I claim the ornamental design for a dental x-ray machine, as shown and described.
FIG. 1 shows a left side perspective view of a dental x-ray machine of my new design;
FIG. 2 shows a front view thereof;
FIG. 3 shows a rear view thereof;
FIG. 4 shows a right side view thereof;
FIG. 5 shows a left side view thereof;
FIG. 6 shows a top view thereof; and,
FIG. 7 shows a bottom view thereof.
The broken lines in the drawings are included for the purpose of illustrating a portion of the dental x-ray machine that forms no part of the claimed design.