CNC Workholding and Fixturing 1 — Questions and Answers
Question 1: What is the primary purpose of a CNC vise stop (work stop)?
- To prevent the vise from moving during cutting
- To provide a repeatable datum for part positioning (Correct answer)
- To increase clamping force on the workpiece
- To align the vise parallel to the X-axis
Correct answer: To provide a repeatable datum for part positioning
A work stop provides a consistent reference surface so that each part loaded into the vise starts from the exact same position, ensuring repeatability.
Question 2: When using a machine vise, why should the workpiece be tapped down with a mallet after initial tightening?
- To work-harden the contact surfaces
- To seat the part firmly against the parallels and prevent lifting (Correct answer)
- To test the vise jaw clamping force
- To check for proper coolant flow
Correct answer: To seat the part firmly against the parallels and prevent lifting
Tapping the part down ensures it is fully seated against the parallels, preventing it from lifting due to upward clamping forces during machining.
Question 3: What type of workholding device uses a magnetic field to hold ferrous workpieces without mechanical clamping?
- Vacuum chuck
- Magnetic chuck (Correct answer)
- Collet fixture
- Step clamp set
Correct answer: Magnetic chuck
A magnetic chuck uses electromagnetic or permanent magnetic force to hold ferrous (iron-containing) workpieces flat against its surface without obstructing the top.
Question 4: In modular fixturing, what is the primary benefit compared to dedicated fixtures?
- Lower upfront material cost per fixture
- Reconfigurability for multiple part families without building new fixtures (Correct answer)
- Faster machining cycle times
- Better accuracy than welded fixtures
Correct answer: Reconfigurability for multiple part families without building new fixtures
Modular fixturing components can be disassembled and reconfigured for different parts, reducing the need to build expensive dedicated fixtures for each job.
Question 5: What is the 3-2-1 locating principle used for in CNC workholding?
- Defining three axes, two planes, and one origin point
- Constraining all six degrees of freedom with the minimum number of contact points (Correct answer)
- Setting three clamp positions for balanced force
- Aligning a part using three dial indicator readings
Correct answer: Constraining all six degrees of freedom with the minimum number of contact points
The 3-2-1 principle uses three points on the primary plane, two on the secondary, and one on the tertiary to fully constrain a part's six degrees of freedom.
Question 6: Why should parallels used in a vise be matched pairs of the same height?
- To ensure equal clamping force on both sides
- To keep the workpiece level and prevent rocking during machining (Correct answer)
- To allow proper coolant drainage beneath the part
- To match the vise jaw height for correct tool reach
Correct answer: To keep the workpiece level and prevent rocking during machining
Matched parallels of equal height ensure the workpiece sits level and stable, preventing rocking that would cause inaccurate cuts and potential part movement.
CNC Workholding and Fixturing 1
What is the primary purpose of a CNC vise stop (work stop)?