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INDEX
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Building Jon Rolfe's Home-brew Faceting Machine,
Part III: The Design of the Facet Head Assembly
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by Cate Harrison and Jon Rolfe
photos by Jon Rolfe
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This month we hope to establish a common terminology for the parts of the facet head assembly, and to describe the functional characteristics of this facet head design. Commercial faceting machines do not necessarily use identical construction
techniques, though there will certainly be some similarities, as the principles of faceting are the same no matter what machine is used. We hope that even those who do not plan to build their own facet head may gain some insight into the faceting process
by better understanding the equipment used. For those who do plan to construct a machine, next month's installment, the final one in this series, will provide you with machine drawings from which any qualified machinist may construct this
assembly.
This facet head design consists of three main sub-assemblies: the mast assembly, the mast follower body assembly, and the quill assembly. The operation is summed up in this way: The mast is supported vertically, perpendicular to the reference plane of
the reference table, and provides for positioning over a wide range of locations upon the reference table (i.e. closer to or further from the lap spindle); the mast follower body fits tightly around the mast and provides for gross and fine height
adjustment; the quill assembly provides for adjustment of elevation angle, as well as holding the dop and providing for the dop's index (rotation) angle about the quill's axis.
![[m2]](http://www.bovagems.com/eclectic/gfx/97_Sep/m2.jpg)
Figure 1 is a photo of one of Jon's completed machines. As you can see, the lamp is mounted at back left, the controls are mounted in a panel on the front, and the lap is on the left side of the reference table. The facet head assembly is at the right.
The mast follower and quill assemblies are rotated away from the lap in this picture, in order to protect it when laps are changed. For faceting, these assemblies would simply be rotated around the mast to a position over the lap with a dopped stone
positioned in the end of the quill. The stop angle for the quill would be adjusted, and then the mast follower body height would be adjusted to cut the facet to the desired depth at the stop angle. Any other facets in that line would be cut to the same
depth and angle with a simple index adjustment.
MORE ABOUT THE MAST SUB-ASSEMBLY
The mast proper is a 1" diameter stainless steel shaft, 9.5" long. It is fitted vertically (the accuracy of this verticality is critical) into a hole bored in one end of a 3/4" thick aluminum tooling plate, 4" x 9". A 5" long 3/8" wide slot is milled in
the other end of the plate. We refer to this plate as the "foot" of the mast assembly.
In a previous article in this series, we described how a hole drilled in the reference table of the facet machine would accommodate a 3/8" bolt. This bolt has a large flat washer to ensure that it tightens square to the table. The bolt passes up through
the reference table, through this slot in the mast foot, and is secured above with a large knurled nut over a flat washer. Because the tooling plate is dead flat, this arrangement permits the mast to be freely positioned upon the reference table, and
then secured in place with verticality assured by simply tightening the knurled nut down upon the mast foot.
MORE ABOUT THE MAST FOLLOWER BODY ASSEMBLY
The mast follower body assembly consist of the following components:
Coarse height stop platform
Mast follower housing
Thompson ball bushing
Micrometer spindle for fine height adjustment
Trunion bearings
The coarse height stop platform is a piece of aluminum plate cut to the "C" shape of a metal hose clamp. This piece of aluminum fits over the mast, and has a tightening screw across the opening of the C. By loosening this screw, we may slide the height
stop to any vertical position on the mast, and by tightening the screw we lock the coarse height stop in place.
The mast follower housing is a machined piece of aluminum. It is bored in the vertical direction to permit the insertion of a Thompson ball bushing, which is secured in place and adjusted for verticality with set screws. The ball bushing permits a tight
fit over the mast with no play, yet permits free motion of the mast follower in the vertical and rotational directions about the mast.
Another, smaller hole is drilled vertically through the mast follower housing, into which is fixed a micrometer spindle with a bearing soldered to the end. When the mast follower body is placed onto the mast, this bearing rests upon the coarse height
stop and supports the mast follower body. Fine height adjustment is obtained by extending or retracting the micrometer spindle. This arrangement is visible in Figure 1.
To visualize a trunion mounting, hold your elbows tight to your sides, and extend your hands straight out in front of you, thumbs up. Now imagine running a shaft from the center of one hand to the center of the other, with ball bearings in the center of
your palms to permit the shaft to turn. That is a trunion mounted shaft, and your forearms and hands are the trunions. The trunions that are machined in the mast follower housing are visible in Figure 1, from the right side, and in Figure 2, from the
front-left side.
![[m6]](http://www.bovagems.com/eclectic/gfx/97_Sep/m6.jpg)
The shaft that passes through the trunions and turns in the trunion bearings in the mast follower body also passes through the back end of the quill assembly, and is fixed to the quill assembly with set screws. This means that as the quill is raised or
lowered in elevation, the trunion shaft moves with it. This is important to note, because the elevation angle protractor is attached to the left side of the trunion shaft (toward the operator when faceting). An indicator on the mast follower body
permits the precise elevation angle to be read.
An angle stop is clamped on the opposite end of the trunion shaft (right side, away from the faceter when faceting). It can be loosened and adjusted to any rotational position about the trunion shaft, and when the quill is released the angle stop comes
to rest against a metal peg that is fixed in the right side of the mast follower body housing. The angle stop is easy to use by clamping it just loosely enough to permit some motion with pressure, and setting it to support the quill at an angle larger
than needed. Then the operator gently pushes the quill down toward the desired angle while watching the protractor angle. At the precise desired angle, stop pushing and tighten the angle stop clamp.
MORE ABOUT THE QUILL ASSEMBLY
The quill consists of the following components:
Quill housing
Quill shaft (not the trunion shaft)
Integral index gear and coupling assembly
Index stop ("pawl")
The quill housing is the part of the quill that is affixed to the trunion shaft with set screws. It is bored out to accommodate the quill shaft, which seats at the back against a ball bearing, and which is supported by a needle bearing at the back and by
a ball bearing at the front. These bearings permit the quill shaft to rotate freely and without play within the quill housing.
The integral gear and coupling assembly fits tightly over the end of the shaft and is secured by set screws. It consists, essentially, of a double ended female coupling to attach dops (male) to the shaft (male), with an index gear silver soldered
perfectly square to the outside middle of it. For each index gear used, a different gear and coupling assembly should be made. As the index gear is serving as a spray guard for the bearings in the quill, it is best to use stainless steel.
Specifications for the index gears and suppliers for them will be given next month with the machine drawings.
Jon uses 1/2" dops, which he machines himself, and constructs his coupling assembly accordingly. For Cate's machine Jon used a slightly different coupling design to accommodate off-the-shelf 1/4" dops. The principle is the same. Dops are secured into
the coupling with a set screw.
With the quill shaft in place and the index gear and coupling assembly screwed in place, the only remaining component is the index stop (also known as a "pawl"). The index stop is a lever, parallel to the length of the quill, which pivots about a support
that is soldered to the quill housing. The back end of the lever rests upon a stiff spring. The front end of the lever has a sharp steel tooth which fits tightly into an index setting of the index wheel. The spring holds the tooth firmly in place, and
yet the operator can release the index stop by compressing the spring to disengage the tooth and permit the index wheel to be spun to the desired setting.
A word about marking the index gear. Rather than marking the index gear with numbers as is done with most commercial index wheels, Jon marks the index settings with a code of dots that correspond to symmetry. For example, there is one main setting with
5 dots, and the one opposite has 4. The two settings at ninety degrees to that have three dots each. The four settings at the 45 degree angles to those four have two dots each. Single dots mark the centers of the eight segments left. This system is
actually much faster to use that counting numbers. To cut a sixteen symmetry pavilion, simply cut a facet at each dotted position. Cate has never cut a stone with any other system, and has found the dop markings to be an excellent aid to thinking about
the geometry and symmetry of a cut, rather than trying to follow cutting directions by rote.
CONCLUSION AND A LOOK AHEAD
As you can see, this design is mechanically very simple, and emphasizes accuracy and sturdiness rather than bells and whistles. Visit Jon's web site (http://www.argo.net/~jonr) to get a look at some of the fabulous
stones that he has cut with his home built machines.
We hope that this installment has given you a good understanding of how this facet head is put together and how it operates. Next month we will provide Jon's mechanical drawings, along with specifications and sources for some of the crucial components
(bearings, bushings, gears, etc.). Please note that in order to build the facet head you will need to know how to read mechanical drawings and how to perform machining operations. As an alternative, you may be able to locate a sympathetic and skilled
machinist to help you out (in exchange for gemstones, perhaps?).
Happy faceting!
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Copyright, 1997 by Cate Harrison and Jon Rolfe
photos by 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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