This operation actually consists of silver soldering and requires practice to become efficient at it. There are two generally accepted methods. One is to pre-tin the carbide tip by applying flux to it and laying it on a graphite block or fire brick. Heat it slowly until the silver solder can be applied and tinned to the surface. Then install it in the saw. The other method is to install the tip in the saw with flux and then heat and apply silver solder. Either method is satisfactory if done properly.
The best silver solder will contain 50% silver and will melt at approximately 1270° F. Solders with higher melting points are generally not satisfactory because they set up stresses in the carbide even in small sections like saw tips.
Flux is very important to a good braze joint, and should be of good quality and not mixed with water to the point that it has lost its effectiveness. Black fluxes are generally the best because they will withstand higher temperatures and they contain fluoride to help clean the work. The presence of flux also helps to surround the joint and prevent oxidation.
The first requirement of a good braze is a good clean recess to put the tip in. Remove all burrs and sand blast, if necessary. The second is to apply flux thoroughly to the recess and the tip. The third is to apply heat at the base of the recess with the torch pointing outward away from the center of the plate. Do not heat too rapidly, and use a torch tip of the size you are capable of handling.
If the carbide tip is not pretinned, add a small amount of silver solder by carefully melting the very end of the silver rod. Now use a small 3-corner file or other small steel rod and push the tip into the recess when the solder melts. Do not overheat. You can seat the tip better by pulling it out and pushing it back into the recess. This allows excess silver solder to escape and also removes any excess flux from the joint. Be sure that the tip and recess are well wetted with braze material.
One major thing, too often overlooked, is the safety factor that should be used when brazing. There are two very dangerous elements involved; fluoride and cadmium. Both of these can injure your lungs or kill you if you do not take precautions to vent their fumes. Do not use silver solder without a fan or other means of ventilation to keep you from inhaling these very toxic fumes.
Another thing to recognize is that the steel plate you are working on has been heat treated to make it capable of doing the job it is intended to do. When you heat the steel behind the tip you draw the temper out of it. Sometimes if you heat too rapidly you create a chill line between the steel you heat and the body of the plate. This causes the whole tooth to break off. Go a little slower and allow the heat to dissipate more gradually and you should not cause these chill line conditions.
A difference in expansion and contraction in the metals involved is of great importance and deserves consideration and understanding. Steel has an expansion rate of .004 thousandths to the inch. Silver solder has an expansion rate of .012 thousandths to the inch. It is very apparent from these two values that expansion has a great deal to do with a good braze joint. With each metal working against the other it is very necessary to do the best job possible and establish a good sound procedure to stick to.
Following is a good sound procedure that works.
1. Set your oxygen and acetylene gauges to a value of 5 pounds. No more than this is required and the flame will be smoother with less blow.
2. Light the torch by opening the acetylene torch valve. Open this valve until the flame separates from the tip. Now back off the valve until a small amount of black smoke appears at the end of the flame. Now turn on the oxygen valve and adjust the flame until there is no feather appearing on the cone of the flame. You now have a neutral flame and the hottest flame that should be used with this size tip. Only two sizes of tips are necessary for brazing saws; number 0 and number 1. Use the # O tip on saws thinner than .095, and the #1 on thicker saws. Never try to use a tip too big for you to handle and don't rush the brazing job. Practice will allow you to speed up the operation.
3. Be sure your tips are clean and not oxidized. Tips treated for brazing are the best. Next, the recess should be clean and free from burrs.
4. Flux all surfaces liberally with a good grade of flux, preferably black flux.
5. Position the tip in the recess and be sure it is located at the desired distance from the sides of the plate to clean up to the dimension required.
6. Apply the heat and silver solder, and wipe the tip into place with a positioning rod.
7. Allow to cool and inspect to be sure that the silver solder is uniformly applied.
If you follow good procedure and are careful you should get good results. Again, be sure that you silver solder in a well ventilated area. Why take chances?
The lower left pieces of carbide have not been tinned. The second pieces have been properly fluxed and tinned. The flux was puddled to form a complete crystal over the surface and the silver solder was applied and it flowed all over the tips under the protective crystal coating of flux.The two images on the right show tips that have been fluxed and heated without spreading out the flux. The lower right-hand piece has been tinned, but had not been properly fluxed first, so you will note that the silver will not flow all over the surface.
|Untinned||Properly Pre-Tinned||Not Enough Flux||Poor Surface Preparation|
These are not the original pictures as the originals were unusable.