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Precision Calipers

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One tool can take all the guesswork out of measuring: a caliper. It's the ultimate in accuracy—for a woodworker, anyway—and it's easy as pie to use. A caliper reads out one precise measurement, clear and simple. Most calipers read in thousandths of an inch. They're designed for machine shops, not wood shops. Who measures wood in thousandths? Musical instrument makers, maybe, but not the rest of us. Fractions are what we want. A new generation of $30 to $50 calipers designed for woodworkers replaces thousandths with sixty-fourths. With one of these gems, you can easily make a measurement to the nearest 1/64 in. And you can go smaller yet, by reading between the lines, to about the thickness of a heavy sheet of paper. That's plenty good enough for wood, which can expand or contract that much each day with a change in humidity. Let's take a look at all the jobs a caliper can do, no matter what kind of scale it has. Then we'll weigh the pros and cons of five kinds of calipers—basic slide, vernier, decimal dial, decimal digital and fractional dial—you'll find in woodworking catalogs. Finally, we'll tell you about the clear winner, a $30 beauty that will pay for itself the first time you avoid a costly mistake.



Width of a Dado
A caliper measures an inside dimension, too, with small “reverse” jaws that can span a gap as narrow as 1/16 in. A caliper is a perfect tool for setting up dado blades. Using the reverse jaws, measure the width of a dado you've made; then use the other jaws to measure the thickness of any shims you must add to or subtract from the dado blades.

 

 

  Thickness
The most common use for a caliper is to measure an outside dimension, such as the thickness of your stock as it comes out of the planer. A caliper is a lot easier to read than a ruler for this everyday job. You can also measure the precise width of a  board you've cut in order to adjust the rip-fence scale on your tablesaw.

 

 

Depth of a Hole
A small bar extends from the end of every caliper for measuring the depth of a hole. This bar may not fit in very small-diameter holes, however. The width of different calipers' bars varies from 1/8 in. to just over 1/4 in.

 

Depth of a Rabbet
Many folks are surprised to learn the back of a caliper can measure the width and depth of a rabbet or dado. In addition, whenever you use a zero-clearance throat plate on your tablesaw, you can measure the height of your blade or dado set with the back of the caliper. This is much easier than standing a ruler on end.

 

 

 

 

 

 

Read a vernier scale by using the zero to measure whole inches and sixteenths. Then find where two lines meet. This is the number of smaller fractional units to add, in this case, 1/128-in. units.

 

 

A vernier caliper is really clever. It has a sliding scale that replaces the dozens of tiny lines you would find on a finely divided ruler. The sliding scale has widely spaced markings that actually read “in between” the 1/16-in. lines. This makes small measurements much easier to read. Here are three simple steps to using a vernier scale. First, read the number to the left of the zero mark on the sliding scale,  including sixteenths (here, it's 1-in.). Next, find the line on the sliding scale that exactly lines up with any line on the fixed scale (here, it's the fourth line from zero). This indicates that the caliper reads 4/128 (or 1/32) past the 1-in. mark. If the fifth mark on the sliding scale lined up better with a different line on the fixed scale, the reading would be 5/128 past the 1 in. mark. The last step is to add the whole numbers, sixteenths and 1/128-in. measurements.

 

Turn the dial so it reads “zero” when measuring a mortise. All calipers can be zeroed out at any position.

 



Measure the tenon. The number on the dial is exactly how much bigger or smaller the tenon is than the mortise. 

A caliper can easily tell you the exact difference in thickness or width between two pieces of wood, without requiring that you add or subtract nasty fractions. This is a great way to fit a tenon into a mortise, a tongue into a groove or a plywood board into a dado.

 

 

Basic Slide 
• Reads in fractions
• Smallest division is 1/16 in.
• Costs $5 to $12
Pros
A slide caliper is inexpensive and small enough to easily slip into a pocket. It doesn't have any spiky points to catch on fabric. Most models read in both inches and milli-meters.
Cons
A slide caliper cannot read the depth of a rabbet or a hole. It cannot be zeroed out to make comparison measurements (see page 41). Its markings are often hard to read. Some models have dimension lines cut in brass or steel, rather than inked-in lines. 
Our Favorit Model
Hartville Tool
(800) 345-2396, www.hartvilletool.com
4-in. brass caliper, #85879, $8.

 

 

Decimal Dial
• Reads in thousandths
• Smallest division is 0.001 in.
• Costs $16 to $30
Pros
A decimal dial caliper is very precise (0.001 in. is one-third the thickness of a dollar bill). By comparison, a basic slide's smallest division of 1/64 in. is about 0.016 in., or 16 divisions on a decimal dial caliper. Roughly speaking, a decimal caliper is 16 times more accurate than a basic slide caliper.
Cons
Few woodworking situations call for this level of accuracy. When you're making two pieces that must fit together, such as a mortise and tenon, the feel of the fit is more reliable than measuring in thousandths.
Our Favorit Model
Woodcraft Supply
(800) 225-1153, www.woodcraft.com
6-in. dial caliper, #01R62, $30.

 

 Vernier
• Reads in fractions
• Smallest division is 1/128 in.
• Costs $13 to $20
Pros
With a vernier caliper, you get plenty of accuracy for the least amount of money. A vernier caliper is named after a two-part scale that reads both coarse and fine measurements (see “How To Read a Vernier,” page 40). A vernier caliper's simple slide mechanism is virtually indestructible, unlike a dial or digital device.
Cons
A vernier scale is not intuitive. You have to learn how to read it and remember how it works each time you use it. A vernier caliper cannot make comparison measurements. Most vernier calipers read in thousandths of an inch, not in fractions.
Our Favorit Model
Lee Valley
(800) 871-8158, www.leevalley.com
6-in. fractional vernier caliper, #88N70.01, $17.

  Electronic Digital
• Reads in thousandths
• Smallest division is 0.001 in.
• Costs $35 and up
Pros
A digital caliper is foolproof and very accurate. It displays one clear number. There's no need for counting lines or mentally adding numbers together, as with a decimal dial caliper. For comparison measurements, you simply push a button to zero the caliper. A short 4-in. model is amazingly compact and lightweight.
Cons
Nobody makes an electronic caliper that reads in fractions—yet. All of them read in thousandths, and going from decimals to fractions can really be awkward. You need a conversion chart or an engineer's head for remembering decimal equivalents. 
Our Favorit Model
Lee Valley
(800) 871-8158, www.leevalley.com
Electronic caliper: 6-in., #88N62.56, $35
4-in., #88N62.54, $33.

 

 Fractional Dial 
• Reads in sixteenths and hundredths
• Smallest divisions are 1/64 in. and 1/100 in.
• Costs $30 to $50
Pros
A fractional dial is clearly the best type of caliper for woodworking. Fractions are our language! This model is our hands-down favorite because it's so easy to read. We prefer the smaller 4-in. version shown here because it fits right into an apron pocket.
Cons
Of the many types of fractional dial faces, some are easier to read than others (see “Types of Fractional Dials,” at right).
Editor's Choice
Highland Hardware, (800) 241-6748, www.highlandhardware.com
4-in. caliper, #168077, $30
6-in., #065008, $37.

A lot of companies have jumped on the fractional caliper bandwagon, but not all their tools are easy to read. Our favorite type, the top dial, has every commonly used fraction up to an inch clearly marked on the outside of the dial. Each unmarked line is 1/64 in. The dial below our favorite isn't as user-friendly. You must count lines in between the major markings to find, for example, the 5/16-in. mark. The bottom dial is only marked in 1/64-in.  increments, and they're on the inner ring of the dial. Which one is 5/16 in. here? You can go cross-eyed, or make a major mistake, trying to figure out both these dials. The key to quickly reading any dial is to treat it like the face of a clock. One revolution is 1 in.; a quarter of the way is 1/4 in. Each major division is 1/16 in.

 

Best: 1/16-in. Fractions 

 

Less good:  1/8-in. Fractions 

 

Less good: 1/64-in. Fractions