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How to Indicate a Fixture
Guide · Updated October 28, 2026
A fixture is only as good as how well it is squared and located on the machine. Indicate the datum features the part locates on, not just the fixture body, so the parts come out where the drawing says.
Check the indicator
A test indicator with a known error gives false sweeps; check it against gage blocks first. The free Gage Block Calculator builds the stack.
Step by step
- Clean everything: Stone the table and fixture base and blow out the T-slots.
- Snug it down: Set the fixture on the table with clamps just snug.
- Sweep the locating edge: Run a test indicator along the locating face or across two dowel pins with the X axis.
- Tap it square: Tap the fixture until the reading is within about 0.0005 in end to end, then tighten clamps evenly.
- Recheck: Sweep again after tightening; clamping often moves it.
- Locate the datum: Sweep a datum bore or pin, or pick up edges with an edge finder.
- Set the offset: Set the work offset (G54 or DRO zero) on that datum and record it on the setup sheet.
Worked example
A fixture plate locates parts on two 0.375 in dowel pins 6.000 in apart along X. Sweeping the pin sides, the indicator reads 0 on the left pin and +0.0012 in on the right. Tap the right end back until both read within 0.0002 in, tighten, and recheck: +0.0001 in. Then sweep the left pin around with a test indicator in the spindle until the reading is even all around, and set X0 Y0 there.
Locating on pins vs edges
A pin or bore can be swept with the spindle for the best location. An edge finder is quick but only good to about 0.0002–0.0005 in. For repeat jobs, add a locating key or bushing so the fixture drops in close every time.
Common mistakes
Indicating the fixture body instead of the locating features, not rechecking after clamping, and cosine error from a test indicator at a steep angle all cause location errors.
Tips
- Vises: indicating a vise true.
- Bores: indicating a bore true.
- Angle error: cosine error with indicators.
FAQ
How square should a fixture be?
Usually within 0.0005 in over its length, tighter for precision work.
Should I indicate the fixture body or the pins?
The locating features the part sits on.
Why did my fixture move after clamping?
Uneven clamping or dirt under the base; tighten evenly and recheck.
Can I use an edge finder?
Yes for moderate accuracy; sweep a pin or bore for better.
How do I make fixture setups repeatable?
Use keys in the T-slots or locating bushings and record offsets.
Related guides and calculators
- How to Indicate a Vise True
- How to Indicate a Bore True
- How to Use a Dial Test Indicator
- Edge Finder Basics
- Cosine Error with Indicators
- Tool Length Offset Basics
- Leadscrew Backlash Basics
- How to Use a Fixture Plate
- How to Use Toe Clamps and Strap Clamps
- How to Use Jack Screws to Support a Setup
- How to Use Toggle Clamps on Fixtures
- All free machinist calculators
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