A dial indicator is not a tool one would typically associate with woodworking, but if you own woodworking machinery, it can provide a high degree of accuracy when setting up and maintaining your machines.
With the use of special holders, dial gauges can be used to set planer and jointer knives accurately after they've been replaced or resharpened. Aligning your table saw blade to the miter slot with anything other than a dial gauge is likely to yield inferior results.
There are two types of indicator gauges suitable for use in the home workshop, the dial test indicator and dial indicator. They both measure distance or movement (deflection), but in slightly different ways.
Dial test indicator and dial Indicator.
Dial test indicator
A dial test indicator uses a finger (lever) with a small ball on the end to measure tiny distances, usually in .0005" increments. On most test indicators, the finger rests at a central point from which it can be moved either forward or backwards a small amount.
The test indicator in the photo on the right will measure in either direction .015" for a total pointer movement of .030". Because the finger has such limited travel, the set-up you're trying to measure must be fairly close (within the total travel of the finger) to start with.
Setting a stationary vice jaw parallel to an axis on a milling machine and centering holes with a rotary table are common uses for the dial test indicator. Since woodworking machines seldom need the accuracy these gauges are capable of, a much better choice for your home shop is the dial indicator or dial gauge.
Instead of a finger, dial indicators use a shaft or plunger to measure deflection. The plunger extends through the body of the indicator, moving the pointer through a series of small gears. The end of the plunger has a removable tip which can be replaced with specialized tips to perform various jobs.
A dial indicator with magnetic stand being used to check for arbor washer runout on my contractor table saw.
The pointer can be "zeroed out" with the graduated scale anywhere along the plunger's travel by turning the outer bezel.
Dial indicators typically measure in increments of .001" with 1 or 2 inches of plunger travel. Each revolution of the pointer marks .100", so a separate smaller pointer is usually set into the dial's face to count the revolutions.
Measurements of less than .001" can be estimated by observing where the pointer lies in relation to the two marks on either side of it. For instance, if the pointer was about halfway between these marks, you could safely guess it was indicating .0005".
In a typical "runout" measurement, as in the above photo, the dial gauge is held securely in a holder of some type (in this case a magnetic stand) and lined up at 90 degrees to the shaft being checked. Any angle other than 90 degrees will yield inaccurate data, so I set the arbor at an angle to allow me to adjust the gauge easily.
You'll notice in this set-up that the plunger is lifted off it's resting point at the top of the gauge. This preloading of the plunger assures it has full contact with the arbor washer at all times. One or two revolutions of the dial pointer is sufficient.
Once set up, the pointer is zeroed to the numeric scale and the arbor turned by hand to observe the deflection. Any movement of the pointer indicates runout. On this particular set-up, the pointer remained at zero, meaning zero runout.
Both types of indicators are available in electronic digital versions.
The back cover of the dial gauge has a lug on the back to allow the gauge to be used with various holders. The lug can be oriented in two different directions by removing the four cover screws and repositioning the cover.
A look inside the dial indicator's back cover reveals the gears and springs.
With the cover off though, we can see the inner workings which allow the gauge to operate as it does.
A precisely ground plunger shaft is supported by bushings in the top and bottom of the gauge body to eliminate any side play through the extent of it's travel.
The linear gear teeth (rack) cut into the shaft turn a small pinion gear, which in turn rotates other gears that operate the main pointer and the revolution pointer. More expensive models use jeweled bearings for the gear shafts.
A return spring attached from the gauge body to a pin on the shaft keeps constant downward pressure on the shaft. To keep the shaft from rotating in its bushings, the end of the pin fits inside a slot molded into the plastic piece next to it. The whole thing is quite simple for something that can measure in the thousandths of an inch.
A dial gauge has a pretty easy life in a woodworking shop, it's not every day you're setting jointer knives or aligning your table saw. Even so, proper care of these precision instuments will help insure they stay accurate and usable.
When not in use, they should be stored away in a safe place, preferably in the case they came in. I remove the back and lubricate the moving parts with light machine oil (3 in 1 oil works fine) periodically as a regular maintenance chore to keep them working smoothly. This is especially important where there is high humidity.
For the average woodworker, the purchase of a dial gauge is probably a once in a lifetime event. A quality gauge will remain accurate and last a long time, and in my honest opinion, one should buy the best they can afford.
Does this mean you shouldn't buy one of the cheaper models?
With today's manufacturing processes, even the less expensive dial indicators are quite accurate. However, given the inferior materials used in their construction, the longevity of their accuracy is a bit of a crap shoot.
Prices vary with quality and manufacturer, but you can usually find a half decent dial gauge for well under a hundred bucks. Here's a link to a page on amazon.com with a large selection of dial indicators, magnetic stands and accessories.
A cheaper import model is still better than not having one at all though, and the time and headaches saved from having properly set up machinery will make your time in the workshop more enjoyable and productive.