CAD to Exact Stop - CNCCookbook: Be A Better CNC'er



CAD to Exact Stop

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, and Solidworks.

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.

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 easily changeable cutting surface that may be used with indexable tooling (See Also Indexable Tooling). Inserts are generally very cheap compared to the cost of replacing the entire cutting tool, and so provide an economical way to use carbide (See Also Carbide) for high performance cutting in machine tools.

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.

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.

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 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.

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.

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.

Some further thoughts on Conventional 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:

• 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.

In general, the home machinist should try to shoot for climb milling.

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.

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.

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.

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 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.

Some further thoughts on Conventional 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:

• 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.

In general, the home machinist should try to shoot for climb milling.

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.

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.

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.

Deflection

Deflection is unintended bending of a tool, workpiece, or machine component. It leads to inaccuracy or perhaps worse if it causes damage to the tool, workpiece, or machine. Rigidity is the counter to deflection.

Delrin: See Acetal

Detent Torque

This is the torque you feel when you twist a stepper motor that has no power applied to it. The detent torque can cause a stepper motor to try to hold its position even when power is switched off, at least to the nearest step. With no power, the microstep position will be lost.

Digitizer

A mechanism for mapping the 3D surface of a real object and producing a computer model suitable for a CAD/CAM program from the model. Digitizing can be accomplished using a Tool Probe (See Also Tool Probe) or Laser Scanner to name just two relevant technologies.

DNC (Direct Numerical Control)

DNC is a protocol, similar to a local area network, used to link together CNC machines on the shop floor. Before DNC, it was necessary to deliver part programs to the machine using other means—originally paper tape, later floppy disks or bubble memory, and lately using USB keys.

DOC (Depth of Cut)

Depth of cut is a measurement of how much material the cutter is removing along one axis. So, a ½” endmill whose end is 0.020” into the surface of the material being cut, either below the face in the Z-direction or along the sides of a slot or pocket in the X or Y directions, has a 0.020” depth of cut.

Obviously it takes more power to run a higher depth of cut. You may need to run at a slower feed rate or spindle speed if you run a greater depth of cut. For CNC machines, it can be more advantageous to reduce the depth of cut, increase spindle and feed rates, and just make more passes to reach the desired depth.

Depth of cut is typically less than the diameter of the cutter, and sometimes much less depending on the rigidity of the machine and cutter.

Dovetail Ways

Sliding ways are used by many machine tools so that their axes may slide against one another. There are four prominent designs:

- Dovetail Ways: Very common on mills, dovetail ways look like the dovetails used in wood joinery.

- Box Ways: Box Ways are rectangular cross section, as opposed to the angular shape of dovetail ways. Box ways are very strong, but they suffer from two shortcomings. First, they involve a lot of surface contact area, so managing friction is key. Second, in order to slide at all, some clearance is required, which results in some slop in their travel. They are the strongest and most rigid design, but they are difficult and expensive to manufacture.

- Prismatic Ways: These are prism shaped ways common to lathes.

- Linear Ways: Linear ways use linear slides as rails and so are much different than the other three styles. The roll on ball bearings as opposed to having more metal-to-metal contact in the other designs. Linear ways will have the least rigidity, but for the price, they are ideal for precision and high speeds.

Drawbar

Drawbars are used to pull collets and toolholders into their tapers. See Also Taper. They may be threaded into the collet, or they may grip the toolholder’s Retention Stud.

Drip Feed

Drip feeding is a technique of feeding the part program to the CNC machine a little bit at a time if the whole program is too large to fit into memory at once.

Drive or Driver

An electronic circuit used to control the current sent from the power supply to a motor and thereby control what the motor does. A Gecko drive is one example of a driver.

DRO (Digital Read Out)

A Digital Read Out or DRO provides a digital indication of the position of one or more axes on the machine. DRO’s typically use linear scales (See Also Linear Scales) to measure the position, but other technologies have been used as well.

In Mach3, a DRO is a numerical indicator within the program that has a number of different capabilities and properties.

Dry Machining

Dry machining is the process of machining without coolant. Often a strong stream of compressed air is used to cool chips. Dry machining can often increase productivity and reduce chip breakage at very high SFM rates. It is also used in some cases because it is less messy or healthier for the machinists.

On the other hand, unless properly applied, it can seriously reduce the life of your tooling. Be sure to check carefully with your tooling supplier about dry machining parameters.

Duplex Bearing

Refers to using two angular contact bearings (See Also Angular Contact Bearing) in pairs. The bearings are usually a matched pair and should not be separated. They’re matched by being ground to match one another, and when used in proper combination, this provides a preload (See Also Preload).

Dwell

An intentional delay to keep a rotating tool in contact with a part, perhaps to improve finish. G04 is the G-Code command to force a dwell between operations.

DXF File

DXF is the Autodesk file format and file extension for Autodesk CAD drawings. It’s also a common format that most CAD and CNC-related programs can read and write.

Edge Finder

A device used to find the exact edge of the workpiece along the x or y axis. Edge finders can rely on electrical contact, a sensitive pressure switch, or be strictly mechanical.

Electrolytic Capacitor: See Capacitor

Encoder

An encoder is an electro-mechanical device used to measure shaft rotation. Often they are optical in nature. They’re used to provide feedback in closed loop control systems. Encoders are often associated with servos.

End Mill or Endmill

End mills are cutters used in milling machines. They superficially resemble drill bits, although it is very superficial. There are a number of characteristics involved in selecting the best endmill for the job including:

- Number of flutes. Endmills may be had in a variety of configurations including 2, 3, 4, or more flutes.

- Roughing or Finish Style: Roughing endmills are good for hogging out material, but not so great for finishing.

- Centercutting: Determines whether the endmills center cuts. If not, the mill cannot be used to plunge cut. See Also Centercutting.

- Material: Endmills are made from a variety of materials ranging from High Speed Steel to Solid Carbide.

- Coating: There are a variety of coatings designed to improve the performance of the endmill.

- Type: Standard, ball nose, double ended, etc.

- Length: Shorter endmills are more rigid (See also Rigidity), longer endmills may be needed to reach.

- Diameter

There are many other characteristics that define an endmill.

E-Stop / Emergency Stop

The control that stops all machine operation. Usually it physically interrupts all power to the spindle and axis drive motors. Once an e-stop has been triggered, a special operation is normally required to resume power.

Exact Stop Mode

Triggered by G61, and cancelled with G64 (see Constant Velocity Mode), Exact Stop mode causes the machine to bring the axes to a full stop between each motion command. This can result in less rounded outside corners. The downside is that if the tool comes to a full stop it will have a tendency to leave witness marks on the workpiece there.

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