Tip #53
Shopsmith Ovararm Pin Router
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Duplicating

Of all the unique capabilities provided by the overarm mode of the routing system, high-speed duplication of complete projects or project components is the most interesting and challenging.

Through the use of shop-made guiding fixtures, you will be able to make an unlimited number of identical pieces, quickly and accurately.

As we explained briefly in the beginning of this chapter, the process works by guiding a pre-cut fixture over a pin which protrudes up from the routing system table surface. When a bit is installed directly above the pin (and in perfect alignment with it), a matching pattern is cut into a workpiece attached to the opposite side of the fixture (Figure 22-28).

Types and Styles of Fixtures
There are two types of fixtures that can be used with the routing system: permanent and temporary.

Permanent Fixtures are more complicated in their design and allow for rapid attachment and removal of workpieces in a repetitive fashion. They are generally used when making five or more of the same project or component.

Temporary Fixtures are usually nothing more than a wooden template of a simple design that is merely screwed to your workpiece. Temporary fixtures often require more time to attach and remove the workpieces than permanent fixtures. This is perfectly acceptable since it makes little sense to spend a lot of time building a complicated fixture that will be used to produce less than five identical projects or components.

When making fixtures, it's important to think about how many times they will be used before deciding how the workpiece will be held in position. If you're planning to make a large number of the same piece, you will want a fixture that allows the rapid attachment and removal of stock. If you're only making one or a few of the same piece, this is less important.

There are a number of different styles of fixtures, determined by the way the stock is held in position and whether you are cutting on the outside, inside or both edges of your workpiece:

Screw-on Fixtures (Figure 22-29) are among the simplest in design. With this style, screws are driven up from the underside of the fixture and into the workpiece to hold it firmly in position during operations. If you have a power screwdriver or variable-speed reversible drill with screwdriver bits, this attachment style works equally well for both temporary and permanent fixtures and is a "must" if you are removing the outside edge of your workpiece.

Drive-on Fixtures (Figure 22-30) feature screws driven up through the bottom of the fixture so they protrude 1/8" to 1/4" above the top surface. The work-piece is positioned on top of the fixture and struck with a hammer or your hand to temporarily "impale" it on the protruding screw points. This attachment style is also ideal for either permanent or temporary fixtures and is a good option when removing the outside edge of your workpiece. Always use sheet metal screws for these fixtures, since they have threads all the way down the shank. Once you've driven the screws through the fixture, sharpen the protruding points with a file.

Clamp-in Fixtures (Figure 22-31) are the most complicated style, yet offer the distinct advantage of quick workpiece attachment and removal. Since they require more time to build than any of the other types, they are usually reserved for situations where you will be cutting-out large numbers of the same item. With these fixtures, the stock is clamped firmly in position during operations by a "floating" bar or cam clamp. Since the workpiece is gripped by the edge, this style of fixture will not allow full-depth cuts around the outside perimeter of the stock.

Drop-in Fixtures (Figure 22-32) are made the same way as clamp-in fixtures, but have no bar or clamps to hold the workpiece in position. Instead, the stock is merely dropped into the frame and cut as it would be

with a clamp-in fixture. Warning: A tight fit of the workpiece is critical to keep it from moving during operations. Again, these fixtures are recommended for high-volume situations and not for projects where full-depth cuts around the perimeter of the stock are required.

Profile Fixtures are usually a combination of drive-on and drop-in styles (Figure 22-33). The most common application for these is the making of fence post tops and similar projects. They usually contain sides to help position the workpiece and protruding screw points to keep it from moving during operations. However, they can also be made with floating clamp bars or eccentric clamps, if you like. If the profile is identical on both sides of the workpiece, the fixture can be profiled on one side only and the stock flipped over to cut the second side.

Double-Stick Tape Fixtures (Figure 22-34) are very simple to make and work very well when the workpiece must be cut around the perimeter and is too thin to grip from below with screws. They can be made with readily available double-stick tape and should only be used when the stock is large enough to hold firmly in position with your hands a safe distance from the rotating bit. They are intended primarily for low-volume production jobs. Warning: Change tape frequently, since wood dust and repeated use will cause its adhesive qualities to fail after a few uses.

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