Haas CNC machine tool and fixture setting and management (6)
Hint: The length of the drill tip depends on the blade angle and the drill diameter. The length of the drill tip is obtained by multiplying the drill diameter by a constant; the value of the constant depends on the drill tip angle (the tip angle of most standard high speed steel drill bits is 118 degrees). Previous Next
For the drill tip angle: 118 degrees 135 degrees 141 degrees drill diameter multiplied by: 0.3 0.207 0.177
Use these constants to calculate the length of the drill tip with an error of only a few thousandths of an inch.
The center drill center drill is a small drill with a pilot point. It is used to machine small diameter holes with a taper wall. 
If the hole position must be kept to a small tolerance, first use the center drill and then use a twist to drill the hole. The center drilled tapered wall surface keeps the twist drill aligned as it begins to drill into the workpiece.
Tip: Many machine tools use this empirical method: if the diameter tolerance of the center hole is not important, the drilling depth should be increased as much as possible. Below 0.375 inches in diameter, the hole diameter using a standard 60 degree center hole will approach the drilling depth. For larger center drills – 0.375 inches or larger – the depth to diameter ratio is greater, so the deviation may range from 0.080 to 0.100 inches.
Reaming reamer is used to remove small amounts of material from the borehole. Reaming drills allow for extremely small aperture tolerances and extremely high surface quality. The hole should be drilled first, leaving a margin of 0.005 to 0.015 inches on the wall of the hole and then removed by a reamer. 
Tip: When reaming, the best size of the hole and the accuracy of the position are as follows: first drill, then boring, and finally reaming.
Hint: The margin for reaming depends on the aperture. In general:
For pores with a pore size less than 1/2", the pore diameter for pores larger than 1/2" is less than 0.0150"
Diameter allowance 0.030"
The type of workpiece material and the method of processing the hole will affect the machining allowance.
Tip: When using the G85 (in, out) fixed-circulation spigot, the highest precision and most uniform surface can be machined. Many people try to save time by using the G81 (drilling) canned cycle, which sends the knife in and exits quickly. The processing speed exceeds G85, but it usually produces spiral marks on the cylindrical surface of the hole. Although this trace is very slight and does not affect the size of the hole, some customers will refuse to accept it because of the appearance of the hole.