121.3.3 - Manual Machining

Mill Workholding

Vise ≠ Fixture

A Fixture is a workholding device used in manufacturing to securely hold and position a workpiece during a manufacturing process. It is designed to hold the workpiece in a repeatable, specific location and orientation so that the tool or machine performing a process/operation can access the workpiece as needed.
- In order for something to be a fixture, it needs to control all 12 degrees of freedom to precisely locate something in 3-Dimensional space:
  1. X+ (X positive linear movement)
  2. X- (X negative linear movement)
  3. Y+ (Y positive linear movement)
  4. Y- (Y negative linear movement)
  5. Z+ (Z positive linear movement)
  6. Z- (Z negative linear movement)
  7. A- (counterclockwise rotation about the X axis)
  8. A+ (clockwise rotation about the X axis)
  9. B- (counterclockwise rotation about the Y axis)
  10. B+ (clockwise rotation about the Y axis)
  11. C- (counterclockwise rotation about the Z axis)
  12. C+ (clockwise rotation about the Z axis)

The 3-2-1 Principle

The 3-2-1 Principle is a method for controlling all 12 degrees of freedom when fixturing an object/workpiece, thus locating a workpiece in 3D space
- This is accomplished by:
  1. Having 3 "pins" on the first plane (typically the XY plane (flat to ground & perpendicular to the force of gravity)
  2. Having 2 "pins" on the second plane (either the XZ or YZ plane)
  3. Having 1 "pin" on the third plane (the remaining plane between XZ or YZ)

Simple clamping devices - such as vises - by themselves are not typically Fixtures, as they do not repeatably locate workpieces. However, there are things that you can use in addition to vises that can create a fixture...

Mill Part Stops

Part stops are small workholding devices that are used to locate and hold a workpiece in a specific position on the milling machine table. They are typically made of steel and can be clamped or bolted to the table.
- They allow the workpiece to be positioned accurately and repeatedly during the milling process. This is essential for precision machining, as even small variations in the workpiece position can affect the accuracy of the final product.
- They can be used in conjunction with vises or other workholding devices to make a fixture by precisely & repeatably locating workpieces in 3-Dimensional Space.
Generally speaking, there are two kinds of part stops:

1 - Fixed Part Stops

AKA Permanent Part Stops, are secured to the milling machine table/vise/other while machining operations are performed. This results in maximum rigidity & precision for locating consistency, but can have clearance issues/concerns depending on the setup & operations performed/tools used.

2 - Removeable Part Stops

AKA Temporary Part Stops, are secured to the milling machine table when needed and removed before machining operations are performed. This results in maximum clearance for the parts/tools, but is also prone to greater locational inconsistency due to the part stop moving repeatedly.

Common Workholding Devices

In addition to your standard milling vises, most workholding devices - things that hold onto the workpiece (part) - can be placed into one or more of the following three categories:

1 - Clamping Devices

V-Blocks

Used for holding cylindrical or round workpieces. They have a V-shaped groove that helps to hold the workpiece securely.

Chucks

Used for holding cylindrical or round workpieces. They have jaws that grip the workpiece tightly.

Hold-Downs

Adaptable, multi-component assemblies used to hold workpieces directly to the milling machine table of other workholding devices.

2 - Indexing Devices

Collet Blocks

Used for indexing small workpieces at common angles (45/60/90 degrees). They have a collet that grips the outside of the workpiece securely.

Rotary Tables

Used for indexing workpieces at precise angles. Typically used in conjuction with clamping devices.

Dividing Heads

Used for dividing machined workpiece features into equal sections. Commonly used for cutting gears and other precision workpieces.

3 - Angular Devices

Angle Parallels

Used for holding workpieces at specific angles. Often used in conjunction with other clamping devices to hold the workpiece securely.

Sine Bars/Plates/Vises

Used for holding workpieces at specific angles. They are used in conjunction with other clamping devices to hold the workpiece securely.

Compound Angle Devices

Used for holding workpieces at two or more angles simultaneously. Often used for precision machining of complex parts and require precise adjustment to achieve the desired angle.

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