Editor's note: An edited version appears in print. Here is the article in its entirety.
It is widely accepted that CNC machines are an efficient way to manufacture many materials, chief among them are wood, plastic, and aluminum. So once you decided that a CNC router is in your future, how do you begin the process of narrowing down the choices to the right one for your business?
In my experience, the criteria that can be divided into 3 main topics: Machine, Software and Support. It’s obvious but worth saying, having a great machine, but programming software that does not fully utilize its capacity is a waste of resources. Conversely having great software that makes efficient programs but running on a machine that cannot precisely execute code also wastes time and money. Finally great software and great machines are not great unless you can get help when you need it.
In many shops the investment in a CNC machine is second only to the building cost. But there is a commoditization occurring at the low end of the spectrum. The cost to own a simple machine now begins below the $5,000 level, making CNC more affordable than ever.
I break the spectrum of machine features into the “physiology” of “bones”, “heart”, and “brains”. The bones are the structural components of the machine (frame, bridge, and bolt on components). Is the frame welded or bolt together? A welded frame will not need to be periodically re-squared. If it’s welded, it is stress relieved. Examine the structure of the frame, the old adage applies, more weight in the frame is generally better for longevity.
Look at the bridge, here excess weight can be a detriment, but steel is much better at vibration dampening, and stiffer than aluminum. And notice the legs of the bridge, all the force of cutting, and all the momentum from movement is borne by the legs of the bridge. Lastly inquire about the components that are bolted to the frame. All of the machine tool builders buy components such as linear guides, rack and pinions, and ball screws. All of the component suppliers have different sizes, and grades which will translate into the usable service life of the component.
There are other features about the frame that need to be examined, chief among them is fixturing. How will you hold the work piece during machining? A work holding system needs to be included into the table design.
If the frame of the machines are the bones, then the spindle is the heart. For routing spindles you will commonly find 2 types, with and without an Automatic Tool Changer (ATC). You must determine the number of different tools needed to complete the work and match the spindle type to the work. But CNC controls have changed what a single tool can do. For example it is possible to use a 5mm router bit to drill 5mm holes, and circular interpolate to make an 8mm hole, and then dado, and finally cut the perimeter of a ¾ think plywood cabinet side (using 2 or 3 passes). This is called the 5mm strategy, and although is not the “ideal” method to cut a nested sheet of cabinet parts, it is much faster than by hand. It certainly works for a small shop with a limited budget, and easily makes a couple of kitchens a month. However more production will require using different tools for each operation thereby cutting faster and decreasing the cycle time.
There are basically 2 types of ATC spindle ISO 30 (or BT30) and HSK63. The difference is how the tool holder is held into the spindle, and while ISO works adequately, the HSK63 is a stronger connection. When it comes to capacity there is a simple calculation. Every 4 x 8 sheet of material can hold all the parts for between 1 and 2 cabinets. It depends on the size of the cabinet and the construction method. For example stretchers use less material than a solid top. The pure cycle time to cut a nested sheet of cabinets can vary widely based upon the tooling used, spindle power, and the rigidity of the machine frame to withstand the cutting forces. As an example using an ATC machine, we’ll say that the actual cycle time is 7 minutes, then add 3 minutes for load/unload. Then the floor to floor time for 1 sheet is 10 minutes. This means 6 cycles per hour could produce between 6 and 12 cabinets. If a “typical” kitchen is 20-25 cabinets, then conservatively, all the parts can be cut in about 4 hours (6 cabinets per hour x 4 hours = 24 cabinets). That’s 2 kitchens per day, or 10 per week. If you need more capacity consider adding a drill block, or increase spindle power to cut faster. A drill block can save about 1 minute per sheet. You may also consider a strategies to decrease loading time. A twin table machine lets you work 1 table while you load/unload the other. There are also automatic devices that can eliminate the human factor of load/unload altogether.
Finally I’d call the CNC control the brains of the machine. In its simplest definition, the function of the control is to keep the machine on the designated cutting path. As you cut faster, and as your accuracy requirements increase, the demands put upon the control will increase. I’ll must admit a personal bias here, I like controls with a big company behind them. Like many other people, I feel like I can fix about anything mechanical (maybe with the help of my local machine shop). But when it comes to electronics, if something stops working, I need to order a new one. I like the idea of a large multi-national companies’ resources to guarantee a part is available when I need it.
I would also include the drive motors in the brains topic. There are two kinds of drive motors, stepper (which are usually open loop), and servo (which have feedback making it possible to be closed loop). Steppers are used on lower cost machines, while servos are the choice for machines where performance is a key consideration.
For example if you plan to machine MDF doors, the repeatability requirements to eliminate steps between tool paths can be in the fourth decimal place (.0005”). Both your mechanics and electronics need to be up to the task to produce those results. “Open loop” controls will have a difficult time with a job like that. So understanding that it’s not common to be given a choice of controls because of the work necessary to marry a control to a machine, so it should be a primary topic as you consider your choices.
Software of some type will be needed to write programs to run the machine. When it comes to software, it becomes more a matter of choice than statements of fact.
I may use a brand of software that I find easy to use and intuitive, but to someone else it feels counter intuitive and difficult to use. Everyone already has their “logic stream” determined by the time they reach their early adulthood. If the software needs input from you in the way that seems logical, that software seems easy to understand, a good fit. Conversely if the input needed does not sync with your logic stream, it will feel clumsy, and hard to use. There is nothing wrong with the software, it’s just not good software for you.
That said, you will need to interview several pieces of software and find one that seems to make sense to you. Then remember, it’s only works as good as you learn to use it. Buy enough training to get you going, do not be “penny wise and dollar foolish” on training. A little extra training up front can prevent a lot of frustration, and scraped parts.
Support for the machine can be as important as the machine or software. It starts with learning to properly and safely operate your machine, but should go well beyond the basics.
Gaining an understanding of the myriad of contributing parameters to a successful and profitable machine tool integration involves many factors. Tool optimization, machining techniques for solid wood, and fixturing small parts, to name just a few. Support should also include having the required replacement parts and technical personal as required to keep your machine in good operating condition. In short, having help where and when you need it.
Today many CNC companies have internet access to your machine and can troubleshoot down to a component level, then overnight a part for you to install. But remote troubleshooting in not a panacea. It’s no secret, the more complicated your machine, the more likely the need for specialized technicians from outside your company. Unless you happen to operate down the street from your CNC machine company, it’s going to cost money to get that tech to your front door. Generally speaking simpler machines are easier to fix and can more reliable in the first place. I understand that this article is brief, so I’m happy to elaborate.
Author: Tom Galzin, is a sales and purchasing rep at New CNC, Holland, MI. For more information contact him at (616) 994-8844 x6, [email protected], or visit NewCNC.com. Read more at WoodworkingNetwork.com.
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