Lec9 Flashcards
Broaching MRR
Rise per tooth (Rt) * Number of roughing teeth in contact with a part (Nrt) width of a broach tool
3 types of saw blades
hacksaw
bandsaw blade
circular saw cold saw
Hacksaw blade properties
straight rigide and of limited length with teeth on one side
Bandsaw blade properties
flexible enough that a long length can be formed into a continuous band with teeth on one edge
What are bandsaw used for
cutting irregular or curved shapes but also straight cuts uniform cutting action result due to evenly distributed load
Circular saw
rigid disk with teeth on the periphery
Saw blade material
tungsten or molbdenum high speed steel high tensile strength alloy steel
Saw blade tooth spacing
proper tooth spacing is required to ensure chips drop off from this space after cutting
saw blade; Blade thickness or gauge
wider and thicker blades desirable for heavy duty work
saw blade; tooth set
refers to the manner in which the teeth are offset
from the centreline in order to make the kerf wider
than the gage
permits the saw to move more freely in the kerf
and reduces frictions and heating
Cutting speed influence on tool life
50% increase in speed 90% decrease in tool life
Feed influence on tool life
50% increase in feed = 60% decrease in tool life
depth of cut influence on tool life
50% increase in depth of cut = 15% decrease in tool life
How to extend tool life
increase feed and depth of cut
but increaseing cutting speed increases productivity and profitiability
Factors to consider when increasing cutting speed
decreases machine time
tools wear out faster and must be replaced
not making money when replacing tool
down time cost incurred
Costing normally based on?
per unit or per batch
factors that affect cost
1 machine tools - suppliers automation working enviroment
2 cutting tools - required performance life operating parameters
3 labour skilled or unskilled
4 overhead - rent electricity
5 machine setup time - for each batch during normal operation
6 loading and unload time - for each unit or batch
7 depreciation and residual values of equipment expressed in unit timer
original cost - residual cost operating life
Non cuttting time percentage total time spent machining
5% - of that incut less than 30% rest position loading gaging idle etc
Machine hourly rate
how much the machine costs to run per hour regardless of the nature of operations
MHR = Depreciation + staff costs + overhead
Fixes cost
cost that is fixed per batch
fixed costs = tooling + setup time x MHR
Variable costs
cost taht varies with batch size expressed per unit
Variable cost = material req + (processing time x MHR)
Total cost
= fixed costs + variable costs x number of units
Trades off are required between
capital cost of machines and tooling
cost per piece or batch
eg CNC more expensive but make products to certain quality much quicker
Compareing raw machining costs
used for selecting new supplier forcutting tools so per per performance
C = non productive cost + machining costs + tool change cost + cost of cutting tools
high efficiency machining range
minimum total cost per piece cutting speed and minimum time per piece cutting speed
Other factors to consider while costing
have to cost and deal with waste, if producing tonnes of waste every year very expensive very quickly
Adv of cutting fluids
coolant lubricant carry away chips and reduce friction reduce temperature maintains tool hardness removal of heat
cutting fluids maintains tool hardness
extends tool life or allows increase in cutting speed
Removal of heat cutting fluids
reduces thermal distortion of the workpiece - improved dimensional control
Effectiveness of coolant
thermal capacity and conductivity of the fluid used
Issues with water as cutting fluid
very effective in reducing temperature
but causes rust and poor lubricant
Oils as cutting fluid
less effective coolant capacity than water
better lubricant
dont cause rust
Normal coolant used
combination of oil and water or water and wax or just water used
Ideal use of cutting fluid
collect and reuse it - good recycling retain 99% of cutting fluid
Economics of machining
majority of time not profitable
A good cutting fluid
helps increase cutting speed
