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. It's probably more than most anybody wants to read. 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, we try to share with you 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 in which it can be positioned or moved, and considerably less friendly for small shops. Some large CNC machines actually require modification to a building's 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 and processes. Lighter tools are more easily configured to the job at hand. They are more energy efficient, costing less to operate. Not to mention, making them and shipping them consumes fewer resources.
ShopBots are engineered of steel and aluminum as relatively light tools (about 800 pounds for a 4x8 PRSalpha ShopBot), but given that our design choice has been made for agility, they have frameworks that are as stiff and rigid as possible. ShopBots' stiff main beams and rigid support members provide good cutting, and - with careful selection of cutters, spindle speeds, feed rates and cutting strategies - they approach the cutting characteristics of much heavier and considerably more expensive machines. Still, a ShopBot can be readily repositioned on the production floor or broken into modules for transport to another location. In small shops, it can be set up in areas that have limited access to heavy equipment, in a basement or above the ground floor.
Table: Bolt Together vs Single Piece Table
For our full-size tools, the table on which the gantry runs is a bolt-together assembly. It is shipped unassembled to reduce shipping expenses and to allow convenient set-up anywhere in your shop. It would be slightly stiffer if welded as a single component; however, experience has convinced us that it is more practical to have a tool that can be easily readjusted with standard tools if ever knocked out of alignment and whose parts can be readily replaced if ever damaged.
Besides the table, our full-size tools ship as two pre-assembled modules: the X-car; and the YZ-car. 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. The fact that a little set-up is required just insures you will have a good understanding of the machine when you're ready to put it to work. (Of course, we can send an installer to your site if you really don't want to be distracted by setting the tool up.)
Note: ShopBot Buddy CNCs are shipped fully assembled, ready to roll off the pallet and be put to work.
Table: Enclosed Table Sides vs Exposed Worker
In developing the gantry system and table for our full-size tools, 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. It also means that any debris from cutting or pieces of a broken bit - if not contained by the dust-collector/guard around the cutter - will be blocked by the table sides rather than flying into an open area.
Linear Motion: V-Wheels vs Re-circulating Bearing Systems
There are a number of schools of thought on which sort of wheels or bearings the axes of a CNC tool should ride. We have used several systems over our 14 years and have come to feel strongly that for full-size woodworking tools, the V-Wheel offers the most practical solution.
Rack & Pinion vs Ball Screw (or spring-loaded machine screw)
The debate over Rack & Pinion vs Ball Screw as the motion mechanism for CNC can get hot and heavy. We think both systems are pretty good, and during our 14 years we've built tools employing both systems. In theory, ball screws offer the advantage of being virtually friction free and very smooth. However, they are also more complex. They require precision alignment. They are very vulnerable to being disabled by debris, so they must be kept very clean. As the length of an axis gets longer, an increasing stiffness in the ball screw is required to prevent the screw from wobbling, and this results in large, heavy and expensive screws (which, in turn, require larger motors to keep up to speed).
Rack & Pinion is a straightforward drive system. Modern grinds of the gearing insure smooth action with limited friction. The system does not require precision installation or alignment and is very impervious to dust and debris. Its components can also be easily replaced if they become worn. While we rarely see significant wear on the rack, pinions do wear, and you can expect to replace them every year or two during normal regular use.
Perhaps the best way to leave you on the Rack & Pinion vs Ball Screw question is to just encourage you to take a look at several $100,000 and up CNC tools. These are tools where the drive mechanism is a small part of the costs so the decision is made with respect to perceived performance. You'll note that these tools use both systems; indeed, it is frequently the case that ball screws are used for one axis and rack & pinion for another. Both work well.
Motors for Motion: Open-Loop Steppers, Closed-Loop Steppers, and Servos
The motion of ShopBots is powered with stepper motors. Steppers are an incredibly precise form of motor that have the unique feature of never producing incremental errors. A stepper is just as accurate at 80" as it is at 8. The motors are inherently digital and thus a perfect match to digital computers and digital cutting.
Conventional open-loop stepper motors such as those employed on our PRSstandard tools are the most affordable solution to producing CNC motion. They have the limitation that, if overpowered -- which usually occurs by attempting to cut too fast or by cutting with a dull bit -- they can lose synchronization with the computer controlling their motion. But, when used appropriately these tools will produce excellent cuts, day-in, day-out.
For heavy duty production work, we recommend our closed-loop PRSalpha tools. These tools use closed-loop stepper motors and drivers. They are called closed loop because they have sensors that continuously monitor their performance. When the forces on these motors become greater or if the are pushed slightly off line, the motors instantly increase force and recover to the correct position. This is similar to the behavior of servo motors, which are also closed loop. Servo motors are a good solution for CNC tools because of their closed-loop control; however, servos can be more complex because they are continuously seeking their targeted location and modifying their location. This means that servos are typically more expensive than steppers and is the second reason why heavy industrial CNC tools are expensive. Be careful of the small servos sometimes offered on less expensive CNC tools as these motors may not provide the speed, power or acceleration you expect. In a closed-loop system, look for something comparable to a PRSalpha, a CNC router than can travel at about 30ips (1800ipm) and have 150-250 pounds of cutting force.
The stepper motors in all of ShopBot's PRS tools are made by the Japanese manufacturer Oriental Motor (OM). OM has been building high-quality motors for more than a hundred years. Their industry-leading technology has been much copied, and it is easy to find budget motors made to look exactly like OM products. However, the copies often fail to live up to the OM performance, which depends on extremely high-grade bearings, precisely cut armatures, dense magnets and carefully wound coils. ShopBot has used only OM steppers, and in the last 12 years, we have shipped almost 24,000 of the motors and have had fewer than a dozen failures. These steppers can be counted on for day-in, day-out production -- absolutely.
The OM alphaStep stepper motor/driver is the system we use in our PRSalpha tools. The alphaStep combines the best of stepper and servo capabilities in a robust and technologically sophisticated system that has been virtually failure-free in the four years that we have been shipping them in our alpha tools. [Read more about alphaStep]
On Developing our own Control System Software and Manufacturing our own Controllers (and on imitation)
We are frequently asked these days about how we feel about the several companies the have imitated our tools in one way or another. Well... we feel good. First, there is the flattery, but more importantly, many of the imitators produce a version of a ShopBot that is primarily different by having chosen differently with respect to one or another of the options described above. As we note, many of the options are equally good, or offer different sets of trade-offs. One company, for example, builds tools that look like ShopBots, but are welded instead of being bolt-together. We do feel that such tool builders can serve a purpose in providing reasonable products to people who do not agree with our design decisions.
Where we do part company is in our belief that the Controller and Control System Software that runs the tool should be well integrated with the tool. We design, manufacture and support our own controllers. And, we develop and support our own 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 two-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.
OpenSyntax Part File Software
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). To 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 language many of the common programming functions from the BASIC programming language. This includes functions for working with variables, logic and program branching 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.)