Although channel setting is one of the more common styles of setting, many jewelers experience difficulty with the process. There are a number of techniques for channel setting. In this article, we will focus on the double undercut. This technique involves undercutting a seat in both walls of the channel.

One of the more critical ingredients to good channel setting is having matched stones. You should match color clarity, size and proportion. The two critical features are the spread (diameter of the stone) and crown height. If the stones are matched in these two dimensions, channel setting becomes much easier. Having a uniform crown height makes it much easier to set the table facets on the same plane. If the diameters are the same, it minimizes the number of burs needed for cutting the seats.

Getting Started

As with any other style of stone setting, the process begins by preparing the mounting to accept the stones. Start by measuring the width and length of the channel in relation to the stones. If the channel is flat, lay the stones in a straight line with a 0.1mm gap between them, and measure the length. This is the minimum length for the channel. If the channel is curved (domed), allow 0.2mm to 0.4mm between the stones, depending on the size of the stones and the radius of the curve. Larger stones and tighter radii require more space between the stones during the layout.

After the length of the channel is set, check the width. The width of the channel is generally between 0.25mm to 0.5mm smaller than the stones, again depending on the stone size. If the channel is wider than necessary, close the width before cutting the seats. Most channels can be closed with parallel jaw pliers. Some mountings will have partitions between the stones that support the channel. It may be necessary to saw through them to allow the channel to close to the necessary width. Solder the partitions back together and polish them out.

If the channel is too narrow, there are a couple of ways to adjust it. First, look at the width of the channel railing. This is the metal on each side of the channel. If there is sufficient metal to allow you to remove some from the inside and still secure the stone, a graver or bur can be used to widen the channel. You can use an inverted cone bur held sideways for this operation. Allow the shank of the bur to glide over the opposite rail and the flat bottom of the bur runs along the line to be removed. If you choose to use a graver, use a knife-edge graver and cut the ends of the channel first.

If the amount of metal that must be removed is small, a flat needle file is a good choice. On the other hand, If you need to remove more than 0.25mm from each side, a flat graver can be used. The purpose for cutting the ends of the channel with a knife-edge graver is to keep the metal from piling in the corners. This way when the graver gets to the end of the channel, the bur of metal flakes off.

If the rail is too thin and you are not able to remove any metal, then the channel will need to be spread. One way of spreading the channel is with a pair of flat nose pliers. The rail can be grasped and pulled outward, or both jaws of the pliers can be put into the channel and then opened to spread the channel apart. A cold chisel can be driven into the channel to force it open, as well.

Before the seats are cut, be sure to remove any tool marks on the inside walls of the channel, and pre-polish it. It is not necessary to polish the top of the rails since these will be hammered down over the stones.

Cutting The Seats

Next begin laying out the stone placement. Take some bee’s wax, and space them evenly along the channel. Use a scribe to mark the location of each stone on the rails. I prefer marking the location of the space between the stones rather than the center point. After marking the location, remove the stones and place them in your stone tray in the same order they were arranged on the channel. If the stones are not well matched in size, it will be easier to set them if you position the larger stones in the middle and the smaller stones at the ends.

If the channel has individual bearings below each stone, they will need to be precut to provide clearance for the pavilion. For this operation choose a 70 degree bearing bur the same width of the channel. This will completely clear the pavilion.

Use a 70 degree bearing bur to cut the seats for the stones. The bur should be about 0.2mm smaller than the stone. Mark the location for the girdle of the stones along the inner walls of the channel (for smaller stones this is about crown height). This can be dove by using dividers to scribe a line the full length of both walls of the channel. This next step is optional, but it is a great aid if you haven’t done much channel setting. Use a #1 onglette graver to cut a groove in the channel walls along the scribed lines to serve as an index for aligning the 70 degree bearing bur.

Tip the bur so one side of it fits into the groove, and the other side is above the opposite rail. Begin cutting at a low speed, taking care not to let the bur drop below the girdle line. As the bur cuts into the rail, stand the bur upright as it clears the opposite side. With the bur standing straight up, make a shallow undercut (about 0.25mm deep) in the groove in the opposite rail. Now check the fit. You will need to tip the stone in the same manner that you cut the seat to fit it into the channel.

If the stone does not slip into the channel, undercut the first side a little deeper. Do not force the stone into the seat. The undercut is at the correct depth when the girdle of the stone lightly binds against one side of the channel and snaps into the seat (for diamonds). For more fragile stones, the stone should barely brush against the opposite wall before dropping into the seat. After checking the fit of the stone, remove it and continue cutting the seats for the remaining stones, stopping to check the fit of each stone as you go. Cut all of the seats from the stones on the same plane.

Setting The Stones

When all of the seats have been cut, file the top rails so they are beveled from the inside downward about 15 degrees to the outside. Put all of the stones in their seats and check the alignment. If a stone is higher than the rest, lower the seat. Of one of the stones is lower, either get a stone that is slightly larger, or cut all of the other seats a little lower.

Once the stones are in proper alignment, begin tightening them. Place a line of bee’s wax over the tops of the stones to keep them from jumping out of their seats when you hammer the rail down. Use a hammer and punch, or a hammer hand piece, and begin to tighten the side with the deeper cut first. This will shift the stones towards the opposite rail and center them in the channel. Don’t completely tighten one side before beginning to tighten the opposite side. When the stones are centered in the channel, alternate sides as you tighten them.

When hammering on the railing, be sure to have the mounting solidly supported from underneath. A steel ring mandrel provides good support for a ring, and a steel bench block is good for a flat pendant. If you are using a Benchmate™, use the steel support bridge that comes with it. Scrape the bee’s wax off of the stones and check them to be sure they are tight.

When the stones are tight, begin shaping the rails. Use either a flat file or sanding disc to square the top and outer sides of the rails. Use a graver (a flat or onglette will work well) to straighten the inner edge of the rails that run over the stones. Use a knife-edge silicon wheel to remove any remaining tool marks along the inner edge of the railing, and a small hard felt lap on a flex shaft with polishing compound to pre-polish the top and sides of the rails. Final buff and clean the piece. Remember to check the stones for tightness after cleaning.