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| CNC MACHINE |
Process Planning:
Define: is the term used to desribe the development of CNC part program
Decision is made based on:
- How will the part be held in the machine
- The use of machining Operation and strategy
- The used of cutting tool
- which machine should be used
Cutting Tool Material
- High speed steel ( HSS) >> aluminium& non ferrus
- Tungsten Carbide >>high silicon aluminium ,steel and exotic
- Ceramic >> hard steel & exotic metal& is coated by titanium nitride
Advantages& Disadvantages:
1. HSS has the following advantages over carbide:
· Save cost
· Less brittle, hard to break during interrupted cuts
· Re-sharpened easily
Disadvantages of HSS:
· Cannot resist as well as carbide or ceramic at high temperature machining
· Cannot cut hard materials properly
2. Carbide
Carbide has three basic types:
a. Solid carbide
b. Brazed carbide
c. Inserted carbide
Carbide has the following advantages over HSS:
· Proper performance at elevated temperatures
· Well hard materials cutting outcome
· Solid carbide tools well absorb the workpiece vibration
· The inserts can be easily changed or indexed when using the inserted cutters
Disadvantages of tungsten carbide:
· Expensive than HSS
· More brittle than HSS , easy to chip during interrupted cuts
· Harder to re-sharpen, requires diamond grinding wheels
3.Ceramic
Ceramic advantages:
· Less expensive than carbide when used in insert tooling
· Can cut harder materials faster and has superior heat hardness
Ceramic disadvantages:
· More brittle than HSS or carbide
· Must run within its given speed parameters specifications, the insert will break
down if the speed low.
Tooling for hole operations
Four basic hole operations on CNC are:
a. Drilling
b. Reaming
c. Boring
d. Tapping
a) Drilling
- have different Types
- twist drill remain on of the common tools for making holes
- HSS drills are the most common
- Cobalt drills have greater heat hardness than HSS drills.
b) Reaming
- Reaming is used to remove small amount of metal from an existing hole as a finishing operation.
- reaming is precision operation which is hold tolerance + or - 0.002 easily
- made with two basic flute designs: straight fluted and spiral fluted
c) . Boring
- Boring removes metal from an existing hole with a single-point boring bar
- boring heads area vailable in two designs: offset and cartridge type.
- Boring bars are available in four material types: HSS, solid carbide, brazed carbide and inserted carbide
d)Tapping
- Tapping is used to produce internally threaded holes
Fixtures
A fixture is technically any device used to hold and support a workpiece machining.
Fixtures may contain:
· Work supports
· Locating pins
· Angle brackets
· Clamping mechanisms
Advantages of modular fixturing:
· Can design custom fixtures to hold unique workpieces
· Reuse the components on another job
· Eliminate extra space
MACHINING PARAMETERS AND OPTIMIZATION
Cutting Speed
- The cutting speed is the edge or circumferential speed of a tool.
- In a machining center or milling machine application, the cutting speed refers to the edge speed of the rotating cutter.
- In a turning (lathe) application, the cutting speed refers to the edge speed of the rotating workpiece.
- Cutting speed (CS) is expressed in surface feet per minute (sfm). It is the number of feet a givenpoint on a rotating part or cutter moves in one minute.
- Proper cutting speed varies according tomaterial used. Generally, the softer the material, the higher the cutting speed.
the spindle rpm necessary to achieve a given cutting speed can be calculated by the formula:
Remember Calculation for rpm:
Rpm= (Cs x 12) / ( D x 3.1416)
just use this formula when for inch
Rpm= ( Cs)/( D x 3.1416)
just use this when D is given in unit m
Rpm= (Cs x 1000)/( D x 3.1416)
use this when D is given in unit mm
Feed rates
Feedrate is the velocity at which a tool is fed into a workpiece. Feedrates are expressed in two
ways.
· Inches per minute of spindle travel
· Inches per revolution of the spindle
For milling applications, feedrates are generally given in inches per minute (ipm). For turning
applications, they are mostly expressed in inches per revolution (ipr). Feedrates are critical to
the effectiveness of a job. Too heavy a feedrate will result in premature dulling and burning of
tools. Too light feedrates will result in tools chipping. The chipping will rapidly lead to tool
burning and breakage.
Milling Feed rates
Feeds in milling are not only depend on the spindle rpm, but also on the number of teeth on the
cutter. The milling feedrate is calculated to produce a desired chip load on each tooth of the
cutter. In end milling, chip load should be 0.002 to 0.006 inch per tooth. The recommended chip
loads for various mill cutters are given in machinists’ handbooks.
To calculate the feedrate for a mill cut, the following formula is used:
F = R x T x rpm
F = Milling feedrate expressed in inches per minute
R = Chip load per tooth
T = Number of teeth on the cutter
rpm = Spindle speed in revolutions per minute
Milling feedrates are also affected by machine and setup rigidity and part geometry.
Chip thickness is another factor which affect feedrates. This is not the chip load on the tooth but
the actual thickness of the chip produced at a given feedrate. Chip thickness will vary some with
the geometry of the cutter (positive rake, negative rake and neutral rake) and should be
maintained in the range of 0.004 to 0.008 inch.
Once a feedrate has been calculated, the chip
thickness it produces should be derived. If chip thickness is out of the recommended range, the
feedrate should be adjusted to bring it to acceptable limit
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Example 1.1:
An aluminum workpiece is to be milled using a carbide inserted mill cutter. The cutter is 1.750
diameter X 4 flute. Find the appropriate spindle rpm and milling feedrate for the workpiece. Recommended chip load of 0.002 to 0.006 inch. A value of 0.004 per tooth is
selected.
ANSWER:
The chip thickness is less than the recommended minimum of 0.004. The feed per tooth is
therefore calculated as using the feed-per-tooth formula. A chip thickness of 0.008 is used.
