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How to Use Adjustable Parallels
Guide · Updated October 28, 2026
Adjustable parallels are two wedge halves that slide against each other so the overall width changes while the faces stay parallel. They measure slots and grooves (set to fit, then mic) and serve as quick adjustable setup blocks.
Set width with blocks
Build the target size with gage blocks and compare. The free Gage Block Calculator builds the stack.
Step by step
- Choose a size: Pick the set that covers the slot or height.
- Loosen the screw: Loosen the locking screw so the halves slide.
- Expand to fit: In a slot, expand until both faces just touch with light drag.
- Lock: Tighten the screw without moving the halves.
- Remove and measure: Mic across the faces.
- Or preset: Set to a mic or gage block stack first when using as a setup block.
- Check parallel: Mic both ends; they should agree within a tenth or two.
Worked example
A slot nominal 0.625 +0.002/−0.000 in. Expand the parallel in the slot until it drags, lock, then mic: 0.6262 in at one end and 0.6261 in at the other. The slot is about 0.6262 in, within limits. To use it as a 0.8125 in setup block, build 0.8125 from gage blocks (0.1125 + 0.200 + 0.500), and set the parallel against the stack with an indicator until it reads the same.
Why the faces stay parallel
The two halves slide along a matching angled face, so moving them changes width without tilting the outer faces.
Common problems
A reading that changes when locking means the screw shifted the halves. Rocking means burrs. Slipping under load means the screw was not tight.
Tips
- Parallels: parallels and setup blocks.
- Grooves: measure a groove.
- Telescoping: telescoping gage.
Worked example: setting to 0.8125 in
Set the parallel by measuring with a micrometer and locking. For 0.8125 in, check against a 0.8125 in gage block stack built from an 81-piece set: 0.1005 + 0.112 + 0.600 = 0.8125 in. Lock and recheck; locking can change the setting by a few tenths.
Use adjustable parallels for height setting and slots, not as a heavy load support.
FAQ
What are adjustable parallels?
Two wedge halves whose combined width adjusts while staying parallel.
How do I measure a slot?
Expand to fit, lock, remove and mic.
Can I use them as setup blocks?
Yes, set to size first.
How accurate are they?
Within a few tenths with careful feel.
Why did my size change?
The halves moved while locking.
Related guides and calculators
- Parallels and Setup Blocks Basics
- How to Measure a Groove or Recess with Gage Blocks
- How to Use a Telescoping Gage
- How to Use Parallels in a Vise
- How to Build a Gage Block Stack
- How to Set a Bore Gage
- How to Check a Square with a Cylindrical Square
- All free machinist calculators
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