Difficult time brazing drill bit to ultrasonic horn

[K_Mitchell]

I have an ultrasonic drill, and need to attach a stainless steel drill bit (hollow tube) to a hardened steel horn. For anyone unfamiliar with ultrasonic drilling, please take a look at the following illustration with instructions for mounting the drill bit:

http://www.free-form.ch/tools/manual-sonic-en.pdf

I received silver solder from a supplier, who also gave me the corresponding flux and recommended a propane torch to join the drill bit to the horn. The first time I brazed anything in my life was yesterday, and I struggled for 3 hours to get the bit mounted. The solder would sometimes melt the first time, but it was nearly impossible to melt a second time no matter how close the flame was. I succeeded twice, but the working time was essentially 0 seconds. I managed to drill a few holes in a 0.5 cm glass plate, but the drill bit snapped off after awhile most likely because the solder was very lumpy and uneven. I do not know the melting point of this solder, but I expect it to be in the 400-500C range. If propane burns at 2500C, why is the solder so difficult to melt?

Next, I bought two different types of silver solder and flux from Home Depot. Both of these melted with ease, and had a working time > 10 seconds. One type was terrible, and the drill bit could be removed by hand after cooling. The second type held the drill bit in place, but the vibration from the drill shot the bit out the end every time I mounted it. Do you think I should try MAPP gas, which burns at 2950C? Will this deform my hardened steel horn?

 

[Spinnor]

I think you you need more heat. Do you live near a radiator repair shop? For a small fee they might show you how to do the job with your tools. The guy in the video above is using an acetylene torch, much hotter then what you are using. Good luck!

 

[K_Mitchell]

Thanks for the help. I watched that video, then watched a few more in series. It seems the technique I needed was to heat up the base of the horn for awhile first, otherwise it may act as a heat sink cooling the solder on top too quickly. Once the horn is a dull red, I move up a little more towards the solder, hitting it with the inner cone of the flame. This melted it for about 2 seconds, which was just enough for me to set the drill bit. I'm drilling holes like a pro now!