Saw filing 101: Grinding the wheels
By Trevor Shpeley
Nov. 21, 2016 - The name of the game in the saw trade is precision. Measurements are to the thousands of an inch. Tension, clearances and speeds are all carefully calculated. It makes sense therefore that the machinery upon which we mount those saws should be equally precise, thus the necessity for grinding bandmill wheels.
In previous columns I stressed the fact that filers in different mills use different techniques to achieve the same goals and that none of them are wrong if they work. This is also the case when it comes to wheel grinds. The one thing almost all filers have in common is that none of them actually want to do one. It’s a long, dirty, tedious job and it isn’t cheap in terms of manpower and equipment.
So why do we do it at all? Well, put simply, if you have worn-out wheels then no amount of fancy benching or tricks with tension are going to keep you from fighting cracks and radically un-level saws until you bring them back into good condition. For most wheels this means grinding once or twice a year depending on the workload of the bandmill.
The first step is to measure the wheels and find out where you are starting from. The traditional method is to use a cloth tape long enough to go all the way around the wheel. You start on an edge and carefully note the measurement on the tape and then do the same for the other edge and the location of the crown. I would call the first mark “0”, the others would be plus or minus whatever they were. Next use a straight edge with a light behind it to determine if there are any hollows.
Another method is to use a Pi tape. This measures the diameter of the wheel rather than the circumference and is far more precise. When you consider that the average crown is roughly the thickness of a piece of paper, you can see the advantage to exact measuring tools. Remember that the Pi tape measures diameter so you have to divide by two to find the size of the crown. To do the actual wheel grind, most filers use a Barnhart wheel grinder which allows you to make perfectly angled grinds in controlled amounts until the wheel reaches the desired shape. The Barnhart’s barrel is squared up to the saw with the help of magnetically mounted dial indicators until the grinder tracks a perfect 90 degrees to the direction of wheel travel and on an angle that results in the desired crown.
The “crown” of the wheel is the apex of the two angles leading to the front and rear edges of the saw. The amount of desirable crown is the subject of much controversy and runs from “no crown at all” to about .012 maximum. You also want the front edge of the wheel to be about .003 larger than the rear edge to make the saw track naturally forward.
When you are ready to go, run the grinding head back and forth starting at the rear until you are grinding evenly all the way from the back to almost the front edge. (You will find a sharp drop at the front that is best left until you grind back the other way.) Then you change the angle of the Barnhart barrel and grind back towards the centre. When the edge is completely clean and the sparking is stopping at your desired crown location, stop and measure the wheel. Never stop and reverse the grinding head right at the edge and don’t try to make a crown just by stopping the grind where you want the crown to be. Both of these practices almost guarantee a hollow in the face of your wheel. Always allow the grinding head to keep moving until the sparking has ceased before heading back the other way.
If everything is how you want it, then congratulations! You’re done. If not, reassess and keep going until everything is perfect. My old head filer used to say to us whenever we would try to get away with a less than precise job, “Good for you, you now have a shiny, worn-out wheel. Start over.”
Wheel grinding isn’t easy; it takes skill and practice to do right. That is perhaps why many head filers choose to use a professional wheel-grinding service. No muss, no fuss and as long as you are clear about your expectations. If, however, you want to have total control over how your saws run and wish your filers to be as well rounded as possible, there is a lot to be said for doing the job yourself. Either way, as long as you get it done regularly, your saws will thank you.
Trevor Shpeley is the head filer for Tolko’s Kelowna division and is currently the financial secretary for the BC Saw Filers Association.