CNC Work Holding & Fixturing 3 — Questions and Answers
Question 1: What is the function of a floating jaw (compensating jaw) in a CNC vise?
- To apply extra clamping force to irregular surfaces
- To self-adjust and distribute clamping force evenly on non-parallel workpiece faces (Correct answer)
- To allow the vise to move laterally on the table
- To lock the moveable jaw in position during tool changes
Correct answer: To self-adjust and distribute clamping force evenly on non-parallel workpiece faces
A floating or compensating jaw pivots slightly to make full contact with both sides of a non-parallel or irregular workpiece, preventing rocking.
Question 2: A cam-lock fixture is characterized by its ability to:
- Apply torque-controlled clamping through a threaded spindle
- Quickly clamp and release a workpiece with a 90-degree handle rotation (Correct answer)
- Clamp from below the workpiece surface
- Automatically re-clamp if vibration loosens the part
Correct answer: Quickly clamp and release a workpiece with a 90-degree handle rotation
Cam-lock clamps convert a partial rotation of a cam into a secure clamping action, enabling fast loading and unloading of workpieces.
Question 3: When a workpiece is clamped in a four-jaw independent lathe chuck on a CNC turning center, the primary advantage over a three-jaw chuck is:
- Faster part loading time
- The ability to indicate and center non-round or eccentric workpieces (Correct answer)
- Greater maximum RPM capability
- Self-centering action for round bar stock
Correct answer: The ability to indicate and center non-round or eccentric workpieces
Each jaw of a four-jaw chuck adjusts independently, allowing precise centering of irregular parts or intentional eccentricity.
Question 4: What is the purpose of a fixture plate (also called a sub-plate or tooling plate)?
- To insulate the workpiece from machine vibration
- To provide a repeatable, pre-drilled mounting surface for quick fixture changes (Correct answer)
- To measure flatness of the machine table
- To hold multiple tools near the workpiece for faster changes
Correct answer: To provide a repeatable, pre-drilled mounting surface for quick fixture changes
A tooling plate with precision-bored holes and threaded inserts allows fixtures and clamps to be mounted quickly and repositioned accurately.
Question 5: In CNC milling, workpiece 'lift-off' during a climb milling pass is most commonly prevented by:
- Using conventional milling instead
- Ensuring clamps are positioned over the supported areas of the workpiece (Correct answer)
- Increasing spindle RPM to reduce cutting time
- Using a lower table feed rate only
Correct answer: Ensuring clamps are positioned over the supported areas of the workpiece
Positioning clamps directly over the support points (parallels, stop pins) counters the upward cutting force and prevents part lifting.
Question 6: A 'riser block' is used in CNC fixturing to:
- Increase vise jaw height to accommodate taller workpieces (Correct answer)
- Set the cutting tool height offset
- Compensate for machine table levelness errors
- Convert metric to inch vise mounting
Correct answer: Increase vise jaw height to accommodate taller workpieces
Riser blocks bolt between the vise base and the machine table, raising the vise so that taller workpieces can be gripped properly.
Question 7: Which best explains why soft jaws are used in CNC production machining?
- They are cheaper to replace than hardened steel jaws
- They can be machined to match the exact contour of a finished or delicate workpiece (Correct answer)
- They grip harder materials more securely than hardened jaws
- They are required for use with hydraulic vises
Correct answer: They can be machined to match the exact contour of a finished or delicate workpiece
Soft jaws are made from aluminum or mild steel and can be bored or milled to conform precisely to the workpiece shape, distributing clamping load and preventing marring.
CNC Work Holding & Fixturing 3
What is the function of a floating jaw (compensating jaw) in a CNC vise?