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Radius Hamerforms- making accurate circular items by hand

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  • Radius Hamerforms- making accurate circular items by hand

    The specific job/project is of less significance imho than the method used. i have never seen any reference to or working example of a radius hammerform. Maybe it's my invention- who knows?

    A Radius Hammerform is a simple and very low cost concept that works extremely well to produce any item with a consistent radius, up to a full circle. Limited equipment is needed to make the radius hammerforms, which could likely be made with a single board or different arrangement of boards. With more practice than I have or a more delicate/accurate working method, very nice items can be made. No powered planishing or abrasive methods were used. Work more conscientiously and you should be very happy with the results. My pending new hammerforms for this project will have tighter wood joints and better finished metal elements without exposed screw heads. Both of these flaws can produce witness marks on the job. Not cool.

    This is a repost of a cowl plenum breather assembly project. It is an ongoing project. I plan to make new versions of these same radius hammerforms to hopefully improve the quality of the results. I have since added a joiner to my wood shop, so can more accurately square lumber before planing it for use as hammerform foundations.

    during the project, I also learned how to more tightly shape metal against a hammerform by turning a flange as the work is driven down over the hammerform. This is important for us novices to know. improvement in results is relatively to achieve by using this simple method. If you're having trouble turning metal over a hammerform while keeping it tightly shaped to the hammerform, try it as described below- it worked well for me.

    When trying to copy & paste, I received a prompt that the character limit for any single post here is 10000 characters, so I'll split it up into a few posts.
    for now, here it is-

  • #2
    This is my first real Metalshaping project, executed circa 2017- 2018 after years of ambitious thoughts.... The thread about the buck is posted under Buck Building entitled Holman Moody Cowl Plenum

    This is a pic of one of the original versions. There were several variations, but this is the general part-

    Some mistakes were made... including one big one. Thankfully none were more fatal than instructive so will be posted to help others avoid the same problems. major goals with the project were
    1. making accurate parts that would fit together
    2. be properly circular and concentric
    3. be of proper assembled height to receive the filter element

    As a sculptor, making accurate circle and/or properly concentric circles can be very challenging because they are instantly perceived by any viewer as success or fail. Since all parts were based upon circular form, I decided to try making radius hammerforms.

    Using scrap hardwood crating lumber planed to dimension and similar scrap steel, a hammerform for the lid was made before the buck was built to see how well it would work. A small hanger bolt with wing nut & washer was used for the center pivot.

    A simple pointer was made to help align the steel shoe so the circle would be true.

    I measured several 1960's OEM Ford air filter housings I have and found them all to be 24g crs. When I asked nearby NASCAR Legend, mentor and friend RockHillWill about sourcing some 24g crs for the project, he generously donated a piece he had. This means a lot to me, since it gives some real connection to racing for the project. We're not sure whether this crs is AKDQ or not, but given the pounding it tolerated, it may be.

    So- A quick run over to Rock Hill & back, some cutting and I was making dents.

    I used a small pad of the same metal to cover the vise grips where it touches the work. I have to protect polished stone while working and it's generally considered same for like to touch like without risking damage. Whether or not it's necessary here, there were no scratches during the project.

    With no prestretching, it got a little out of shape so some hammer and stump work plus a little wheeling and all was back to level

    Then I started on the beading around the edge. This was done without a bead roller. Not too difficult but I expect the next one to be better.

    Once the radius hammerform had been proven viable, I made the actual buck (previous thread) and then made two more radius hammerforms. There were three in total- one for the lid, one for the top half and one for the bottom half. Once the wood was dimensioned by saw & planer, I laminated (screws) pieces side by side.

    Then I used a router with trammel, pivoted by the hanger bolt, to develop the necessary profiles into the wood for both open space and proper register for the metal shoes.

    To develop the metal shoe portions of the hammerforms which would take the greatest abuse, I cut and welded various pieces of mostly 1/8" and 1/4" scrap steel.

    Since I have no metal lathe or mill, I broke out the red makita drill with grinder stone and an eye bolt tensioner from an old screen door. Pretty sure this method is from the same school as MP&C Robert McCartney's vise-mounted jigsaw.... After roughing in the built-up welds by hand, using that hanger bolt as the pivot helped me get a reasonably consistent form on my built-up welded metal for the most dramatic profiles.

    In the end, there were three radius hammerforms that worked very well.

    Next came the major mistakes portion of the project.....

    I started on the top half of two parts that are held together by a folded seam. Clamp the blank on the hammerform and start swinging. Trying to think through shrinking the folded edge to the buck and then having to stretch it back into a flange, I jumped into first working the metal over the edge.

    Following Peter's advice, I used a polished flipper and all was going well. To help with puckers and shrinking, I thought about the HVAC crimpers on the shelf. It kinda worked but I learned those lines NEVER come out. Mistake #1.

    The bigger issue was that I could not get the part to tightly conform to the buck. It flared away from the hammerform and buck. The metal was so abused I didn't think more tucks & shrinks could be done without cracking.

    These parts made it to the Redneck Roundup and led to some head scratching about how to do it better. I got another piece of known plain 24g crs as a remedy but never cut it. In the meantime, I took this apparently junk top half and tried to see how much stretching it would take as an experiment for the bottom half. It stretched a lot without failing, approx 2 1/2" deep. Of course I stretched it in the easiest way I could reach it. It was junk anyways...(actually not- more on that later)

    Very cool. I had thoughts of making a separate 16" open element filter top and bottom as souvenir of my failed first attempt, somewhat matching the old Ford 14" breather on the wall.


    • #3
      Between the 2018 Redneck Roundup and Labor Day weekend, I bought a set of the first 5 books by Timothy Barton. In Book #5, hammerforms are discussed.

      In order to keep a piece of metal tight against a form when it needs to end with a flange, it is recommended to turn the outer edge of metal in the opposite direction. In simple terms, it will look like a gutter or channel. Then a corking tool can be used (I used a hardwood tool) to form the metal tightly against the hammerform. I believe this allows the metal to be stretched to the form instead of being shrunken to it, which is what I thought needed to happen but didn't know how to accomplish.

      Needless to say, it worked great. When I started the next blank for the top, it was finished almost as quickly as it started and turned out pretty good. As a result, there's not as many pics as before.

      It went so fast that I thought I might have time to do the bottom half. Sitting there not wanting to cut new metal and looking at the failed top half from earlier, I realized that if I flipped the failed top half over, it would already be bent in that upturned channel around the perimeter. It was already scrap, so I gave it a shot. I'm glad I did. Here's the failed top half-

      This is the same part, after about an hour of work-

      For a completely flogged piece of metal, it turned out pretty well. I literally flipped it inside out and only had two small cracks on the edge. Very fast to accomplish, too. The surface would probably be smoother if I had any idea this experiment was going to work out so well. I really didn't think it was going to be any kind of success.

      I hit these with the gas torch and a scrap of trimmed metal for filler and that seemed to work well.

      Now that the top and bottom were getting close, I spent some time trying to smooth things. A couple of dollies (including one nice comma dolly I got from Dan Shady at the RR because I thought it looked exactly right for this project) and a polished flipper/slapper I made from an old file and surfaces are much better.

      These will be painted, not polished. No abrasives yet and they're not perfect, but are getting better.

      While I still had a long but uneven edge on the top half, I needed to turn a flange on the top part (per original) of consistent width. I made a tipper with a stand-off length on one side equal to the desired flange width. Another experiment, so I used a piece of scrap hardwood. It worked good enough that I would make others in more durable metal.

      After the top flange was turned, the bottom was trimmed and trial fitting proceeded until things were acceptable.

      I had an ongoing concern about how flat the pieces were & weren't, especially the top. they were close and I thought/hoped things might relax once I cut out the hole in the top half. Patience was the right approach. When I cut out the top and turned the flange, it settled down a lot.

      Assembled, most of the fit is pretty good at present. The only problem is the filter protrudes above the top near the front of the assembly, up to 1/4" max. The bottom ring where the filter sits is not adequately level, so I'm going to stretch it a little deeper before I finish joining the top & bottom halves.

      Some of the originals had spot welds to supplement the crimped flange. I'll add those after the fit is resolved and more is planished smooth. There will be more posted as it wraps up, but this is it for today.

      update- I have since learned that the arrangement of the flange & fold-over crimped seam was the opposite of what I produced. I suspected it was but only confirmed this after I made mine. All the more reason to make new pieces and add these to the eye candy (or scrap) collection.

      Hammer and learn.....


      • #4
        I like your idea of the turnbuckle and the drill. I can likely use something very similar to grind the brake shoes for the proper arc on A models. There was a machine made for this, a brake Doktor, but they are scarce and expensive. Nice progress on your cowl induction!


        • #5
          Good stuff ,... the next one is always going to be better , and the one after that better still . You are getting the idea well done!
          Peter T.