Metric machine screws and nuts are used in conjunction with one another in order to fasten metal parts together securely. To ensure proper fit and quality of life, make sure to use the Hillman Metric Flat-head Phillips Machine Screw with nuts of similar thread size and finish. Fasten the flat head machine screws with a screwdriver to reduce risk of over-tightening. The length of a machine screw is measured from under the head to the point of the screw.