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Loading...Beginner’s Guide to CNC Machining in Wood – Understanding the Machines, Tools, and Software
Ralph Bangall
CONTENTS
Introduction
Gallery of CNC Projects
How to Use This Book
Frequently Asked Questions
Chapter 1: Getting Started
Setting Up a Workspace
Choosing Your First Machine
Alternative CNC Machine Formats
Basic Operations
Choosing Bits for Your CNC
Chapter 2: Basic Fittings
Making a Spoil Board
Making Your Own Clamps
Chapter 3: Practical Shop Projects
Push Sticks
Zero-Clearance Throat Plates
Safety Guards
Chapter 4: Shaping and Lettering
Carved SignDecorative Luminary
Tea Trivet
Chapter 5: Combining Techniques
Logo Luggage Tag
Wine Bottle Spaghetti Measure
Custom Coaster Set
Chapter 6: Inlays and Carvings
Box Lid Fox Inlay
Ibis Photo Carving
Grapevine Relief Carving
Glossary
Sources
GLOSSARY
This glossary is not meant to be regarded as an official list of definitions. I
have created it to clarify terms as I use them in this book. Like many
sciences and professions, terms used by CNC programmers and operators
have sometimes evolved to be different than the common usage. Use this
glossary as a reference when reading to understand unfamiliar terms as you
encounter them.
Allowance – When the toolpath is automatically offset from any raster line
by a predetermined amount. Useful for inlays and for making minor
adjustments to holes and pockets without needing to re-draw the part.
Bit – The cutting tool mounted into the router or spindle of the CNC
machine. This may be specially manufactured for use in CNC machines but
does not need to be. Almost any common router bit that is not bearingguided can be used in a CNC. (See also tool.)
Bridge – Component that moves forward and backward along the frame of
the CNC. It holds the head, which moves side-to-side across the bridge. The
front-to-back and the side-to-side motions make up the X- and Y-axis, but
which is which may vary between machines.
CAD – Computer Aided Design; a computer program that allows the user
to create drawings in scale with dimensions.
CAD/CAM software – Computer programs that combine both drawing
functions with toolpath functions to allow programming CNC machines in
one software package.CAM – Computer Aided Manufacture; computer programs that allow for
associating physical milling operations to individual details contained
within a vector drawing.
Cartesian coordinates – A system of locating objects within a field by
defining points across a plane. First described by René Descartes in the 17th
century, the system is how all machines can work accurately within a
known area. Starting from a point chosen to be zero, all positions can be
plotted by moving known distances in known directions. The system allows
all points to be accurately returned to when needed. (See also coordinate.)
CNC – Computer Numerical Control; Guiding machine processes and
movement using software programs. The term has come to mean the
machines themselves even more than a method of control.
Collet – A clamping piece used to hold router bits within the spindle or
router motor. Most common in the US are ¼″ and ½″ (6.4 and 12.7mm)
diameter, but other sizes may be available.
Coordinate – The measurement of distance in any axis from the center
point. Coordinates are generally used as “absolute” numbers that reference
back to the zero point, where a “relative” move is covering a specified
distance from the current point. (See also Cartesian coordinates.)
Datum – The anchor point of a CAD drawing or CNC program. It is
defined as 0 in all three axes and all items in a drawing or movements in a
program relate to it. The datum can be reset, and all other points will move
in relation to the new datum. (See also start point.)
DXF file – Just as G-codes are a universal standard language to operate
different types and brands of CNC machines, the “.dxf ” file format allows
designs from different CAD programs to be easily and accurately shared.End mill – A rotary cutting tool used in CNC machines. The term “end
mill” is usually associated with metalworking tools rather than wood, but
most CNC router bits can be considered end mills.
Feed rate – The speed at which the router bit is moving across the part as it
is cutting. This is usually stated in “inches per minute” or “mm per second.”
It is different than the RPM that the bit is spinning at. (See also RPM.)
Fillet – Radius added to a square corner or edge. On corners, it can be used
to round the 90° angle to the radius you select. But because router bits
cannot cut square inside corners, many CAD/CAM programs can
automatically add a “reverse fillet” opening and extra radius into the corner
to cut beyond the radius of the bit, removing the rounded corner.
Flat depth – When carving with a V-bit, the depth of cut is variable and is
defined by the bit meeting both sides of the vector being cut. The “flat
depth” command allows for limiting the depth of the carving, preventing it
from cutting deeper than desired.
G-code – A text file created from the CAD/CAM software that actually
runs the CNC machine. This is a universal set of instructions that can be
adapted to nearly any model of machine. Common commands all begin
with the letter “G,” which is where the name comes from. (See also post
processor.)
Going home – Typically, this is a button command on a controller that
automatically returns the machine head to the current zero or start point.
This may or may not be the machine’s mechanical origin point.
Hard home – Most CNC machines have a mechanical origin point that
cannot be changed where all operations are computed from; I call this the
hard home. It differs from other home positions because it cannot be easily
changed, if at all. (See also home.)Head – Component that holds the router or spindle to which the tool is
attached. This section can move in the X-, Y-, and Z-axis.
Home – Refers to the mechanical origin of the machine itself, preset origin
points that can be selected, or a predetermined start point for a particular
program. (See also hard home, saved home, or temporary home.)
Homing – An automated process the machine may need to go through to
physically find the mechanical origin point upon system startup.
Interpolate – Milling a hole to diameter using a bit that is smaller than the
hole. The cut starts in the center and the bit curves outward to the final
diameter. This improves the bit’s life by reducing overheating and allows
for easily adjusting the size of the hole being milled.
Jog – Manual movements of the head. This can be done by pressing a
button and visually moving the head, or by entering a set distance for the
head to move on command.
Lithophane – Originally porcelain art in which shallow figures are made
visible by light shining through. In CNCs, this can be accomplished by
carving photo-negative images in acrylic, showing the image when backlit.
Max carving depth – V carving toolpaths can cut quite deep if vectors are
far apart. An overall depth can be set to limit the depth of the cut.
Onion skinning – Technique for holding workpieces that leaves a very thin
(1/32″/1mm) layer of material behind by intentionally cutting too shallow.
Tends to leave a cleaner edge on the part. (See also tabs.)
Post processor – A translation program that takes all of the data within the
CAD/CAM program and rewrites it into a G-code file that is tailored to a
specific type of CNC machine. This allows any CAD/CAM software to
create G-codes that will properly control a wide variety of different CNC
machines just by selecting the correct post processor.Raster – Art files that are made up of pixels such as bitmaps and JPEGs.
Rasters cannot be read nor used in programming CNC machines, they must
be converted to vectors. Programs exist that can convert rasters to vectors,
and most modern CAD/CAM software packages have some level of rasterto-vector conversion. (See also vectors.)
Relief groove – A cut made to remove much of the waste material from the
path of an undercut bit. Relief grooves make for cleaner cuts and less
overheating when forming an undercut groove. (See also undercut bit.)
Router – The motors from handheld routers often used to power the tools
in an entry-level CNC machine as opposed to a dedicated spindle. (See also
spindle.)
RPM – Revolutions per minute; a measurement of how rapidly the router
bit is spinning as it cuts. (See also spindle speed.)
Saved home – Home positions that remain in the memory—even through
system startup—until the operator resets them. (See also home.)
Scarf joint – Originally a shipbuilding term, a scarf joint joins two planks
end-to-end using a long taper rather than a butt joint. With inlays, angling
the edges of both the pocket and the inlay hides minor variations in the
perimeter.
Secondary processes – Manufacturing steps that happen after milling on
the CNC, including cutting tabs to remove parts from the web or adding
threaded inserts into holes machined on the CNC.
Shank – Body of a router bit or end mill that is clamped into a collet when
held in a router or spindle motor.
Single line fonts – In CNC programming, single line fonts are typically
made from vector lines rather than the outlines from which TrueType lines
are made. (See also TrueType fonts.)Spindle – A motor that is purpose-built for use on a CNC machine. It has a
collet to hold bits and serves the same function as when a handheld router
motor is used. (See also router.)
Spindle speed – Essentially the same as the RPM setting on a handheld
router, this is the rotational speed of the motor turning the router bit. (See
also RPM.)
Spoil board – A secondary surface added to the machine bed of a CNC.
The primary purpose is to allow router bits to cut through stock without
damaging the bed of the CNC, hence the name; the board is there to be
“spoiled” instead of the machine. The spoil board can also be used for work
holding and be surfaced to provide an accurate Z-axis reference. (See also
table mill.)
Start point – Also called the “datum,” it is the base location from which all
the features of a drawing or locations in a program are calculated. It is
typically zero in the X-, Y-, and Z-axis. (See also datum.)
Tabs – Small, uncut sections left around the perimeter of a part to hold it in
place but allow it to be easily separated from the surrounding web. (See
also onion skinning.)
Table mill/surfacing – The process of using the CNC to mill a spoil board
or other fixture to create an accurate Z-axis reference surface. (See also
spoil board.)
Temporary home – A home position that can be set manually at any time.
It is not saved if the system is shut down or a different home is selected.
(See also home.)
Tool – Another word for the router bit used in the CNC for cutting
operations; also known as “tooling.” (See also bit.)Toolpath – The set of instructions that control how a cut is made: the tool
to use, depth of cut, feed rate, tool offset, and similar information.
Toolpaths are divided into a number of categories that determine which
parameters can be set and how. Common toolpaths include:
Profile toolpaths are typically used for cutting parts out.
Pocket toolpaths cut recesses in the stock where all the selected area is cut
to the same depth.
VCarve toolpaths incise letters and graphics using a V-bit. These are highly
automated to mimic hand carving.
Drilling toolpaths automate the creation of holes where the proper vectors
are or in patterns that can be selected.
TrueType font – Letters made up as outlines rather than single lines.
TrueType fonts work very well for V-carving. (See also single line fonts.)
Undercut bit – Slot-cutting bits that cut a larger interior opening than at the
surface. (See also relief groove.)
Vector – Lines drawn between known points in a CAD drawing. Because
all of the movement commands within a G-code program need specific Xand Y-coordinates, CAD/CAM programs require vectors for the drawings
rather than image files, such as bitmaps or JPEGs. (See also rasters.)
Web – When parts are cut out of a larger blank, the leftover material from
the borders and between the parts is known as the web because it holds
everything together.
Work holding – How stock is clamped to the machine during milling. It
can be as simple as double-faced tape or as complex as vacuum hold-down
fixtures.Working envelope – The area that the machine router bit can actually reach
during operation. This is always smaller than the machine bed size.
Zero point – This term usually refers to the start point or origin of a
program. (See also start point.)
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