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INDEX
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Building Jon Rolfe's Homebrew Faceting Machine
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by Cate Harrison and Jon Rolfe
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Low-budget lapidarist that I am, I was excited and intrigued when I discovered Jon Rolfe's website. In addition to a devilish sense of humor, Jon's pages treat the eyes to a feast of beautiful faceted gems, all of
them cut on Jon's home-built faceting equipment. Now I had come across various plans for makeshift faceting set-ups using dowel-rods and plastic protractors, and had sketched quite a few novel designs. But when I saw the precision gems that Jon had cut,
I knew that his was no rickety makeshift arrangement. Jon and I became acquainted over email, and eventually Jon convinced me that I was capable of taking on this highest pinnacle of lapidary self-sufficiency: building my own faceting machine.
Jon's web site contains information that can help you construct your own faceting machine, particularly if you have machining experience. For me, however, there was too much of a gap between what I already knew and what I would need to know. With a lot
of reading, emailing with Jon, reading some more, and puzzling things out, I was slowly able to get to the point where I knew what I needed to know to begin. The purpose of this 3-part series of articles is to explain the process of constructing a
faceting machine in greater detail than Jon's original pages have done, using my own learning process as the starting point for a technically competent amateur builder. Jon has constructed additional figures and diagrams which will be appearing in this
series, and which may eventually be placed permanently at his site or otherwise available directly from him.
Why build your own?
Why build one's own faceting machine? For me, as usual, one issue was money. Commercially available faceting machines run from around $1000 to $4000 or even more. Another issue that Jon immediately pointed out was quality assurance. By building your
own, you are able to make all the decisions as to cost/benefit tradeoffs. You can use a 1" stainless steel mast if you choose to, and all stainless in the facet head. Or, if you need to save money, you can decide which compromises are bearable.
For example, I chose to use a 3/4" thick sheet of cast acrylic for the table of my first faceter, stiffened with aluminum angle underneath. Eventually I will replace it with milled aluminum, and that will be an easy upgrade to make. One final issue has
to do with understanding and appreciating the technology involved. Had I purchased my first faceter, I would not have been forced to understand the electrical circuitry or the dynamic components of it. As it is, I not only understand these aspects well
enough to build them, but I also have several ideas for future upgrades that might be expensive options or simply unavailable for a commercially produced machine. Instead of a unitary machine to be happy with or not, I have instead a system of components
that I understand and can tinker with and upgrade as my budget and expertise grow.
Why not build your own?
As Jon says on his gem pages, this is an advanced project. You will need to be able to machine metal parts very precisely, for one thing, so if you don't have a metal lathe you will need the goodwill of someone who does. You will also need to build a DC
power supply and motor speed controller. The version that we describe here is fairly simple, and you may feel free to get as fancy as you like, provided you know what you're doing. However, if you have never taken electronics, you will need to read a
great deal more than this series of articles before you start monkeying around with potentially lethal voltages. This is no kidding. With a bachelor's in physics and a few military electronics courses under my belt, I still had quite a bit of reviewing
to do before I was able safely to build this simple controller, as I hadn't touched electronics for six years.
If you do not have a knowledge of electronics and electrical safety, DO NOT attempt to build or wire the circuit described here until you acquire that knowledge.
"Builder Beware..."
Jon has urged me to make an additional cautionary statement. This project is NOT a good way to get a "free" faceter. Many, many hours of work are required to complete the project, including a few costly hours of a machinist's time if you must pay
someone to do the machining. As Jon pointed out to me, if your goal is to get faceting, and your only holdup is cash, then your time might be better spent just getting a part-time job to buy an off-the-shelf faceter. There are "hidden" costs that don't
show up in a parts list. For example, I bought a soldering iron to make my wiring more secure. I also paid to have my baseplate drilled, even though acrylic is easily cut with wood-working tools, because I do not have a full-size drill press at home and
the holes for mounting the motor and the mast must be accurately positioned and square to the surface.
On the other hand, someone with a well-equipped home shop, including basic electronics experience and tools and an accurate metal lathe, could probably complete this project over the course of several weekends, with only an outlay for raw
materials.
To sum it up, be honest with yourself about your capabilities and facilities. Even if you decide that this project is not for you, we hope that you will learn some interesting and helpful things by following along with us.
Subsystems
I thought of this project as a three-subsystem assembly.
The first subsystem consisted of:
- the motor
- the supporting flanges for the laps
- the circuitry to control and power the motor
The second subsystem consisted of:
- the table of the machine, which serves as the reference plane for all angles and upon which all components are mounted
- a wooden base on which to mount the table and electronic controls
The final subsystem was:
- the facet head and mast assembly
Subsystem 1: The motor, lap mounting hardware, and circuitry
Motor
In Jon's design, the laps are directly driven rather than belt driven. As Jon explained, any belt system introduces the potential for wobble and inaccuracy. Instead, he machines and mounts hardware to support the laps directly onto the motor shaft.
This does require a DC motor with a large diameter shaft and very well sealed bearings. Jon recommends a DC motor capable of 1/5 HP with a minimum 1/2" shaft. Says Jon: "1 HP is about 740 Watts. The motors I always use are around 80-90 volts. So
that at 2 amps, they are rated around 180 Watts (volts times amps equals watts), or >1/4 HP." He later added, "All commercial faceters use brush-type DC or Universal (AC/DC) motors. The latter run beautifully on DC. They love it. I once built a lap
for someone out of a gigantic surplus vacuum cleaner motor. Ball-bearings, you know! :-)"
More of Jon's comments about motors: "You keep forgetting that the unit will NEVER be used at its full rating. The reason for using a big motor is mechanical ruggedness and fast momentary acceleration to recover from an applied load, and in faceting,
those loads are tiny. Also, a massive armature acts as an energy storage flywheel, and gives smooth operation without slowing down when the stone first contacts the lap. I mean, this is DELICATE GENTLE work we will be doing. You'll see! :-) If you
were hogging agate bookends, it would be an entirely different matter. The motor is used so far below its torque rating that heat will never be an issue. I have never seen one get above 40-50 C., and I have used big ones, small ones, Permanent magnet
one, shunt wound ones, and series wound ones, and have never seen one get hot. Really!"
As it was at this time too rainy to scavenge a dry DC motor in my local scrap yard, I found the following motor in the C&H Sales Co. catalog [(800) 325-9465]:
"900 RPM AMETEK #116536-00. Permanent magnet. 40 VDC. Ball bearing. Reversible. 1153 rpm no load speed. No load current 0.27 amp DC. 900 rpm@ 120 oz-in load @ 3 amps DC. For servo/generator applications the
specifications are as follows: KT is 44 oz-in per amp. KE is 32.5 V per 1000 rpm. Max current before demagnetization is 15 amps. Dimensions: 4" max. dia. X 5" long. Shaft 5/8" dia. X1-3/8" long. Tapped holes on front plate. Stock #DCM9005
$39.50."
Jon's response was: "120 Watts is about 1/5 HP. It will do just fine, and I like the 5/8" shaft." After I sent him the motor in order for him to machine the arbor to fit the shaft, he had the following comments about it: "Hey, that is one SERIOUS motor!
I just got home and connected it up to the speed control components I have saved for you. I think you will be VERY happy with it. With a brute force motor a fancy SCR speed control is not needed for this faceting application. The only caution is to be
careful with the splash guard, and make sure dust does not get in, as the bearing is not sealed that well. But bearings are cheap! So tomorrow I will remachine the face of the mounting flange so the dings and dents are smoothed out and it is perfectly
perpendicular to the shaft. Then I will machine the shaft with a 1/4-20 thread in the center and a hub/flange so it will take standard 1/2" faceting laps, and make a retainer. This way, you can use cabbing laps on it, too! To do this, I shall have to
disassemble the motor."
Arbor
Jon graciously did all the machine work for this project. You will need to find an equally gracious machinist, if you are not suitably equipped yourself. This is what Jon did to the motor and shaft in order to make it suitable for mounting the laps:
- Machined the shaft to 1/2"(0.4997")
- Turned the commutator so it's new bright copper
- Tapped the shaft 1/4-20
- Machined the mounting flange [face of the motor] dead flat to remove dings and burrs
- Machined a stainless steel flange [upon which the laps will rest]
- Assembled a retainer and spacer out of odds & ends
The spacer is a close fit to the shaft and holds the stainless lap flange at the right height above the motor's face. I planned to mount the motor beneath a 3/4" thick plexi sheet, so the flange had to support the laps at least that distance from the
motor's face. The laps will rest upon the flange, which is press-fit to the shaft on top of the spacer. There is another spacer to fit over the lap, and the retainer is a knurled upside-down-cup shaped screw, about 1" diameter, that screws into the
1/4-20 tapped end of the shaft, with the edges of the cup pressing down on the lap to hold it flush against the flange below.
Figure 1 is a diagram by Jon Rolfe which shows in cross section how the motor mounts beneath the plexi plate, and how the spacer, flange, lap, and retainer are arranged upon the shaft and
in relation to the plexi plate.
Here is what Jon has to say about the arrangement: " Your arbor is made for a 1/2" bore, but with a 1/4-20 thread in the center so you COULD use cabbing attachments, too. My arbors have a 1" step, then a 1/2" step, and a 1/4-20 thread in the motor shaft.
The motor shaft has been accurately turned to 1/2 inch so that standard laps can be used. The laps sits on the flange. Over the lap is placed a spacer ring, which is held in place with a 1/4-20 knurled screw. In this way a wide variety of thicknesses
can be accommodated. Most commercial laps are 1/4" thick. Also, a riser can be used for faceting the girdle facets and preforming the stone. The flange is machined to as close a fit as possible, so it is quite tight. This is the critical surface to
which the laps are referenced. Slop here can cause hop and runout. The spacer is not as critical for diameter, though both top and bottom must be parallel. Spin it all you want! When a lap wheel is clamped on, then the whole thing tightens."
Speed Controller/Power Supply
When Jon returned the motor to me with the arbor hardware installed, I was ready to build the power supply and speed controller circuitry. Figure 2 is a circuit diagram by Jon Rolfe that
illustrates the very simple but perfectly adequate circuit used in this machine. This circuit varies the speed of the motor by varying the DC voltage applied to it. Note that it is possible to build a (more complex) speed controller that will control
speed by varying the amperage applied to the motor instead of the DC voltage. That approach would have the advantage of maintaining constant torque. However, the motor that I purchased already has way more torque than I will ever need. As Jon put it,
"An active speed and torque controller is not essential. Honestly, constant torque is like mink mud flaps on a car...not really necessary, but nice to have."
Jon had some of the electronic components for this circuit lying around (the reversing switch, a variac, and a full-wave diode rectifier). The rest I purchased from Allied Electronics, reachable at 1-800-433-5700 and at http://www.allied.avnet.com. As Jon says, "Call and get their big fat catalogue! An electronics supplier I have used since there were TUBES!!"
As the circuit diagram shows, household AC voltage is supplied through an on/off switch ($5 for a nice toggle switch from the local hardware store) to a variable transformer (Variac). Instead of a Variac, a heavy duty lamp dimmer may be substituted.
The AC variable output of the Variac or lamp dimmer is then run to a step-down transformer, the output of which must not exceed the rated voltage of the motor. Figure the maximum AC voltage output of the Variac or dimmer (in my case, 120 VAC), and select
a transformer with the appropriate turns ratio. I purchased one (rated for 4 amps to give me plenty of margin) from Allied that would step 112 volts down to 36 volts, which means the transformer has a 3:1 turns ratio. Whatever AC voltage comes out of the
Variac will be reduced to one-third. The idea is that you must not exceed the maximum voltage rating of your motor, but a lower voltage is okay and will provide for slower speed. Jon had the following comments about use of a lamp dimmer in this
application: "Many dimmer switches do not like inductive loads... in theory. But I have used them for years, and if they burn out, so what? They are only $5.00 or so, and not $200 like a sophisticated speed controller. And they burn out, too!"
At one point I suggested using a resistor to drop the voltage, instead of a second transformer. Here was Jon's response: "I would not use a limiting resistor to get a voltage drop to 40 volts because that would limit the CURRENT. We want fast speed
recovery under load, and it is best to leave the currents and the RL Time Constant alone."
The voltage coming out of the step-down transformer is still AC, and now must be rectified to DC. Jon had a full-wave diode bridge rectifier lying around, but you can easily purchase them from Allied as well. Make sure that your rectifier can handle
the maximum voltage and current. Jon's comments on rectification: "After rectification, the voltage increases because AC is measured as RMS voltage, and the rectifier integrates the peak voltage. Some overdrive will not hurt the motor, as you will NEVER
run it flat out. I cannot imagine faceting anything that requires 2,000 RPM."
An AC current looks like a sine wave. The full-wave rectifier takes the bottom half of the sine wave and flips it up, so the current amplitude varies with time like a series of hills next to one another. The hilly profile is referred to as "ripple", and
over time it could slowly de-magnetize the magnets in a permanent-magnet type of motor. In order to smooth out the ripple, you must connect a capacitor in parallel across the output of the rectifier. Jon says, "you can use a capacitor of only a few
hundred MFD, BUT it MUST be at least 100 Working Volts rated( For your 40 Volt motor), and preferably 200. There are inductances present which in some conditions can cause spikes exceeding the rating of the capacitor if it's only 50VDC. Be careful about
connecting electrolytic capacitors. If they are connected in reverse, they will explode. Watch the + and - markings". I was worried about the danger of residual charge in the capacitor, and it is wise to treat all capacitors with respect. However, as
Jon pointed out, the motor is always connected to the capacitor through the reversing switch, and will discharge the capacitor within seconds when power is removed from the circuit. The capacitor I purchased from Allied was rated for 200 VDC and 850
microfarads. That was adequate to allow for the 1:1.414 voltage increase that occurs in full wave rectification.
After the rectifier and capacitor, we now have nice, clean DC voltage. I wired this DC voltage to the direction reversing switch (a spare supplied by Jon, but available at any hardware store for a few dollars) , the output of which connects to the leads
of the DC motor. My motor was reversible, but according to Jon, " One does not have to worry about whether the motor is reversible or not because it only takes a little wiring to make any DC motor reversible. Just a switch, really. The polarity of the
armature is reversed to provide either Clockwise or CCW rotation."
A word about grounding. I used a heavy duty 3-lead power cord to supply household power to this machine. The powered leads went through a toggle switch as described above. I ran the green ground wire to the metal faceplate of the variac, and also
soldered a connection from it to the metal case of the motor. The metal faceplate of the variac connected to the aluminum front plate to which I mounted all the switches (as explained in the next article), grounding them as well. This way, as Jon puts
it, "Ground is never switched off. If you use a single pole switch, switch the black lead (hot one). White is 'neutral'. Black kills."
I used 16 gauge wire in this project. As Jon said, "You will never operate continuously at even an amp. I had an ammeter on one of my machines because I used it as a load meter to avoid pressing too hard. It was unnecessary, but I tried it. Even at a
full start, it never hit 5 amps. That would be 10 amps at 40 volts."
In order to keep everything tidy, I followed another of Jon's suggestions and made all of my splices using heat shrink tubing. Then I protected my wire runs and kept them neat within the base by enclosing them in some black split conduit made just for
that purpose. The conduit is 3/8" in diameter, and mounts nicely in clips that I screwed to the wood base. More details will appear on that in the second article in this series.
My machine has controls for:
Motor Direction | Motor Speed (the Variac knob) | Main Power.
If you have visited Jon's web page, you'll see he has more switches on the front of his machine. From the left, Jon's switches are for:
Coolant valve | Lamp | Fwd/Reverse | Speed | Main Power
Jon's coolant valve controls the flow of water "from a 1/8 tubing connected to the Street! It goes through a 0.2 micron HEPA filter, is used once, then is drained away." Pretty nifty -- but for now I'll just be using a drip bottle filled with distilled
water.
My machine also does not currently have a lamp built into it, as I have a very nice halogen drafting lamp on my workbench. Here's how Jon wired a lamp in and attached it to his machine: "I used a McMaster-Carr $30 high intensity 75 watt lamp. Take the
power off the output of the main line switch, run the black lead through a switch to the lamp. The gooseneck screws into a 1/8 pipe threaded hole. Attach a ground wire to the gooseneck where it goes into the base." [CH: McMaster-Carr has no 800 number
that I can find. Their Chicago number is (708) 833-0300.]
Part I Conclusion
Next month Jon and I will tell you how to construct the faceting machine reference plane table and a wooden base to house and support the unit. At that point, you will be able to mount a purchased facet head, one of Wykoff's Calibrated Jamb Pegs, or any
facet head of your own design. Then, in the third installment of this series of articles, we will talk you through Jon's facet head design.
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Copyright, 1997 by Cate Harrison and Jon Rolfe
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Cate Harrison designs, fabricates and sells original jewelry designs using gemstones and beads. She teaches classes in jewelry and bead techniques at The Bead Lady in Champaign, Illinois. Her popular bead-netted vase kits are among the work that she sells
through her website. "My business is definitely moving in the direction of more lapidary work, as my experience grows. Lapidary has opened up a fascinating new world for me."
Cate Harrison's website is at http://s.psych.uiuc.edu/~charriso/willowdale.html and she welcomes email comments and questions at charriso@s.psych.uiuc.edu
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