Digital Faceting Machine

Jeff R Graham shares his opinions on various topics in Just Ask Jeff. See his answer to Questions you need to ask when buying a digital Faceting Machine?

18 Minute Read

Questions you need to ask when buying a digital Faceting Machine?

Rubellite Tourmaline/digital faceting

As any faceter looking to buy a new faceting machine has learned… There is a new digital world out there. Many old names in the faceting machine business are updating to digital read outs. Note I did not say digital faceting machines.

Also there are new machines on the market and they are all digital or at least partly digital hybrids to start with. It is important to make a distinction between digital read outs, which are basically a hybrid machine and a true digital faceting machine. The two are NOT the same thing.

Only the machines that generate their data digitally can truly be called digital faceting machines. The other machines are a mixture of analog and digital read outs, which should really be called hybrids.

On these hybrids, unfortunately the generation of the angle is analog. They are generating an analog signal using a digital voltmeter and potentiometer. An analog machine of this type is NOT a true digital machine and voltmeter/potentiometer machines are very vulnerable to linearity, heat, wear, temperature variables, and so on.

In a true digital machine (using an encoder), the angle is generated digitally. That is the digital machine is using "1" and "0" to generate the data. The angle is literally counted by pulses. The Digital Ultra Tec and OMNIe are machines which use an encoder are the only true digital faceting machines at this point in time that I am aware of. There will probably be more in the future, because the true digital method is by far superior to any thing else.

How does an encoder work? An encoder works by using a laser to count marks on an optical disk, the number of marks determine the accuracy (the more marks the finer the resolution/accuracy). When you turn an encoder on and reference zero, the encoder literally counts the marks and is 100% accurate, it is truly digital and is not effected by any of the problems that plague potentiometer/voltmeter machines. An encoder either counts or it does not count, that is it. Also because the encoder is zero referenced every time it starts up, it does not have the wear and linearity issues that a potentiometer/voltmeter machine has.

Here are some basic rules to live by when considering a digital or hybrid faceting machine:

1. IMPORTANT - Is the faceting machine in question a true digital machine and does it have the advantages and accuracy of digital? What is the resolution (how many marks in other words) if it is an encoder? Or is the machine in question an analog hybrid? As I have said the true digital machine is by far superior. Be sure and know the difference, because the performace is significantly different between the type of machines.

2. IMPORTANT - Just because a faceting machine is digital and reads out to .01 of a degree. Do not assume the read out and faceting machine is accurate to the level that it indicates. It might be, but probably is not. Most hybrid digital faceting machines in my opinion and experience are not nearly as accurate as the manufacturers would like you to think. Just because a manufacturer is making high accuracy claims does not mean the machine really is that accurate. Manufactures have been known to… exaggerate their claims some times. Or worse yet in my opinion they play the "do not ask, do not tell…" game. They know their machines are not as accurate as they claim and just are not talking about it. That is one major reason I have written this article, do not assume anything or let a manufacturer play this game. Ask questions and expect good answers.

3. Manufacturers also have quality control issues and other problems (besides design) that can significantly effect the machine quality. So pay attention to the machining and assembly. Yes, there can be a little issue or problem from time to time that happens to any maufacturer, but look at the over all quality and machining. If you are not sure compare machines and ask… You want quality.

4. Ask questions (see below). If a machine manufacturer fails to answer any common sense technical questions or says some thing stupid like that is "top secret". My advice is to immediately remove that manufacturer and machine from your list of potential faceting machines to buy. Any manufacture that will not answer basic technical support questions is some one that does not want your business and probably not being truthful.

Note: A manufacturer may not want to be completely explicit about all the technical aspects, there are a few things that could be considered proprietary… or some things the manufacture may not want to educate his competition about (this is natural and understandable). But in general any manufacture should at least be willing to discuss the technical basics and be willing to answer most technical questions as well as give you basic part numbers and specifications that you can check.

5. ANY electronics manufacturer or electronics supply store can very easily not only tell you how to make a digital read out. Most of them can and will sell you off the shelf parts to make your own digital read outs. For that matter most Radio Shack stores can sell you the basic parts and supplies. Digital hybrids read/outs (note these are different than a true digital machine) are not secret or even difficult, so if a manufacturer fails to be honest and up front about answering any reasonable technical questions you (as a potential customer) may have about their product… Well the manufacturer in my opinion is not being honest and does not want your business.

OK, at this point you have lots of questions or should have. I am not going to get too technical here, I will try to keep things simple and cover the basics. The majority of digital faceting machine heads are hybrids, at least at this point in time and are using volt meters coupled with potentiometers to achieve the angle read outs.

Note: The new OMNIe is an encoder machine and truly a digital faceting machine, which is completely different from a potentiometer machine and I will be covering it in a future article.

Here is an article written by Jon Rolfe a well know internet person who designed and produced a digital head and then sold the right to the head to Graves who are using it for their new digital hybrid head.

Note: The article that this link goes to has been moved/removed by Jon Rolfe. I am guessing like the other manufacturares he does not really want people to ask these questions. My advice is contact him directly and ask any questions you may have on his machine heads. Also ask why he removed the article?

Here is a direct quote from Jon's (Batt Laps) article about the accuracy or the lack of potentiometers… Note the original article has been blocked on the web. I wonder why? Perhaps the accuracy of later claims on some faceting machine heaads are not what they should be? That would be my guess and opinion.

What is a potentiometer? What do they do? Here is a link to some of the most commonly used potentiometers in faceting heads. Several of the hybrdid machines I have examined are using this exact one or some thing very close.

The first questions some one interested in buying a digital hybrid head should ask are.

What are the specifications of the potentiometer used in this digital head? If an encoder is used… What are it's specifications?

What is the brand/part # so the specifications can be looked up? Always check.

What kind of potentiometer is it? A single turn? A ten turn? What is the manufacturer's resolution of the potentiometer in question (often called variance and given as a percentage)?

These are absolutely critical questions.

Here is why. As you probably know, not all things are created equally. In almost all cases the more expensive some thing is, the better, or in the case of a potentiometer… The more expensive the potentiometer is, the higher the quality and specifications and more accurate it is.

What do I mean? Well basically the more expensive potentiometers have much higher quality specifications and ratings, which translates into accuracy. This is not very surprising really, things in life all tend to be this way. You generally get what you pay for. As you can see in the pictures below. A potentiometer is basically a tube/post wrapped with wire which metal brushes touch. The brushes are set on basically a screw adjustment that allows the brushes/wipers and their relationship touching the outer wrapped wires to be adjusted. This adjustment is done mechanically with the screw, as the brushes run up and down (or in/out) the brushes read different electric currents (this is where the voltmeter comes in, read below) establishing a voltage which is calibrated as an angle.

Below - A picture of a post and brush Potentiometer.


Here are some potentiometer basics.

First lets talk resolution which in our situation means angle accuracy. The resolutions is dependant on how accurately the brushes/wipers touch how many wires. This is basically something that is figured by the manufacturer and it is called variance/accuracy and almost always given as a percentage.

Two common types of potentiometers are being used in faceting machines.

A single turn potentiometer is basically like what is on your stereo volume control knob. Basically the knob (potentiometer) turns from left (no volume) to right (full volume). One stage or turn, usually less than 340 degrees.

A 10 turn potentiometer does basically the same thing but in 10 stages. So say your stereo has 100 ohms of power, basically what you have is 10 stages of 10 ohms as you adjust the knob. In theory.

However there is a fly in the ointment here… It is called linearity. Basically with a 10 stage potentiometer every stage should be 10 ohms. But this depends on the linearity of the potentiometer. If the linearity is say +/- 2%, then the ten stages could be anything from 9.8 ohms to 10.2 ohms just depending on the equipment and the temperature.

Note: Temperature is a critical factor and on almost all potentiometers the manufacturers will give a performance variable by percentage usually depending on the temperature. The way around the tempature variable is to have a more expensive potentiometer that has a heating/cooling element in it to maintain a constant temperature. This of course is a much more expensive potentiometer

So what is the basic difference between a single turn potentiometer and a 10 turn potentiometer? There is no difference in resolution what so ever, assuming the same brushes and wire wrap count are being used (or performance percentage). Where there is a difference is the repeatability. In a 10 turn potentiometer you have 10 divided stages of current. Which as you can see is more inherently repeatable. So basically a simple inexpensive potentiometer like many faceting machine manufacturers are using is what is called a single (or ten depending) turn wire potentiometer. In general these potentiometers have an accuracy of +/- 2%. So what you have for accuracy just counting the potentiometer, not all the other parts and variables that make up the faceting machine head is… 2% of 90 degrees = 1.8 degrees. So using a potentiometer with a +/- 2% accuracy the actual angle maybe off as much as plus or minus 1.8 degrees. That is the accuracy of the actual potentiometer given by the manufacturer. Note there is a 3.6 degree range of error.

Note: This of course is not taking into consideration the rest of the variables that will effect accuracy of the entire faceting machine head, like the accuracy of the voltmeter, the digital read out, and just quality of machining and design, as well as assembly.

It is very important to also realize that the back lash greatly impacts the repeatability of the potentiometer. The repeatability of an inexpensive potentiometer is basically very poor. Just think what the odds are of getting the brushes in the same place twice if they have say 1.8 degrees of typical back lash? Not very good right? If you cannot "repeat" the setting because of backlash, then there is obviously no repeatability. A ten turn potentiometer will help but it will still not be very repeatable depending on the acutal specifications of the part.

Also remember that as you change directions (up and down angle adjustments in our case) that you turn the potentiometer… the brushes can and often are flopping back an forth, causing the brushes to be out of line to each other which only makes the backlash that much greater.

Note: Because of the what I would call "flop" of the brushes it is always best to set a potentiometer from one direction by at least a 1/2 turn or 10 degrees depending on the potentiometer. Using this method the brushes will all be as lined up with each other as possible and it will elininate some back lash, not all.

Below - A picture of a post and brush Potentiometer backlash. See the amount of the brush/wiper and the bend touching the coil? That is back lash. Remember that the brushes in this picture are running in and out on the coil, the view is from the end. Think of this like brushing your teeth from sider to side. What happens when you change direction? Right the brush bristles flop.

Backlash of brushes

Accuracy depends on the quality and cost of the potentiometer used in the faceting machine. Basically an inexpensive potentiometer is generally used in faceting machine heads (I know of several major brands using them). The cost is around $36.00 and has an accuracy of basically +/- 2% which translates to 1.8 degrees, as applied to the 90 degrees that are used to set angles on a faceting machine. EVEN though the digital read out maybe reading to .01 accuracy. The electronic parts inside the head that are making the actual angle reading are only capable to +/- 1.8 degrees and maybe as bad as 2 or more degrees off, if a cheap potentiometer is being used. This of course is why you want to know what the potentiometer specifications are… A digital read out only gives an arbitrary number and is only as accurate as the potentiometer being used is… as well as the other parts.

In other words the angle on the digital read out is only as accurate as the accuracy/specifications of the parts that are generating the data.

How accurate can a potentiometer be? Depends on price, but here is an example. A larger pot Omega Engineering model RP103-360 with a backlash rating of .01 and a linearity rating of 1% is available. This potentiometer has quality wiper/brushes made from precious metal. This potentiometer is currently $400.00. This potentiometer is one of the best and will give accurate read outs to the .01 degree (maybe better).

Note: These prices are as of December 2006, electronics change as we all know so the prices may vary.

But the big question here is. How many faceting machine manufacturers are using the $36.00 potentiometer and how many are using the $400.00 potentiometer? A quick price check on the faceting machines will answer this question. If a faceting machine costs say $1800.00 total. Do you think that manufacturer is using a $400.00 potentiometer??? I doubt it very much. When in doubt ask the manufacture what potentiometer they are using. If the manufacturer does not want to answer with part numbers and facts… well you got your answer anyway. They are using the cheap parts and potentiometer.

Note: Price varies from manufacturer to manufacturer, but the price of the head or complete faceting machine as the case may be is a good indication of what parts and costs.

So this is why customers need to ask for potentiometer specifications and part numbers. What exactly are the manufacturers of the machine(s) you maybe looking at and considering buying using for parts?

Next question is. What volt meter is being used? Why is this important?

Several other parts are used internally to give the digital read out information. Not just the potentiometer.

The volt meter is one of the internal parts being used to get an angle reading and voltage is used to determine the angle. Again, this boils down to the specification of the voltmeter, which in turn is determined by the cost of the volt meter. If the faceting machine in question is using an inexpensive volt meter, the accuracy is usually around 0.1 Volt, some times not even that good. If a more expensive volt meter is being used then of course accuracy improves. For true accuracy and repeatability the volt meter would have to be capable of at least .001 Volt, maybe better. But the point here is if the manufacturer is using an inexpensive volt meter, the meter itself is not capable of a high degree of accuracy and it does not matter what accuracy the potentiometer being used is, if this is the case. The volt meter cannot deliver the accuracy, so it makes no difference what the quality of the other parts used to get the angle reading are.

The next question is what is the specifications and accuracy of the digital read out itself? Just because the digital read out is capable of showing .01 accuracy on the display does not mean the display is accurate to that place. What I am trying to say is that again most inexpensive digital displays while showing/displaying .01 on the read out are really only accurate to .1 on the read in real life. Most inexpensive digital displays are not capable of being more accurate than .1 even though the actual display may read .01. In other words the display does not have the required resolution and is incorrect.

There are of course other factors that will influence the accuracy of any digital read out. One of the major factors is of course the quality of the machining and assembling of the rest of the faceting machine. Most of these quality issues are easy enough to see visually if a person has some machining experience and takes the time to look closely.

Note: The internal electronics are not where they can usually be easily seen or verified by part numbers, which is why it is so important to ask the manufacture for the specifications. Not only to verify accuracy claims but to know what parts and specifications might some day be needed for repairs.

Also design has a lot to do with accuracy, but this will vary from machine to machine. So I will not get into it here, but I will in future machine reviews. Remember that all of these parts, specifications and factors can substantially impact the digital accuracy of any faceting machine and that often the lower quality parts and specifications can add up together to make a digital hybrid machine much less accurate than advertised. In many cases digital hybrid faceting machines using potentiometers may only be accurate to +/- 1 to 2 degrees. Buyer beware.

Do not think that is a big deal? Well most modern Quartz faceting designs have a 1/2 degree to 1 degree cushion above the critical angle of Quartz. So a faceting machine that is say 1 degree off could very easily put the stone below critical angle. This of course would be a huge problem.

So in closing. Any one thinking of buying a digital or a digital hybrid faceting machine should ask these basic questions from any manufacturer.

What are the specifications of the potentiometer being used?

What are the specifications of the voltmeter being used?

What are the specifications of the digital read out being used?

In the case of a truly digital faceting machine…

What are the specifications of the digital read out being used?

What are the specifications of the encoder being used?

ANY manufacturer should be up front and honest about these parts and specifications. The parts should all be quality and match the manufacturer's advertised claims of quality and accuracy.

If ANY manufacturer(s) fail to answer any of these questions fully, acts secretive (says "top secret"), or just fails to answer the questions. My advice is that manufacturer has some thing to hide and any customer not getting good answers to these questions should look else where for a digital or digital hybrid faceting machine.

Do not assume that a digital hybrid faceting machine is more accurate or even better accuracy wise than any other faceting machine. Generally I would say that at least potentially a true digital faceting machine, followed by a quality hybrid digital, and then an analog faceting machine, are the most accurate in that order. The accuracy of course depends on the quality of parts used.

True digital faceting machines are potentially the most accurate, it depends on the manufacturer and the quality of the parts and design, but hybrids do have the potential to be very accurate also. It is just that most of the hybrids I am aware of the manufacturers are not using quality parts.

In many cases a digital hybrid faceting machine maybe much less accurate than and old dial or protractor faceting machine. As I have noted, many of the hybrid manufacturers are not using quality parts and they are not being up front in my opinion about the real accuracy of their machines. So be sure and ask the right questions.

The accuracy of any digital faceting machine depends entirely on the quality and specifications of the parts used and of course the design and machining.

Jeff R. Graham

The late Jeff Graham was a prolific faceter, creator of many original faceting designs, and the author of several highly-regarded instructional faceting books such as Gram Faceting Designs.

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