# Conservation of Angular Momentum in different cases

Conservation of Angular Momentum (AMC) can be applied when there is no external torque on the system which is isolated. However, my physics teacher gives me a sum that

Two discs are pressed against each other until they stop slipping on each other. Find angular speed of the bigger disc when they stop slipping. Disc 1(mass m, radius r) and Disc 2(mass m/2, radius 2r). Disc 1 had angular speed $\omega$ initially, whereas Disc 2 was at rest initially

I tried to apply AMC in this case since there is no external torque in this two disc system, but my teacher says that Angular Momentum is not conserved since there is an external torque acting on it. I fail to understand what is the external torque in this case. I don't need the answer to this question, but the reason why AMC cannot be applied, and what external torque acts on this body