measuring tools: compasses
Post on 26-Jun-2015
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Measuring Tools: Compasses
I like to set my compass with a steel rule. I set the steel point into the etched graduation at the 1" mark and then adjust the drawing point to the desired radius (plus 1"). Although the steel point of a compass generally does an adequate job of holding its place, I've found that making a slight starter hole with an awl helps keep the point from wandering as the compass is rotated to mark the circle or arc.
Here's a quick way to convert a circle to a hexagon or lay out six equally spaced holes (such as for spindles or mounting holes). With the compass still set to the desired radius, place the steel point on the perimeter of the circle and rotate the compass until it scribes a mark across the perimeter. Then move the steel point to this location and scribe another mark. Continue like this all the way around the circle - you'll end up where you started.
DIVIDERS AND CALIPERS
A spring divider differs from a compass in that both legs hold metal points instead of a single point and a pencil or pencil lead. The legs of the dividers are opened or closed by adjusting a knurled nut on one of the legs. Although you can use dividers to scribe circles or arcs, they are more commonly used for layout to "divide" (hence the name) or step out equal distances, often referred to as stepping off a measurement.
DIAL AND SLIDE CALIPERS
Thought of mostly as machinists tools, dial and slide calipers can be quite useful in the woodworking shop. Although you rarely need to measure in thousandths of an inch, these precision tools are great for measuring small parts and checking the thickness of a work-piece. Slide calipers are not as easy to read as dial calipers. Whoever came up with the idea of adding the dial deserves a medal, in my opinion. The bodies of slide calipers are usually metal, whereas you can find dial calipers with either metal or plastic bodies. The latest in calipers takes advantage of digital technology and offers a numeric readout (though these can be quite pricey).
I've been using dial calipers for years to measure the thickness of stock. I use one so regularly for this that it resides in my planer stand. Once you get used to using one of these, you'll wonder how you ever lived without it. Look for one with a dial that has 1/64" measurements as well as thousandths.
Most dial and slide calipers can also be used as very accurate depth gauges. Just position the end of the caliper over the hole to be measured. Open the sliding jaw until the rod on the end of the caliper bottoms out in the hole; then read the dial.
Finding the center of a workpiece is a common layout task. It's so common that a number of tool manufacturers make plastic center finders, like the one shown in the top photo, specifically for this task. These simple tools have lips on two adjacent sides to quickly position the workpiece.
To use a center finder like the one shown here, press the edges of the workpiece up against the lips of the center finder. Then butt a pencil or marking knife up against the center cutout and draw along this to mark a line on the workpiece. Next, rotate the work-piece 90 degrees and make another mark. Where the lines intersect is dead center.
Trammel points (also called a beam compass) come in handy when you need to draw a large-diameter circle or arc. Trammel points are a set of steel points that can be mounted on virtually any length beam, as long as it's straight. The heads are held in place by tightening a knurled knob on top of each trammel point. Some versions include an accessory head that accepts a standard pencil, in case you'd prefer a pencil line to a scribed line. Bridge City Tool Works manufactures a gorgeous tool that features a beam with a built-in metal rule.
Just like the panel gauge, trammel points require two hands for use. One hand holds one trammel at the pivot point, while the other hand moves the opposite head in a graceful arc or circle to mark the work-piece. Keep the points razor-sharp and you'll need to apply only light pressure to mark your workpiece.
If you don't own a set of trammel points and need to draw a large-diameter arc or circle, try this crude but effective substitute. Simply drill a pair of small holes the desired distance apart in a thin stick. Use an awl or a brad to temporarily hold one end at the pivot point, and insert a pencil or awl in the other hole to scribe or mark the arc or circle. Naturally this isn't anywhere near as accurate as a good set of trammel points, but it'll do in a pinch.
DRAWING SMALL CURVES
If you're drawing-challenged, like me, you'll find a set of French curves a real boon when it comes time to lay out graceful curves for a project. French curves are available individually or in sets in a wide variety of shapes and sizes. Most are made of sturdy acrylic and may be clear or tinted. I prefer the clear curves, as this allows me to easily see the grain so that I can position the curves to maximize interesting grain patterns. French curves are available in most woodworking catalogs and at any art store.
I most often use French curves to create patterns or templates (such as the cabriole leg). Slide the curve up and down along the workpiece until the desired curve is found. Then trace around the curve with a pencil or marking knife (I generally use a pencil, since a marking knife can cut and nick the plastic). French curves can also be used to lay out ovals. Here again, position the curve until the desired shape is obtained, and trace around it. A pair of light pencil marks on the curve will help you align it for the remaining quadrants.