• In core drilling, a drilling operation used to enlarge an existing hole of diameter D(hole), depth of cut is given by d = 1⁄2 (D(drill) – D(hole)) where D(drill) is the drill diameter, and D(hole) is the diameter of the hole being enlarged.
8. Drilling time • Drilling time (T) can be given by the equation; T = L / f N; Where f is the feed (mm/rev) N is the rotational speed (rpm) L is the sum of hole depth, approach and over travel distances. *The approach is usually considered as 0.4D while over travel ranges from 1 to 3mm.
9. Drilling operations
• Drilling is used to drill a round blind or through hole in a solid material. If the hole is larger than ~30 mm, its a good idea to drill a smaller pilot hole before core drilling the final one. For holes larger than ~50 mm, three-step drilling is recomended; • Core drilling is used to increase the diameter of an existing hole. • Step drilling is used to drill a stepped (multi- diameter) hole in a solid material.
• Counterboring provides a stepped hole again but with flat and perpendicular relative to hole axis face. The hole is used to seat internal hexagonal bolt heads. • Countersinking is similar to counterboring, except that the step is conical for flat head screws. • Reaming provides a better tolerance and surface finish to an initially drilled hole. Reaming slightly increases the hole diameter. The tool is called reamer;
• Center drilling is used to drill a starting hole to precisely define the location for subsequent drilling. The tool is called center drill . A center drill has a thick shaft and very short flutes. It is therefore very stiff and will not walk as the hole is getting started. • Gun drilling is a specific operation to drill holes with very large length-to-diameter ratio up to L/D ~300. There are several modifications of this operation but in all cases cutting fluid is delivered directly to the cutting zone internally through the drill to cool and lubricate the cutting edges, and to remove the chips
10. Reamers • The reamer used in reaming operation has similar geometry as for twist drill . • The difference in geometry between a reamer and a twist drill are: The reamer contains four to eight straight or helical flutes, respectively cutting edges. The tip is very short and does not contain any cutting edges.
11. Drill materials • Drills are normally made of high-speed steel (HSS) but carbide-tipped drills, and drills with mechanically attached carbide inserts are commonly used in many operations, especially on CNC drilling machines
12. Apparatus needed during drilling operation • Normally, holes produced by drilling are bigger than the drill diameter and depending on its applications; the drilled holes will subjected to other operations such as reaming or honing to better surface finish and dimensional accuracy • There are also several apparatus needed during the drilling operation as shown below: Drilling machine Center punch Hammer Center drill Twist drills Coolant Vernier caliper Two flute drill set: i. Center drill; ii. Countersink drill; iii. Counter bore drill; iv. Drill various diameter
Part II. Welding Processes
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