A method for joining a plastic workpiece to a further workpiece, comprising the following steps: (a) Providing a plastic workpiece having an abutment surface and a projection, wherein said projection projects away from the abutment surface and has a front surface, (b) Providing a further workpiece having a first surface, a second surface opposite to the first surface, and a through hole which connects the first surface to the second surface and which is defined by a circumferential hole wall, wherein in the hole wall an undercut is provided, (c) Arranging the plastic workpiece relative to the further workpiece, such that the abutment surface of the plastic workpiece abuts on the first surface of the further workpiece and the projection extends through the through hole, (d) Providing a friction tool which is configured for rotation about an axis of rotation, which has a base surface extending transversely to the axis of rotation, and which has a pin extending along the axis of rotation away from the base surface, wherein the pin at its end remote from the base surface has a friction surface which extends transversely to the axis of rotation, wherein the pin is formed as a frustum of a cone and has a conical lateral surface which connects the friction surface to the base surface, wherein the radius of the pin at the level of the friction surface is referred to as, r 1, wherein the radius of the pin at the level of the base surface is referred to as r 2, wherein the radius of the through hole is referred to as R, wherein the distance between the friction surface and the base surface is referred to as h, wherein the thickness of the further workpiece is referred to as t, wherein the following parameters a,b,c are defined as: a = r 1 R, b = r 2 R, and c = h t, wherein the friction tool and the further workpiece are formed such that a is between 0,35 and 0,5, preferably between 0,4 and 0,45, further preferably 0,425, such that b is between 0,5 and 0,75, preferably between 0,6 and 0,65, further preferably 0,625, and such that c is between 0,6 and 0,9, preferably between 0,7 and 0,8, further preferred 0,75, (e) Rotating the friction tool about the axis of rotation, (f) Linearly moving the friction tool in parallel to the axis of rotation towards the projection, so that the friction surface contacts the front surface of the projection, (g) While rotating, further linearly moving of the friction tool along the axis of rotation, so that the pin penetrates the material of the projection, plasticizes said material, and laterally displaces said material into the undercut, (h) Retracting the friction tool from the projection. (a) Providing a plastic workpiece having an abutment surface and a projection, wherein said projection projects away from the abutment surface and has a front surface, (b) Providing a further workpiece having a first surface, a second surface opposite to the first surface, and a through hole which connects the first surface to the second surface and which is defined by a circumferential hole wall, wherein in the hole wall an undercut is provided, (c) Arranging the plastic workpiece relative to the further workpiece, such that the abutment surface of the plastic workpiece abuts on the first surface of the further workpiece and the projection extends through the through hole, (d) Providing a friction tool which is configured for rotation about an axis of rotation, which has a base surface extending transversely to the axis of rotation, and which has a pin extending along the axis of rotation away from the base surface, wherein the pin at its end remote from the base surface has a friction surface which extends transversely to the axis of rotation, wherein the pin is formed as a frustum of a cone and has a conical lateral surface which connects the friction surface to the base surface, wherein the radius of the pin at the level of the friction surface is referred to as, r 1, wherein the radius of the pin at the level of the base surface is referred to as r 2, wherein the radius of the through hole is referred to as R, wherein the distance between the friction surface and the base surface is referred to as h, wherein the thickness of the further workpiece is referred to as t, wherein the following parameters a,b,c are defined as: a = r 1 R, b = r 2 R, and c = h t, wherein the friction tool and the further workpiece are formed such that a is between 0,35 and 0,5, preferably between 0,4 and 0,45, further preferably 0,425, such that b is between 0,5 and 0,75, preferably between 0,6 and 0,65, further preferably 0,625, and such that c is between 0,6 and 0,9, preferably between 0,7 and 0,8, further preferred 0,75, (e) Rotating the friction tool about the axis of rotation, (f) Linearly moving the friction tool in parallel to the axis of rotation towards the projection, so that the friction surface contacts the front surface of the projection, (g) While rotating, further linearly moving of the friction tool along the axis of rotation, so that the pin penetrates the material of the projection, plasticizes said material, and laterally displaces said material into the undercut, (h) Retracting the friction tool from the projection.