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Magnetic Base Indicator Setup
Guide · Updated October 27, 2026
Most indicator readings that drift come from the setup, not the indicator. A mag base holds an indicator in place, and these habits keep the reading honest.
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Step by step
- Place the base: Put it on a clean, flat steel surface and switch the magnet on.
- Keep it short: Keep the arm as short as possible with the joints close to the base, then lock every joint.
- Set the preload: Load a dial indicator about a quarter turn, or a test indicator a few divisions.
- Align it: Put the plunger perpendicular to the surface, or keep a test indicator stylus near parallel to it, to avoid cosine error.
- Check repeatability: Lift and reset onto the same point several times. The reading should repeat.
Tips
- Tap the base lightly. If the reading moves, the setup isn't rigid.
- Fine-adjust with the base's fine-adjust knob, not by bending the arm.
- Verify the indicator itself: dial indicator and test indicator checks.
Related: comparator stand setup, edge finder basics.
Quick shop check
Keep the indicator arm short: each joint and extension adds spring. A setup that reads 0.0003 in when you push lightly on the base is too flexible for tenths work. Set preload about 1/4 to 1/2 turn of the dial. For test indicators, keep the stylus nearly parallel to the surface to limit cosine error.
Worked example: a reading that drifts
A dial test indicator on a magnetic base creeps about 0.0005 in while sweeping a vise jaw. Wipe the base and the mounting surface so no chips sit under the magnet, mount it on a flat, solid surface, and shorten the arm so the indicator sits close to the post. Tighten the central lock, keep the stylus nearly parallel to the surface, and re-zero. Sweep again in both directions; the reading should return to zero at the start point, which shows the setup is rigid.
FAQ
Why does my indicator reading drift?
Long arms, loose joints or a weak magnet seat. Shorten the arm and lock every joint.
How much preload should I use?
About a quarter turn for dial indicators, enough that the reading stays on the dial both ways.
What is cosine error?
Reading at an angle under-reads the real movement. Keep the plunger in line with the movement.
How do I check an indicator?
Against gage block stacks. Build them free at gageblockcalc.com.
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General shop information only — follow your gage manufacturer's instructions and your quality system.