Not everyone will want to read this section, andcertainly they don’t need to. It's probably more than most anybodywants to read. But for those interested in the design andengineering choices made in configuring and producing ShopBots, weprovide the following information. In particular, we’ve tried toindicate areas where affordability dictated compromise and how wehandled it, and areas where performance priorities remained paramount.Basically, we try to share with you why a ShopBot looks and works theway it does.
Light but Stiff
Quick project I made over the weekend for carburetor adjustment drivers for my chainsaws, I tend not to misplace tools if they have a spot to go. The wood is sugar. Driver Booster is a program designed to update all the drivers for audio, video, USB or other devices on your PC that have become obsolete and that may adversely affect the performance of your computer or cause system errors. This tool lets you search for and identify outdated drivers, and will automatically download the updates you are missing. ShopBot Tools, Inc. Is closed New Year’s Day, Memorial Day, July 4th, Labor Day, Thanksgiving Day and the following Friday, and Christmas Eve and Christmas Day. Outside of normal office hours, we regularly check for support requests, email and phone messages.
From a machining point of view, it is always betterto be heavier. Weight provides substance, mass and rigidity. Thesefactors lead to smoother cutting, but these factors also lead to highercosts, because all aspects of the motion system must be beefier. Thedrive system -- motors and their electronics -- are a particularchallenge because their costs increase exponentially with the mass ofthe system they need to move.
In addition, there is a practical problem withheavy. It makes the tool less portable, more limited in thelocations in which it can be positioned or moved, and considerably lessfriendly for small shops. Some large CNC machines actually requiremodification to a building's foundation for their installation.Such machines do not fit well into the current competitive emphasis onflexibility and lean production techniques, which emphasize theimportance of being able to readily reconfigure production flow andprocesses. Lighter tools are more easily configured to the job athand. They are more energy efficient, costing less to operate. Not tomention, making them and shipping them consumes fewer resources.
ShopBots are engineered of steel and aluminum asrelatively light tools (about 800 pounds for a 4x8 PRSalpha ShopBot),but given that our design choice has been made for agility, they haveframeworks that are as stiff and rigid as possible. ShopBots' stiff mainbeams and rigid support members provide good cutting, and - with carefulselection of cutters, spindle speeds, feed rates and cutting strategies- they approach the cutting characteristics of much heavier andconsiderably more expensive machines. Still, a ShopBot can bereadily repositioned on the production floor or broken into modules fortransport to another location. In small shops, it can be set up inareas that have limited access to heavy equipment, in a basement orabove the ground floor.
Table: Bolt Together vs Single Piece Table
For our full-size tools, the table on which thegantry runs is a bolt-together assembly. It is shipped unassembledto reduce shipping expenses and to allow convenient set-up anywhere inyour shop. It would be slightly stiffer if welded as a singlecomponent; however, experience has convinced us that it is morepractical to have a tool that can be easily readjusted with standardtools if ever knocked out of alignment and whose parts can be readilyreplaced if ever damaged.
Besides the table, our full-size tools ship as twopre-assembled modules: the X-car; and the YZ-car. These componentsare easily mounted onto the table in a few hours. We believe thatit is considerably more practical to ship our tool in modules ratherthan as a bulky and vulnerable single structure. The fact that alittle set-up is required just insures you will have a goodunderstanding 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'twant to be distracted by setting the tool up.)
Note: ShopBot Buddy® CNCs are shipped fullyassembled, 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 ourfull-size tools, we’ve explored several configurations. We settledon one that offered affordability and stiffness, and at the same timeoffered operators and observers an extra shield of protection fromflying debris or material that might break loose during cutting andmachining operations. We call these our Safety Sides™. Itmeans that our tools are most conveniently front loaded and/or endloaded, but this system seems to work well for the work flow of mostshops. It also means that any debris from cutting or pieces of abroken bit - if not contained by the dust-collector/guard around thecutter - will be blocked by the table sides rather than flying into anopen area.
Linear Motion: V-Wheels vs Re-circulating Bearing Systems
There are a number of schools of thought on whichsort of wheels or bearings the axes of a CNC tool should ride. Wehave used several systems over our 14 years and have come to feelstrongly that for full-size woodworking tools, the V-Wheel offers themost practical solution.
Rack & Pinion vs Ball Screw (or spring-loaded machine screw)
The debate over Rack & Pinion vs Ball Screw as themotion mechanism for CNC can get hot and heavy. We think bothsystems are pretty good, and during our 14 years we've built toolsemploying both systems. In theory, ball screws offer the advantageof being virtually friction free and very smooth. However, theyare also more complex. They require precision alignment.They are very vulnerable to being disabled by debris, so they must bekept very clean. As the length of an axis gets longer, anincreasing stiffness in the ball screw is required to prevent the screwfrom 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 isvery impervious to dust and debris. Its components can also beeasily replaced if they become worn. While we rarely seesignificant wear on the rack, pinions do wear, and you can expect toreplace them every year or two during normal regular use.
Drivers Shopbot Tools
Perhaps the best way to leave you on the Rack &Pinion vs Ball Screw question is to just encourage you to take a look atseveral $100,000 and up CNC tools. These are tools where the drivemechanism is a small part of the costs so the decision is made withrespect to perceived performance. You'll note that these tools useboth systems; indeed, it is frequently the case that ball screws areused for one axis and rack & pinion for another. Both work well.
Motors for Motion: Open-Loop Steppers, Closed-Loop Steppers, andServos
The motion of ShopBots is powered with steppermotors. Steppers are an incredibly precise form of motor that havethe unique feature of never producing incremental errors. Astepper is just as accurate at 80' as it is at 8. The motors areinherently digital and thus a perfect match to digital computers anddigital cutting.
Conventional open-loop stepper motors such as thoseemployed on our PRSstandard tools are the most affordablesolution to producing CNC motion. They have the limitation that,if overpowered -- which usually occurs by attempting to cut too fast orby cutting with a dull bit -- they can lose synchronization with thecomputer controlling their motion. But, when used appropriately thesetools 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 aremade by the Japanese manufacturer Oriental Motor (OM). OM has beenbuilding high-quality motors for more than a hundred years. Theirindustry-leading technology has been much copied, and it is easy to findbudget motors made to look exactly like OM products. However, thecopies often fail to live up to the OM performance, which depends onextremely high-grade bearings, precisely cut armatures, dense magnetsand carefully wound coils. ShopBot has used only OM steppers, andin the last 12 years, we have shipped almost 24,000 of the motors andhave had fewer than a dozen failures. These steppers can becounted on for day-in, day-out production -- absolutely.
The OM alphaStep stepper motor/driver is the systemwe use in our PRSalpha tools. The alphaStep combines the best ofstepper and servo capabilities in a robust and technologicallysophisticated system that has been virtually failure-free in the fouryears that we have been shipping them in our alpha tools. [Read moreabout alphaStep]
On Developing our own Control System Software and Manufacturing ourown Controllers (and on imitation)
We are frequently asked these days about how we feelabout the several companies the have imitated our tools in one way oranother. Well... we feel good. First, there is the flattery, butmore importantly, many of the imitators produce a version of a ShopBotthat is primarily different by having chosen differently with respect toone or another of the options described above. As we note, many ofthe options are equally good, or offer different sets of trade-offs.One company, for example, builds tools that look like ShopBots, but arewelded instead of being bolt-together. We do feel that such toolbuilders can serve a purpose in providing reasonable products to peoplewho do not agree with our design decisions.
Where we do part company is in our belief that theController and Control System Software that runs the tool should be wellintegrated with the tool. We design, manufacture and support our owncontrollers. 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 thesame as the industry standard G-Code file format. Yet, there isactually very little difference between G-Code and ShopBot Part Filecode in principle. They are each just a list of the coordinatesthat the tool moves through during cutting.
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Historically for us, the story goes like this:About 12 years ago as ShopBot was getting started, we were trying todesign an interface that worked intuitively and was easy to rememberfrom one session to the next. We wanted a user to be able to usesimple commands from the keyboard to move the tool around, and it madesense to use these same commands inside the part files. At thattime, we took as our model the interface to AutoCAD and GenericCAD andother design programs that used a two-keystroke command sequence toenter commands, with the keys being reminders of the command (and accessto the command menu pull-downs). For ShopBot, that meant that to move in the X axis we would use a command like [MX; for movein X axis], tojog in the Y axis a command like [JY], to make a 3D moveit would be [M3] and so on.
We pondered using G-code format for entry ofinstructions at the keyboard, but these seemed awkward and more complexthan the instructions needed to be. Indeed, G-code was designed inan era when machine tools were controlled by punch tape and when thedevice receiving the instructions was considerably more primitive thantoday's PCs. Additionally, today's CAD/CAM approach to toolpathdesign is different to the manner in which early NC tools that dependedon G & M codes were programmed.
Having decided on the two-keystroke commands as away to work from the keyboard, it made sense to us to stick with thesame format for Part Files. The two-keystroke commands would be easy forusers 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 theprimary reason for using the format that we do.
This is why we ended up with the ShopBot Part Filelanguage. There are a couple of additional relevant points to alsoconsider.
OpenSyntax Part File Software
Additionally, we wanted our Part File language to beeasily programmable. You can write a Part File for ShopBot usingany ShopBot keyboard command, just as you would enter it at the keyboard(in fact, keyboard commands can be automatically recorded and turnedinto files). To make the Part File language as open and flexibleas possible, we wanted to add further programming capability to it.While G-code can be programmed to a degree, it is quite awkward andnon-intuitive. We added to the ShopBot Part File language many ofthe common programming functions from the BASIC programming language.This includes functions for working with variables, logic and programbranching and functions for reading and writing files and displayinginformation. These functions are implemented in a manner that willbe familiar to anyone with any kind of computer programming experienceand will allow consider custom control of our tools and interactionswith them. Our goal is to make our software as open as possible interms of being put to use for any kind of special or custom purpose inour customers shops.
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(We recently had a workshop before the Camp ShopBotin NJ at which we illustrated the functionality of programming withinthe ShopBot Part File language and the way in which ShopBot functionsand capabilities can be accessed by outside software.)