Soft Jaws vs. Back Bolt Fixtures: A CNC Workholding Comparison.

When it comes to CNC workholding, the method you choose directly impacts efficiency, precision, and the complexity of the parts you can produce. While traditional soft jaws are a workshop staple, newer technologies like the back bolt fixture plate offer a compelling alternative. This comparison breaks down the advantages and disadvantages of each system to help you decide which is right for your workflow.

The Soft Jaw Method

Soft jaws are a versatile and widely used workholding solution. They are typically made from a softer material than the workpiece, such as aluminum or mild steel, and are machined to create a custom-fit profile for a specific part.

A three-step diagram of a typical soft jaw workflow. Step 1 shows a part after its first setup, held by excess stock. Step 2 shows the part flipped into soft jaws. Step 3 shows the final part after the excess stock is machined off.

Advantages of Soft Jaws

  • Versatility: Soft jaws can be machined to fit almost any part geometry, making them ideal for complex or irregularly shaped workpieces.

  • Precision: By machining the jaws on the same machine that will produce the part, you can achieve a high degree of concentricity and repeatability.

  • Surface Protection: The softer material of the jaws minimizes the risk of marring or damaging the surface of the finished part.

  • Accessibility: Soft jaws are a common and well-understood workholding method, with a wide range of available blanks and materials.

Disadvantages of Soft Jaws

  • Time: Consuming Setups: Creating custom soft jaws for each new job can be time-consuming, especially for low-volume or one-off parts.

  • Material Waste: The "picture frame" or excess material left after the first operation is often scrapped, leading to material waste.

  • Limited Rigidity: The softer material of the jaws can sometimes lead to less rigid clamping, which may not be suitable for heavy or aggressive machining operations.

  • Storage and Organization: Managing and storing a large inventory of custom soft jaws for repeat jobs can be a logistical challenge.

The "Picture Frame" Method: Ultimate Workholding Rigidity

The core advantage of the back bolt system is its ability to use an intentional "picture frame" for unparalleled workholding stability—a complete reversal of how excess material is typically viewed.

Instead of trying to minimize the excess material around the part, this method uses it as a feature for rigid clamping.

A three-step diagram of a Back Bolt Fixture Plate workflow. Step 1 shows a part after its first setup has run. Step 2 shows the part flipped and loaded on a back bolt fixture plate. Step 3 shows the finished part still attached to the picture frame that is mounted on the Back Bolt Fixture Plate.

How It Works:

  • Setup 1: In the initial operation, a pocket slightly larger than the final part profile is machined into the excess stock, extending down to the part's top surface. This pocket includes several internal tabs that will be exposed during the second setup. The hold-down holes and locating posts are in the excess stock outside the picture frame pocket.

  • Setup 2: After this, the workpiece is flipped and secured to the Back Bolt Fixture Plate using the prepared holes. This arrangement provides unobstructed access to the top and all four sides for the more complex second operation, as clamping forces are applied from below. Once the part is removed, the remaining excess stock forms what is called the "picture frame."

Advantages of the Back Bolt System

  • Unobstructed Access: 

    • Unobstructed Access: It provides full access to the top and all four sides of the workpiece, making it ideal for 3-axis machines to perform multi-sided work.

    • Reduced Setups: Complex parts can be finished in fewer setups, saving significant time and reducing the potential for errors from re-fixturing.

    • No Jaw Marks: Since there are no clamps touching the finished part, it eliminates the risk of marring the surface.

    • Excellent for Thin Parts: It provides uniform, low-stress support, which is perfect for machining thin or delicate workpieces that might warp under traditional clamping pressure.

  • Improved Rigidity: Bolting the part directly to the fixture plate provides a more secure and rigid setup, enabling more aggressive machining and tighter tolerances.

  • Faster Setups: Once the initial programming is done, setting up repeat jobs is as simple as bolting the part to the fixture plate, dramatically reducing setup times.

  • Simplified Workflow: The back bolt system streamlines the entire machining process, from CAM programming to final production.