Mech 2020 Exam two

Lead

Linear distance travelled in one revolution

Pitch

• Distance between crest to crest

• most commonly equal to lead

Start

• Number of individual helical threads

• most are single

Multiple start threads

• More thread engagement

• More travel rotation

• Less effort to to turn (lambda is greater)

Single start thread

L=p

Double start thread

L=2p

Triple start thread

L=3p

Thread designations (english): ex. 1/4’’ 20 UNC-2A.

• 2A = Medium tolerance (class of fit or thread tolerance)

• UNC = course pitch (thread series)

• 20 = threads per inch (pitch = 1/threads per inch)

• 1/4’’ = Major thread diameter (inches)

Thread designations Metric: ex. M12×1.75-4h-LH

• M= metric

• 12 = nominal diameter (major dimeter in millimeters)

• 1.75 = pitch (millimeters)

• 4h = class of fit (thread tolerance class symbol)

• LH = left-hand

Threading - Cutting Taps

• Taper tap gets threads started

• plug tap gets threads formed slightly deeper

• Bottom tap gets threads formed as deep as possible

• create chips that interferes with tapping

Threading - FORM taps

• oll/form, or deform material

• Thread forming is the internal analog to thread rolling

• No chips

Hand tapping: Mill

Using the quill as a guide and support

Hand tapping: Lathe

Using a center in Tailstock as a guide and support

Threading: Taps

• Make sure you have enough thread engagement for bolt

• Must bottom tap blind holes

• Minimum depth is diameter

External Threading: Die

• For external threads

• One side cuts deeper threads

External Threading: Rolling

• Cold forming process

• Preferred for volume productions

  • Less waste

  • High speed

  • Enhances strength

  • Accurate thread forms

  • smooth surface finish

• Cons

  • forms/rolls are very expensive

  • Need highly ductile material

  • Cant have cavities in raw stock

  • Limitations on depth and diameter of threads

Threaded Fasteners

• Use wedging action to clamp parts together

• Should screw into mating part a distance equal to at least 1.5 times thread diameter

Insert

• special form of nut or internal thread

• provide strength in soft metals and plastics

• Used to replace damaged or stripped threads

Thread-forming screws

• Produce thread in part as they are driven

• Eliminates costly threading operation

• Self drilling variation eliminates hole-making and aligning operations

Thread-cutting screws

• cut threads into material when driven

• hardened

• Join heavy gage sheet metal

Drive screws

• Hammered into drilled or punched hole of proper size

• Result in permanent assembly

Drilling Technique: Spot facing

• provides flat/smooth surface for fastener

Fastening Techniques

• proper thread length

• 2 threads past nut

Washers

• used to evenly distribute load or increase area of contact

Types of Fits

• Clearance fit

• interference fit

• transition fit

Clearance fit

• Hole >shaft

• maximum clearance

• minimum clearance

Interference fit

• Hole <shaft

• Maximum interference

• minimum interference

Transition fit

• hole = shaft

• maximum clearance

• maximum interference

Design for Manufacture

• Define the problem

• Research

• Ideation/concept formulation/brainstorm solutions

• concept/solution/idea evaluation

• concept/solution/idea selection

• Prototype

• Test

What makes a good design

• simple

• manufacturable

• cost-effective

• reliable

G-codes

• Instructions to a CNC machine

• Codes that start with G or M

• N is a code line number, will not cause action

N

line number

G

motion

X

horizontal position

Y

vertical position

Z

depth

F

feed rate

S

spindle speed

T

tool selection

M

Miscellaneous functions

I and J

incremental center of an arc

G00

Rapid positioning in given directions

G01

-Linear interpolation

-needs a feed rate command to execute (F)

G02

-Circular Interpolation

-R value or I and J center point values of a circle

-full arc or quadrant input

G03

Circular Interpolation Motion CCW

G20

Inch Input

G21

-metric input

Canned cycles

• A predefined string of variables which allows for a predetermined sequence of events

• Simplifies input and reduces program size

Cutter Compensation

• The ability to adjust the cutter location with offset values in the controller

• G40, G41, G42

M00-M01

Program stop/optional program stop

M02/M30

End of program/ EOP Rewind

M03

Spindle on CW

M04

spindle on CCW

M05

Spindle stop

M06

Tool change

M08/09

coolant on/off

Modal code groups

• allows for multiple codes on a single line, but only one code from each group per line

Surface per minute

SFM = 0.26(RPM)(Diameter)

RPM

=(SFM)/(Diameter) * 3.82

Inches per revolution

IPR

Inches per minute

IPM = RPM * IPR

Feed, Speed, and MRR: Rigid Tapping

• RPM = (SFM/Diameter)*3.82

• IPR = 1/TPI

• IPM = IPR*RPM

Feed, Speed, and MRR: Milling

• MRR = Feed rate(IPM) DOC axial (in) * DOC radial (in)

• RPM = (surface feed *12)/piD

Feed, Speed, and MRR: Lathe

• MRR = Radial DOC (in) Feed Rate (ipm) * work circumference (in)

• Radial DOC = (D1-D2)/2

• Feed per revolution (ipr) = (surface feed)/RPM