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Why a ShopBot Looks and Works the Way It Does ...
Not everyone will want to read this section and
certainly they don’t need to. But for those interested in the design and
engineering choices made in configuring and producing ShopBots, we
provide the following information. In particular, we’ve tried to
indicate areas where affordability dictated compromise and how we
handled it, and areas where performance priorities remained paramount.
Basically, here’s why a ShopBot looks and works the way it does ...
Light but Stiff --
From a machining point of view, it is always better
to be heavier. Weight provides substance, mass, and rigidity. These
factors lead to smoother cutting. But these factors also lead to higher
costs, because all aspects of the motion system must be beefier. The
drive system -- motors and their electronics -- are a particular
challenge because their costs increase exponentially with the mass of
the system they need to move.
In addition, there is a practical problem with
“heavy”. It makes the tool less portable, more limited in the locations
it can be positioned or moved too, and considerably less friendly for
small shops. Some large CNC machines actually require a foundation for
their installation. Such machines do not fit well into the current
competitive emphasis on flexibility and lean production techniques,
which emphasize the importance of being able to readily reconfigure
production flow.
ShopBots are engineered of steel and aluminum as
relatively light tools (about 800 pounds for a 4x8 PRTalpha ShopBot),
but given that design choice has been made, to have frameworks that are
as stiff and rigid as possible. ShopBots are rigid enough to cut well …
and careful selections of cutters, spindle speeds, feed rates, and
cutting strategies and will increasingly approach the cutting
characteristics of much heavier machines. Moreover, a ShopBot can be
repositioned on the production floor, or broken into modules for
transport to another location. For small shops, it can be set up in a
room with only a small door, in a basement, or above the ground floor.
Closed Loop Steppers --
Stepper motors such as those employed on a
PRTstandard ShopBot are a more affordable solution to producing CNC
motion than are servo motors. They have the limitation, that when
overpowered they can lose synchronization with the computer controlling
their motion. Servo’s, which can also be overpowered, have the advantage
of being able to detect a failure and alert the operator. However, when
used appropriately, stepper motors provide highly accurate positioning
(intrinsically more accurate than a servo) and reliable motion and
positioning, and represent a technology that has been used industrially
for 50 years. The stepper motors in a PRTstandard tools are made by
Oriental Motor (OM), a leading Japanese producer of high-quality stepper
motors. In the last ten years, ShopBot has shipped almost 18,000 of
these motors and have had fewer than 10 failures. They can be counted on
for day-in, day-out production.
ShopBot’s high-performance PRTalpha CNC tools use
alphaStep motors and drives, also made by OM. These are technically
advanced, closed-loop stepper motors that combine the precision and fast
acceleration of steppers with the closed-loop features of servo motors.
AlphaStep motors help make our PRTalphas one of the fastest CNC tools
under $100,000 offering impressive production capabilities in an
affordable tool (positioning speeds up to 1,800 inches/min; cutting
speeds up to 600 inches/min). For our performance tool we considered
servos, but became convinced that the overall performance of the OM
alphaStep system and its reliability made it a more desirable option.
For performance, these advanced steppers were not a compromise.
R&P vs Ball Screw --
TABLE: Enclosed Table Sides vs Exposed Worker --
In developing the gantry system and table for our
tools (floor models, not the BenchTop), we’ve explored several
configurations. We settled on one that offered affordability and
stiffness, and at the same time offered operators and observers an extra
shield of protection from flying debris or material that might break
loose during cutting and machining operations. We call these our ‘Safety
Sides’ ™. It means that our tools are most conveniently front-loaded
and/or end-loaded, but this system seems to work well for the work flow
of most shops.
TABLE: Bolt Together vs Single Piece Table --
Our table is a ‘bolt-together’ product. It is
shipped dis-assembled to reduce shipping expenses and to allow
convenient set-up anywhere in your shop. It would be stiffer if made in
a welded version. We provide plans for this table on the web site for
those who would like to use the measurements to fabricate their own
table, either to save money or to produce a stiffer structure.
Modular Assemblies – Besides the table, out tools
ship as 3 pre-assembled modules: the X-rails; the X & Y gantry; and the
Z-axis (the vertical axis). These components are easily mounted onto the
table in a few hours. We believe that it is considerably more practical
to ship our tool in modules rather than as a bulky and vulnerable,
single structure.
Our Own Controllers and Integrated Software --
The ShopBot Part File Format --
If you haven't noticed it yet, ShopBot Part Files
(the files of moves that cause your tool to cut and machine) are not the
same as the "industry standard" G-Code file format. Yet, there is
actually very little difference between G-Code and ShopBot Part File
code in principle. They are each just a list of the coordinates that the
tool moves through during cutting.
Historically for us, the story goes like this. About
12 years ago as ShopBot was getting started, we were trying to design an
interface that worked intuitively and was easy to remember from one
session to the next. We wanted a user to be able to use simple commands
from the keyboard to move the tool around, and it made sense to use
these same commands inside the part files. At that time, we took as our
model the interface to AutoCAD and GenericCAD and other design programs
that used a two keystroke command sequence to enter commands, with the
keys being reminders of the command (and access to the command menu
pull-downs). For ShopBot, that meant that to move in the X axis
we would use a command like [MX; for move in X axis], to
jog in the Y axis a command like [JY], to make a 3D move
it would be [M3] and so on.
We pondered using G-code format for entry of
instructions at the keyboard, but these seemed awkward and more complex
than the instructions needed to be. Indeed, G-code was designed in an
era when machine tools were controlled by punch tape and when the device
receiving the instructions was considerably more primitive than today's
PCs. Additionally, today's CAD/CAM approach to toolpath design is
different to the manner in which early NC tools that depended on G & M
codes were programmed.
Having decided on the two-keystroke commands as a
way to work from the keyboard, it made sense to us to stick with the
same format for Part Files. The 2-keystroke commands would be easy for
users to read and work with if they needed to modify their Part Files,
or if they wanted to write a file from scratch. This was the primary
reason for using the format that we do.
This is why we ended up with the ShopBot Part File
language. There are a couple of additional relevant points to also
consider.
1. It takes only a couple of seconds to convert a
G-code file to a ShpoBot
Being Open vs Open Source Software --
Its hard not to love "Open Source" software,
particularly if you're like those of us ....
Additionally, we wanted our Part File language to be
easily programmable. You can write a Part File for ShopBot using any
ShopBot keyboard command, just as you would enter it at the keyboard (in
fact, keyboard commands can be automatically recorded and turned into
files). Too make the Part File language as open and flexible as
possible, we wanted to add further programming capability to it. While
G-code can be programmed to a degree, it is quite awkward and
non-intuitive. We added to the ShopBot Part File languange many of the
common programming functions from the BASIC programming language. This
includes functions for working with variables, logic, and program
branchcing and functions for reading and writing files and displaying
information. These functions are implemented in a manner that will be
familiar to anyone with any kind of computer programming experience and
will allow consider custom control of our tools and interactions with
them. Our goal is to make our software as open as possible in terms of
being put to use for any kind of special or custom purpose in our
customers shops. (We recently had a workshop before the Camp ShopBot in
NJ at which we illustrated the functionality of programming within the
ShopBot Part File language and the way in which ShopBot functions and
capabilities can be accessed by outside software.)
Off-the-Shelf Parts and Simplified mechanisms (e.g. Rails) --
On being imitated --
We are frequently asked these days about how we feel
about several companies having imitated our affordable CNC products.
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