An assembly for a drug delivery device, the assembly comprising: a housing unit comprising a housing for receiving a drug reservoir, and a cap, wherein the cap is releasably securable with respect to the housing, wherein the assembly has at least two different states, i.e. a capped state, where the cap is secured with respect to the housing and an uncapped state, where the cap is released from the housing, e.g. disconnected from the housing, wherein the cap comprises an outer member which is arranged to be gripped by a user for releasing, e.g. disconnecting, the cap from the housing, wherein the outer member is rotatable relative to the housing to release the cap from the housing, wherein the assembly comprises a mechanical interface which is configured to convert rotational movement of the outer member relative to the housing into axial movement of the outer member relative to the housing when switching from the capped state to the uncapped state, the mechanical interface comprising or being formed by at least one guide track which is configured to guide the movement of the outer member relative to the housing when the assembly is switched from the capped state to the uncapped state, wherein the mechanical interface comprises at least one interaction feature, wherein the interaction feature is in mechanical cooperation with the guide track, characterized in that the guide track has at least a first sloped region and a second sloped region, wherein, when the assembly is switched from the capped state to the uncapped state, the first sloped region guides the movement of the outer member before the second sloped region guides the movement of the outer member, wherein the first sloped region and the second sloped region are adjusted to one another such that, when the first sloped region guides the movement, and the outer member is rotated by an angle A the outer member is displaced axially away from the housing by a first distance, wherein, when the second sloped region guides the movement, and the outer member is rotated by the angle A the outer member is displaced axially away from the housing by a second distance, wherein the second distance is smaller than the first distance. a housing unit comprising a housing for receiving a drug reservoir, and a cap, wherein the cap is releasably securable with respect to the housing, wherein the assembly has at least two different states, i.e. a capped state, where the cap is secured with respect to the housing and an uncapped state, where the cap is released from the housing, e.g. disconnected from the housing, wherein the cap comprises an outer member which is arranged to be gripped by a user for releasing, e.g. disconnecting, the cap from the housing, wherein the outer member is rotatable relative to the housing to release the cap from the housing, wherein the assembly comprises a mechanical interface which is configured to convert rotational movement of the outer member relative to the housing into axial movement of the outer member relative to the housing when switching from the capped state to the uncapped state, the mechanical interface comprising or being formed by at least one guide track which is configured to guide the movement of the outer member relative to the housing when the assembly is switched from the capped state to the uncapped state, wherein the mechanical interface comprises at least one interaction feature, wherein the interaction feature is in mechanical cooperation with the guide track, characterized in that the guide track has at least a first sloped region and a second sloped region, wherein, when the assembly is switched from the capped state to the uncapped state, the first sloped region guides the movement of the outer member before the second sloped region guides the movement of the outer member, wherein the first sloped region and the second sloped region are adjusted to one another such that, when the first sloped region guides the movement, and the outer member is rotated by an angle A the outer member is displaced axially away from the housing by a first distance, wherein, when the second sloped region guides the movement, and the outer member is rotated by the angle A the outer member is displaced axially away from the housing by a second distance, wherein the second distance is smaller than the first distance.