Prucha Reissues Prewar Specs One-Piece Flange

By Jaroslav Prucha, with help from Martin Mikulas with Donald Nitchie

WorkshopJI am enchanted, probably like all banjo aficionados in the bluegrass community, by the appeal of vintage instruments, and predominantly by those made in the Gibson Kalamazoo Factory in the prewar years. While visiting the legendary Jim Mills’s showroom during IBMA back in 2013, Jim showed me a prewar one-piece flange in excellent condition, which happened to be in his collection at the time. Jim pulled it out of the display case; I gave it a brief survey, and knew right away that purchasing it was a must, even though it was not inexpensive. ‘This will be the next banjo part we will produce!’ I remember thinking. I was standing there holding a beautiful, 80-year-old flange - made by the Doehler Die-Casting Company. That moment was probably the ultimate impetus for me, as I had been considering a possibility of onepiece flange production before; and not only because we couldn’t rely on the production of our previous flange supplier for many reasons. Of course, we had been considering the possibility of finding another available product in the market; we tested a couple of flanges from several producers, but we turned that idea down because of our high insistence on quality. So the toughest decision was still before me.

New batch of our raw Prucha flangesProduction of a banjo flange mold and the die-casting process itself require a huge investment. The mold costs tens of thousands dollars and you can’t count on the return of your investment for at least ten years! Finally, after returning home, we were trying to find corporate agreement (which means family agreement...) on weather we’d be able to handle such an investment. Either because of a desire to beat the challenge or an abrupt brain blackout that suppressed realistic thinking, I ended up deciding to go for the flange production. And then it started for real. Endless and patient work while measuring the original prewar flange and producing the mold. Many sleepless nights spent with the flange samples. Unending measuring, comparing, disassembling old prewar banjos, examining the tiniest details, and examining prints or imprints which can reveal secrets about manufacturing methods, casting processes, finishing or plating during the prewar years. Every piece of knowledge we would need later on during the nickel plating tests— meaning the method, direction of polishing, etc. Measuring and comparing my samples to flanges available on the market today. Tracing deviations and differences from the prewar pieces. Flanges, disassembled banjos lying everywhere, in my workshop, at home in our dining room, living room…God, I was in Paradise, but my family hated me!

Our new flange nickel and rawThe first step was making a rough blueprint of the flange and contacting the several companies capable off making the zinc alloy molds in the Czech Republic. I ended up with the time-tested and also the most expensive company that produces molds for the Czech automobile plant Skoda. We had to create exact 3D software models according to precise measures taken from the prewar flange. My goal was to create a new flange—an exact copy of the flange from the 30’s of the last century. It had to be the best prewar reissue flange on the market ever. A flange that would be always available for the customers who require periodic supply of constant top quality parts as well as our other banjo parts that we make.

I had to study available materials and samples which could be compared to the surviving blueprint from the prewar era (from 1928–29) because even the original flange or rather the mold underwent several changes. I was trying to take into account later findings about the fact that the first generation of prewar one-piece flanges tended to pull, break or warp. It led me to analysis of different metal compositions of the zinc alloy called zamak.

Old Gibson flange  with DDCC trade mark on my  blueprintFrom the tool machinery professional point of view, the flange had a very complicated shape that required elaborate consultancy on the shape and size of the sprues [passages through which a molten material is introduced into a mold], runners, overflow wells and venting holes, ejector pins and cooling channels. All these features, together with the injection unit, influence the final structure of the casting, which has to be compact. All casting flaws and porosity would be clearly visible after surface finishing and nickel or gold plating.

Due to recent advanced technologies I was able to watch a software simulation of the die casting process after the mold construction had been designed by the main engineer. During the simulation, critical profiles causing difficult flow of injected alloy can be discovered, which may cause faster cooling process and imperfections of the final cast. This way it is much easier to design shape and size of sprues, gating cuts, size of overflow wells and venting holes.

The next logical step was to finalize the mold design, approve required production measurement tolerance and finally launch the mold production. Believe me, one of the toughest tasks was to persuade the management of the company I entrusted myself to that not only automobile industry demands measuring within a hundredth or a thousandth of a millimeter accuracy (or 0.001–0.0001 inches). And that even banjo players or builders (“pluckers” as the engineers called them) insist on measuring accurately, and one tenth or a hundredth of a millimeter is a huge tolerance for them. I guess that not everyone knows that the mold itself is a huge 24” by 24” by 12” block of steel weighing over 1200 pounds. In April 2014, after six months of the project, we were ready to carry out our first test pouring. Despite the competence and experience of the construction team, we discovered the mold had to be redone. They failed to calculate correctly the tolerance for contraction, so the final cast was smaller by one millimeter in diameter. After the next test pour, which took place in July, it was clear we were on a right path. A few other tests followed in order to fix subtle corrections and mold adjustments, as well as to make the pouring process perfect. Finally, we had the cast without flaws and defects.

At the beginning of September, almost a year from the project kick-off, we were able to pour the 200 piece batch of the New Prucha Flange. In the meantime we had to make tools for cutting off the sprues that remain on the cast after removal from the mold, cleanup cut-ins of the oval holes and neck cut-away on the flange. We had to agree upon and test the proper technology of how to turn outer and inner diameter on a CNC lathe, for which I had to design a special clamping unit that also allowed me to make sure that after machining, the flange holes for hooks would run evenly with both inner and outer diameters. Perfect fitting to a banjo rim is a key operation that allows a flange to function properly. When it is fitted to the rim loosely it may cause deformation or warping. One of the biggest challenges was the polishing and perfect nickel plating processes. Everything had to be worked out in detail and elaborately finalized; we wanted our flange to be absolutely the best in the current market. We wanted it to look as close as we could get to the old one. We’re proud that the entire project was driven by our study of old technologies and working processes, gathering all available information and respecting the tradition that you can touch and feel any time you hold the old prewar flange in your hands.

Prucha Flange

Mold production is very difficult and patient machinery work—it is actually my original profession. I believe that combination of my professional machinery skills and deep knowledge of banjo construction is the reason we are offering the top quality onepiece banjo flange today. I am amazed by the skills and dexterity of the craftsmen and machinists in the old days who couldn’t take advantage of today’s technologies and I am still astonished by the perfection of their work.

We presented our New Prucha One-Piece Flange during 2014 IBMA in Raleigh, NC, and after initial positive reviews we expect extensive interest. And I can’t wait to face another challenge!


Prucha Flange