In Search of the Perfect CNC Part 1

Is your shop ready for a CNC? After 21 years of hand fabrication, Reid Signs finally decided to take the plunge. The decision to buy a router was easy. I have been considering it for at least 10 years and knew the first minute I saw one that I would eventually have one in our shop. The allure of actually making signs while at home sleeping is a very tasty vision indeed.

The hard part was figuring out when to do it and ultimately, which brand to buy! If there ever was a reason to put this decision off, it was that one. I’ve been around the sign business for a long time and have seen the same advertisers year after year tout the benefits of these incredible machines.

This repeated exposure has given me a good idea of who sells what, for how much and with what features. Armed with a briefcase full of cash handcuffed to my wrist, I decided to venture a little deeper and see who isn’t readily available to our little sign niche.

The woodworking trade is a huge industry whose craftsmen do similar types of work as those of us in the sign industry. They care about tolerance, edge quality, reliability, support, and cost just as we do. Because their industry is larger than ours, I decided to see who their big CNC players were and compare against our beloved brands.

The ink had barely dried on my research strategy when I found out about an industrial woodworking exposition going on right here in Seattle! This was going to be perfect. I checked the exhibitors and found a whole bunch of CNC names I’d never even heard of. So, Dustin King, our top fabricator, and I headed off to see what we could find out. If you have never gone to a show like this, you should check one out. The amount of automation available to a sign shop is absolutely staggering and two days worth of looking around left us staggering too.

OUR OUTLOOK
I took a close look at our own shop before deciding to buy just to make sure that the timing was right. Reid Signs has spent the last five years really getting on top of digital printing and now that the learning curve is pretty much over with, I theorized that this would be an ideal time to conquer a new challenge. Having the time, interest, and mental capacity to undertake something like this is an important assessment you will want to make about yourself and your business.

Kathy, our bookkeeper, and I analyzed reports generated by our accounting system and saw that financially, we were in good shape. I also saw that the amount of CNC work we sub out is not huge, but measurable, so this expense would become captured revenue once we brought it in-house. I looked at the physical size of our shop and drew up floor plans showing the accommodation of a large table and necessary walkways around the unit. Our present and anticipated workload definitely agreed with our decision to buy, and we had plenty of areas of our operation that would immediately benefit from increased efficiency and volume.

Once we determined that our company and staff were ready, we turned our attention to choosing the machine features best suited for our application. This is where we found ourselves faced with the big daddy of CNC questions:

BALL SCREW VS. RACK & PINION
Sounds like a good card for a boxing match!

There are several drive systems widely used in CNC routers. Rack and pinion and ball screws are the two predominant systems I have heard about the most, but there are a couple others too.

Nearly all machines use a ball screw in the Z-axis because the length of travel is very short and accuracy of cutting depth is almost always critical. Lower cost machines use leadscrews, which are a plastic nut connected to a screw, and are sometimes used interchangeably with ball screws. There is a big difference between the two as ball screws are very efficient at transferring power, while leadscrews are very inefficient because of the high amount of friction between the nut and screw. This slows down the 3D cutting speed significantly.

This is a very simplified overview. Please note that it is very important to do your own research and come to your own conclusions!

Rack and Pinion - Advantages are that they can be extremely fast and they are a very stiff and rigid style of drive system. They require low maintenance, have excellent resistance to dust and chips and are excellent for medium and large CNC tables. The disadvantages are that they can contain backlash to a degree that is determined by the quality of the table you invest in.

Ball screws - Advantages are supreme accuracy and no backlash. They operate with very little friction and are very stiff in design. They are also good for operating environments requiring 100 percent duty cycles (that means they can run constantly without a bathroom break).

Disadvantages are that they are slow and are problematic in tables that have long travel. It is hard to support a long ball screw without significant sagging. While this isn’t something you should talk to your doctor about, it may be something to consider when researching a CNC router.

The spindle needs to be supported which means lower maximum speed due to resonance and lower loads or the system can be designed with a fixed spindle and rotating nut to help compensate. Ball screws are also more sensitive to dust and they are expensive.

Timing belts - The advantages are that they are fast and require little maintenance. They are very resistant to chips and dust and they are inexpensive. Disadvantages are that they have large deflection and vibration, especially across long travel.

Leadscrews - The advantages are that they are adjustable for low backlash and are tolerant to chips and dust. Disadvantages are their slow speed and low duty cycle (bathroom breaks ARE required for this system).

Like the ball screws, they are problematic in systems with long travel. Leadscrews also create high friction and are a very inefficient drive system. If the system uses materials like Teflon, that is a plus for the lead screw design as it reduces friction.

The next bout in our event is the battle between:

SERVO VS. STEPPER
Stepper Motors - is street slang for Step Direction Driven Motor. I remember the old days, shootin’ dice in the alley with my homeys talking about Steppers and Servos... those were the days... but I digress.

Advantages of stepper motors are that they are the lowest cost solution for machine builders. They are maintenance free, they’re repeatable if the job is sized right, and they require no tuning. You just plug ’em in and they’re good to go.

Disadvantages are that they are an open loop system, have a degraded edge quality of cut pieces, have resonance within the mechanics, and suffer from higher wear due to the steps.

Steppers have lower acceleration. This is a big issue if you want to do 3D machining as there are a lot of changes in direction required in the Z-axis. If stepper motors are accelerated too hard they stall, like a car would. If they run low on torque, they can briefly stall and then resume again at the wrong position with the control system not knowing it even happened. Comparatively, stepper motors are considered to be an outdated technology overall.

Stepper motors utilizing microstep technology have all of the same advantages mentioned above. They also have many of the disadvantages too.

Servo Motors - When you’re talking about the really good servos, you’re really saying True Closed Loop Digital AC Servo Motors.

There are a few different types of servo motors available on the market and not all of them are the same. Servo motors by nature, always know the position they are at so that can be configured for maximum acceleration. They don’t suffer from stalling because when the motor requires more power, the amplifiers know that the machine isn’t where it is supposed to be, and will shut down if it doesn’t get there in time.

DC Servo Motors - Advantages are that they are the lowest cost way to offer a servo motor on a machine and they are a closed-loop system.

Disadvantages are that they cost 200-300 percent more than stepper motors. They require maintenance in the form of replacing coils and they need to be cleaned of dust. They also require tuning after a change of components.

True Closed Loop Digital AC Servo motors - Advantages are they are a true closed-loop-system, require no tuning after change of components, have extremely high levels of acceleration and torque, and have super smooth motion due to high encoder resolution, and they’re maintenance free.

Disadvantages are that they are expensive and they have a slower sample rate when compared to analog servo amplifiers when used with analog servo controllers.

After reading everything above, I’m sure you’re left with one burning question: What is the difference between Step Direction Control, and True Servo Control?

True servo control closes the position loop and tells the controller where the motors are located at the controller instead of at the amplifier. A step direction servo motor uses a stepper controller with smart amplifiers and closes the loop on each motor separately resulting in no coordination between all three axes.

Get it? Me neither. Bottom line is that a step direction servo system is actually using a stepper controller to drive a servo motor.

Now do you get it? Me neither.

When I was out looking at machines, this was something that they didn’t do on the industrial level, expensive machines in the 100k range. Studying the high end market and seeing which features begin to fall off as the machines get better is a good way to judge features that maybe should be on the “extra research needed” list. True servo seemed to be the best bet for us for a couple of reasons.

With a true servo controller all loops are closed together, giving the best 3D performance. It’s also good for doing diagnostics because the technician can see all of the motor information from the controller when connected through the internet.

This speaks directly to the beauty of remote support capability that we’ll get into later.

OTHER CONSIDERATIONS
Fixturing: A smaller consideration, but no less important, was deciding how we would hold down our material. Everyone we have talked to warned us that holding down (or fixturing) your parts will prove to be a bigger challenge than we think.

Heeding that advice, we opted for an 1,100-pound, 12-hp Becker vacuum motor that produces an immense amount of holding power (28.2'' HG or 60 mbar abs).

It looks like a jet engine and sucks through a sheet of MDF (your spoil board) while it holds down a second sheet (the material you wish to cut). When you’re dealing with vacuum, choose pressure over flow because it holds smaller parts better.

We opted to not get a T-Slot table for a couple of reasons. Clamping devices which fit into a T-Slot will hold the edge of a sheet, but aren’t so good for the center of an “A”, for example. There are exceptions to everything, of course. A super high powered vacuum unit will essentially do the same thing but you don’t have to set clamps. You just throw your board down and it sticks. The other reason we decided not to get the T-Slot table is that the machine we chose has a phenolic surface that can be drilled and tapped. We’re going to outfit our table with a number of mechanical fixturing attachment options that will operate using simple hex nuts.

Table controls: Next on our list of features were the controls for the table. We wanted our controls to be on the table itself. We also wanted to be able to carry the hand-held controller anywhere around the table where we might find ourselves working. Being able to make adjustments on-the-fly was a feature we really were excited about. We can watch the spindle as it cuts the material and adjust the feed and speed dynamically and see what effect our adjustments are having on our edge quality. Without this feature, we would have to guess at a setting, run to the table, decide we didn’t like it, run back and change it, run back to the table… gets me tired just thinking about it!

Click here for Part 2, in which we’ll discuss some of the important options and peripherals needed to get up and running...

In Search of the Perfect CNC Part 2