Combo (Slotted - Phillips) truss head screw dimensions according to ASME B18.6.3 standard.
The truss head has a low rounded top surface with a flat bearing surface, the diameter of which for a given screw size is larger than the diameter of the corresponding pan head.
Some important notes from the ASME B18.6.3:
- Thread dimensions for the No.0000, No.000 and No.00 sizes are also included in the standard.
- The general data about the head height, bearing surface, gaging of recess, slot, length, tolerance on length, length of thread, points, finish, workmanship and designation of machine screws are given in the standard.
Note 1: Dimensions in this calculator are in inches.
Note 2: Machine screws shall have a definite underhead fillet large enough to ensure that full fastener strength is achieved. The radius of the fillet under countersunk head screws shall be no greater than 40% of the basic screw diameter. The radius of the fillet under truss heads and number 6 sized pan heads shall be no greater than 25% of the basic screw diameter. The radius of the fillet under all other head styles shall be no greater than 15% of the basic screw diameter.
Note 3: Due to the underfill of material caused by the combination recess, the minimum slot depths are approximately 10% lower than slotted pan heads.
Penetration Gaging: Penetration gaging is a test to determine the suitability of recesses in the heads of screws. Penetrations that are too deep indicate the possibility of a thin section between head and shank of screw, a weakness that might result in twisting-off screw heads during tightening of the screws. Screws with shallow penetration might result in production problems such as reaming of recess or excessive wear on driver bits.
Wobble Gaging: Means to determine the compatibility of recesses in the heads of screws with mating screw drivers, and will indicate the point where deviations in the recess contours affect satisfactory driver engagement. Recesses that exhibit excessive wobble characteristics will result in poor screw driveability because of driver camout prior to have normal torque level, damage to recesses, and/or accelerated driver wear.