Drilling and Holemaking

Lecture Video

Twist Drill Example

Introduction

• Drilling is a common process, almost 1/2 of all chips are from drilling
• Two ways of cutting: rotate drill or rotate workpiece
• Because of confined cutting, coolant is important
• Lubrication
• Cooling
• Chip removal

Drilling Tool Selection

• There are many types of drilling tools and methods.
• Selection is based on:
• Hole Diameter
• Hole Depth
• Tolerance Required
• Material Type and Conditions
• Number of holes needed

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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.

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Short Hole Drilling

• Ratio of depth to diameter
• Short holes are drilled in one motion
• Rule of thumb for holes that can be drilled in one motion: 3 or 4 diameters for drills up to one inch. 2.5 diameters for larger holes.

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Deep Hole Drilling

• Peck drilling - Repeated withdrawal of the drill to clear chips from flutes.
• Rule of thumb: 3X drill diameter for first peck, then 2X then 1X until through.
• Deep hole drilling is more difficult
• Hard to keep hole straight
• May require guides, special drills
• Through-tool coolant often helpful/required

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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.

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Twist Drill Geometry

ISO Drill Nomenclature

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.

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Note: Parabolic flutes are often recommended for soft materials such as aluminum.

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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

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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

Counterboring tools

Other types of drill bits

Piloted Drills

Spade Drills

Subland

Step Drills

Multistep

Gun Drills

Trepanning Drills / Rotabroach

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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.

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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.

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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)

Rohan von Indien at English Wikipedia

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.

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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

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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

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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