I already have cut with these formblocks. I'll add a few more clamps to the table - I currently set the formblock in my bench vice while hammering. I'm using a deadblow mallet and try to set the radius first before bringing it in. I suspect that much of the bow I see now is more from the hammering when the rib is out of the formblock, and I'm trying to get that last 10 degrees or so of bend due to the springback. Maybe I'll get more aggressive with the hand seamer for these. I've also only tried the smaller ribs... perhaps I can try this with a center rib and see if I get better results. The trailing edge is a little harder to clamp down tight.

        Thanks again, and next time I'll remember to post in the right spot.

        4-Place Model 'B' Serial 1529B (with many years to go...)


        • #5

          What I saw in you rib is that as you were forming the rib, the backup block was not tight enough against the form block. The radius of the flange to the rib is way larger than 1/16 inch. The end of the rib looked fine. I don't think that small bolts will give you close enough rigidity to form the ribs properly.

          I have a large vise (cheap one) that I clamped my short form blocks in as I formed them, For the center ribs, I clamped the form blocks to a work bench that is built on top of a 2x6 frame and no over hang at the edges. So when the ribs are clamped against the bench, I have a very stiff structure backing the form block. I used 3/16 dowel pins to align the form blocks and the rib blanks.

          I glued a print of Drawing #7 (The Wing Rib From Block Layout) to a piece of 1" thick hard maple. Having the drawing as part of the from block was really handy.


          • nborer
            nborer commented
            Editing a comment
            Totally agreed that the radius at the end of the rib wasn't from the radius in the formblock, but from not enough clamping force.

            I transferred all of the notations from drawing #4 (that's what the Mylar sheet is in the model "B" plans) to the master formblock. I glued the sheet on when I made the formblock, including using a nail to locate the jig pin holes and help locate the spars, and then peeled the sheet off. I then duplicated the markings from the drawing onto the formblock.

        • #6
          I use #10 bolts and fender washers to hold the blocks together--- BUT--- the main clamping force is a wood workers vice with 3/4 oak jaws-- AND 2 or 3 of the heaviest woodworking
          bar clamps I can find. The # 10 bolts just keep the two halves aligned. They contribute little to clamping.
          When you apply the liquid wood hardener--- use LOTS of it. Use a LOT around the edge and inboard about 2 inches. A little (one heavy coat) out in the center. I used all most a whole
          can on my nose rib form. Its like phenolic now ! I couldn't aster the bob stick either. It must use a side of the brain I don't have ! :-)
          I have been 100% pleased with the hardened MDF form blocks and 3 inches of rubber push pad. followed by the RV cornering tool -- and then fluting.