Plain Talk on CNC, Part II

June 6, 2005

SPINDLES AND SAWING
   A few years ago, the first saw attachment to a CNC appeared to cut out sink holes and cut off large pieces of stone. Everyone had one to show at the next large stone fair, including OMAG. In reality, the idea was bad to begin with, and applying it definitely proved it.
   The saw attachment needed a cupped blade to do sink holes; because of the size of the saw blade, it made it useless to do sink holes that were not very large ellipses and circles, so nearly all kitchen sinks and vanity sinks couldn’t be cut.
   As far as just using the attachments to saw off pieces of stone to ready your countertop for profiles, I think it’s a terrible waste of time. The saw has to travel much slower than a finger bit, so you are losing time.
   It’s more-efficient to do all of your prep work on a bridge saw. I had one company once that didn’t have a saw, and – even though I recommended against – they got a saw attachment to slice up their work. In a short amount of time they bought an Accu-Glide rail saw for pre-cutting.
   If you’re cutting with a CNC, sink cutouts and other big pieces falling away rarely damage suction-cup hoses. I do recommend keeping a few pieces of wood around to slide under fall areas to keep the table from getting dinged up. It’s the surface that your suction cups need to achieve a good seal, and it’s worth protecting them.

MOVING A MACHINE
   A machine that’s a single unit, or features unibody construction, is easier to move for sure – but moving any CNC is never fast and never inexpensive. There is a lot to moving a CNC, and it’s not a job for amateurs. I’ve seen highly mechanical people (who were also involved in the installation) do it successfully, but it requires the right tools and knowledge.
   Most single units are limited, by the way, to how many parts can be placed on the table. I have a few clients who rarely have less then three parts going at once.

LASER SCANNING
   Also called reverse engineering, it’s a process where a laser (almost always mounted on the CNC) scans a model by making passes back and forth over it while slowly moving forward in small increments. This process creates what is sometimes referred to as a cloud of points that additional software uses to create a virtual “skin” that allows you to shrink or enlarge the size to what you desire, and then develops the ISO code needed to have the machine create the work.
   The challenge you’re looking at is your desire to completely finish a piece, which is determined by the size of the tool you plan to use. If you want an extremely finished look, you must use a tiny tool.
   Now, consider taking a tool the size of a Dremel bit over a fireplace mantel; the pass could take several days to run. Also consider that, to get to where that tiny tool can work on detail, you need to run at least two larger tools ahead of it to clear away material. People who go for the completely finished look are sometimes spending over a week of run time to complete a slab-sized piece of work on the finishing tool alone.
   For reaching under items like carved flower petals and other relief pieces that are curved backward, you’re also stepping up from a normal three-axes machine to a five-axes machine. A big jump in machine, price, and knowledge are required, and I would never suggest to anyone that they move directly into buying a five-axes machine without having years of experience on a three-axes machine.

HOW MANY AXES?
   A normal CNC doing countertop work is really a two-and-a-half axes machine, and is commonly referred to as three-axes machines in our industry. When you are hear a machine is three-plus axes, find out exactly what those extra axes really are.
   A rotary tool store rack, tilt head, or tilt table capable of doing drainboard work technically constitute an extra axis, but these aren’t things that allow moving of the spindle head in extra directions to allow “five-axes” work. Be sure of what you really need to do your work and what you are getting. Do the homework and call lots of people, and ask lots of questions.

AUTOMATIC TOOL STATIONS
   The more tool stations you have, the more edge profiles and other tools you can hold at one time. OMAG machines, for example, can hold 36 tools – and, to be honest, it’s a convenience and that’s all.
   Be aware that it can take up to seven tools to make a profile edge, and it may take you two to five minutes to walk to the machine and physically change them – and then another minute to change the file at the machine.

WATER PRESSURE AND TREATMENT
   The largest tooling manufacturer would like 58-78 psi of water. I am concerned about this being tough on the seals of the spindle.
   A CNC will use 12 to 13 gallons of water a minute. My experience tells me that if you have the machine on so all of the water is flowing, and you can hold your waterline pressure at 45 psi (or, even better, 50 psi), you will do just great.
   Remember this is pressure you get while all of the other machines in the shop are running. This is an amount that the average water lines won’t usually deliver and does require some research on your part on how to handle it.
   As far as water conditioning, OMAG’s own specs require water delivered to the spindle to be filtered to 20 microns, and outside water filtered down to 50 to 30 microns. Remember that anything of any size is actually adding abrasives to your water and possibly hindering your attempts to polish. I have seen people pumping water onto tools that was so dirty they were adding a bigger grit than the actual grit of the tools.
   Water filtration is something that may not be mentioned in the same conversation as buying a CNC machine. It should be if you want your CNC – and all of your machines – to run right.

TOOLING
   Consider something in your mind – a spinning tool moving in a linear motion around a linear part. Now consider the nature of abrasives, with various sizes of grit that grind to a determined finish. To be an abrasive it requires manufacturing processes that ultimately create a tool similar to just what you see on a sheet of large-grit sandpaper – there are high and low points all over it. The same is true on the tools, only we are working at the micron level.
   I have created finishes on CNCs that have literally awed people, but are there some linear lines? Of course there are, although you may need the right light and a magnifying glass to see them. The idea is to use tooling that makes the lines impossible for all except a trained professional to see. Different tools do have different effects and histories of how well and long they can make those incredible edges.
   There really isn’t an “average” cost per foot on tooling. This answer depends on your shop and how you handle things; every operator is different about how they use the machine. Some people, for example, are faster on killing the edges or touch ups.
   The CNC itself doesn’t care about the size or shape of a tool; it simply does the job. A fancy edge takes the exact amount of time as the simplest edge, given it is using the same manufacturer of the tooling. Any tool that comes to a sharp edge such as an ogee requires a human to touch it up on the top and bottom edge so it’s not left sharp. An exception is a bullnose that would require no hand work when finished.

SUCTION CUPS
   The amount of suction cups required with a CNC depends on the part size. Different-size parts require different amounts of suction cups. I recommend that you get as many as it takes to get a good solid hold on the part. If you try to use too few, you may move the part while cutting it, or – just as bad – create a condition that is causing premature wear on your tools.

TABLE SIZE
   I have no idea who has the biggest table in the industry. I think it’s more-important to make sure you can put a whole slab on the table. Many islands require a whole slab; I’ve made an island that required two complete slabs.
   The table on my countertop machine that I like the most is 78” x 150”; the actual working area of that table depends a little bit on the tools. At each edge is an electronic limit switch that keeps the spindle safely over the table. The center-line of the tool must stay within that limit – so if you had a 3” tool, subtract 1.5” from all sides to determine your working area.

CNC CONTROLS
   It’s probably a personal taste, but I really like having the CNC control on a pendant so you can move it around. You can move it closer for a better view of the work and still have it right there with you to be able to be in complete control.
   While a CNC machine usually has Windows-driven controls on-board, that’s not where the CAD-CAM software goes. It’s installed on a PC, and files are sent to the CNC’s computer via networking.

A FINAL NOTE
   Look at the machines you may be considering. How are they built? Does it appear simple or difficult to load the material on and off the table? How thick is the material that the machine is built with? How big is the spindle? How big are the tool cones that hold the tools? Considering the force created by the grinding of granite, do you want the spindle and tool cones to be big and solid?
   Think about the purpose of your business, and then decide what you’re really trying to find. Everyone wants to do the stuff that looks cool – but is that where you are going to make your money?
   Jerry Kidd is the North American Technical Representative for OMAG S.r.l., a stone machine company headquartered in Zanica, Italy. Kidd, who’s worked with CNC machines for more than 17 years, is based in western Colorado.