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Speeds and Feeds for Milling and Drilling 03/19/12
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Module - Speeds and Feeds for Mil ling and Dril ling
Objectives
• compute the speed, feed, and depth of cut for milling
• compute the speed and feed for drilling
• describe methods used to make threads
• compute the speed and feed for tapping
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Overview
Prior to writing a program, the following must be calculated
• the speed (rpm) of the cutter
• the feed rate (how fast the cutter moves through the material)
•
the depth of cut
Speed
• cutting speed is the velocity of the cutting edge
• depends on the material you are cutting and the material that the tool is made from
• cutting speed greatly affects tool life
Calculating Speed
• you set the machine's speed in revolutions per minute (RPM)
• in manufactures’ data or other handbooks, the cutting speed is specified in surface
feet/minute (SFM) or meters/min
Calculation relating RPM to CUTTING SPEED (V)
• procedure
• find desired cutting speed in SFM from manufacturer’s or machinist's data book
• compute RPM
RPM = (12 * SFM) / (π * tool_diameter)OR
RPM = 4 V / D (easy to remember in the shop)
where D is in inches, V is in feet/minute
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Feed Rate
• feed rate is how fast the cutter is fed through the material
• you need to specify the feed rate in inches per minute (ipm) or mm/min
• the feed rate from a handbook is usually specified as the chip load per tooth in inches per
tooth (ipt) or mm/tooth• the chip load is the amount of material that each tooth of the cutter removes in each
revolution
• too high a chip load can lead to broken cutting edges because the force on thecutting edge will be large
• too low a chip load can lead to dull edges since the cutter tends to rub on thematerial rather than shearing it off
Calculation relating FEED to CHIP LOAD
• procedure
• find desired chip load per tooth from manufacturer’s or machinist's data book
• compute feed rate
feed rate = chip load per tooth * number of teeth * RPM
where chip load is in inches/tooth or mm/tooth
Depth of Cut
• axial depth of cut has the least affect on tool life - maximize it in order to minimize machiningtime but don’t go so deep that you break the tool or stall the machine!
• depends on radial depth of cut (width of cut), tool diameter, rigidity of setup and machine,
and available horsepower • to determine depth of cut, you must look at the volume of material removed since it affects
how much pressure (and force) is on the tool and how much horsepower it takes to turn thetool
• in general, the smaller the radial depth of cut, the larger the axial depth of cut
• when the radial depth of cut is large such as when you are slotting or roughing apocket with large stepover, and you are using the suggested chip load, the depth of cut is smaller than the tool diameter. The rule of thumb for our shop is
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depth of cut = 1/3 x too l diameter
Note: This is conservative for regular milling. Some shops may go higher.
• when the radial depth of cut is small, you can go much deeper. For example, for a
finishing cut where the tool is taking 0.020” (0.5mm) from a wall, the depth of cutcould be several times the tool diameter.
Other Considerations for cutting
• you should normally remove most of the material by taking roughing cuts which have a highmaterial removal rate but do not machine accurately due to higher tools loads and may notgive the required surface finish.
• you will then take light finishing passes to remove the remainder of the material (this givesa more accurate surface due to lower loads and deflections and a better surface finish)
• finish cuts generally remove from 0.010” to 0.030”
•
some materials (e.g. stainless steel, titanium) have a very thin but hard surface layer dueto work hardening. On these materials, you must take heavier finish cuts so that the cutpenetrates below the hard surface layer into the softer material.
• on a CNC machine where there is very little backlash, you should climb mill when finishingto get the best surface finish. This requires you to select the correct feed direction. In somematerials, especially where the surface is hard or tough on the cutting edges, it is better touse conventional milling.
• you should create lead-in and lead-out moves
• for a lead-in, the cutter should plunge to cutting depth while it is away from the finishedsurface (in air if possible) then feed into the material tangential to the first cutting path
• for lead-out, the cutter should move away from the part tangential to the last path beforemoving up
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Milling cutters
A milling machine can perform a variety of tasks. Each task requires a different type of cutter. Asample of popular cutters is shown below.
flat end mill • most popular cutter
• used for profiling, pocketing,slotting
• available in high speed steel(HSS) or carbide
• most popular have 2 to 4cutting flutes
• many configurationsavailable (single ended,short, long, double ended)
• regular and high helix
availableBall nose endmill
• used mainly for cuttingsurfaces
Carbide insertend mill
• replaceable inserts are costeffective
• available in large sizescompared to solid end mills
Carbide insertface mills
• used for facing (flattening) asurface
Carbide insertball nose endmills
• used for cutting surfaces
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Speeds and Feeds for End Mills
The following tables are examples of recommended speeds and feeds for HSS and carbide endmills. Check with your tool supplier for recommendations or for other tool types.
Cutting Speed for End Mills
Material
HSS
Speed
(SFM)
Carbide
Speed
(SFM)
Aluminum / Aluminum Alloys 300 600
Soft Cast Irons 200 250
Medium Cast Irons 125 150
Hard Cast Irons 65 80
Brass/Bronze 200 200
Coppers / Copper Alloys 150 300
Magnesium 300 800Nickel Alloys 75 150
Free Machining Stainless Steels 100 150
Work Hardening StainlessSteels 50
75
Low Carbon Steels 150 200
Medium Carbon Steels 100 100
High Tensile Steels (35-40 Rc) 50 75
Tool Steels (40-50 Rc) 40 60
Titanium 80 100
Plastics 300 600
Notes on speeds and feeds:
• the speed shown is a good starting point, you can increase the speed by up to 100%when taking light finish cuts
• for slotting applications where a full cutter width of material is being removed, reduce thespeed to 75-80% of the value shown
• when milling steels, chip color indicates correct speed (tan – good, blue or dark – toofast, white – too slow)
Tool
diameter
Chip Load
(inch/tooth)
1/16" 0.0002 - 0.0005
1/8" 0.0005 – 0.001
1/4" 0.001 – 0.002
1/2" 0.002 - 0.004
3/4" 0.004 - 0.006
Note: Recommended chip load varies with
material type. The chip loads shown in thetable are suitable for softer materials. For
hard materials, reduce the chip load up to
50%. The chip load may need to be
reduced further if end mills are long or the
setup lacks rigidity.
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Class Exercise
Compute a speed (rpm) and feed (ipm) for the following situations (all tools ½” diameter):
Tool Material Speed (rpm) Feed (ipm)
HSS end mill, 2 flutes Aluminum 6061-T6
Carbide end mill, 2flutes
Aluminum 6061-T6
HSS end mill, 2 flutes Steel 125 BHN
Carbide end mill, 2flutes
Steel 125 BHN
For the following situations, would you mill clockwise or counterclockwise in order to climb mill ?
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Drilling
Drills are available in a variety of styles and materials. The most common drill is a high speedsteel twist drill with a 118 degree drill point angle. Solid carbide twist drills are available whichallow for faster drilling speeds and longer tool life at increased tool cost. Insert drills (areplaceable carbide insert) can also be advantageous in certain situations.
Drill Point
Most drills have a 118 degree angle which is fine for general purpose use in softer materials.Hard materials require a blunter drill point (e.g. 150 degrees) while soft materials like plasticsmight use a sharper point (e.g. 90 degrees)
Drill Depths
The depth of a hole on a drawing is the depth for the full diameter hole. For a blind hole, the
bottom of the hole will not be flat because the drill tip is not flat. When you write a CNCprogram, you program to the tip of the drill. Therefore, you have to calculate the height of the tipof the drill and add it to the hole depth. For through holes, the drill should pass completelythrough the part. To calculate the depth, add the drill tip amount plus a breakthrough amount of approximately 0.1”. Depths of drilled holes are not precise so you rarely need to calculate tomore than a couple of decimal points.
Drill tip calculation
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Center drilling
Center drilling forms an entry point for subsequent drill operations. It is important when:
• the subsequent drill has a drill point (twist drill, spade drill)
• the hole size is small (generally < 1/2") since larger drills don't deflect as much
•
the hole must be precisely located
The hole must be center drilled deep enough so the drill tip does not make contact before theedge of the drill.
Spot Drilling
If a chamfer is desired on a hole, the hole can be spot drilled before drilling. Spot drilling willalso help to locate the hole so there is no need to center drill. A spot drill has a 90 degreeincluded tip angle which produces a 45 degree chamfer.
To determine the spot drill depth, first determine the diameter of the top of the cone that resultsfrom the spot drilling operation. For example, if you are drilling a 1/2" hole and want a 0.050"chamfer on the hole, the spot drilled hole diameter is 0.5 + 2x0.050=0.6 inches. The tip depth ishalf this amount or 0.3 inches.
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Threading
Common methods of forming threads include
• taps
• thread mills
Taps
standardhand tap
• Most common especially for hand tapping
• Used for machine tapping as well
spiral pointtap
• pushes chips ahead of tap
• good for through holes
spiral tap • draw chips out of the hole
• good for deep, blind holes
The end of a standard tap is tapered (chamfered). This is desirable for manual tapping as ithelps to align the tap to the hole and start the thread cutting process. The 3 types of standardtaps are:
• Taper Tap – first 8 to 10 threads are tapered. Used for threading through material and
for the first threading pass in a blind hole.
• Plug Tap – first 3 to 5 threads are tapered. Used for threading through or for threadingcloser to the bottom of a blind hole.
• Bottom Tap – first 1 to 2 threads are tapered. Used for threading to the bottom of ablind hole.
Depth for tapping
When you compute the tap depth in CNC, you must account for the tapered portion of the tap.For example, if you are tapping a through hole with a plug tap, you must go at least 5 threads
beyond the hole depth in order to get fully formed threads. For a 3/8-16 tap, this would be 5/16of an inch.
For a blind hole
• you cannot form threads right to the bottom of the hole since each tap has taper
• you can make the drilled hole deeper than required so the threaded portion is thedesired amount
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For example, Tapmatic (a producer of tapping attachments) recommends that the hole bedrilled at least Chamfer Teeth + One Pitch + 1mm beyond the required thread for clearance.
Example:1/4-20 bottom tap, 0.5" deep
Chamfer Teeth = 2 x pitch (.050) = 0.10Chamfer Teeth + one pitch + 1 mm = drill depth clearance0.10 + .050 + .039 = .19 inchesso, drill 0.5 + 0.19 = 0.69 inches deep if you require a 0.5” deep threaded portion
Thread milling
Another common method used to make threads is the thread mill. It looks similar to a tap but isoften used for larger (about >0.5”) or for external threads. The tool cuts the threads by movingin a circular path around the area to be threaded while also moving down.