Greg Parker from Ann Arbor, Michigan asked:
What percent silver was your wire? What brand and part number? How did you get your miters good enough to use silver? Without any lugs, the gaps would be even more critical. How did you build a fillet with high-silver-content wire? That sounds a bit fishy to me, unless the silver content was actually quite low.... Professional framebuilders, what say you?
That would be me, Greg, and I¹ve been following these various topics regarding KOF fillet brazing frames with some interest and at times amusement. Let me first shed some light on silver brazing a fillet. It is possible to do this but it¹s not likely when using the latest tubing like Reynolds 853. This is because they have designed it to be tig welded and have combined the alloy¹s so that when it is heated over 1600 degrees, it gets stronger. Silvers¹ melting point is 1200 or so, so it¹s greatly adde d cost wouldn¹t take advantage of the tubes properties. The most common silver used by framebuilders with lugs is 56% cadmium free (I use 45% with cadmium because it flows better but I will die younger for it). The temperature range with either of them between the liquid and solid state is very narrow meaning that it doesn¹t have good ³build up² qualities - as soon as it gets liquid to melt, it will also start to run away. Keeping th e filler in between the liquid and solid state is how we control the build up of the filler so enough of it stays in the right place to create the shape we want. Now 35% silver has a broader temperature range between the liquidus and solidus states (I don¹t know why the brazing industry makes those words sound Latin instead of using plain English. Perhaps it¹s to market it¹s users as seeming to be intelligent.) so it will build up more easily. This silver can be used to fillet braze more easily than the typical ones we use. Of course it¹s broader range also means it gets to a higher temperature at the top end. The most likely candidates for fillet silver brazing was older thinner walled tubing like Columbus KL which wasn¹ t very commonly used. It¹s thicker walled cousins SL and SP could withstand the higher temperatures of brass better so the trouble and expense of using silver wasn¹t as necessary.
Fillet brazing is more common today than before among some KoFs because of the challenges presented by using a threadless stem instead of a quill stem . In my opinion, it looks better not to have lots of head tube above the top tube (which a threadless design requires). By putting a bit of slop (not compact style steep slop) to the top tube, it can place the top tube closer to the top of the head tube (keeping the length head tube the same) so it doesn¹t have that extended head tube look which I personally don¹t prefer. Remember this extra height is necessary for those of us that have more weight and less flexibility than we had in our youth and, as a result, need a higher handlebar height than the racing crowd this stuff was designed for . Keep in mind that another controlling variable is the need to keep the top of the top tube low enough so we can stand over it. There aren¹t many lugs available to accommodate this newer design variation. Also thrown into thi s mix is carbon forks which usually have 1.125² steerers that require a bigge r diameter head tube. These factors eliminate use of the older lugs. So an easy and common solution is the fillet braze. I also like to file my fillets. I can get close to an end look by brazing alone but think perfection can only be reached by filing. I also think it is easier to fillet braze than use lugs but of course I insist on filing even investment cast lugs a lot to get the effect I find most attractive.
Greg, I¹m not very far away from you in the southwestern part of Michigan. Come and see me if you want to learn how extremely accurate miters can be made. I¹ve got a super nice mitering device sitting on my Bridgeport mill. I¹ve also got a few nice old frame around that are mine or customers frames in the painting process. I like visitors.
Doug Fattic
Niles, Michigan