The ornamental design for a handpiece for a dentaldevice, as shown.
FIG. 1 is a bottom view of the handpiece for a dental device showing my new design;
FIG. 2 is a right side view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is an upper-right side view thereof;
FIG. 5 is an upper-right-front perspective view thereof;
FIG. 6 is a rear end view thereof; and,
FIG. 7 is a front end view thereof.
The broken line showing of an accessory cord in FIGS. 1 through 6 is included for the purpose of illustrating an environmental structure and forms no part of the claimed design.