Indicator Gauges & Squareness Checks

An indicator does not tell you the size of anything. It tells you how much one thing differs from another: this side of a shaft from that side, this end of a vise jaw from the other end, this part from a master. That comparison is how you find setups that are out of line by less than the thickness of a hair.

What You Will Be Able to Do

• Pick between a dial indicator and a dial test indicator.
• Set up a magnetic base so the indicator does not move on its own.
• Avoid cosine error by keeping a test indicator's lever at the right angle.
• Check runout and report total indicator reading.
• Tram a vise, and check squareness and parallelism.

Two Tools That Are Not the Same

Read a dial the way you read a clock. The big needle shows small steps. The small revolution counter, if there is one, counts full turns. Most dials rotate so you can set zero wherever you start. A balanced dial runs both ways from zero, such as 0-50-0, so plus and minus readings are easy. A continuous dial counts up around the face.

Before each use, press the plunger or lever and let it return a few times. The needle should come back to the same spot. If it sticks or does not return, stop and have the indicator checked.

Mounting It Right

Most bad indicator readings come from a loose or springy setup.

1. Clean the surface under the base, then switch the magnet on. A magnet only holds on steel or iron. On a granite plate or aluminum fixture, use a heavy stand or a clamp.
2. Keep the arm short. Every extra inch adds spring.
3. Snug every joint on the arm.
4. Preload a plunger indicator, pressing it in about a quarter of its travel so it can read both ways.
5. Touch the base lightly and watch the needle. If tapping the bench moves it, the setup is too loose or the floor is shaking.

Keep the plunger in line. A dial indicator's plunger must point straight at the surface, square to it. If it is tilted, it reads wrong.

Cosine Error

A test indicator's tip swings in an arc. It reads correctly only when the lever is close to parallel with the surface, so the tip moves straight up and down against the work. If the lever is tilted, the dial shows more movement than really happened.

True movement = dial reading x cosine of the lever angle.

Rule of thumb: keep the lever within about 15 degrees of the surface. Swivel the indicator body, not the work. When you are simply trying to bring a setup to zero, a little cosine error matters less, because you are chasing no movement at all. When you report a number, it matters.

Checking Runout

Runout is how much a surface moves in and out as a part turns about its axis. You read it as total indicator reading (TIR), which is the highest reading minus the lowest in one full turn. Some prints call this full indicator movement (FIM).

1. Support the part on its axis: between centers, in V-blocks, or in the machine that spins it.
2. Bring the indicator square to the surface and preload it.
3. Turn the part one full turn by hand, slowly.
4. Note the high and low readings.

Worked example: the needle runs from -0.0007 to +0.0011 in one turn. TIR is 0.0011 - (-0.0007) = 0.0018 inch. If the shaft is simply offset from the axis, its center is off by half of that, about 0.0009 inch.

Tramming a Vise

Tramming means lining up a vise or fixture with a machine axis. If the fixed jaw is crooked, every part in the vise is cut crooked.

Warning: Do this with the spindle off. Keep clear of the table while it moves, and follow your shop's rule for lockout when you reach into the machine.

1. Clean the table and the bottom of the vise. Stone off any burrs.
2. Bolt the vise down snug on one end and just lightly on the other, so it can pivot.
3. Mount a DTI in the spindle and bring the tip against the fixed jaw near the tight end. Set zero.
4. Move the table along the X axis to the other end of the jaw, and read the dial.
5. Tap the loose end with a dead-blow hammer to take out about half the error.
6. Go back to the start and do it again, until the sweep is in tolerance.
7. Tighten both bolts a little at a time, then sweep again. Tightening often moves the vise.

Target: 0.001 inch over 6 inches of travel is a usual target for general work. For precise work, hold 0.0005 inch or better.

Checking Squareness

Squareness asks whether two surfaces meet at 90 degrees.

With a Machinist's Square

1. Clean and deburr the part and the square.
2. Set the square's base flat on the reference face and slide the blade gently up to the other face.
3. Hold it up to a light. Any gap shows as a line of light.
4. Slide feeler gauges into the gap to find the thickness that just fits.
5. Report the gap and the length, such as 0.002 inch over 6 inches.

The check is only as good as the square. It also can't tell you much about a surface that is bowed. Use the indicator method for anything tight.

With an Indicator on a Surface Plate

1. Stand the part on a clean surface plate, with the reference face down.
2. Set an indicator on a height stand so it touches the side face near the bottom. Set zero.
3. Bring a known square, such as a cylindrical square, up to the indicator first if you want a true reference.
4. Slide the stand, or run the indicator up the part on a height gauge, and read the change from bottom to top.
5. Report the change over the length you traveled.

The part must sit flat. A chip or a burr under it tips the whole part and shows up as error.

Checking Parallelism

Parallelism asks whether two surfaces stay the same distance apart.

1. Rest the part on a clean surface plate with one surface down.
2. Set an indicator on a stand, and zero it on the top face at one corner.
3. Slide the stand across the plate so the tip sweeps the top face.
4. Read the highest and lowest values over the whole face.

Read the pattern as well as the number:

Finding an Edge or a Center

Indicators also find a reference before cutting. To pick up a bore or a round boss on a mill, mount the indicator in the spindle and turn the spindle by hand around the feature. Move the table until the reading stays the same all the way round. The spindle is now on the feature's axis. Long machining runs warm a machine up, so pick up your references again after long runs if the job is tight.

Common Mistakes

Check Your Understanding

1. In one full turn of a shaft, the needle reads from -0.0003 to +0.0009. What is the TIR?

0.0012 inch.

2. You sweep a vise jaw and read 0.0012 over 6 inches. It is a general job. What do you do?

You are close but outside a usual 0.001 target. Tap the loose end to take out about half, sweep again, tighten, and sweep once more.

3. A DTI's lever sits at about 40 degrees to the surface, and it reads 0.004 inch of movement. Is the real movement more or less than 0.004?

Less. The dial overstates movement. Re-aim the lever close to parallel and measure again.

4. Parallelism across a plate won't repeat. The same pass gives a different answer each time. What do you check first?

Check for a loose joint on the stand, and for dirt under the part or the base.

Key Takeaways

• Indicators compare. They do not give a size by themselves.
• A short, rigid, clean setup comes first.
• Keep a DTI's lever within about 15 degrees of the surface. A steep lever reads high.
• Runout is reported as the highest reading minus the lowest in one turn.
• Always re-check a vise after you tighten it.
• A square shows a gap. An indicator shows how much, and where.