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Nesting CNC Buyers Guide

High production rate Nested Based Manufacture (NBM) is finally taking foot in South Africa.

Seven years ago, as Woodtech, we put the courage of our convictions into Nested Based Manufacture. Having had the privilege of international travel and seeing the methods that were taking root in other more developed countries, we took the decision to forgo the then current common wisdom and develop the in-house technology for Nested Based Manufacture (NBM) and the manufacture of CNC routers. Taking it slowly we commissioned more and more machines to the point where today we have more than 40 CNC machines running in South Africa and others in Africa and Australia. We have implemented CNC routers in the manufacture of MDF doors, Shopfitting applications, Office furniture applications, and Kitchen carcass manufacture. The success is proven and our customers will attest to that. Competitors can no longer tell customers that Nesting “doesn't work” because the results now talk for themselves. Some of our customers have expensive Beam Saws now standing idle while the Nesting CNC router does up to 120 boards of cutting and drilling per day.

We have seen some unusual decision making by some clients and as a result many people do not get the full benefit of their investments, or make wrong choices and see a lot of loss before they get full benefit.

So this buyers guide is our attempt to assist customers to make informed decisions and get the best advantage from their hard earned money. We will do this in the form of a Q & A ( questions and answers), then a brief discussion.

Why Nested Based Manufacture?

Quite simple. For 90 % of the market, it is the most efficient method to process boards for the cabinet trade. Common methods have most people thinking that they must aim at owning a Beam Saw and a Component CNC machine. They are told of how many boards a Beam saw can cut in a day. This is a half truth! Please hear this one out. While there are applications where the majority of the components are of similar size and you can cut boards in stacks of 4 or 6 boards, 95% of all beam saws in the market are cutting only one board at a time! Any beam saw cutting average size parts is going to take approximately 15 minutes to cut and will probably demand two operators to manage it. A CNC Nesting router will cut THAT SAME NESTED LAYOUT on average in 3 to four minutes WHILE THE OPERATOR DOES SOMETHING ELSE! With an included board lifter he does not even need an assistant. This is not an idle claim. We have machines cutting and drilling melamine boards at 90 to 120 boards PER DAY. Think about that. Components cut and drilled on one machine with no extra handling ready for the edgebander in less time than a beam saw and with fewer factory staff. It would sound like a no-brainer and yet many factory owners struggle to grasp it. And the investment is almost exactly the same. In a board processing company where sizes are always varying, each board layout will be different, and a Nested Based CNC router will cut those boards up quicker than any other method and with optimal accuracy and minimal handling.

But a Nested Based Router cannot do side boring

Well actually, neither can a Beam Saw. You have to take almost all the panels from a beam saw and put them through a second process on a component CNC for boring. But with nesting, you cut and drill on the same machine, and faster, much faster. You have much less coordination of parts between two machines. So you want side boring. Yes it can be done on a Nesting CNC. It is just not smart to try to do it on a Nesting machine because it is time wasted in which an expensive machine is used to do the job of a low cost machine while it could be used to process a few more boards in the same time frame. The difference is that in the Nesting scenario only fixed shelves and cleats (rails) need side boring and therefore much fewer components need to be put through a second process compared to the traditional Beam Saw setup.

But that is only valid if you follow traditional dowel assembly methods. Nesting opens up other options which do not require side boring.

So how difficult is it to create the code for the nested layouts?

Well this is dependent on the software that is used to feed the machine. The software should be able to generate the required layouts in minutes. Also, it should not be done on the CNC itself. An expensive machine like a CNC router should be cutting all the time. The layouts should be created on a computer in the office and fed to the CNC via network. All the operator should need to do is open the project, load the board and press start. If the code is being Generated at the CNC, it effectively becomes the most expensive office computer ever. The CNC must do what it is good at i.e. cutting and boring, and not be handicapped by tasks that should run on a cheap office computer.

What is meant by single point of entry?

Basically, it is a way to reduce mistakes and reduce operator time consumed. The information should be entered into the system once only. After that all processes should use that information. As soon as information is required to be typed in a second time, for example by a CNC operator, it means that there is risk of error. This implicates driving up cost of the project. If an operator on a component machine has to select and load the right file for each panel he processes, he will not only consume time while the machine stands still but ultimately make mistakes from time to time.

Another terminology is “Screen to Machine”. The ultimate in this concept is to have software that allows you to design your finished product and then with a few keystrokes, produce the machine code and send it to the relevant machine. Here again, fewer errors can creep in if this is done with nesting based manufacture.

But some nesting CNC's are so cheap and other are so expensive. Who is ripping us off?

Well if only it was that simple. Let us start by saying that the steelwork that makes up the bulk of the machine is relatively cheap compared to the technology which drives the steelwork. So why do some use expensive technology if others can do it with cheaper technology? Well that is the key to deciding what is right for you. In an ideal CNC, you will have a machine that is absolutely rigid, has no deflection owing to cutting force, and has instantaneous speeds with infinite acceleration rates and absolutely zero backlash. Well that just obviously does not exist, so everything becomes a compromise and it really becomes a question of just how much compromise is enough or too much. That is where one starts to consider things like motor types, drive methods, guide rail quality, overall mechanical design etc so we will look at these in the next few questions.

Why the hoo-haa about motors? steppers vs servos, brushed vs brushless, DC vs AC?

Short answer, critical if you care about your tools and product finish. Basically, the slower you run your machine, the quicker you will burn out your cutters. The ideal speed is as fast as possible without compromising the cut finish or over stressing the cutters ( breaking them). By losing a small amount of speed a cutter reduces its life dramatically. Adding 25% more speed can sometime double the number of meters a cutter can cut before it requires sharpening. That is, as long as you don't snap the cutter in the process.

How can you compare acceleration from one machine to another? The easiest is by looking at how “snappy” the machine reacts. When comparing two machines, If one looks slugish compared to the other, but both are calining the same speed capability, then the difference is probably in the acceleration rates. Manufacturers do not specify acceleration rates, probably because they are so badly understood. Another way is look at it while it is cutting and try to see how far it travels to reach full speed. The shorter the better.

So what is the implication of this. Well take for example a straight line cut. If the head is standing still the machine has to be accelerated until it reaches the cutting speed. When at the cutting speed it has to be decelerated in order to stop on time. The implication of this is that while it is speeding up and slowing down, it is running slower than the ideal speed and the cutter is wearing at a much higher rate then necessary. This is where things start to change when comparing cheaper machines to more expensive ones. The next implication about acceleration is the ability to control the consistency of cut when the machine is accelerating or decelerating. If the deceleration is to abrupt for the mechanical design of the machine or for the motors or drives, it causes bouncing or shudder. The more rigid the machine the less elasticity. But a low powered motor on a heavy machine, or an elastic drive chain, can also cause bounce. So the key is in getting the balance right. So let us look at the individual components.

Okay, so how important is speed and Acceleration?

There are vast differences in the way each type works and more importantly, there is an even larger price difference between them.

The cheapest type is Stepper motors. They probably cost 10% of the price of a top line brushless servo motor. They are cheaper because they are easier to drive and don't use feedback. That means that the driver has no idea whether the motor is actually at the point it has been instructed to be at. Feedback is done by means of electronic encoders. Encoders will normally know their position up to 1 / 8000 of a revolution. That is typically within 2 to 5 microns on a CNC router. The encoder will report back any deviation from the planned position and the drive will rectify it. Stepper motors typically do not do this.

Stepper motors are very good at low speed operations as they have their maximum torque at low speed and the torque falls away as the speed rises. They are then not able to run at the high speeds of Servo motors. Stepper motors also have an inherent elasticity in them which can be observed when attempting rapid decelerations. This reduces the ability of steppers to hold a reliable position especially with heavy rigid hardware. Stepper motors can be heard by the distinct and loud whining noise they make when they are running.

The next in range are Brushed DC servo motors. Suffice to say that while they are better than steppers and are more expensive they are not as good as brushless motors. They do have encoder feedback but do not have the same instantaneous torque response of brushless motors. Another factor in brushed motors is that one day they will require having the brushes changed.

The top range motors which you will find on all top level CNC machines and robots is Brushless motors. These can be either DC brushless or AC brushless. AC brushless is more common but DC brushless is considered by some to be better. Woodtech uses both AC and DC brushless motors on their various CNC routers. While we can produce routers with stepper motors, we steer our clients away from this unless their application is specifically better suited to stepper motors. One application that does lend itself to steppers for example is CNC plasma cutting where the torch does not have any cutting force on it and the speeds do not subject the machine to undue flex. Precision plasma however would again be recommended to go with Brushless servos.

So Brushless Servo motors give the highest consistent torque response through the speed range and fastest response times as well but they are the most expensive by far. The bottom line is that when you are paying so much for the rest of the system, why compromise here when the result will directly affect performance, accuracy and tool life.

What other issues must I consider?

One other issue is the quality of the linear rails. We have tried a few producers of linear rails and have come back to using the Rexroth linear rails and guides or equal. Rexroth is part of the Robert Bosch group of companies. Cheaper products look the same and seem to perform the same initially, but the better products will last much much longer, will maintain high specification, and give a lifetime of failproof use.

The next issue is the drive system. We started out using a belt driven mechanism. Belts tend to bounce when exposed to high inertial loads. We have since moved on to low backlash planetary gearboxes and thus been able to more than tripple our effective acceleration rates.