A back bolt fixture plate is a type of fixture plate where the workpiece is secured from the bottom using screws that thread into the fixture plate. This is different from traditional methods where clamps or bolts are applied from the top.

Key Difference:

A back bolt fixture plate secures the workpiece from the bottom using screws, while traditional methods typically apply clamps or bolts from the top.

Advantages of this Difference:

• Greater Accessibility: Bottom-up clamping provides unobstructed access to the top and sides of the workpiece, facilitating easier machining and setup.
• More Uniform Clamping Force: Distributes clamping pressure evenly, minimizing workpiece distortion or bowing, especially with thin or delicate parts.
• Reduced Interference: Eliminate the need for top-mounted clamps or bolts, preventing potential collisions with cutting tools and improving machining safety. Additionally, the back bolt fixture plate system ensures uninterrupted machining without the need to stop and adjust or remove clamps during the process.

In essence, a back bolt fixture plate offers a more accessible, secure, and interference-free workholding solution compared to traditional methods.

Advantages of using a back bolt fixture plate include:

• Excess stock becomes the fixture itself.
• Uniform clamping force prevents warping, especially in thin parts.
• Thin-floor parts are not squeezed or bowed by vise pressure.
• Allows for more aggressive machining.
• Reduces the need for soft jaws.
• Enables quicker machine setups.
• Ideal for short-run parts.
• Eliminates the need for complicated fixtures on short-run parts.
• Holds thin and odd-shaped parts without special fixturing.
• Grid size is 1/4 inch apart.
• Tab sizes are small and easily removed.
• Can be clamped in a vise at 90 degrees for side features.
• Requires less material than bolt-down style fixture plates.
• Simplifies programming.

Back bolt fixture plates are ideal for machining non-ferrous materials such as aluminum, brass, plastics, and other softer metals.

This is done to minimize the possibility of the part being loaded incorrectly onto the back bolt fixture plate. If the part locating posts are not sliding into the locating counterbores, the operator is attempting to load the part incorrectly. 

This radius serves as a clocking mark, providing a quick visual check for two purposes:

1. Part Alignment: When loading the part onto the fixture plate, the clocking mark on the part should align with the clocking mark on the fixture plate. If they are not aligned, the part is loaded incorrectly.
2. Fixture Orientation: The clocking mark should be positioned on the left-hand side when the fixture is loaded into the vise. This ensures consistent and correct orientation for machining operations.

The .250 grid pattern on the fixture plate was chosen primarily for two reasons:

• More Hold-Down Locations: The smaller grid pattern provides a greater number of options for placing hold-down bolts. This flexibility is especially useful for complex or irregularly shaped parts, as it allows for optimal positioning of the bolts to securely hold the workpiece during machining.
• Reduced Material Usage: Compared to a conventional plate with larger, .500 hold-down holes, the .250 grid pattern uses less material. This translates to cost savings and a more efficient use of resources.

In essence, the smaller grid pattern offers a balance of flexibility, efficiency, and cost-effectiveness, making it a key feature of the back bolt fixture plate design.