Drilling and Holemaking
Twist Drill Example
Introduction
Drilling Tool Selection
Hole Types
•A “through” hole goes completely through a workpiece.
•A “blind” hole is drilled only to a certain depth.
•“Interrupted” holes intersect with other holes within a workpiece.
Short Hole Drilling
Deep Hole Drilling
Twist Drill
A twist drill is a round, end-cutting tool with one or more cutting lips and one or more helical flutes.
•Twist drills are made of high speed steel or carbide for drilling hard and abrasive materials
•Some are “indexable-insert” drills- steel drills with one or more carbide cutting edges seated in them.
Twist Drill Geometry
Twist Drill Geometry
Drill Shank
The twist drill is held and driven by the shank. These shanks can be straight for use in collets or drill chucks, or tapered for mounting directly into machine spindles.
remove chuks with chuck wedges (video)
Flutes
Flutes on the drill bit are passageways that permit chips to exit the hole. They may be straight or helical. The helix angle of the drill’s flutes will vary according to the material being drilled.
Note: Parabolic flutes are often recommended for soft materials such as aluminum.
Cutting Lips
The cutting lips are the part of the drill bit that does the actual cutting.
Chisel Edge
The chisel edge is the point of the drill web. Does not cut as well as the cutting lips. Often, a pilot drill is necessary. Pilot diameter should not exceed the web diameter of the larger drill.
Drill Web
The web is the solid part of the drill bit along the axis of the tool.
Margin
The cylindrical portion of the land which is not cut away to provide clearance. Some drills have double or triple margins.
Point
The drill point is formed by the two cutting lips and does the work of metal cutting. The standard drill point angle is 118° with a clearance angle of between 10° and 20°.
Rake Angle
The rake angle of a typical twist drill changes from the innermost to outermost edge of the cutting lip.
Drill Sizes
Fractional
Number
Letter
Metric
ANSI/ASME B94.11M-1993
Twist Drills
This standard covers Nomenclature, Definitions, Sizes and Tolerances of High Speed Steel, Straight and Taper Shank Drills, Combined Drills and Countersinks, Plain Bell Type, In Both Inch and Metric Sizes.
Also...Drill rod in these sizes can be purchased economically
See Chart
Twist Drilling Cutting Action
Twist Drilling
Brittle Material (cast iron)
Center and Spot Drilling
Special Hole Geometry
Countersink
Counterbore
Counterdrill
Spotface
Chamfer
Tapered Holes
Other types of drill bits
Gun Drilling
•For long holes such as those found in gun bores, gun drills are used. The length of the hole requires that coolant be delivered through the shaft of the gun drill to the cutting front.
image: wikipedia.org
Gun Drilling
•The coolant also serves to eject chips from the cutting area and to move them back and out of the hole entrance.
image: wikipedia.org
Gun Drilling
Trepanning
This involves the making of a circular cut using a hollow core cutting tool. The result is a cut in the form of an outer ring while a central core of material drops through and leaves the full hole.
(core-stay drilling, annular cutters, hole saw, laser)
Trepanning Video
Friction Drilling (Flow Drilling)
Cutting Parameters
•Cutting speed
Measured at the periphery of the drill body and expressed in surface feet per minute
•Feed/penetration rate
The distance of travel into the stock per unit of time and expressed as inches or millimeters per minute. The harder the material, the slower the cutting speed and feed rate.
•It is easy to burn up a drill bit.
Hole Finishing - Reaming
http://upload.wikimedia.org/wikipedia/commons/8/83/Machined_Holes.svg
Reamers can be chucking reamers or hand reamers. Adjustable reamers are available.
Reaming Stock Removal
Roughing
Ranges from .003 at .0625 hole diameter to
.018 at 1.0 inch diameter
Finishing
Ranges from .002 at .0615 hole diameter to
.014 at 1.0 inch diameter
Hole Finishing - Boring
Boring
Boring is an internal turning operation that enlarges and finishes an existing hole. Boring uses a bar with a single-point cutting tool, or a tool with two or three edges. Boring can improve a hole's geometry and location, since the hole shape is primarily determined by machine tool motions.
Hole Finishing - Roller Burnishing
Hole Finishing - Roller Burnishing
Roller burnishing is a polishing operation which smooths irregularities on the hole wall and often produces a mirror-like finish.
Machines for Holemaking
Sensitive Drill Press
Radial Drill
Gang Drilling Machines
Self-Feeding Drilling Units
Multiple-Spindle Drilling Machines
Lathes
Mills
Specialty drilling machines
Holemaking and improving processes
http://upload.wikimedia.org/wikipedia/commons/8/83/Machined_Holes.svg
Precision Hole Making - Tarkka Video
Drilling Formulas