Band saw blade silver brazing, some rambling on
Have had fairly good success joining blades using silver brazing (high temperature silver solder without lead) on regular high carbon blades.
Bi-metal blades was another story and most blades would break with very little use. Frustrated started making changes and plan was to improve anything I could and then go backwards and see what the one big thing was that caused the failures.
First thing was making a grinding jig to make precise scarf joints. Believe the scarf surface is about 3 and half time the blade thickness. The jig fits on surface grinder but think a fixture could be made where a hand held angle grinder could do a good ‘nough job.
Was using a shear to cut the blades to length. Close inspection showed that the shear was distorting the blade so started cutting the blades using an abrasive cut off wheel. This worked good and there was no sign of the blade “cupping”.
Thought these two improvements should bring success. Cutting up an old ¾” wide blade and doing a bunch of test samples showed only some improvements and not much better then a fifty-fifty chance of a good joint.
There was two types of failures. One was improper annealing and breaking of the blade close to the joint. Sanding or polishing the joint to a bright color and slowly bringing the heat up with a hand held small propane torch to get an oxide color seemed to work good. This was done while still clamped in “soldering” fixture. Annealing without being clamped in place did cause some poor joints that failed.
The other was failure at the silver brazed joint. Studying the joint failures showed there was was too thick of silver brazing in the joint.
Tried the blacksmith method. Put sliver of silver braze in joint, fluxed and squeezed joint shut with hot tongs. This made a nice thin joint but there was lots of burnt flux which was hard to clean off and it required holding the tongs tight for a long time for the joint to cool enough. Letting pressure off of tongs before joint had completely frozen was a sure way to cause a weak joint and failure.
Continuing in another post due to 4 photo limit per post.