In the past customers have often asked us for our advice when considering the purchase of a CNC routing machine. There are many things to consider but from a machine perspective some of the main points are listed below.
Drive System
There are 3 types of drive systems available for CNC routing machines. A brief description of each is outlined below:
Rack and Pinion – Works best when you require larger machine area, generally greater than 1.5 metres. They are a lower in cost option and a good starting point if the machine is your first.
Lead Screw – These are more accurate than rack and pinion. If you are dealing with a larger workpieces, you may not need it. They don't perform as well as ball screws do for high production environments, or persistent and long process durations. Be that as it may, they function well for jobbing applications.
Ball Screw – Are the most expensive for a reason. You can work machines fitted with ball screw much harder and cut larger material sizes with more prominent accuracy and productivity. In saying that the one drawback of ball screw drive systems is the potential for spindle whip. We are working on a technical article for a future post explaining exactly what this is and how it can effect machine stability and tool performance.
Servo VS Stepper Motors
Normally stepper motors have 50 – 100 magnetic poles. Servo engines, as a rule have only 4-12. Because of the higher number of magnetic poles in stepper motors, the accuracy and precision of axis positioning is greatly improved. Stepper motors cost less and are simpler and more cost effective to repair or replace. Servo motors on the other hand need an encoder to provide the same. While more uncommon, the upside of servo motors is they are generally more efficient and cost effective for longer, heavier workloads.
Machine Weight
There was a long held belief that a heavier machine was more effective at reducing vibration therefore increasing feed rates and hence productivity. With improvements in technology, CAD and computer based testing, where structural designs can more effectively evaluated for frame stiffness and rigidity under various loads, this may no longer be the case. In light of this fact the better question to ask might be “How is this machines design better able to reduce vibration in high load work environments”. Another thing to consider for a machine of greater weight is the shipping cost for obvious reasons.
Spindle Interface
The two options typically available in CNC routing machines are ISO30 or SK30 and HSK. While both are suitable HSK is a better option for heavy load or faster production speeds, something we call HSN or High Speed Nesting. There are other factors to consider when considering HSN machining and I have earmarked the subject for a future blog post so stay tuned for that.
Software Packages
There in an increasing number of options available to the CNC owner in this area from both machine manufactures with in house solutions and third party providers. With so much choice it can be hard to know which is best. It is important to think about the type of work you currently do and ask yourself if you intend to continue doing the same work or expand the range of work you do to the point of offering contract service to other industry participants. The key thing to remember here is that what works for one company might not be the right option for you. Another thing to consider is the cost structure, is it one time payment or ongoing annual payments.
Clamping System
Tool clamping, or chucking systems are often viewed as just a device for holding the cutting tool which in turn does all the work, therefore not that important. But this attitude could not be farther from the real story. Clamping devices probably have the most impact of all the variables on tool performance in CNC routing and therefore directly impacts the ongoing performance of the machine. There are a number of chucking types to consider including:
Spring collet type (ER / OZ)
Hydro (fluid)
Heat Shrink (Special equipment required)
Tribos or Power Shrink (Special equipment required)
The most common is the ER spring collet type and if used correctly and maintained is suitable for all operations including HSN in high volume environments and is extremely cost effective when maintained correctly. Other systems can be prohibitively expensive and provide negligible increases in tool life and machine efficiency in most work environments.
Tool Quality
Quite simply you get what you pay for. There is a strong correlation between the cost of tools and performance. Cheaper tools tend to not last as long and can break more regularly. The advice here is to find a supplier that not only can provide the right quality at a competitive price but can also provide advice on optimizing the machine performance.
