MTFCA Forum

Certain high carbon steels can do that, even if they're not exactly heat treated. The heat & work of drilling can cause the steel to harden. You'll most likely have to choose another spot to drill, since the spot you tried has now been work hardened. A very sharp drill will help. Maybe try a carbide tipped drill.

And such is the reason I balance flywheels with the triple-gears in place.

Consider a carbide drill bit

Jerry VanOoteghem wrote: ↑

Tue Feb 19, 2019 9:05 pm

Certain high carbon steels can do that, even if they're not exactly heat treated. The heat & work of drilling can cause the steel to harden. You'll most likely have to choose another spot to drill, since the spot you tried has now been work hardened. A very sharp drill will help. Maybe try a carbide tipped drill.

Followed your suggestion and got them all balanced.

If you can drill into a triple gear with a regular drill bit, you should be suspicious that it isn’t properly heat treated and maybe use a different gear. The hardness of the triple gears is such that you should have to use a solid carbide drill, endmill, lathe bit, etc to remove material from them. They are not hard all the way thru, but are hard to a certain depth. I use a 3/8” solid carbide end mill to “drill” into them for balancing. Generally when I get maybe about .040” to .060” deep then they cut easier. That generally seems to be the depth of the heat treating.

Maybe you are running your drill too fast you didn't say that you slowed it down. The gears would be cut before hardening then the gear is heated red and quenched with oil to harden and it may not be consistently hard thought. Some gears may chip, wear or break they are not made to an exact science just general. Try advancing the drill size in smaller increments.

The riveted 3 piece gears are more problematical to balance. Not all pieces are perfectly centred before the gears are cut. When the three components are put together there can be quite a variation in their weight. Off centre pieces can be a bonus when balancing the gears, as the off-centre part is the bit removed when machining them. Balancing these unbalanced gears on a flywheel assembly is less than satisfactory. The orbit in which they rotate and their own rotation on the pins will greatly affect the total balance. This is a problem of much lesser magnitude when using the totally machined one piece gears.

Hope this helps.
Allan from down under.