A set of parts for assembling a three-dimensional, 3D, printer for 3D printing a specimen therewithin, the set of parts comprising: - an enclosure (1100, 1200) for receiving a controller of the 3D printer, the controller receiving instructions, when the 3D printer assembled from the set of parts is in use, for changing a definite 3D location of a hot end in relation to a specimen and for selectively extruding material from the hot end to print the specimen; - a hot end holder component for holding the hot end at the definite 3D location within the 3D printer when the set of parts is assembled into the 3D printer; - a specimen holder component for receiving the specimen within the 3D printer, the specimen holder component comprising a y-axis rack and pinion mechanism (1522, 1320) for moving a sliding plate component along a y-axis thereby determining a y-axis coordinate of the definite 3D location, at least a y-axis rack section of the y-axis rack and pinion mechanism (1522, 1320) being provided in a single material, the single material being used for the specimen holder component and the y-axis rack section; - an x-axis component comprising an x-axis rack and pinion mechanism (1310, 1320) for moving the hot end holder component along an x-axis along the enclosure (1100, 1200) over the specimen holder component thereby determining an x-axis coordinate of the definite 3D location, at least an x-axis rack section of the x-axis rack and pinion mechanism (1310, 1320) and structural elements (1330, 1305) of the x-axis component being provided in the single material; and - a z-axis component for moving the specimen holder component along a z-axis along the enclosure (1100, 1200) thereby determining a z-axis coordinate of the definite 3D location when the set of parts is assembled into the 3D printer, wherein the single material is used for structural elements of the z-axis component, thereby providing a single component comprising a z-axis rack and lateral sides of the specimen holder component. - an enclosure (1100, 1200) for receiving a controller of the 3D printer, the controller receiving instructions, when the 3D printer assembled from the set of parts is in use, for changing a definite 3D location of a hot end in relation to a specimen and for selectively extruding material from the hot end to print the specimen; - a hot end holder component for holding the hot end at the definite 3D location within the 3D printer when the set of parts is assembled into the 3D printer; - a specimen holder component for receiving the specimen within the 3D printer, the specimen holder component comprising a y-axis rack and pinion mechanism (1522, 1320) for moving a sliding plate component along a y-axis thereby determining a y-axis coordinate of the definite 3D location, at least a y-axis rack section of the y-axis rack and pinion mechanism (1522, 1320) being provided in a single material, the single material being used for the specimen holder component and the y-axis rack section; - an x-axis component comprising an x-axis rack and pinion mechanism (1310, 1320) for moving the hot end holder component along an x-axis along the enclosure (1100, 1200) over the specimen holder component thereby determining an x-axis coordinate of the definite 3D location, at least an x-axis rack section of the x-axis rack and pinion mechanism (1310, 1320) and structural elements (1330, 1305) of the x-axis component being provided in the single material; and - a z-axis component for moving the specimen holder component along a z-axis along the enclosure (1100, 1200) thereby determining a z-axis coordinate of the definite 3D location when the set of parts is assembled into the 3D printer, wherein the single material is used for structural elements of the z-axis component, thereby providing a single component comprising a z-axis rack and lateral sides of the specimen holder component.