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How to Use Jack Screws to Support a Setup
Guide · Updated October 28, 2026
A machinist jack is a small screw-adjusted post. It supports an overhang or the heel of a clamp so the part does not bend, rock or chatter under the cutter. The rule is to support, not lift.
Check the height
Set fixed supports to height against gage blocks. The free Gage Block Calculator builds the stack.
Step by step
- Find the overhang: Look for areas that will be cut but are not on a support.
- Place the jack: Put the jack under the part close to the cut, on a clean table.
- Indicate the top: Put an indicator on top of the part over the jack.
- Run the jack up: Raise the jack until the indicator just starts to move, then stop; back off to zero if it moved.
- Lock it: Tighten the jack lock nut if fitted.
- Clamp near supports: Place clamps directly over supports or jacks.
- Recheck: Recheck the part with the indicator after clamping.
Worked example: how much does an overhang move?
A 0.500 in thick, 4.00 in wide steel plate hangs 3.00 in past the vise jaw. A 50 lb cutting load at the end bends it as a cantilever: δ = F × L³ ÷ (3 × E × I). I = b × h³ ÷ 12 = 4 × 0.125 ÷ 12 = 0.0417 in⁴. δ = 50 × 27 ÷ (3 × 30,000,000 × 0.0417) = 1350 ÷ 3,750,000 = 0.00036 in. Small, but it springs back and forth with every tooth, which is chatter. At 6 in overhang, L³ is 8× larger: about 0.0029 in. A jack under the end turns the cantilever into a supported beam.
Support, do not lift
If you push the jack up hard, you bend the part. After it is unclamped it springs back and the machined face is no longer flat. Use an indicator so the jack only touches.
Under clamps
Put a jack or step block under the heel of a strap clamp, and the clamp should be level or the heel slightly higher. The clamp bolt should be closer to the part than to the heel so most of the force goes into the part.
Jacks vs fixed blocks
Parallels, 1-2-3 blocks or gage blocks are fixed heights and cannot over-push. Use jacks where the height is odd or the surface is rough.
Common problems
Machined face not flat after unclamping: jack pushed too hard. Chatter at the edge: no support near the cut. Jack loosens: no lock nut; tighten it or use a fixed block.
Tips
- Clamps: toe clamps.
- Blocks: 1-2-3 blocks.
- Indicate: indicating a fixture.
FAQ
What is a machinist jack?
A screw-adjusted support post.
How tight should a jack be?
Just touching; check with an indicator.
Where do I place it?
Under the overhang near the cut and under clamps.
Why does my face come out not flat?
The jack lifted the part.
Jacks or parallels?
Fixed blocks when the height fits; jacks for odd heights.
Related guides and calculators
- How to Use Toe Clamps and Strap Clamps
- How to Use 1-2-3 Blocks for Setups
- How to Indicate a Fixture
- Parallels and Setup Blocks Basics
- How to Set Up a Milling Vise
- How to Use Transfer Screws to Locate Holes
- How to Use Toggle Clamps on Fixtures
- All free machinist calculators
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General shop information only — follow your machine and tooling manufacturer's instructions and your shop's safety rules.