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Parallels and Setup Blocks Basics
Guide · Updated October 27, 2026
Parallels lift a part in the vise so it sits above the jaws, flat and at a known height. A part that isn't seated is the most common cause of tapered and out-of-parallel parts.
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Step by step
- Pick the height: Choose the parallel height so the cut is clear of the jaws, with enough grip left.
- Clean everything: Clean the vise floor, the parallels and the part.
- Snug the vise: Close the vise gently.
- Tap the part down: Tap it with a dead-blow hammer while tightening.
- Check: The parallels should be clamped tight. Paper strips under each corner should not pull out.
Care
- Store them in pairs and in their case.
- Stone off burrs.
- Check a matched pair against each other with an indicator.
Related: checking a 1-2-3 block, indicating a vise.
Quick shop check
Common precision parallels are matched in pairs to about ±0.0002 in in height and parallel within about 0.0002 in over length. To check a pair, wring them on a surface plate and sweep both with a test indicator; a difference over 0.0002 in means they should not be used as a pair for precision work. Tap the part down with a dead-blow until both parallels are pinned.
Worked example: a part that rocks on parallels
A 0.75 in thick plate is set on a matched pair of 1 in parallels in a vise. After snugging the jaws, one parallel still slides when tapped. Lightly tap the part down with a soft-faced mallet near that parallel until it no longer moves, then try both parallels with your fingers; both should be pinned. If one stays loose, the part is likely not flat or a burr or chip is under it: open the vise, wipe the jaws, parallels and part, deburr the bottom edges, and reclamp before machining.
FAQ
Why does my part rock on parallels?
Burrs, chips or a jaw lifting the part. Tap it down and clean everything.
How do I check parallels?
On a surface plate, with an indicator over the length, comparing them to each other and to gage blocks. Build any reference size free with the Gage Block Calculator at gageblockcalc.com.
Thin parallels or thick?
Use the lowest height that clears the cut. The part grips better.
Can I use 1-2-3 blocks as parallels?
Yes, for taller setups.
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General shop information only — follow your gage manufacturer's instructions and your quality system.