Cutting Tool Design
Machining
Changing form and dimension of a component through metal removal.
Elements of Machining
Key to success?
Chip Formation Basics
•Cutting tool must be harder and more wear-resistant than the workpiece
•Interference between tool and workpiece designated as feed and depth of cut
•Relative motion & force
Chip Formation
Chip Formation
See:
Chip Formation
Type I - Discontinuous or Segmented Chip (video 2)
Continuous Chip Formation
Type II - Continuous Chip
Can have built-up edge
Can be serrated, saw-tooth like.
Basic Terminology
Side View (of part)
Top View
Compare side cutting-edge angle to Lead Angle (below)
Lead Angle
Increasing the lead angle removes the same volume of metal at the same rate but thins the chip and reduces tool wear.
Tool Bit Grinding
Generic wedge-shaped tool
Orthogonal Cutting Theory
•Rake Face
–Surface the chip flows across
•Flank
–Surface that forms the outer boundary of the wedge
•Rake Angle
–Angle between the tool face and a line perpendicular to the cut workpiece surface
•Clearance (relief) Angle
–Angle between the tool flank and the workpiece
Cutting Zones
Rake Angle
Has a lot of influence on the behavior.
Increasing the rake angle tends to:
Reduce cutting forces.
Reduces BUE formation
Weakens tool
Encourages continuous chips
Can cause chatter with a large depth of cut
Shear Angle
Influenced by rake angle and friction
Shear and its normal (like friction)
Chip Thickness
Ratio is an indicator of shear angle
Friction Angle
Isolate the friction surface...
A circle fits nicely on a right triangle. The diameter is the hypotenuse.
Shearing and Shearing Normal
These forces in equilibrium
See it as two friction problems with the same result. “Friction” from the shear and the actual friction.
Cutting Forces
Three Component Dynamometer
Tool post dynamometer
Combining Px and Py into Pxy
From the Toolholder’s Perspective
The machine itself resists this force...
Pz is the Cutting Force
Pxy is the Thrust Force
Pxy is the combined action of the
feed force (Px) and the outward radial force
Changing Friction
Fundamentals of Cutting Tool Design (Second edition). SME
Note: You can measure the chip thickness easily and compare to the feed rate.
Relief Angle(s)
As the tool advances, it has to get out of the way.
Also, there is elastic recovery behind the cutting edge.
Primary and Secondary Relief Angle
Primary and Secondary Relief
Manipulating Factors
•Velocity (Speed)
–Affects temperature
•Size of cut
–Feed rate, depth of cut, lead angle
•Tool Geometry
•Tool Material & Coating
•Cutting Fluids
•Workpiece Material & Geometry
Built Up Edge (BUE)
Chip Breakers
Chipbreaker Geometry
Ideal chip is 6 or 9 shape
Breakers generally forces chip into a tighter radius. But - chips that are wound too tight (corrugated chips) can cause premature tool wear.
Can be
Integrated
Clamp-on
Two types
Groove
Designed for a limited range of speed and feed
Clamp
Cutting Tool Guidelines
•Rigidity
•Strength
•Chip Disposal
•Avoid Uneven Motions
•Avoid Chatter (see)
–Fewer teeth, higher velocity
–Small nose radius. Depth of cut should be larger than nose radius
--Sharp edge
About Low and Neg. Rake Angles
Coolant and Lubrication
Coolant and lubrication
Ideal Chip Formation
Types of Tool Wear
Or: Carbide Depot
Failure is when the tool can no longer produce parts to the required specifications
See: Sandvik.coromant.com
Mechanisms of Tool Wear
Rotating Cutters
Rake angle is determined by offset from center axis.
Clearance has to happen fast (on periphery) because the tool and shape of cut is round.
Rotating Cutting Tools
D-Bit is a simple case
Rake angle
Primary clearance angle
Conical clearance
End Mill Geometry
Still a wedge-shaped cutting edge.
Primary and secondary reliefs ground in.
Sharpening affects size.
End is typically cupped in.
Peripheral Geometry
End Mill Geometry
Other Cutting Tools - Drills
Helical Flutes
Allows easier entry of cutting face into the material. (Slicing action rather than chopping)
See: Carl A. Bergstrom, Weldon Tool Company 1918
Twist Drill Geometry
Twist Drill Geometry
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.
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.
Tap Geometry
Chip Formation in Other Processes
Roughing Mills
Note chip formation
Plastic Strain FE Model
Cutter Grinding
BBC - Material Removal Processes
& Cutting Tool Materials
Review & More Detail
Grinding the Periphery of an End Mill
Review & More Detail
Stephan Gotteswinter’s single lip cutter grinder
Review & More Detail
Making Insert Cartridge
Review & More Detail
Clickspring’s Dovetail Cutter
See Book
Cutting Forces - Merchant’s Circle
Involves three triangles whose vertices lie on a circle.
Things known:
Fc and Ft (from force measurements).
Rake angle.
Thickness of chip and depth of cut.
The circle can solve any of the other unknowns.
Let F = Frictional force
N = Normal to frictional force
Fs = Shear force
Fsn = Normal to shear force
Fc = Cutting force or tangential component of force
Ft =Thrust force or feed force
β = Friction angle
μ = Coefficient of friction = tanβ
Fc and Ft are along and normal to the direction of velocity.