Plain Talk on CNC, Part I

By Jerry Kidd
   Editor’s Note: Not long ago, the author – a technical representative for an Italian stone-machine manufacturer – received a detailed inquiry from a customer in Turkey concerning CNC machines. Several thousand words later, he’d written a response that reflected his lengthy experience in the field, with plenty of common-sense advice for anyone considering a CNC. What follows is the first of two parts on some personal and interesting observations on owning and running CNC equipment.
   To be honest, it’s pretty easy to set up the machines in whichever style you chose to run parts with.  Basically there are three different styles.
   Fixed zero: This is probably used the least, but it’s very simple.  There is a fence, or rails, or stops on two ends of the material that allow you to literally bump the stone into a definite repetitive solid stop that never moves.
   Floating zero: This is commonly used. It allows the user to position the stone (or, hopefully, stones to be more-productive) in any desired position to get the optimum part-placement and efficiency of the machine.  This process sounds more difficult to people at first, but once they do it a few times it is really quite simple and easy.  People that struggle to get the machine running on their own usually evolve to fixed zero, and people that are trained by professionals usually are using a floating zero.
   Laser-positioned: There are at least three different laser-positioning systems that can usually be found in the back sections of the stone-industry magazines.  These systems claim to save close to 30 percent of the normal setup time.  I believe this is a strong possibility. 
   Most manufacturers are not providing them with the machine because of the initial cost of the laser.  The machinery market is very competitive now, and anything that adds to the price of the machine is heavily scrutinized. 
   The idea of the laser is to take the same geometry drawings that you must create in the machining CAD-CAM software and project that or those drawings right onto the table so you can quickly set the suction cups of the stone into position.  These drawings that we create can be exported as a DXF file (the most-common file type, along with DWG, for industrial use).  This may take a minute when you are first using it and are moving a little slower.
    As far a lining the stone up, the real question is: How accurate and square is your saw?  If you are only cutting the front edge of a countertop stone, you can literally split the laser so half falls on the stone and half goes on the table.  The projection is in approximately 1/8”-wide lines, so it’s not difficult to accurately locate the laser. 
   Imagine a table with three parts on it.  The suction cups (no matter the shape) are projected onto the table; after they are placed and the suction lines are attached, the parts are projected that need to fit onto those suction-cups areas.  No guess work at all, and no limitation of a fixed stop that is sometimes in the way;  just locate the suction cups and parts on the laser lines, turn the suction cups on, load the part programs, and then run the parts. 
   This is not a good machine/ bad machine issue.  The real question is: Does the machine come with the capability to measure the tools automatically and adjust the machine for tool wear when found? 
   All of the tooling manufacturers suggest an exact amount of thousands of an inch that the tools should be positioned from each other to achieve optimum finish and wear.  If your machine has a device to measure the tools, you can plug in those exact physical relationships from one tool to the other in your software; then, you’re ready to run after performing your measurement program so your tools are staggered in an ideal relationship to each other. 
   I recommend that if you remove one set of tools from their holders, you measure them again.  If the tapered portion of the holder is only a few thousandths of an inch different to another holder, it could cause changes that would lose the ideal relationship from tool to tool.  This is why it’s cost-effective to have tool holders (or cones) for each set of tools. 
   Having a system that measures the tools creates an opportunity to have your tools perfectly adjusted in one to two hours.  Not having such a system requires you to make test cuts on a piece of stone and use instruments to actually measure the amount of difference between tools.  To make these measurements, it’s necessary to make test cuts with the tools coming out a few inches from each other, so have something to gauge both vertically and horizontally.  I have seen this process take almost all day to perform. 
   Worse yet are operators who make a pencil mark on the stone, run the next tool, and stop to see if all of the pencil mark is off of the test stone.  How many thousands of an inch did it take to remove that pencil mark?  The tool manufacturers give exact values so you can achieve optimum finish and wear of the tools.  If you are using measuring tools or a pencil two months later, how much tool wear do you need to adjust for?
   There is a difference in quality of these devices, so ask the manufacturers about their accuracy.  (Ours at OMAG, by the way, is plus/minus .00005”.)
   I have one on a Profiler installed in 2001 that is in perfect health today.  I have another machine that was built in 1998, been abused horribly since it was purchased, and is on its third spindle. 
   I believe a spindle’s life is dependant on its care and use.  Clients performing the maintenance, and using their machines intelligently instead of crashing them all of the time, can expect a long spindle life.  I’ve been a machinist since 1976 and a CNC machinist since 1978 – and based on my experience, I would not consider rebuilding one. 
   Cooling the water flow through the spindle is really dependent on how the spindle is built. At OMAG, for example, we build our own spindles on the expectation that the water temperature is whatever your water system delivers, so we don’t require additional cooling.
   Ultimately, my personal belief is spindle construction with proper maintenance and minimizing crashes is what determines spindle longevity.  Even with those machines run by people who do not seem to care or pay attention, my worst-case scenario is an older design that lasted two years through incredible punishment.
   One other thing to note about spindles is actual performance speeds. OMAG’s delivers 15 HP at any speed of 100-9,000rpm. Others may show larger HP output, but possibly at maximum rpm.  A check with the tool suppliers shows that almost all granite tools are built to cut at 4,500rpm, and polish at 3,000rpm or slower.  And that’s the level that you need to know power delivery to make good comparisons. 
    There are many different software packages in the industry for CNC, and nearly all of them were developed in other industries and adapted for stone work. My bottom line is find the machines at the trade shows that interest you the most, and then have them walk you through the software – and see for yourself just how easy it looks. 
   Please don’t be fooled by nice graphic displays.  Look for how many details it takes to accomplish the job.  One thing I would suggest is have them show you what it takes to create a new set of tools to work with and how you apply them.  This alone can be crucial. 
   Most countertop software is programmed for two-axes restricted 2D.  There are some sinks that this type of software can do, but they must have the exact same radius coming down the side walls of the bowl all of the way around the sink.  An oval sink with the normal compound radii on the two sides cannot be done with this software.
   A 3D software allows you to move three axes at a time, and this software is what’s required to do most of the sinks that people like to produce.  The difference usually is that 3D software is about triple the cost of 2D software.
   I guess it is time to ask you what you really are making your money on. Are countertops the real production of your shop, or do you have the time to take possibly a good part of a day to get out a sink?  I have many countertop clients that would all love to do the fancy things, but the reality they face is the fact that their business is countertops. 
   Unfortunately, because of the costs of things in the United States, you can almost always buy a sink bowl cheaper from a foreign country than you can produce it here.  Sad but true.
   The software I use is made in the United States; there’s technical support from California, so there is no huge time zone difference in getting help.  It’s software that is still openly made for metal work, but I personally find it is the simplest for countertops. 
   My reality is that I need software that is simple for the clients to use, and therefore causes me the least amount of follow-up technical support.  The sooner that the current client is running confidently, the sooner I’m heading for the next client and getting them trained.
   Any CNC should be able to do any size of radius and then move right into normal shaping without hesitation.  The CNC doesn’t care what shape it cuts; it just cuts. 
   One note on completely round pieces – the smallest I’ve produced is an 18” circle.  The issue is that vacuum power is rated in square inches.  Big suction cups have much greater square inches than small ones, regardless of the fact that they are all run by the same vacuum pump and deliver the same amount of suction.  I am sure that, with the right configuration, a much-smaller round could be made, but I haven’t done any testing to find out just how small.
   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.

This article first appeared in the May 2005 print edition of Stone Business. ©2005 Western Business Media Inc.