CNC Dictionary: Tangential Control to Workshift Offset

CNC Dictionary: Tangential Control to Workshift Offset


Tangential Control

A Mach 3 feature that lets the A-axis rotate so that a knife or other cutter is kept aligned with the direction of travel.


There are many tapers associated with CNC including ISO, BT, CAT, R-8, Morse, Jacobs, SK, and HSK.  Unlike almost all other tapers, R-8 and Morse can refer to both a collet and a toolholder shank.

Most commercial CNC machines in the USA use a CAT taper which is a modified NST 30, 40, or 50 taper that uses a pull stud and a groove in the flange. The machine pulls on the pull stud to hold the holder in the spindle, and the groove in the flange gives the automatic tool changer something to hold onto.  The CAT taper is an ANSI standard.  It is called CAT because it was a standard created by Caterpillar, at one time the world’s largest consumer of CNC machines, when they grew tired of dealing with the large number of standards that existed at the time.

HSK holders are a more recent attempt to improve on the CAT taper, and they are gaining slow acceptance.

The R-8 taper was introduced on the Bridgeport manual milling machines.  It is generally used on smaller CNC machines.  The Morse tapers are more popular in Europe as an alternative to the R-8.

There are a number of links with specs for Machine Tapers on our Reference Sources page.

Tapered Gibs

There are two types of gibs, straight gibs and tapered gibs. Straight gibs are adjusted by screws spaced out along the length of the gib. The screws push the gib in to create more contact with the sliding mechanisms. Tapered gibs use one or two screws. The screws are located in each end of the tapered gib. One screw acts as an adjustment while the other screw acts as a locking mechanism. Because tapered gibs are wider on one end than the other, they slide in or out creating more or less contact between the sliding mechanisms.

Tapered and Straight Gibs

Tapered gibs are simpler to adjust because you don’t have to try to balance out the force on all the screws as you would with straight gibs, and they often provide better contact and more uniform wear. Higher end machines generally use tapered gibs in preference to straight gibs, although not exclusively.

See also Gibs and Gib Adjustment.

Tapping Head

A tapping head is a device that goes into a machine spindle (could be a mill or a drill press) for the purpose of matching the spindles speed with the motion that is ideal for tapping. Tapping heads have clutches, gears, and may have reversing mechanisms. If equipped with a reversing mechanism, the head will reverse the direction of rotation when pressure is released so that the tap may be withdrawn. Use of a tapping speed provides a quick and efficient means of driving a tap into the workpiece.

A Tapping Head…


A word that selects a specific tool in a part program.

TEA: See Tool Engagement Angle


Tempering is a heat treating process that is used to reduce the hardness of a material to a defined level. Often it may not be desirable to use the full hardness possible with the allow, perhaps because it will be too brittle or too hard to work. Tempering goes back after the initial hardening treatment and raises the material to a particular temperature associated with the desired hardness.

THC: See Torch Height Control

Thread Milling

Thread milling is a method for producing a thread on a milling machine. To cut a thread using a mill, helical interpolation (see also “Helical Interpolation”) is used to describe the helical motion of the threads. A single point cutter is used to cut one or more threads.

A lot of useful information on thread milling is available in the Vardex Thread Milling Handbook.

Timing Belt

A toothed or ribbed belt where the teeth run all the way across the width of the belt.  The corresponding pulley has grooves that fit the teeth on the belt.  This is a common and very successful mechanism by which a drive motor may be connected to a leadscrew.  This is typically done to move the motor to a more convenient location or to allow there to be a ratio between the motor’s rotation and that of the leadscrew.


A workholding apparatus that stands up high like its namesake and is used to attach parts to its vertical sides. Tombstones can also be mounted horizontally on an indexer or 4th axis for even more flexibility and access to the various sides of a part. Other names for Tombstones include pedestal fixtures, tooling towers, tooling columns, and fixture blocks.

Toolchanger or Tool Changer

A device that allows a Machining Center to store and automatically switch between multiple tools.

Tool Engagement Angle (also called “TEA”)

The amount of circumference of the cutter, measured in degrees, that is involved in cutting at a particular point in the toolpath. TEA goes up rapidly in corners, and the higher the TEA, the more a tool is stressed. There are CAM Toolpath Strategies that take this into account and attempt to equalize TEA’s all over the toolpath to allow higher overall speeds.

Tool Length Offset

An offset used to account for variations in tool length.  It can be advantageous to set up all the tooling so that the same offset may be used for all tools, eliminating confusion about whether the proper tool length offset has been set up for a particular tool.

An important relationship to keep in mind is:

Total Height = fixture height + work height + tool height

Tool Nose Radius and Tool Nose Radius Compensation

The tool nose radius represents the radius on the nose or cutting edge of a single point tool for a lathe.  Compensation for tool nose radius is an offset to ensure the proper cut takes place based on the tool nose radius of the tool in use.

Tool Offset:  Another name for Tool Length Offset

Toolpath or Tool Path

The toolpath is the series of coordinate positions that determine the movement of a tool during a machining operation, or literally the path the tool follows as the CNC machine guides it through the work. Toolpaths may be manually generated by the programmer or automatically generated by a CAM program using an appropriate Toolpath Strategy as specified by the programmer. See also CAM.

Toolpath Simulation

An animation display that shows what the toolpath will be to produce the part.  Toolpath simulations are useful debugging and informational tools for the development of part programs.

Tool Post: See “Posting”

Tool Probe:  See “Touch Probe”

Tool Table

A tool table is a feature in CNC controller software such as Mach 3 that provides a list of tools along with information and parameters on how the tools are to be used.

Tool Pre-Setter

A device whereby the tool is advanced until it triggers the pre-setter to determine where the tip of the tool is and thereby set the appropriate offset.  Pre-setters are basically switches mounted out of the way but within reach, perhaps near the limit of travels.

There are some clever ways to create a pre-setter, for example, by completing an electrical circuit, perhaps between the toolholder and the chuck or vise holding the workpiece.

Pre-setters eliminate the need for the operator to measure tool offsets and enter them into the system.  They are also often used to check whether a tool was broken during an earlier operation so that the machine can be stopped and the operator can replace the broken tool.

There is information on implementing a tool pre-setter under Mach 3 on my CNC Tips & Techniques page.


Toolsetters are devices used to precisely set the overall height of a tool when in its toolholder.  CNC machines need to know how far the tool sticks out with precision and these machines help make that measurement consistent.

An alternative is a tool pre-setter, which measures the tool length on the machine.  The advantage of a toolsetter is the tools can all be set up in the holders ahead of time, which may save time on the machine.

Torch Height Control

A Torch Height Control or THC is used in plasma cutting applications to maintain a constant height over the work.  This enables the torch to track surface irregularities and warpage that can occur during the cutting. They work by measuring the tip voltage of the torch, which will be proportional to the gap between the torch and the material.

Toroidal Transformer

A toroidal transformer is a transformer wound on a round toroidal core.  See also Transformer.  They are favored over rectangular transformers because they are somewhat more efficient.


Torque is a measure of the angular force needed to produce rotation.

Touch Off

To determine the exact location of a feature or tool’s tip by touching the tool to the feature or a known location.  A sensor may be used, or often, a piece of paper of known thickness is placed between the tool and the feature and the tool is slowly advanced until the paper is just trapped between the two.  Advance too quickly and the tool or feature can be chipped.

Sensors include Touchsetters (see also Touchsetter) and dial indicator-based devices specially designed for the job. In some cases it may be possible to complete an electrical circuit when the tool contacts a particular touch point or even the work material, assuming it is a conductive metal.

A touch off is typically used either to establish a work offset or a tool offset. The type of touch off will also govern the appropriate technique. If one is determining a work offset, perhaps to establish Work Zero, the sensor must either be of known height and sitting on the workpiece, or the paper method could be used, or a circuit could be completed with the workpiece. If one is using a Touchsetter to establish a tool offset, the touchsetter is typically mounted in a fixed location on the tool and a macro in the part program is called each time the tool is changed to reset the offset.

Touch Probe

A touch probe is a sensitive device that can tell with a very high degree of precision when the probe touches another object such as the workpiece, vise, or part of the machine.  They typically have a small tip with a ruby or sapphire.  They have a variety of applications such as digitizing.

Touchsetter:  See Tool Pre-Setter

TPI:  See “Turns Per Inch”

Tramming a Mill or Vise

Tramming a mill is the process of adjusting a moveable head until it is square with the table.  See also Squaring.

Tramming a vise is the process of adjusting the vise until the jaws are square with the desired axis.

Tramming is normally performed with the aid of a dial test indicator or dial indicator and some sort of holder such as an Indicol to attach the indicator to the spindle.

Hints on how to tram in a mill or vise.


A transformer is an elecronic component used to transform voltages.  Depending on how many windings are on the primaries and secondaries, voltages may be increased or decreased.  Transformers are a common component in power supplies, and may be either conventional rectangular designs or round toroidal designs.


The distance a particular axis can move is referred to as its travels.


A type of boring that involves cutting only a ring and removing a solid plug of material from the hole.

A trepanning tool…


The science of friction, lubrication, and wear when two contacting surfaces are in motion relative to one another. contains a lot of useful reference information on Tribology.


T-Lathes are intended to spend all of their time facing. Most do not have a power feed in Z.


T-Slots are a common mechanism for attaching vises, rotary tables, and clamps to the table of a machine tool.  They resemble an inverted T.  A nut of the same shape slides into the T-Slot and studs are threaded into the nut from above.

Turcite: See Acetal

Turning Center

A Machining Center (see also) that can spin the workpiece in order to cut it.  More generically called a lathe.

Turns Per Inch or TPI

The number of turns required to advance a leadscrew one inch.


The component of a lathe that holds a number of cutting tools.  Analagous to a toolchanger on a mill.

Twisted Pairs

Twisted pairs are a wiring technique used to increase noise immunity (See Also Noise). The idea is to twist a signal wire around it’s corresponding ground wire. A noise spike will affect both wires equally, and so the ground potential will rise along with the signal. Since most circuits only consider the difference between the two, the noise will be ignored. CAT5 LAN wiring is a typical twisted pair system that uses 4 twisted pairs or 8 conductors. If the corresponding grounds are not paired with their signals, there is little noise immunity provided by a twisted pair approach. The twisted pair approach is commonly called a Balanced Circuit.


An undercut on a mill is when the cutter has to reach under a shelf, so that the cutter is working on a surface not visible directly from above.  Obviously the cutter must stick out from the shank in some way and there must be clearance to get the cutter into position under the shelf.

Unipolar Stepper or Driver

Stepper motor controllers can be either bipolar or unipolar. The term bipolar means current can be supplied to the windings of the stepper motor in either direction. The alternative configuration is unipolar (see also Unipolar), which involves using a center tap and only half the windings at a time. Bipolars have the advantage of producing 40% more torque for a given number of windings, all other things being equal. The controller for a bipolar motor is slightly more complex than unipolar, and the bipolar may run hotter, but in general, bipolars deliver higher performance.

Step motors are typically 4, 6, or 8 wire devices. Bipolars have no center taps, so if there are only 4 connections, the motor is a bipolar only design.

Up or Uphill Milling: See Climb Milling


Universal Serial Bus.  This standard has largely replaced the old parallel port standard as a way for PC’s to communicate with other devices as well as the older RS232 serial standard (See Also RS232).  Unfortunately, most CNC software and breakout board capabilities have not kept up with some notable exceptions such as ModIO (See Also ModIO).

USB Key: See Also Dongle

Variable Helix Endmill

A unique endmill design that tries to serve as both a roughing and finishing endmill (See Also Roughing and Finishing). One approach on a 4 fluted endmill is to have 2 flutes the smooth finish style and 2 flutes the serrated rougher style. Another approach varies the helix angle (See Also Helix Angle) to achieve greater versatility.

Being able to use the same cutter for roughing and finishing can save time in a manufacturing process, which makes it worth the higher cost of variable helix endmills.

Vector Drive

VFDs or “Variable Frequency Drives” are great gadgets that go between the power source and a motor and allow the motor’s speed to be continuously varied. They do this by means of something called “Pulse Width Modulation” where they are essentially varying the frequency of the current going to the motor. Thing of the frequency as governing how many “pushes” the motor gets. 60 Hz or 60 cycles per second is normal. A typical Hitachi 3HP VFD can deliver 0.5 to 360 Hz, which is quite a speed range!

Enter a new and better gadget, which has the really sexy name “Vector Drive.” A white paper by Reliance Electric gives the full details, but in a nutshell, these drives are called vector drives because they understand the vector relationship between magnetizing current and torque producing current and they do the right thing with that.

What does doing the “right thing” buy us? Well, several things are better for vector drives than plain old VFD’s:

  • A VFD can maintain constant HP over a 2:1 rpm range, but a Vector Drive has a much broader 4:1 range.
  • A VFD can maintain a speed accuracy of 1%, but a vector drive will keep it to 0.01%, which is similar to the accuracy of a servo.
  • Lastly a vector drive will deliver about 50% more starting torque to get a motor moving, so they are good for applications that have to start and stop suddenly and accurately.

Are we beginning to see why modern CNC machines like vector drives for spindles?

What’s the downside? Largely cost. An equivalent vector drive probably costs 20 to 30% more than a VFD of the same horsepower capacity and it will require you to install a tachometer input to give it feedback on what the spindle is doing.

Vertical Machining Center

A Machining Center (See Also Machining Center) intended for mlling operations whose spindle is vertical.  Think of them as very sophisticated CNC milling machines.

Vertical Turning Center

A Machining Center (See Also Machining Center) intended for turning operations whose spindle is vertical.  Think of them as very sophisticated CNC lathes.

VFD (Variable Frequency Drive)

A device used to vary the speed of an AC motor by varying the frequency of the electrical current sent to the motor.  They are a very effective way to control spindle speeds in that respect.  Many models have an input that allows the CNC control program to vary spindle speed as needed. See also “Vector Drive”.

VMC (see “Vertical Machining Center”)

Wall Wort or Wall Wart

A small power supply that is attached to the plug and sits directly on the wall outlet.  They have very low current capability but are useful to power breakout boards and other logic that does not require much current.

Way Covers

Way covers are used to protect the ways and leadscrews from chips, coolant, and debris produced during machining operations.  They reduce wear and preserve accuracy.  See also Bellows and Clock Spring Covers.

Wear Offsets

An offset that allows for the slight adjustment of tool tip location to account for part deflection, tool wear, etc.

Whipping (for leadscrews)

Whipping is the tendency for leadscrews or other long objects to whip back and forth when rotated while unsupported at one end or when too long and thin for the supports provided at one or both ends.  To reduce whipping, choose a larger diameter leadscrew or support it better.

Wiper (for leadscrews and ways)

A wiper is a piece of felt, rubber, or other material that is used to keep out chips, coolant, and other debris from sensitive metal-to-metal contact areas of ways and leadscrews in order to reduce wear and ensure proper operation.


In the G-Code language, a Word consists of a letter code and a numerical value.

Work Envelope

Effectively, the region where the cutter can reach on a CNC machine.  It is determined by the amount of travel the axes have in relation to one another.

Work Hardening

The tendency for the workpiece to become harder as it is machined.  Many allows of stainless steel work harden.  It is important to know whether the material you are machining work hardens and to take appropriate steps when machining work hardened material.  In essence, you want to cut through the area that hardens quickly, before it has a chance to harden very much.

Work Offset

Work offsets allow the coordinate system to be offset from Machine Coordinates for convenience.Work offsets may be selected by G-Codes G54 to G59. The origin as offset from Machine Zero by a Work Offset is called Work Zero. See Also Origin, Machine Zero, and Work Zero.

An important relationship to keep in mind is:

Total Height = fixture height + work height + tool height


What you are machining.

Workshift Offset

An offset that shifts the machine spindle the same amount for every single tool.

Work Zero

The location the part program regards as the origin it will begin at each time.  It is different for each part program and part of the design of the program. Work Zero is a new origin established using Work Offsets (See Also Work Offsets) to shift the origin from Machine Zero. Work Zero often corresponds to a point called out on the print for the part. Also called Program Zero.

For simplicity, a lot of CNC programmers like to set their Work Zero Z = 0 to be the top of the material being machined. That means any positive Z coordinates are clear of the workpiece and any negatives are cutting travels. This is a convenient convention in many cases but is by no means the best possible approach to every situation.

When you draw your part in the CAD program, be sure to have a point on the part that corresponds to Work Zero = X0Y0Z0.

X-Axis: See Axes

Y-Axis: See Axes

Z-Axis: See Axes

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Recently updated on April 29th, 2024 at 12:06 pm