CAD (Computer Aided Design or Drafting)
CAD programs are basically drawing programs used to create
a plan or drawing of a part. There
are a lot of different CAD programs. Some
of the most popular include Autodesk, Rhino3D (See Also Rhino3D), and
Solidworks.
CAD Drawing of a Bench Tapper
Made in Rhino 3D...
CAM (Computer Aided Manufacturing)
CAM is software that can generate
G-Codes from a drawing. Often it
is an add-on module for whatever CAD program you or using (VisualMill
for Rhino3D is an example), or perhaps the CAM
program incorporates its own CAD capability (OneCNC
would be an example).
CAM programs make it much easier
to generate complex G-Code part programs automatically, rather than having
to write the codes by hand. There
are many specialized CAM programs as well, such
as programs that may be used to generate G-Codes that produce a router
carving based on a bitmap (VCarve is an example).
Canned Cycle
A canned cycle is a pre-determined cycle used to simplify
CNC programming. For example, a
peck drill cycle (see Peck Drilling) makes it easy to implement peck drilling. Canned cycles are somewhat specialized to the
particular controller and machine. Common
canned cycles included various drilling, boring, and tapping cycles.
Capacitor
A capacitor is an electronic component that stores up
charge and will release it when called to.
Think of it as a reservoir for smoothing fluctuations in electrical
currents. Electrolytic capacitors
have particularly large capacities and are used in DC or linear power
supplies (See also Linear Power Supply) that are used in conjunction with
stepper and servo motors.
A Rectifier Board Atop An Electrolytic
Capacitor to Make A Linear Power Supply...
Carbide
Carbide is an extremely tough compound (usually Tungsten
Carbide or Titanium Carbide is the compound) used as a cutting surface
in machine tools. It is valuable
because it resists wear, heat, and is extremely rigid.
The entire cutter may be made of carbide as in the case of a carbide
end mill, boring bar, or twist drill, or a Carbide Insert (See Also Carbide
Insert) may be used in indexable tooling (See
Also Indexable Tooling) so that the cutting surfaces may be easily
changed as they wear.
Carbide can and should typically be run at much higher
speeds and feeds than high speed steel.
Carbide Inserts
An insert (See also Insert) made of carbide (See also
Carbide). These are the most commonly used type of insert.
Cartesian Coordinates
Named for mathematician René
Descartes, Cartesian coordinates provide a system for identifying the
relative locations of points in space. Locations are specified by a set
of numbers, with one number for each dimension (or axis if you are thinking
CNC, See Also Axes). Each dimension is orthogonal to the others, meaning
it is at right angles. Dimensions are usually lettered from X to Z for
three dimensional. The number specifies how far from the Origin (See Also
Origin) to travel along a particular dimension. The sign of the value
determines which direction to travel from the origin.
CAT Taper: See Taper
CAT5: See Taper
C-Axis
The C-Axis is the rotational axis about the Z-Axis.
Center Cutting
A center cutting end mill is one that has the ability
to cut with the center of it’s bottom face. This is crucial if you intend to plunge the
end mill, which is to send it straight down the way you would a drill
bit. Some end mills are not centercutting,
and the cannot plunge straight down as a result. Often, a larger non center cutting end mill
is not center cutting because there is a coolant passage down the core
of the tool that allows coolant to be injected directly into the cutting
action instead of coming from above.
Centerline Programming
Writing a CNC program where the toolpath is specified
as the centerline of the mill cutter or the virtual tip in turning. The
actual path is then generated from the centerline using tool offset and
toolnose radius information. See Also Tool Offset and Toolnose Radius.
Chain Drive
Chain drives are an option for moving the axis on gantry-style
machines such as routers and plasma cutters.
See Also Belt Drive
for a similar application.
Chamfer
A chamfer is beveling applied to an edge so it will not
be sharp. This can be done for
reasons of appearance and feel, or to avoid weakness for stress. For two surfaces that meet at right angles,
imagine a chamfer at a 60 or 45 degree angles that just
takes the edge off for a very short distance.
Chamfering is a common machining operation, and it can also be
useful as a means of deburring sharp edges.
There are tools specially made for chamfering, especially
chamfering holes. I like Zeo's zero flute chamfering tools for holes.
Charge Pump (See "Safety Charge Pump")
Chatter
Chatter is a vibration or sound that comes from the machine
tool under certain conditions. It
interferes with proper cutting and produces cutting errors and bad surface
finish. Generally, it is a harmonic
vibration or natural resonance. It
can be triggered through improper setup or operation of the machine. Frequently, changing the spindle speed, depth
of cut, or feed rate can eliminate the chatter.
It is generally not advisable to continue cutting with chatter
present.
Chip Auger or Chip Conveyor
Chip augers and conveyers are features of machining centers
that are used to help manage the huge amount of chips and debris these
machines can produce in a short period of time.
They deliver the chips to a hopper that may then be used to cart
the chips off for disposal or recycling.
Chip Breaker
A chip breaker is a feature on a lathe tool or insert (See Also Carbide
Insert) that is designed to cause the chip to break off by exerting force
on it. The chip breaker causes the chip to curl until it breaks off.
Chip Load or Chipload
Chipload a measure of the actual thickness of chip that is produced in the
machining process. It is measured in thousandths (i.e.: 0.010)
per tooth and may be influenced by the spindle speed, number of cutting
edges on the tool, and the feedrate of a CNC machine. The number of cutting
edges or flutes on a cutting tool determines how the chipload
is divided. A single edge tool takes all the chipload
during a revolution, while a multiple edge tool divides the load over
multiple edges.
Chipload is an important factor
in tool life because it dictates how much heat will be carried away from
the cutting edge. Better heat dissipation directly relates to increased
tool life. The formula for determining chipload
is:
Chipload
= feedrate / (rpm x #cutting edges)
A higher chipload will carry
away more heat because the larger chips carry the heat away. To increase chipload,
increase feedrate or decrease the rpm or number of cutting edges. All things considered, a 2 flute endmill offers a greater chipload
than a 4 flute endmill running at the same spindle
speed.
Note that higher chiploads
will also result in a less fine finish, so a trade off is necessary. Machinery’s Handbook has tables of optimal chiploads. Just to
give an idea, a chipload of 0.005” might make
sense for roughing steel while 0.001” to 0.002” is better to finish.
Chuck
A chuck is commonly
a workholding device, although some types of collets, such as ER, are
used primarily for toolholding and hence their collet chucks are also
for toolholding. Chucks grip round objects almost exclusively, although
they can grip square or hex objects, and a 4-jaw chuck with independent
jaws has quite a range of shapes it can grip. Chucks are designed to rotate
the workpiece or tool along an axis. They are most commonly used on lathes,
on rotary tables, or on the mill, as a collet holder or drill chuck for
twist drills.
Chucker or Chucking Lathe
A lathe without
a tailstock.
Chopper Supply: See PWM
Circular Interpolation
G-Codes support three common types of motion:
- Rapids (G0): Moving
the tool as quickly as possible, preferably without cutting (although
the machine usually doesn’t enforce this) in order to get to the next
location where cutting is desired.
- Linear Interpolation (G01): Linear interpolation is smoothly moving multiple
axes of the machine so the tool follows a straight line as closely as
possible. Since the line is usually
not aligned with an exis, this requires motion
from more than one axis to “stair step” together to produce this diagonal
motion.
- Circular Interpolation (G02/G03): Circular interpolation
is smoothly moving multiple axes of the machine so that the tool follows
a circular arc as closely as possible.
Since the axes only move in straight lines, a series of tiny lines
are used to simulate an arc. G02
causes the motion to be in a clockwise direction, while G03 is counter-clockwise.
Note that if the tool can follow an arc, a smaller
endmill can be used with circular interpolation to rough bore a larger
hole. This is a common practice in CNC. Likewise circular interpolation
might be useful to run a chamfer around the edge of a hole that's larger
than the chamfering tool.
Climb Milling
Depending on the direction of cut relative to the rotation
of the cutter, a milling operation may be said to be either climb milling
or conventional milling:
Note that in this illustration, it is the workpiece moving
and not the spindle. If the spindle moves, the "Climb" and "Standard"
labels would be reversed. Think of the spindle as a pinch roller that
either helps the workpiece move faster in the direction its already moving
(that's climb milling) or slows it down by turning "against"
that motion.
It's called "climb" milling because it will
tend to make the mill climb up the workpiece.
Some further thoughts on Conventional Milling (also called
"Down Milling" or "Downhill Milling"):
- The
width of the chip starts from zero and increases as the cutter finishes
slicing.
- The
tooth meets the workpiece at the bottom of the cut.
- Upward
forces are created that tend to lift the workpiece during face milling.
- More
power is required to conventional mill than climb mill.
- Surface
finish is worse because chips are carried upward by teeth and dropped
in front of cutter. There's a lot of chip recutting.
Flood cooling can help!
- Tools
wear faster than with climb milling.
- Conventional
milling is preferred for rough surfaces.
Some further thoughts on Climb Milling (also called "Up
Milling" or "Uphill Milling"):
- The
width of the chip starts at maximum and decreases.
- The
tooth meets the workpiece at the top of the cut.
- Chips
are dropped behind the cutter--less recutting.
- Less
wear, with tools lasting up to 50% longer.
- Improved
surface finish because of less recutting.
- Less
power required.
- Climb
milling exerts a down force during face milling, which makes workholding and fixtures simpler.
- Climb
milling reduces work hardening.
- It
can, however, cause chipping when milling hot rolled materials due to
the hardened layer on the surface.
There is a problem with climb milling, which is that it can get into
trouble with backlash if cutter forces are great enough. The issue is
that the table will tend to be pulled into the cutter when climb milling.
If there is any backlash, this allows leeway for the pulling, in the amount
of the backlash. If there is enough backlash, and the cutter is operating
at capacity, this can lead to breakage and potentially injury due to flying
shrapnel. For this reason, many shops simply prohibit climb milling at
all on any manual machines that have backlash. Some machines even came
equipped with a "backlash eliminator" whose primary purpose
was to enable climb milling and its attendant advantages.
One way to think of it is to consider the concept of chip load. This
is a measure of how much material each tooth of the endmill is trying
to cut. Typical values for finish work would be 0.001 to 0.002" per
tooth. For roughing work, that might increase to 0.005". Now in the
worst case, climb milling may grab the table and slam the work into the
cutter by the full amount of backlash during the instant when a single
tooth is cutting. You can therefore add the backlash to the chip load
to see what your new effective chip load might be in this worst case.
Suppose you are roughing 0.005" per tooth and have 0.003" backlash.
In the worst case, your chipload will soar to 0.008". That's probably
not the end of the world, but it is a strain. Now suppose you have an
older machine with 0.020" of backlash and are running an 0.005"
chipload. If the worst happens there your chipload will soar to 0.025",
which is probably going to break the endmill or something else.
The second thing to consider is whether cutting forces are strong enough
to pull the table through the backlash in the first place. A lot will
depend on the exact cutting scenario together with your machine. If you've
got a fancy low friction linear way machine, it can grab easily. If you've
got a lot of iron in the table, and maybe you're running with the gibs
tightened a bit, it'll be harder. There are ways to calculate the cutter
force, but in general, smaller end mills, less depth of cut, lower feeds,
and lower spindle speed will all reduce the cutting force and make it
less likely the cutter can drag the backlash out of your table and create
a problem.
In general, the home machinist should investigate climb milling carefully
and responsibly. Don't run afoul of the backlash problems that can lead
to spectacular failure. Know what your backlash is, and therefore what
kind of chipload you can run before getting into trouble. Run at
lower chiploads to leave more margin for error, and reduce DOC, feed rate,
spindle speed, and endmill size to reduce the cutting forces.
Clock Spring Cover
A clock spring cover is a springy spiral of material
that is wrapped around a leadscrew to protect it from chips and debris. It will contract and stretch to keep the screw
covered as the axis moves back and forth.
An alternative type of cover is a bellows (See also Bellows).
Closed Loop
A motion control system in which there is feedback that
tells the controller whether the system was able to move in the desired
way after it was commanded to do so. In
CNC applications, a closed loop is typically created by using feedback
from optical encoders that measure how far a shaft has turned.
Close loop systems are often referred to as servo systems.
They are more complex than open loop systems, but they offer higher
performance and they are not subject to lost steps, which can greatly
improve their accuracy and reliability. To get some idea of how closed
loop compares to open loop, you may want to visit my Stepper/Servo/Backlash
Simulator Page.
Coarse In-Position
Another name for Constant Velocity Mode. G64 sets Constant
Velocity Mode. Another way to view this mode is at the end of each move,
the controller checks how far from the expected position it is and compares
it to a coarse or fine boundary. G64 (Constant Velocity) uses a coarse
boundary while G61 (Exact Stop) uses a fine boundary. See also Constant
Velocity and Exact Stop.
Cogging
Cogging is the tendency for a motor to have variable
torque levels at different points in its rotation due to the internal
configuration of the motor. Cogging
is undersirable in a CNC drive motor, and sophisticated (and
more expensive) motor designs try to minimize it. A good servo driver can significantly reduce
the amount of cogging through encoder feedback.
Cogging is usually only a factor when one tries to use
a DC motor never intended to be a servo as a servo by coupling an encoder
to it. This is not to say it can’t
be done, just that performance will not be in the same class as a purpose-built
servo motor.
Collet
A collet is a device used to hold a tool or workpiece. There are many different collet standards including
5C (invented by Hardinge, commonly used with
lathes), ER, R-8 (the Bridgeport
standard), and so forth. The collet
works as a clamp, gripping whatever has been inserted more and more tightly
as the clamp is pressed against a taper to squeeze it shut. This pressing action may be due to a drawbar
pulling the collet into the taper (5C and R-8, for example), or a threaded
cap that pushes the end of the collet more deeply into the taper (ala
ER collets).
A lot of machinists say that collets will give a better
surface finish than setscrew mill holders because they grip the endmill evenly all around the circumference. A
lot of information on collets is available on Dr Spiff's Collet Compendium.
Combination Lathe
A lathe with a tailstock. See also "Chucker".
Also used to refer to a lathe that may be run as CNC or manual.
Constant Surface Speed or CSS
On lathes, Constant Surface Speed is the technique of
varying the spindle speed as the tool approaches the axis of rotation
so that the speed the cutter moves over the workpiece is constant. The speed must therefore increase as the cutter
approaches the centerline.
The advantages of Constant Surface Speed are longer tool
life, more consistent surface finish, and better cycle times. It also simplifies programming as the programmer
can think in terms of surface speed rather than rpm.
The G96 code turns on CSS, while G97 turns it off. As such, G96 is a mode. It is desirable to cancel it as soon as possible
when finished cutting as it will slow things down if used during rapids.
Constant Velocity Mode
This is the default operation for G-Codes, but is cancelled
by a G61, which causes the machine to enter “Exact Stop” mode (see Exact
Stop). In constant velocity mode
the machine does not pause between motion commands, but just keeps right
on going. Some lookahead (see
also Lookahead) processing may be done for optimal
results. This can result in some
rounding of corners, which is why Exact Stop mode is provided. Note
that constant velocity really doesn't mean the machine travels literally
at a constant velocity at all times, it just means it tries to avoid abrupt
acceleration and deceleration.
Another name for Constant Velocity Mode is Contour
Mode. Constant Velocity Mode is better for contouring because it encourages
a smooth interpolation between points making for a better surface finish.
Contactor
A contactor is a heavy duty relay capable of carrying
high currents such as those used by motors.
See also "Relay".
Contouring
Movement in two or more axes at the
same time in order to produce a smooth continuous surface or curve.
Contour Mode
Another term for Constant Velocity Mode (See Also Constant
Velocity Mode). Constant Velocity Mode is better for contouring because
it encourages a smooth interpolation between points making for a better
surface finish.
Contour Turning
A CNC lathe operation where a smooth continuous curve
is machined. It is analogous to
contouring on the mill, and may also be called “Profiling”.
Conventional Milling
Depending on the direction of cut relative to the rotation
of the cutter, a milling operation may be said to be either climb milling
or conventional milling:
It's called "climb" milling because it will
tend to make the mill climb up the workpiece.
Some further thoughts on Conventional Milling (also called
"Down Milling" or "Downhill Milling"):
- The
width of the chip starts from zero and increases as the cutter finishes
slicing.
- The
tooth meets the workpiece at the bottom of the cut.
- Upward
forces are created that tend to lift the workpiece during face milling.
- More
power is required to conventional mill than climb mill.
- Surface
finish is worse because chips are carried upward by teeth and dropped
in front of cutter. There's a lot of chip recutting.
Flood cooling can help!
- Tools
wear faster than with climb milling.
- Conventional
milling is preferred for rough surfaces.
Some further thoughts on Climb Milling (also called "Up
Milling" or "Uphill Milling"):
- The
width of the chip starts at maximum and decreases.
- The
tooth meets the workpiece at the top of the cut.
- Chips
are dropped behind the cutter--less recutting.
- Less
wear, with tools lasting up to 50% longer.
- Improved
surface finish because of less recutting.
- Less
power required.
- Climb
milling exerts a down force during face milling, which makes workholding and fixtures simpler.
- Climb
milling reduces work hardening.
- It
can, however, cause chipping when milling hot rolled materials due to
the hardened layer on the surface.
There is a problem with climb milling, which is that it can get into
trouble with backlash if cutter forces are great enough. The issue is
that the table will tend to be pulled into the cutter when climb milling.
If there is any backlash, this allows leeway for the pulling, in the amount
of the backlash. If there is enough backlash, and the cutter is operating
at capacity, this can lead to breakage and potentially injury due to flying
shrapnel. For this reason, many shops simply prohibit climb milling at
all on any manual machines that have backlash. Some machines even came
equipped with a "backlash eliminator" whose primary purpose
was to enable climb milling and its attendant advantages.
One way to think of it is to consider the concept of chip load. This
is a measure of how much material each tooth of the endmill is trying
to cut. Typical values for finish work would be 0.001 to 0.002" per
tooth. For roughing work, that might increase to 0.005". Now in the
worst case, climb milling may grab the table and slam the work into the
cutter by the full amount of backlash during the instant when a single
tooth is cutting. You can therefore add the backlash to the chip load
to see what your new effective chip load might be in this worst case.
Suppose you are roughing 0.005" per tooth and have 0.003" backlash.
In the worst case, your chipload will soar to 0.008". That's probably
not the end of the world, but it is a strain. Now suppose you have an
older machine with 0.020" of backlash and are running an 0.005"
chipload. If the worst happens there your chipload will soar to 0.025",
which is probably going to break the endmill or something else.
The second thing to consider is whether cutting forces are strong enough
to pull the table through the backlash in the first place. A lot will
depend on the exact cutting scenario together with your machine. If you've
got a fancy low friction linear way machine, it can grab easily. If you've
got a lot of iron in the table, and maybe you're running with the gibs
tightened a bit, it'll be harder. There are ways to calculate the cutter
force, but in general, smaller end mills, less depth of cut, lower feeds,
and lower spindle speed will all reduce the cutting force and make it
less likely the cutter can drag the backlash out of your table and create
a problem.
In general, the home machinist should investigate climb milling carefully
and responsibly. Don't run afoul of the backlash problems that can lead
to spectacular failure. Know what your backlash is, and therefore what
kind of chipload you can run before getting into trouble. Run at
lower chiploads to leave more margin for error, and reduce DOC, feed rate,
spindle speed, and endmill size to reduce the cutting forces.
Conversational CNC
Conversational CNC is a method of using a CNC machine
that does not require G-Code programming.
Instead the operator interacts with a set of menus to define what
the machine will do. The menus
are often organized like Wizards around basic operations for the machine
such as turning, threading, facing, boring, or grooving on a lathe.
Conversational CNC is very convenient on machines that
are used for one off jobs because they save the time of programming on
simple jobs.
Coolant
Liquid, air, or mist injected at the workpiece and cutting
tool in order to evacuate chips, provide cooling, and provide lubrication. Proper use of coolant can prolong tool life,
allow higher cutting speeds, and improve surface finishes.
Coordinates: See Cartesian Coordinates
Coordinate Rotation
A G-Code, G68, enables coordinate system rotation for
milling machines and G69 cancels it. It can be advantageous to rotate
the coordinate system in a variety of situations. For example, perhaps
the work is rotated slightly and is not square to the X, Y axes. It may
be easy to simply measure the angle it is off by and rotate the coordinate
system to compensate before starting the part program.
For many controllers, coordinate system rotation is an
additional cost option.
Coordinate Word
An axis letter followed by a numerical value signifying
a coordinate value. For example, X24.7500 is a coordinate word in the
G-Code language.
Counterbore
Counterboring is a common machining operation. It is
often used to recess a bolt head down into the hole. The are counterbore
cutters specifically made for the task, or some folks plunge cut with
endmills. The endmill will not leave quite as flat a hole as a true counterbore,
but it is often good enough.
Counterweight
A counterweight is often used to offset the weight of
a vertical axis to provide for smoother operation. Counterweights can literally be constructed
of weights, sometimes supported with compound leverage to reduce the weight
required, or from other mechanisms such as gas springs.
Crash
The unhappy case where the machine makes unintended
contact of some kind, perhaps with the workholding components. Always
run a new part program with offsets (See Also Offsets) set to "cut
air", in other words cut above the workpiece and any workholding,
and do so slowly. You can use the feedrate override (See Also Feed Rate
Override) to slow things down, as well as using Single Block mode
(See Also Single Block) to manually control execution. Functions like
Toolpath Simulation (See Also Toolpath Simulation) and Soft Limits (See
Also Soft Limits ) are an aid to testing out part programs and controlling
their ability to do damage.
Another source of crashes can sometimes occur in closed loop systems
(See Also Closed Loop) if the encoder loses its connection. In this case,
an axis can suddenly accelarate to maximum speed in an effort to "catch
up" to an encoder that has quit responding. The results are often
a spectacular crash.
Cutter Force: See "Cutting Force"
Cutter Offset
The predetermined distance from the
surface of the workpiece where the cutter can be moved rapidly between
cutting operations. See
also R-Level.
Cutter Radius Compensation
An offset used to compensate for differences in tool
diameter.
Cutting Force
The force exerted by the tool on the workpiece, and in
an equal and opposite reaction, on the machine itself. High cutting forces can potentially cause deflection,
which can lead to inaccuracy and chatter. Ultimately, it can even break the cutter.
Cycle Start
The control on the panel that begins
program execution, or resumes execution of a program that has been paused.
Cycle Stop
The control on the panel that halts
program execution.
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