Ladder Diagram
A ladder diagram is an electrical schematic drawn according
to a convention that has the two ladder "rails" being power
supply lines and the "rungs" of the ladder between circuitry
that passes between the two lines. The left line is normally hot and the
right line is ground. The practice is used extensively for machine control
systems and is useful because for troubleshooting one simply checks out
each "rung" until the problem is found. A circuit drawn in this
way is sometimes referred to as "Ladder Logic" or "Relay
Logic".
Some great examples of ladder diagrams and more detail
may be found on the Seattle
Robotics site.
Ladder Logic: See Ladder Diagram
Lapping
Lapping is a process of rubbing two mating surfaces together
with an abrasive grit in between to ensure they fit smoothly and closely
together. Lapping does not necessarily
ensure flatness, in fact it may achieve just
the opposite effect unless carefully applied.
Laser Interferometry
Laser interferometry is a very high precision method
of measurement that is commonly used to align commercial CNC tools.
Lead
The axial distane the nut advances in one revolution of the leadscrew. The
lead is equal to the pitch times the number of screw starts. See Also Screw
Starts.
Lead Accuracy
The faithfullness of actual distance
travelled versus what it theoretically should be given the number of turns
made. All leadscrews have some inaccuracy. Leadscrew mapping is a generic
software technique used to compensate for it assuming there is an accurate
map of the errors.
Lead-in / Lead-out
The terms "lead-in" and "lead-out"
(also leadin and leadout or lead in and lead out) refer to how a CNC program
approaches and leaves the part before cutting. Typically, these are parameters
used by CAM software (See also CAM) to automatically generate the part
program needed to machine a part.
Lead Angle
Lead Angle refers to the angle of engagement
for cutters, usually milling cutters such as face mills. A 90 degree face
mill cuts a square shoulder, while a 45 degree facemill leaves an angled
shoulder. The more angle from the vertical, the greater the lead angle.
Why use a higher lead angle? Because it allows a higher feedrate. A 45
degree facemill can be fed at 1.4 times the feedrate of the 90 degree
facemill and will generally deliver a better surface finish while doing
so. This is because of the relative chip thickness or chipload of one
cutter versus the other:
Notice that when fed 0.5" the actual
chip thickness of the 45 degree cutter is only 0.35", hence the higher
feedrates...
Leadscrew
A generic device for translating rotary
to linear motion. There
are many different kinds of leadscrews including ACME and ballscrews (See
Also ACME and Ballscrew).
Leadscrew Mapping: See Ballscrew
Mapping
Limit Switch
A sensor used to detect the end of travel for an axis. Limit switches may also function as Home Switches.
They are typically mechanical switches (such as microswitches),
Hall Effect devices, or optical devices.
In order to reduce the number of inputs a CNC controller
must handle, limit switches may be ANDed or ORed together. For example, normally closed limit switches
would be ANDed together by wiring them in series so that if any of them
is tripped, the whole circuit is tripped.
If the switches are normally open, they may be ORed together, by
wiring them in parallel, so that if any are closed, the whole circuit
is tripped. It is up to the CNC controller to determine
by context (i.e. which axis it was trying to move when the limit was tripped)
which limit was actually tripped when they’re all ganged together like
this.
Linear 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.
Linear Motion
Motion in a straight line and all those components and
technologies associated with that motion.
Linear Power Supply
A Linear Power Supply converts AC current to DC current
that has been smoothed, but that may not be regulated to a precisely controlled
voltaged (See Also Regulated Power Supply).
Linear Power Supplies are simple and are typically used to power
driver motors such as servos and stepper motors.
A Simple Linear Power Supply
With Transformer, Bridge Rectifier, and Smoothing Capacitor...
Linear Scale
A linear scale is an electronic device used to measure
a position along a line. Linear
scales are commonly used in DRO’s (See Also DRO) and sometimes to provide
feedback information in sophisticated closed loop systems (See Also Closed
Loop).
A Linear
Scale Used as DRO...
Linear Slide
Linear slides are components that may be used to create
Linear Ways (See
Also Linear Ways).
They consist of a rail and a truck that rides on the rail using
ball bearings between the two to create minimum friction and maximum accuracy.
Linear 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.
Live Tooling
Powered tooling, such as a drill, that
may be held in the turret of a lathe.
Load Meter
Load meters are common on CNC machines. They give the
operator one way to gauge how hard the machine is working, which is something
that's harder to do on CNC where there is no manual "touch".
Usually load meters are just ammeters that measure the amount of current
the spindle motor is drawing. Some machines also offer load meters on
the individual axes, for example to measure how hard it is to feed.
Load meters have a lot of interesting uses. For example,
during roughing, the load meter can be used to tell when feeds and speeds
and DOC are aggressive enough to be near the machine's capacity. Another
use is to observe the load meter while drilling a hole. If the load increases
the deeper into the hole the machine gets, it is an indication that chips
are not being evacuated well enough. It may be time to fool with the coolant
delivery or look at other steps to improve chip evacuation.
Lookahead
Lookahead is a software feature whereby the CNC control
program looks at future G-Codes it will need to execute in an effort to
plan better what it should be doing. If
the G-Codes ahead call for sudden slowdown or acceleration, the system
may want to begin that change earlier for better performance.
Lost Steps
Lost steps are a problem that can occur when using open
loop stepper systems. Because the
system is open loop, there is no feedback to tell the controller whether
the axis actually reached the position it had been commanded. If there is a problem of some kind, ranging
from a machine crash to just simply not enough power to get to the destination
as quickly as commanded, the machine will go on thinking it is in a different
position than it really is. The
result is a bad part or a worse crash.
There are a couple of things that can be done to reduce
or eliminate the likelihood of lost steps.
First, the machine and program can be designed or operated very
conservatively. Use slower cutting
and travel speeds. Install larger
step motors that are capable of higher performance.
Second, a closed loop system can be employed to provide feedback
to the controller on whether the axis moved as it was commanded to. This involves either replacing the step motors
with servo motors equipped with encoders, or adding encoders to a step
motor-based system, which in the end makes it a servo system by definition
anyway.
To get some idea of the impact of lost steps on
machine accuracy, you may want to visit my Stepper/Servo/Backlash
Simulator Page.
Lovejoy Coupler (see "Oldham Coupler")
Mach 3
Mach 3 is a very popular CNC Controller that runs on
a PC under Windows and is sold by Artsoft.
Machine Coordinates
The absolute coordinates or position
of the axes. Machine coordinates
are always 0 or positive.
Machine Reference: See Machine Zero
Machine Zero
Movement along an axis in the farthest
possible positive direction. It is synonymous with the Home
Position of the machine. Many machines will have Home Switches at this
location so that the axes can be commanded to move until the Home Switches
are triggered, telling the machine it is exactly at Machine Zero. See
Also Home Switches. Other common names for Machine Zero are Machine Reference
and Reference Point.
Machining Center
A CNC tool that can perform multiple
operations in the same setup with automatic tool change.
Manual Pulse Generator or MPG
MPG’s are devices that are used to simulate the handwheels
used on manual machines. They usually
consist of a dial connected to an encoder with a suitable mechanism to
provide detents so there is tactile feedback to the operator when the
MPG is turned.
MPG
on my Lathe's Pendant...
M-Codes
The M-Codes are words defining miscellaneous operations
in the G-Code language. For example,
they command tool changes, turn on or turn off the coolant or spindle,
etc. M-Codes are usually fairly specific to the controller, and are
not well standardized. Some controller programs, such as Mach 3, will
even let you define your own m-codes for special purposes and then reuse
them.
MDI (Manual Data Input)
Manual entry and execution of G-Codes
from the keyboard.
Microstepping
A technique used to increase a stepper
motors smoothness and position it to finer degrees by using varying amounts
of current in adjacent windings to create more steps than the motor would
normally have. In essence, the
motor is delicately balanced between steps.
Microsteps can be down to 1/32 of a step or even much less. It is also useful in controlling stepper motor
resonance.
Microstepping is a function of the stepper motor driver,
rather than the motor itself. Gecko
drivers, for example, offer microstepping.
It is important to remember that the holding torque of a microstep
is much less than that of a full step, so you can only microstep if you
are using a fraction of the motor’s capacity.
For example, with 16 microsteps, there is less than 10% of the
holding torque at a microstep that there would be on a full step.
The value of microstepping also falls off rapidly as the motor’s
speed increases. For this reason,
it is usually not wise to assume that the resolution of a system is equivalent
to the full resolution of a microstep.
Microswitch
Microswitches are simple electrical switches that make
contact with very little travel, making them useful as home and limit
switches. See also Home Switch
and Limit Switch.
Mist Cooling
Mist cooling employs droplets suspended in a compressed
airstream as coolant (See Also Coolant).
They are a good alternative when flood cooling is impossible.
ModBus : See ModIO
Mode
In G-Code programming, a mode is a programmed function
that stays in effect until it is cancelled or substituted by another function.
Model: See Solid Model
ModIO
The ModIO
is a ModBus device made by Hohman Designs.
It provides Mach3 and other programs with a lot more inputs and
outputs using a USB port.
Moglice
An epoxy resin
with embedded PTFE (See Also PTFE and Acetal) that is used to refurbish
or line machine ways. See www.moglice.com.
Motion Control
Motion control is the science and technology of controlling
motion. The components used to
convert a manual machine to a CNC machine are all motion control components.
Motor Driver: See Driver
MPG: See Manual Pulse Generator
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