Manufacturing Processes Chap. 22b - Drilling & Holemaking

Holemaking Holemaking is one of the most important operations in manufacturing. When are holes used? To locate or position parts. Attach parts. Hold parts together. Lighten the weight of parts. Relieve stresses. As inspection cavities. Cooling and Lubrication.

Drilling Drilling is the major and most common hole making process. Apparently a simple process but is quite complex. Drilling tool has two cutting edges. Cutting takes place inside the workpiece. Chips exit via hole filled by drill. Additional heat is therefore generated due to friction.

Drilling Hole diameters done by drilling are generally oversized. Amount depends on tool and equipment quality and type. Drilling is used for rough cutting or to produce a hole with limited accuracy. Two chips are created. Drill has diameter D, rotating at N rpm, feeding axially.

Drilling Fundamentals 1. Find V and fr from tables (depend on material and size of hole being drilled.) 2. Establish RPM: N = (12 V) / (P D) V in surface ft/min at outermost pt. on drill, D in inches. 3. d.o.c. = fr / 2 4. L = depth of hole + Allowance (A~D/2) 5. Cutting Time CT = (L + A) / (fr N ) 6. MRR = Volume / CT = (PD2 / 4) / (fr N) (in3/min)

Material Removal Rate (MRR) in Drilling. Thrust Force Acts perpendicular to the hole axis. Cannot be too excessive or tool will break, workpiece will distort. Depends on workpiece material, feed, rpm, drill diameter, drill geometry and cutting fluids. Must know torque to estimate required power. Power dissipated = Torque x Rotational Speed (in rads/sec)

Typical Twist Drill Parts / Parameters Shank Body Overall Length Flute / Flute Length Land Helix Angle Helix Lead Point Angle

The Twist Drill Point: Body Flutes: Actual cutting edge, angled. For harder materials, a flatter point angle is used. (usually 118o) Body Determines size of hole to be machined Flutes: Spiral grooves along the length of the drill. Chips travel upward along flutes. Serve as passageway for cutting fluid to reach cutting edges.

The Twist Drill Lands Shank Material between the flutes. Give tool strength. Shank Fits into spindle or chuck of machine. Can be straight or tapered.

Twist Drill Geometry Key geometry components are: point angle lip-relief angle chisel-edge angle helix angle (usually 24o) Proper Geometry needs to be selected to: Produce accurate holes. Minimize drilling forces and torque. Optimize drill life. Tables allow you to select proper geometry.

Drill Geometry Effects Lip relief angle too small: thrust force, generated heat, wear increase. Lip relief angle too large: cutting edge can chip or break. Twist drill generally has 2 flutes and 2 cutting edges. Can have a: low helix (more gradual spiral) chips are smaller - good for shallow holes, thin metals. High helix (more acute flutes around drill body) chips curl and flow to surface better- used for deep holes.

Drill Types Twist Drill: most common. Step Drill: Can produce 2-3 diameters in a hole in a single operation. Counterbore: To enlarge a hole for part of its depth - has a pilot which follows an existing hole for proper alignment. Countersink: To provide a recess area with tapered walls around the top of a hole - allows screw to lie flat with part surface.

Drill Types Center Drill short, rigid drill used to produce indentations to locate the center of a hole before actual drilling is done. Prevents walking action of point. Straight fluted drills Good for hard materials. Drill does not bite into the material. Usually used with shall holes / sheet metal.

Drill Types Gun Drilling Originally developed for gun barrels. Used for drilling deep holes Deep holes have depth:diameter = 3:1 or larger) Have special name now: BTA Drills (for Boring Trepanning Association) Cutting fluid is forced under pressure along longitudinal hole in drill body which cools and flushes chips out the flutes. Have single lip cutting action.

Drill Types Hole Saw Combination Drills Step Drills Subland Drills Can produce larger holes in thin material. Has a drill in center to guide tool. Combination Drills Can drill two or more diameters, or Can drill & countersink and/or counterbore at the same time. Step Drills Single set of flutes, ground to two or more diameters. Subland Drills Have separate set of flutes on the same body.

Drill Types Spade Drills Used for holes 1”+ in diameter. Usually need an existing hole, but can drill directly. Body is made of regular steel. Drill blade is ground and attached to body. No flutes are required.

Selecting Drill Types Guide for selecting drill type (if cylindrical shape is desired); see figure 24-13.

(See plastic card issued by Mr. Wright.) Drill Nomenclature (See plastic card issued by Mr. Wright.) Numerical (also called Wire Gauge) Series: #97 (.0059") to #1 (.228") Letter Series: A (.234") to Z (.413") Fractional Series: 1/64" to 1 1/4"(in 1/64" increments) to 1 1/2" (in 1/32" increments) straight shank. Millimeter Series: 0.05 mm up in 0.01mm increments.

Drilling Practice Need to prevent drill “walking”; initial indentation allows for tool to stay in position better. Usual (and most accurate) procedure is: Center Punch or Spot Drill (to mark location of hole) Center Drill (to start hole, prevent “walking” action) Drill (for roughing) Ream (to finish hole) Bore (to enlarge with precision / finish hole if too large for regular tools)

Tool Holding Usually drill is placed in a drill chuck. Adjustable over a large range. Jacobs: have three steel fingers adjustable with a key. Universal: two clamps adjust to secure drill. Collet: A taper action secures drill. Quick change chucks: for high production. See figs. 24-14, 24-15.

Drilling Machines Drilling can be done on many machines: lathes, vertical milling machines, boring machines. Drill Press - most common. Designated based on size that can be set on table (6" to 50").

Drill Press Components: Power Head - holds motor & drive system Sliding Head - Contains spindle & feed control Spindle - drives chuck & tool Feed & Speed controls - controls tool feed and rpm Work Table - holds workpiece / fixture Column - carries head Base - supports entire machine body

Other Drilling Machines Types: Bench-type press (small diameter holes). Radial drills (for large workpieces) Horizontal Drills Gang drills (with multiple spindles in line). Turret drills (have multiple spindles attached to a head).

Other Drilling Machines Radial Arm CNC Turret Gang

Boring Definition: Tool: Boring bar. Creation of circular internal profiles in hollow workpieces. A preexisting hole must exist. Tool: Boring bar. Long, single point tip set up on a boring head. Must have stiffness to minimize deflection, vibration and chatter. Boring can be done an a lathe, or mill. Precision boring is performed on jig borers.

Other Operations Counterboring: Countersinking: Spot facing: provides an enlarged cylindrical hole with a flat bottom for bolt head or nut. To enlarge a hole for part of its depth - has a pilot which follows an existing hole for proper alignment. Countersinking: Makes a beveled section at the end of a drilled hole to provide a proper seat for a flat-head screw or rivet. Spot facing: Provides a smooth bearing area on a rough surface at hole opening normal to the axis.

Other Operations

Reaming Definition: Operation used to make an existing hole dimensionally more accurate and improve its finish. Hole must have been previously produced i.e. by drilling.

Reaming Reamer Finishing tool Has a rigid body an multiple cutting edges Produces holes with very high accuracy Multiple cutting edge tool. Removes very little material. 0.008" minimum material removal on a hole diameter.

Tapping Used to produce internal threads. Tool is chip-producing with multiple cutting teeth. Must remove chips properly given the small clearances involved. Use cutting fluid and periodic reversal / removal of tap from hole.

Thread Nomenclature See (Fig. 22.16) American National Thread System (Unified System): US, Canada, UK. ½ - 20 UNF - 2 - A a b c d e a: Nominal Size (Major diameter) b: TPI c: Thread Series d: Thread Class e: External/Internal Thread

Thread Nomenclature ISO General Purpose Screw Thread Form (more universal standard) M 6 x 0.75 - 5 g 6 g a b c d e f g a: Metric Thread b: Nominal Size (mm) c: Thread Pitch d: Tolerance Grade of Pitch Diameter e: Tolerance Position of Pitch Diameter f: Tolerance Grade of Crest Diameter g: Tolerance Position of Crest Diameter

Designing for Hole Manufacturing Design so drilling take place perpendicularly to locating surface. Otherwise tool will deflect. Match hole bottoms with standard drill-point angles. Avoid-flat bottomed holes. Through holes are preferred over blind holes. If holes are very large, design a pre-existing hole made by forming or casting and then finished off by boring. Drill blind holes deeper for subsequent reaming and tapping.

Designing for Hole Manufacturing Drills have a high L/D ratio and can produce deep holes. Care must be taken to ensure accuracy and prevent drill breakage. When HSS drills wear out, they are reground. If done improperly, original drill geometry is lost.