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Dial indicators and dial test indicators with a mini-lathe

Dial indicators Introduction

NB Not to be confused with Thread dial indicators

Here is another classic example of a metalworking concept so simple and so obvious, it is a wonder that the whole world does not know it. Like all such things, when you start in the world of metal working, you certainly have NOT heard of these dial indicators. You do not know the wonderful solution dial indicators present to problems: If fact, you probably didn't even know the problems existed in the first place.

Let us take the most simple example. Put a piece of round stock into your 3 jaw chuck on your metalworking mini-lathe. Use the lathe to turn the stock into a nice cylinder with a nice faced end.

Then undo the chuck and remove the job. Then try and put it back in again. You will find that it is very easy to tighten the chuck with the axis of the cylinder un-square to the ways of the lathe. As the lathe turns, the cylinder will wobble - its sides are not concentric with the rotational axis of the lathe.

In fact, by eye, it is very difficult to position the job in the chuck, and tighten the chuck so that there is no wobble. Even if you get it close, when you bring a tool up to the side of the cylinder, it will show the eccentricity, and only contact with the job for part of a revolution - once you have taken off a mill of metal, the cylinder will once more be true to the rotational axis of the lathe... but not true to your faced end.

This is why it is very important to perform various jobs without removing the part from the lathe so that the surfaces that need to be concentric to each other do infact turn out this way.

Using Dial indicators

Enter the indicators. An indicator is a device which measures small deviations from the position of rest. There are two types. "Dial Indicators" are typically have a piston that moves up and down. As the piston or plunger moves, the needle on the dial indicates how much it has moved. Hence the name dial indicator. Clever, eh?

Dial indicators typically have a "throw" of about 1 inch of movement.

Another, more precise device is the Dial Test Indicator. This has a finger mechanism that swings back and forth like a very small pendulum. It will only move a small amount, usually only a few millimetres, but, that is all you need. This finger is much much more useful than the plunger type. It lets you "indicate" on small holes and at strange angles. Since getting a finger type dial test indicator, I have not used my plunger type dial indicator.

These devices a used by resting the moving part (finger or piston) against your part. As the part rotates in the lathe, the needles will swing back and forth as the cylinder wobbles in the lathe. To align the part properly in the lathe, turn the chuck by hand until the dial indicator gives the biggest reading. Then tap the part with a copper headed hammer (I use a scrap bit of copper) to remove some of the error. Rotate the part again. The swing of the dial indicator should be less. Keep repeating this until the error shown on the dial indicator is less than 0.001". At this point the part is as centered as it is going to be - more so than the accuracy of the rest of your lathe.

Dial Indicator

Dial Test Indicator

Usually you must get a mechanical arm with a magnetic base to hold the dial indicator firmly in place.

In the workshop there are countless uses for dial indicators. Tramming mills. Aligning work in the milling vice. Setting up work in a fixed steady or testing the runout of a lathe. The possibilities are endless.

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