If you are at this point, you probably already understand the basics of a CNC System. Just to make sure, here are the primary parts of a CNC system (flat bed panel cutting type):
- The Support Table
- The Gantry
- The Cutting Tool
- The Control Motors
- The Motor Driver, Indexer, and Controller
- The Safety Equipment
- The Cut File
- The CAM Software
- The CAD Software
It may be counterintuitive, but I feel that it’s easier to understand all the pieces of a Computer Numerically Controlled (CNC) system in a reverse order of work flow. Generally, the part designer has an idea for a product and eventually a part is finished out on the shop floor. To explain this process in reverse order, the starting point would be support table that the system sits on and explain backwards to the software that the designer initially used.
Also, I find that most people are so focused on the whistles and bells of the software and control of a CNC system that the working tool becomes an afterthought. This would be similar to choosing your company’s work truck based on the display of the instrument panel. Yes, all parts must work together as a whole, but there seems to be many systems on the market that have wonderful, highly graphic, and incredibly complex software packages driving a somewhat marginal cutting tool.
One last thing before I start, this isn’t a textbook for understanding all the concepts of engineering necessary to be a competent CNC designer. I am a professional engineer with a mechanical engineering degree and enough years of experience. Although you may or may not have the same training as I do, I will try to explain things without getting too technical but, at times, I can’t help it if I do. The following is simply an engineer’s list of opinions and recommendations.
THE SUPPORT TABLE
Like almost every other tool in a typical shop, the primary purpose of this CNC is to cut material into parts, period. Precision and speed rely on the stoutness of the equipment used. Cheap and flimsy tools compromise accuracy and production. The table should be STRONG and HEAVY. It’s not only the supporting foundation, but it’s the inertia base which will absorb and attenuate vibration. It has to rigidly support the weight of the material to be cut and the cutting equipment. It should not flex or loose accuracy under normal operation. At the end of a rapid move by the gantry, the table shouldn’t shake or scoot around your shop.
In addition to the daily abuse from vibration and forces from the cutting process, it’s quite likely that the table will get hit by your forklift, used as a walking platform to change the lights above it, used as an assembly table for other projects, and moved around as you company grows or changes locations.
CNC panel routers come in two styles: either the gantry is fixed and the panel moves below it or the panel is fixed and the gantry moves above it. There is much debate between manufacturers about which is more rigid and is pretty dependent on which one they are selling. From a practical standpoint, the decision boils down to which is heavier and harder to move, the gantry or the panel. For simplicity, this document focuses on fixed panel, moving gantry models.
Tables that are bolted together are subject to loosening during use. It’s an option which is prevalent on entry level machines because it’s the easiest for the CNC manufacturer. A bolted table doesn’t require completion by the manufacturer, is therefore easier to ship, and both of which keeps costs down.
Also, once it’s bolted together it will likely never be unbolted. If you have to move it or sell it, it’s just easier to do so in one big piece. So having a table that you can later disassemble is not really a strong selling point.
If you are cutting all the way through the material to cut parts out, your material will not sit directly on the CNC table. There will be some sacrificial support between the table and the material. Very often this is referred to as a Spoil Board. This will be securely affixed to the table and is expected to be partially cut into during normal cutting operations.
Unless you need a system which cuts without pushing sideways on the material, such as a laser, water jet, or plasma cutter, you will need some method to hold the material in place.
How the material is held in place is likely one of the most overlooked aspects in selecting equipment. Getting material on and off the CNC is a major part of production and requires considerable consideration. There is not one system that will work for all conditions so it’s a smart thing to know your needs and the available options.
One of the most popular systems marketed by CNC manufacturers is a vacuum system. Why? For most customers it could be used in some fashion… and it’s a great item to sell. Vacuum pumps and blowers are expensive and have the potential to be nicely profitable for a manufacturer.
The idea is that the material is placed on the vacuum equipped table, vacuum is applied and the material is pulled down onto the table, and the material is held in place by the friction between the material and the vacuum board (or the perforated spoil board on top of the vacuum board).
Vacuum Problem #1 ‘ If you loose the vacuum seal, the material moves.
The simplest form of this system is a single vacuum area which constitutes a single sheet of material. However, as parts are cut from the sheet, air passes through the open cut and undermines the vacuum. This is sometimes referred to as a ‘bleed board’ system. There are a couple of options to counter this:
Don’t cut all the way through ‘ This is a great option if you don’t need parts cut out, such as milling three dimensional shapes or engraving signs. It’s not really a productive option for cabinet makers.
Use part-specific localized vacuum ‘ In this model the vacuum openings are specifically located not to intersect a cut path. This can be done with specifically designed vacuum boards or vacuum cups or pods. If you plan to cut the same part all day long, a vacuum board specific to that part is a good entry level choice. However, you will need to change the board with every part change. A vacuum pod system typically has a universal vacuum board but with movable pods. Each part change will not need a vacuum board change, but will need the pods to be relocated for that part.
Use a vacuum pump or blower with lots of extra capacity ‘ Don’t try to over-think the problem, just let the air bleed and suck the heck out of it! Well… this doesn’t always work very well and of course, extra capacity means larger pump ($$$), larger electrical equipment ($$$), and more energy use ($$$). You get the idea.
Vacuum Problem #2 ‘ You can only cut one sheet at a time.
A vacuum system has it’s place and is the best choice for many users… just make sure it’s your best option.
An alternative to pulling down on the material from below with vacuum is to push down from above with a press roller. This works well for multiple sheets and doesn’t have any of the problems associated with vacuum systems. However, it requires a substantial gantry system, the material must be stiff enough to push back, and has the potential to mar the finish of the material.
If you are cutting four sheets of plywood at a time to cut internal structures for fiberglass boats, this is an option to look into. If you are making cabinets, it’s not a good option.
Screwing the material to the spoil board is the simplest and cheapest method. If you just wanted to get started cutting sheet stock without investing any money in any other system, this would be the way to go. You just need a cordless drill driver, and six to eight screws around the outside edge of a 4 x 8 sheet and you are ready to go.
The downside is that you can’t cut all the way out to the edge. Otherwise, you may end up with a partial sheet of uncut and unsupported material and you risk cutting into a screw.
In a previous company that I was a part of, this was the way we operated the CNC 95% of the time. The cutting patterns left 1-1/2″ to 2″ of the outside edge of the sheet intact and cut parts from the sheet to account for the fact that the sheet was held in place by the edge. What I mean to say is that you cut parts from the center of the sheet out, or from one short end to the other… never from the outside edges in (you don’t want to loose connection with the remaining part of the sheet material still screwed to the table).
A step up from screwing down a part is to clamp down the part. Clamps, either manual or pneumatic, can be used in a variety of ways. It is possible to clamp down a sheet of plywood around the edges similar to the method using screws. If you set up the CNC with special jigs to hold previously cut or uniquely shaped parts for second operations, clamps are likely the best way to hold those parts.
The major detractor to clamps is that they are potentially in the way of the cutter path. Not only do you need to be concerned with avoiding cutting into a clamp, you need to watch any positioning or homing moves. Crashing into a clamp with the cutter is destructive.
So now the material is sitting on a table and is held firmly in place. If you were going to cut this material with a hand tool, you would be ready to go. Unfortunately, your arms are short, get tired after a while, not strong enough to freehand perfectly straight cuts, and are attached to a body that would rather be doing something else.
The gantry is the general term for the section of the CNC that holds the cutting tool and moves around the table to do actual work. Similar to the table, accuracy is dependent on strength and weight. The gantry should not be so flimsy that it bends or twists under cutting pressure.
Many manufacturers use a few performance figures as selling points. One is cutting speed. However, the faster the cutter is pushed through the material, the greater the bending and twisting force transmitted to the gantry. Have you ever pushed a 1/2″ diameter router bit through 3/4″ oak plywood at 200 inches per minute? How hard did you push? Imagine pushing that hard on the tool hanging from the gantry. Do you think you could deflect the the whole get-up 1/16″ or more?
The Linear Motion Equipment
Since we exist in a three dimensional space, we need to concern ourselves with three axis of the CNC. For most equipment, the X and Y axis defines the plane of the table. Some manufacturers use the long end as the X axis while others use a short end depending on their table orientation. Usually, it boils down to where the operator stands. As you stand at the front of the machine (whichever side that is) the Origin (or Home) will be at the left corner. The X axis will increase from left to right; the Y axis will increase away from you. The Z axis is the up and down movement of the tool. Additional axes are named for a multiple heads, head rotation, head tilt, etc.
The gantry will move in each of these three dimensions on a set of linear rails. The accuracy of the gantry is dependent on the straightness and rigidity of these rails. It is difficult to recommend any one type of rail, but a potential buyer should ask the following questions:
- What is the expected life of the bearings and rails?
- How are they replaced?
- How much does it cost to replace them?
- Are they protected from damage?
- How much maintenance is required of them?
The gantry and the linear motion equipment will need to resist forces imposed by the cutting operation. To maintain accuracy, the equipment must be designed so that it is rigid. Additionally, the equipment must be designed to minimize torque. Although similar, these two statements are not necessarily the same.
Consider an exaggerated CNC with a gantry that is two feet tall and the bit of the router must then reach down two feet to cut into the material. Let’s also guess that it takes 25 pounds of force to push the bit in a cutting operation. Since torque is the product of the perpendicular force multiplied by the moment arm (the distance from the bit to the head attachment point), the resulting torque on the cutting head is 50 foot-pounds. This results in some quantity of deflection of the cutting head. This torque is then transmitted to the gantry which results in some deflection of the gantry.
To minimize torque and provide the same cutting force, the moment arms should be as short as practical. If a gantry with four inches of clearance will cut 100% of your work, it’s not in you best interest to consider only those machines that can clear at least 10 inches.
In simple terms, the drive motor pushes the gantry in one direction and the resistance from the cutting operation pushes equally in the other direction. Those two opposing forces try to push the gantry over. The linear motion equipment (the bearings and rails) must hold the gantry upright. This same principle applies to the head as the cutting forces try to twist the head off of the gantry.
The manufacturer must take this into consideration when determining drive location for the drive that moves the gantry along the table. Since it’s the drive that fixes the location of the gantry, the placement of this with respect to the height of the gantry determines the moment arm. Some manufacturers place the gantry drive low or beneath the table in an effort to protect the drive. The result of this is that they have increased the torque seen by the linear motion equipment by increasing the moment arm.
AND ONE LAST THING (and this drives me crazy); too many CNC designs do not have enough bearings to be effective. For example, the head of the gantry is usually connected to two parallel rails. If there are only two roller bearings on the top of the top rail and two roller bearings on the bottom of the bottom rail, this is almost always not nearly enough! For every rail, there should be a bearing on opposite sides each pulling the other tight against the rail. There is a lengthy explanation based on resultant torque arms, but I’ll skip that here. In essence, four one-sided bearings don’t have the geometry to provide enough resistance against the twisting about the face of the gantry’s head. So, for the two rail setup, there should be at least eight roller bearings holding on the head. The other acceptable option is to use extruded rails which have bearing assemblies that envelope the rail; then you could get away with only four.
THE CUTTING TOOL
I mentioned this before in other sections, but the following is really important so I will reiterate it here:
Speed and Feed: The thickness that you cut with each pass is the Feed. Speed is obviously how fast you are pushing the cutting edge. A router bit is essentially one, two, or more cutting edges on a rotary cutter. There is an optimum speed and feed for all cutting materials.
The speed is limited by the material properties, heat generation, and tooling and for aluminum and steel and it’s about 100 feet per minute. Wood likely has a speed limitation too, but you will sooner hit other limitations and I’ll explain shortly.
Feed is another way of indicating the thickness of the chip generated and usually for milling aluminum is about 0.007 of an inch, for steel is about 0.004 of an inch, and for wood is somewhere in that range (based on the tool diameter). What is really important is that the thickness of the chip plays a huge role in how fast the tool gets dull. First; the chip actually acts a coolant. If you don’t take off enough material, there isn’t enough mass there to cool the tooling and the tool overheats. Second; if the tool can cut 0.008 of an inch just fine, a feed of 0.004 of an inch would result in twice as many cuts required and wear the tool twice as fast.
Using a 1/4′ bit, each revolution the cutting edge travels 0.79 inches or 0.0066 feet. With that bit spinning at 18,000 rpm this equates to 1,178 feet per minute which is acceptable for wood. Now let’s calculate feed: If we want a 0.008 inch chip from a single flute bit, the tool needs to advance 0.008 inches each revolution and at 18,000 revolutions per minute that equates to 144 inches per minute. But most bits have two flutes so the tool has to advance twice as fast so that each flute gets a 0.008 inch chip. This would result in 288 inches per minute. The limitations are now going to be router (or spindle) horsepower, how strongly the material is held down, and how strong the bit is.
Depending on the strength of the 1/4″ bit, it may simply break at that speed if you are trying to cut all the way through 3/4″ plywood in one pass (multiple passes are counter to production, so that would be unacceptable). So a larger, stronger bit is in order; like 1/2″. This results in a larger horsepower requirement from the spindle.
By the way, a “router” is a single speed (or slightly variable speed) hand held tool designed to do intermittent work with bearings designed to resist forces imparted by the operators hands. A spindle is a three phase motor with much larger and stronger bearings, and when coupled with a variable frequency drive, is capable of reverse rotation and a speed range of between 100 to 18,000 rpm. In a CNC environment, a router is a disposable item and is barley adequate. If you plan on actually cutting materials, just skip the router step and get a spindle. You will end up there anyway.
THE CONTROL MOTORS
Obviously, the motors do all motion control work. I noted the basics of the control motors, screw drives, rack and gear drives, steppers vs. servos, and defined Resolution and Accuracy elsewhere so I will skip it here. However, I have iterated through several designs and ultimately come to a few points which must be addressed in all designs:
A gantry must be held in position on both sides of the table! For a CNC router where the work being cut resists and imparts a force back on the gantry, the gantry will twist or rack if driven (or held) only on one side. So both sides each need a drive screw or gear and rack. Those setups that use a single drive centered beneath the table aren’t adequate. Either a shaft or a second motor is required to drive the opposite side. My recommendation is a shaft: once adjusted, it holds the gantry in orthogonal position and keeps it from racking, even after the power is turned off.
The Z-axis (tool up and down) will drop once the motor is de-energized if considerations are not taken. For manufacturers that drive them with a rack and gear, a counter spring is used which hold the tool up when the power is off. A screw, with its higher gearing, generally won’t spin and allow the tool to fall simply by the inherent friction. This is why screw drives are most commonly used on the up and down axis.
THE MOTOR DRIVER, INDEXER, AND CONTROLLER
This is the part of the system which appears to be the most confusing. There is a lot of information and products available on the internet and some products provide some or all of the components necessary to actually move the motor. But basically, there are three components.
One quick note before I start here. Digital signals are just an output that is either on or off. In other text, you may read +5 volts or 0 volts, high or low, 1 or 0, on or off, closed or open, yes or no. If you find yourself glazing over when you read ‘… the driver sends a high signal, blah blah blah…’ just think of turning on and off a light switch; it would be the same principal.
The motor driver is a device that takes digital signal inputs and a higher voltage source input and provides the appropriate higher voltage output to the motors. This higher voltage energizes the motor windings and moves the motor. There are usually two control inputs to this device: Direction and Step. Direction is simply that and is usually something like on for forward and off for reverse. Step is a signal to turn the motor one increment (usually 200 increments or steps per motor revolution). When this turns on, the motor moves one step, then off, it moves another step. Turning this signal on and off at a given frequency rotates the motor at a given speed. (Yes, this input style is the same for both stepper and servo motors, although their output style between the two is different.)
The indexer generates the on and off signals to the motor driver. Its input is usually a command like ‘move the axis 5 inches forward at 100 inches per minute’. Internally, it knows that 5 inches is 3,000 steps and 100 inches per minute needs 1,000 on and offs per second, then sends these signals to the motor driver.
The controller is the software or device that reads and interprets the program and sends the appropriate commands to the indexer. It is the device that has the operators interface.
On an entry level PC based system using a printer port, the controller is the software that is running on the computer, the parallel printer port is the indexer, and the driver is the device directly connected to the motors that is in a separate box, usually with the motor power supply. The next step up beyond entry level is to replace the parallel port (due to its frequency limitations) with a stand-alone indexer (usually located to that separate box). Keep throwing money at this part of the system and you get a dedicated and specifically built controller as opposed to the PC. However, a PC based system, running something as entry level as DOS, and using a printer port is surprisingly effective.
Two things that need to happen in an emergency situation are that the motion control motors de-energize (so you can push it off or away by hand) and the spindle stops. Of course a nice big button on the wall or control cabinet is a necessity; it needs to be supplemented with a pull chain or cable accessible anywhere around the machine. That button on the wall doesn’t help with your shirt gets caught up in the motion control screws and you’re trapped.
THE CUT FILE
Now that the machine and control system is in place, it needs something to do. A cut file is a simple text list of motions to perform and is usually as follows:
G01 Z-1.000 F100
G01 X4.000 Y4.000
G02 X4.000 Y8.000 I2.000 J0.000
Cut files are almost always in a text format, use simple commands, and can be written or modified by hand if needed. Of course you need to know the commands to do things manually, but I’m not going to explain all that here. Just know that there is a text file which gets read by the controller to make things happen.
THE CAM SOFTWARE
This Computer Aided Manufacturing (CAM) software primarily performs that automated task of writing the cut file using some sort of drawing. Sure, you could write a text file like that shown above, but it’s easier to click a mouse and tell the software that you need this line and this arc cut. Or even easier, tell it to cut all the shapes you see on the screen. Good CAM software will figure out tool offsets, optimized paths, provide warnings if you plan to do things beyond the capabilities of the CNC, etc.
THE CAD SOFTWARE
The Computer Aided Drawing software is just that, drawing software. Most of the time, its functionality is included with the CAM software. I’ve listed it here separately because it really is two different phases of the process. You need to draw the parts, and then you need to use those files for the CAM process.
What is really interesting is when all of the CAD/CAM process gets automated, and there is all kinds of software that can do that. For instance, some software will take a drawing full or parts, figure out how to nest them (arrange them on the sheet to minimize waste), and then create the cut file. Or, as typical in the heating and air conditioning industry, you simply input the dimensions of the sheet metal fitting or duct that you need, and the software draws, nests, writes the file, starts the CNC, and cuts the parts. It’s a question of how much money you want to spend.
JUST A LITTLE MORE ON SOFTWARE
The CAD and CAM software you select will either make you love CNC work or hate it. It is also the most fundamental thing to overlook… you shop for a CNC looking at the machine itself. But on a day-to-day basis, it’s the software you’ll be mostly working with! You better get something that works for you. This area is also the place where you can waste a lot of time and money… so research, ask others, try free demos.
What’s interesting to me is that the quickest combination I’ve ever used includes a really old CAM program that runs in DOS. I’ve used AutoCAD for two decades and I do all my part drawing and part offsets with that. I then export the cutting paths to a DXF file and use the simple program to manually select which path, starting where, and going in what direction. I so far haven’t found something that works for me faster. Of course, I’ve never spent thousands of dollars on a completely automated package that offests, nests, and outputs directly to my controller. I’m not recommending one thing over another, but just pointing out that there are lots of options.