Glossary

Several of the Books also have glossaries. See also, CNC Cookbook Dictionary and CNC Mentor Glossary. Many entries were initially copied from the latter under a creative commons license.

A

A-Axis[1]

The A Axis usually refers to the fourth axis of a CNC machine and is usually a rotational axis. The fourth axis sits underneath a typical X,Y,Z setup. Picture a stepper or servo controlled lathe on the table of a typical 3 axis machine. That is your 4th axis. It is important to understand the difference between A axis and B axis. The A axis is the rotational axis along the Y axis. While the B axis is the rotational axis along the X axis. So the picture below demonstrates an A axis setup.

A-Axis (photo courtesy of CNC Mentor)

Absolute Coordinates[2]

Absolute coordinates are expressed relative to a fixed position (in most cases the homing switches of your machine). Relative coordinates on the other hand are relative to the current position of the cutting tool (think zeroing your tool at some arbitrary point on the table). Helpful Gcodes in this regard are the following:

• G90 command places the part program into absolute coordinate mode.
• G91 cancels absolute coordinate mode and places the controller software (i.e. mach 3) into relative coordinate mode.

Accuracy vs Precision[3]

Accuracy refers to how close an actual measurement is to its true value. For example, a hardened rod is very accurate if it is supposed to be .0001 and it is in fact .0001. That is high accuracy. Precision on the other hand refers to the repeatability of a measurement no matter how accurate it actually is. Using the same example, there is high precision in a line of 50 hardened rods that all measure .0015 even though they are supposed to be .0001. In this case there is high precision but low accuracy.

Arbor[4]

An arbor (American English) is synonymous with the term mandrel (also spelled mandril). An arbor can be either a workholding device or a toolholder. For example you may read in a catalog, drill chuck (includes R8 arbor). The arbor here refers to the portion that adapts the spindle of your mill to the actual drill chuck.

Axis (cnc)[5]

In Geometry there are three axes of movement (X, Y, Z). It is therefore confusing at first to hear of a 5 or 6 axis CNC machine. How can there be more than 3 axes? In CNC terminology an axis is either a linear or rotary freedom of movement.

B

Backlash

Any kind of unexpected play in an axis due to clearance or looseness of mechanical parts. Check for it by moving an axis in one direction, stopping the unit, then noting how much movement is necessary in the opposite direction to begin moving the machine.

Ball Endmill[6]

Ball end mills are end mills that have a half spherical bottom (examples pictured below). They’re typically employed in 3D profiling (as opposed to profiling or pocketing. For example, carving out a 3D chess piece) because they don’t have a flat bottom and hence tend to smooth out the steps. They have some unique cutting cutting problems at the bottom because the cutting speed varies all along the ball. The cutter is moving faster along the areas of the ball with larger diameter.

Bounding Box[7]

A bounding box is used in CAD drawing to get a rough estimate of how much volume a 3D object contains. A bounding box is defined as the smallest box that can be drawn and still contain the object(s) you are “bounding.” So in the picture below the bounding box cube has been roughly drawn around the sphere giving an approximate volume of the object. This is useful in CNC when trying to figure out how large of stock to need to start with in order to mill out the model.

Bounding Box (photo courtesy of CNC Mentor)

C

C-Axis[8]

The C Axis is the rotational axis around the Z axis. In and of itself a machine with just C Axis capability would be pretty useless. But if you couple it with B Axis movement you have a 5 axis machine with incredible versatility.

Image of axes, including rotation (photo courtesy of CNC Mentor)

Center Cutting Endmill[9]

A center cutting end mill is distinguished from a non center cutting end mill in that it has the ability to plunge the end mill in much the same way one would plunge a drill bit. End mills with coolant passages down the center of the bit are not centercutting and consequently cannot plunge straight down. Non-center cutting end mills can not make plunge cuts but clear chips better & it can be sharpened easier. Center-cutting (left) & non-center cutting (right) type end mills. Center-cutting end mills can make plunge cuts.

Center-cutting (left) & non-center cutting (right) type end mills. (photo courtesy of CNC Mentor)

Centerline Programming[10]

Centerline programming refers to CNC Gcode written in such a way that the toolpath is programmed along the centerline of the mill cutter (or the virtual tip in turning). Since it is rare that we actually want to cut this way, the actual path is generated using the centerline as a reference and calculating the offset via the tool offset and toolnose radius information.

Chatter[11]

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.

Chipload or Chip Load[12]

Chip load is the thickness of a chip measured in thousandths (i.e.: 0.010) per tooth that is removed by one cutting edge of the tool (Chip load is also sometimes measured as feed per tooth). Chip load is an important factor in tool life because it determines how much heat will be carried away from the cutting edge. Better heat dissipation directly relates to increased tool life which is one of the primary reason for the use of coolant.

Circular Interpolation[13]

Circular Interpolation is combining the movement of two linear axis’ into a smooth arced or circular motion. Since an axis can only move linearly, intellegent coordination of axis’ are required to create an arc. G02 causes the motion to be in a clockwise direction, while G03 is counter-clockwise. Circular interpolation requires an endpoint, a feed rate, a center, a radius, and a direction of movement.

Climb Milling[14]

A milling cutter can cut in two directions: Conventional and Climb. Climb Milling on the other hand moves the cutting tool the same direction that it wants to travel. Each tooth engages the material at a definite point, and the width of the cut starts at the maximum and decreases to zero. The chips are disposed behind the cutter, leading to easier swarf removal. The tooth does not rub on the material, and so tool life may be longer. However, climb milling can apply larger loads to the machine, and so is not recommended for older milling machines, or machines which are not in good condition. This type of milling is used predominantly on mills with a backlash eliminator or ballscrew setup. The nature of climb milling is that it will pull the workpiece into the tool and so if there is any backlash in the table, that “slop” will be sucked into the workpiece with dangerous results. <to do copy in images, duplicate text in Climb vs. Conventional milling, make link, edit further>

Collet[15]

A collet is a mechanical device used to hold a tool or workpiece. Some of the more popular collet standards include:

• 5C Collets (Invented by Hardinge, commonly used with lathes)
• ER Collets (DIN 6499) R-8 (The Bridgeport standard)
• 16C
• 3C
• MT (Morse taper 0-7; DIN 228-1)
• 3J
• WW

The collet works by squeezing the tool or workpiece more and more tightly as the clamp is drawn up against a taper to squeeze it shut. This compressing by drawing into a taper is usually accomplished via a drawbar (really just a long bolt) pulling the collet into the taper (5C and R-8 both use drawbars), or a threaded cap that pushes the shoulder of the collet more deeply into the taper (ER collets use this method). <to do copy in images, note which style is typical for Dremels>

Contouring[16]

Contouring is very simply the process of cutting smooth a smooth continuous curve or surface in CNC. Contouring by definition is impossible to do with manual mills and lathes since it requires the simultaneous movement of two or more axis to perform the operation.

Conventional Milling[17]

A milling cutter can cut in two directions: Conventional and Climb. Conventional milling moves the cutting tool opposite the direction it wants to travel. The chip thickness starts at zero thickness, and increases up to the maximum. The cut is so light at the beginning that the tool does not cut, but slides across the surface of the material, until sufficient pressure is built up and the tooth suddenly bites and begins to cut. This deforms the material, work hardening it, and dulling the tool. The sliding and biting behaviour leaves a poor finish on the material. Surface finish is also poor because chips are carried upward by teeth and dropped in front of cutter. There is therefore a lot of chip recutting. Coolant can help carry away some of those chips and increase the quality of the finish. Conventional milling creates upward forces that tend to lift the workpiece during face milling. <to do copy in images, duplicate text in Climb vs. Conventional milling, make link, edit further>

Conversational CNC[18]

Conversational CNC is a type of G-Code programming. There are actually three types of programming methods, manual programming , conversational programming (which is also called shop floor programming), and computer aided manufacturing (CAM) system programming. Each has it’s place and application. Conversational CNC can most easily be thought of as a G-code “wizard.” For example, one wizard might be for cutting a circular pocket. The wizard asks for critical input data like feedrate, diameter of pocket, step size, diameter of cutting tool, etc and then outputs G-code straight to the control software. Conversational CNC tools are usually best used for one-time machining operations.

Coordinate Word[19]

In standard G-code a coordinate word is a section of code that begins with an axis letter followed by a position. For example, Y2.0175 is a coordinate word that may be part of a larger block of code.

Crash[20]

A crash is the unfortunate condition of “crashing” your tooling into a limit switch, workholding or some other unintended obstacle. It can result in broken tooling or broken machines, but on a ShapeOko, in the X and Y-axes, normally just results in skipping steps on the belt. There are several methods of avoiding costly crashes:

1. Perform dry runs
2. Use simulation software
3. On untested parts files cut s-l-o-w-l-y
4. Setup soft limits

Crashes can sometimes be a result of hardware failure. For example an encoder can stop responding and in an effort to catch up to the non-responsive encoder, the motor will accelerate to full speed resulting in some rather spectacular crashes!

Cutting Force[21]

Cutting force refers to the force exerted on the workpiece by the tool. High cutting forces can potentially cause deflection, inaccuracy, chatter, and broken tooling. Cutting force increases with cut depth, material hardness, and friction coefficient. Cutting forces are inversely proportional to rake angle. Power required increases with the feed rate.

Cutter Radius Compensation[22]

Cutter radius compensation allows a program to be written without considering the size of the cutter being used. Three G codes are used to control compensation G40, G41 and G42. They are group modal.

• G40 cutter compensation off, centre line programming.
• G41 cutter compensation to the left of the programmed path.
• G42 cutter compensation to the right of the programmed path.

Cutter Offset[23]

Cutter offset is the distance from the surface of part to bottom of tool along the z axis. In practice cutter offset is a predetermined distance from the surface of the workpiece that allows for the safe and rapid movement of the cutting tool between cutting operations.

D

Depth of Cut (DOC)[24]

Depth of Cut is how deep the tool is under the surface of the material being cut. The depth of cut will determine the height of the chip produced. Typically, the depth of cut will be less than or equal to the diameter of the cutting tool. It takes more power to run a higher depth of cut and so slower feed rates and/or spindle speeds are usually necessary. The depth of cut is almost always less than the diameter of the cutter.

E

E-Stop / Emergency Stop[25]

An e-stop or emergency stop is the control that stops all machine operation in the event of a crash, runaway machine, or some dangerous or potentially dangerous situation. True emergency stop devices cut power to spindles, drives and any other powered element of a machine so that all sources of potential danger can be eliminated with a button.

Endmill[26]

An endmill is a type of cutting tool used in industrial milling applications. It is distinguished from the drill bit, in its application, geometry, and manufacture. While a drill bit can only cut in the axial direction, a milling bit can generally cut in all directions, though some cannot cut axially. Endmills are used in milling applications such as profile milling, tracer milling, face milling, and plunging.

F

Face Mill or Face Milling[27]

A face mill is a type of mill cutter that contains multiple cutting teeth and is often used to remove large amounts of material. In face milling, the cutter is mounted on a spindle having an axis of rotation perpendicular to the workpiece surface. The milled surface results from the action of cutting edges located on the periphery and face of the cutter.

Finishing Pass[28]

A finishing pass is usually the last pass over a part characterized by higher spindle speeds and a shallower depth of cut in order to improve the finish of a part and increase tolerances.

Flutes[29]

The term flute refers to the groove on the periphery of a cutter that allows for chip flow away from the cut.

L

Lead-In and Lead-Out[30]

The terms “lead-in” and “lead-out” refer to how a CNC part program approaches and leaves the part when cutting. Most CAM programs have parameters for describing the type of approach and exit strategy that will be employed in cutting a part. These cutting strategies can range from directly plunging and retracting the tool (no lead-in or lead-out) to arching toolpaths which “kiss” the origin after a soft lead-in. These cutting strategy decisions vary based on workholding, fixtures, material choices, etc.

Limit Switch[31]

The term limit switch is a generic term for a sensor or switch at the end of an axis which is placed there to trip an emergency stop situation if the axis for some reason travels that far. In a perfect world, limit switches would never be triggered, but in the real world they are critical to avoid machine self destruction. NEVER run a CNC machine without limit switches on both ends of all axes fully operational. Limit switches can be made from microswitches, hall effect sensors or optical switches. Because inputs on CNC machines are often times limited, it is a common practice to tie the switches together so that all limit switches trigger use the same input.

P

Peck Drilling[32]

Peck drill is a “canned cycle” drilling operation in which the bit advances into the hole retracts to clear chips and/or flood the hole with coolant and then advances further, retracts, etc. Peck drilling is often used for holes that are three or four times deeper than the drill diameter.

Pocket Milling[33]

Pocket milling is an interior recess that is cut into the surface of a workpiece. Pockets may be round, rectangular or an arbitrary shape.

Pocket Milling (illustration courtesy of CNC Mentor)

S

Stepover

The offset from one toolpath to the next adjacent toolpath.

Rule of thumb: The stepover should be between 1/3 and 1/10 of the tool diameter.

Great tutorial from CNC Cookbook - How To Choose a Stepover