How to Use the Back Bolt Fixture Plate

A Step-by-Step Guide to Using Your McChine Fixture Plate

 

Welcome to the official guide for the McChine Back Bolt Fixture Plate System. This guide will walk you through our recommended Pocket & Release Method, a stable and efficient process designed to help you get the most out of your fixture plate.

This method is broken down into two main stages: preparing the stock in Setup 1 and finishing the part on the fixture plate in Setup 2.

Part 1: CAM & Design for Setup 1

A technical diagram showing a part within its transparent stock, with callouts labeling key first-operation features like hold-down holes, locating posts, tabs, and nesting surfaces.

Success with the "Picture Framing" method starts with a solid digital setup in your CAM software. This process turns your finished part model into a manufacturable component by designing the workholding features directly into the stock.

We use a pre-configured Manufacturing Template in Fusion 360 to standardize this process. This template is a huge time-saver, as it already includes our machine vises, the Back Bolt Fixture Plate, and pre-programmed CAM operations for the essential features like drilling, tapping, and post-machining.

Here are the key steps to prepare your model for the first machining setup:

  1. Create the 'Setup 1' Component

    Your first step is to create a new component. This component will be a model of what the stock will look like after the first setup is machined—essentially, the finished part still nested inside its "picture frame."

    • CAM Note: This is critical for your toolpaths. When you create a new Setup in Fusion 360's Manufacture workspace, the "Model" selection must include two components: your final part component and this new "picture frame" component. This tells the software to recognize all the geometry you intend to machine.

  2. Position the Part: In your CAM setup, place the finished part model inside your stock component. The bottom face of your part should be positioned **0.200"** deep from the top of the stock. This critical depth perfectly accommodates the 0.150" deep pocket and the 0.050" tall locating posts.

    1. Pro Tip: Create a new body from your stock and assign it a transparent or "glass" appearance. This allows you to clearly visualize the finished part nested inside the stock you'll be machining in Setup 1.

  3. Add Mounting Features:

    • Hold-Down Holes: Add the through-holes that will be tapped for **8-32** threads. These are what secure the workpiece to the fixture plate.

    • Locating Posts: Around two of the hold-down holes, model two small posts that are **Ø0.186"** (±0.001"). These will be machined to a final height of **0.050"** and are essential for locating the part accurately on the fixture plate.

  4. Add a Clocking Mark: Machine a small, distinct feature like a chamfer or notch on one end of the stock. This provides an unmistakable visual cue to ensure the part is not loaded backwards in Setup 2.

  5. Create the 'Picture Frame' Sketch & Pocket

    The easiest way to create the pocket is with a single, comprehensive sketch. This sketch will define the pocket's main boundary, the corner reliefs ("mouse ears"), and the holding tabs all at once.

    1. Create the Base Sketch: Start a new sketch on the nesting surface. The primary feature of this sketch is a profile that is offset **0.010"** outward from the perimeter of your part.

    2. Add Features to the Sketch: Before you finish the sketch, add the following modifications directly to your offset profile:

      • Corner Reliefs ("Mouse Ears"): In each internal corner, add a relief radius. This radius must be larger than the radius of your finishing endmill, and its center point should be offset **0.010"** from the part's actual corner.
                                  Technical diagram illustrating corner reliefs, also known as 'mouse ears.' These looped corners in a CNC pocket allow a cutting tool to fully machine a part's corners.

      • Holding Tabs: In 2 to 4 strategic locations, modify the sketch profile so it moves **0.010"** inward from the part's edge. Each tab section should be between **0.050"** and **0.150"** long.

    3. Cut the Pocket: Once your sketch is complete, use it to perform a single subtractive cut. This cut should be **0.150"** deep, creating your finished picture frame pocket.

  • Set Part Orientation & Pocket Depth: Position your part within the stock. We recommend setting the bottom surface of your part at a depth of 0.200 inches from the top of the stock. This accommodates the 0.150-inch deep pocket and the 0.050-inch tall locating posts.

  • Define the Pocket: Model the pocket around your part at the specified depth. This pocket should be slightly larger than your part's final profile (e.g., .010" larger).

  • Add Tabs: In 2 to 4 strategic locations, each .050" to .150" long, modify the pocket so that it cuts inside the final part profile (e.g., .010" inside). These will become your strong, reliable holding tabs.

  • Add Hold-Down Holes: Add holes that will be threaded (e.g., 8-32 thread) to accept the Back Bolt Assemblies.

  • Add Locating Posts: Add two small posts that are .186" in diameter (+/- .001"). These will be machined to a height of 0.050".

  • Add a Clocking Mark: Machine a small, unique feature on the end of the stock to visually confirm the correct orientation in Setup 2.

Part 2: Setup 1 - Machining the First Side

Raw stock material loaded in a machine vise on parallels, ready for Setup 1.

In this setup, you will machine the stock in a standard vise to prepare it for the fixture plate.

Goal: Machine all the features you designed in the CAM stage.

  1. Secure your raw stock in a standard machine vise.

  2. Run your CNC program to machine the pocket, the threaded hold-down holes, and the locating posts.

  3. Machine any internal features or pockets that are part of your final design.

A workpiece shown in a machine vise on parallels after Setup 1 is complete.

At the end of this stage, your part is still fully contained within the stock, but it is now perfectly prepared for the second and final setup.

Part 3: Setup 2 - The Complete Workflow

This is where the magic happens. This section covers preparing the plate, mounting the part, and securing the entire assembly in your machine vise for the final operations.


An annotated diagram showing the proper setup of the McChine Back Bolt Fixture Plate in a 6-inch machine vise, with callouts for the parallels, stock, solid jaw, and the stop pin used for repeatable locating.

 

Video Demonstration: Part and Fixture Loading

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Full Video Transcript

In this video, we'll demonstrate how to load a part onto the McChine Back Bolt Fixture Plate and then how to properly secure the fixture plate in your machine vise.

Part 1: Preparing the Fixture Plate

  1. Bring your fixture plate and 8-32 cap screws to your computer.
  2. Reference your CAM model to identify the correct hole locations for your part.
  3. Insert the cap screws into the plate from the top side. This keeps the row and column labels visible for easy reference.
  4. Flip the plate over to the back side. Using another cap screw, push each mounting screw out from the back, leaving the "pushing" screw in its place to hold the part during assembly.

Part 2: Mounting the Part

  1. Take your part stock from Setup 1 and carefully inspect the nesting surfaces for any burrs.
  2. Turn the fixture plate on its side and align the clocking mark on your part with the corresponding mark on the side of the fixture.
  3. Move the part into position over the screws. It's easiest to start by lining up the two Ø0.186" locating posts that have tapped holes.
  4. Carefully start threading the screws to pull the part onto the fixture.

Troubleshooting Fit Issues

Stop immediately if the part does not slide easily onto the Ø0.1875" locating holes in the fixture plate. Forcing it can damage both the plate and your part.

If the part doesn't fit, here are three things to check:

  1. Clocking Marks: Confirm the marks on the part and fixture are properly aligned.
  2. Post Diameter: Use calipers to verify the diameter of the Ø0.186" posts on your part.
  3. Bolt Pattern: Double-check that the screw placement on the plate matches your CAM model.

Part 3: Securing the Part

  • Secure all screws until snug. A good rule of thumb is to tighten them by feel, about a quarter-turn past hand-tight.
  • If you use a torque wrench, tighten to 14 in-lbs.
  • NOTE FOR PLASTIC: When threading into plastic parts, tighten just enough to pull the nesting surface down. Over-tightening will strip the threads.

Part 4: Loading the Fixture into the Vise

  1. Place two clean parallels into your machine vise.
  2. For a quick visual check, ensure the clocking mark on the fixture is on the left-hand side.
  3. Set the fixture plate on the parallels with the side locating pin facing the solid vise jaw.
  4. Slide the fixture plate until the pin makes firm contact with the solid jaw.
  5. Tighten the vise and use a dead blow hammer to seat the plate firmly on the parallels.
  6. Finally, check that the parallels are secure by trying to push or pull them. They should not move.

Learn More

We have now demonstrated the process of loading the part onto the Back Bolt Fixture Plate and loading the fixture plate into the machine vise.

For more information on the Back Bolt Fixture Plate system, to purchase one, or to see more guides, please visit mcchine.com.

Like and subscribe for more videos!

 

Step 1: Prepare the Fixture Plate

A McChine Back Bolt Fixture Plate with the 8-32 Back Bolt Assemblies installed, showing the threaded ends ready for a workpiece.

  1. Reference Your CAD Model: Open your CAM program and look at your Setup 2 assembly. This will show you exactly which holes in the grid pattern require a Back Bolt Assembly.

  2. Insert Bolts from the Top: To easily identify the correct row and column, insert the required Back Bolt Assemblies into the plate from the top side.

  3. Seat the Bolts: Take a spare screw and, from the bottom of the plate, push it into a hole to pop the Back Bolt Assembly out. The cap screw head will now be properly seated in its recessed pocket. Use the bolt you just removed to pop out the next one, and so on, until all the required bolts are correctly seated.


Step 2: Mount the Part

The two .186" diameter locating posts you machined are also threaded 8-32. These are your starting points for accurately mounting the part.

  1. Inspect for Burrs: Before mounting, carefully inspect the nesting surface of your workpiece (the side that will sit on the fixture plate). Use a file or deburring tool to remove any burrs left from Setup 1. A perfectly flat surface is critical to ensure the part sits flush and is machined accurately.

  2. Identify the Locating Rows: Look closely at the top of your fixture plate. The rows that accept the .186" diameter locating posts are easily identifiable by the engraved lines that run between each hole. This feature helps you quickly find the correct starting points.

  3. Pro-Tip for Easy Alignment: To avoid frustration, lay the Back Bolt Fixture Plate on its side.

  4. Check Orientation: Before aligning the posts, orient the workpiece so the clocking mark you machined in Setup 1 faces the corresponding clocking mark on the fixture plate. This provides a quick visual check to ensure the part is not loaded backward.An annotated diagram showing how the clocking mark on the part aligns with the clocking mark on the Back Bolt Fixture Plate for a quick visual check of proper orientation.

  5. Align the First Post: Insert an Allen wrench into one of the Back Bolt Assemblies in a locating row. Use the wrench to guide the post to the screw and loosely start the thread.

  6. Align the Second Post: Repeat the process for the second locating post.

  7. Secure and Torque the Part: Snug down the two locating screws first, then install and tighten the remaining Back Bolt Assemblies to fully secure the part. Apply the correct torque to ensure a secure hold without damaging the threads in your workpiece.

      • Using a Torque Wrench: Tighten each screw to 14 inch-pounds (in-lbs) for materials like aluminum and brass.

      • Tightening by Feel: If you don't have a torque wrench, tighten about a quarter-turn past hand-tight.

      • ⚠️ For Plastics: Do NOT torque to 14 in-lbs. Tighten just enough to feel the part pull down flat against the plate to avoid stripping the threads.

    • ⚠️ Warning: If you feel any resistance when inserting the posts, stop immediately. You may be trying to mount a .186" post into a .177" through-hole. The posts should slide in smoothly into the designated locating rows.

  8. Final Verification (First Part Run): Before moving to the vise, perform this quick sanity check to ensure your physical setup perfectly matches your CAM model.

    • Open your CAM model for Setup 2.

    • In the model, identify the first empty column to the left of your part and note its identifier.

    • On your physical fixture plate, confirm that the same column is empty.

    • Do the same for a row (e.g., the first empty row above the part).

    • This quick visual comparison is the best way to confirm your part is located exactly where the program expects it to be, preventing costly errors.

Step 3: Mount the Fixture in the Vise

Properly seating the fixture plate ensures your part is perfectly flat and level for the final, high-precision cuts.

An annotated diagram showing the Back Bolt Fixture Plate mounted in a machine vise, with callouts for the parallels, stop pin, and solid jaw.

  1. Place the fixture plate on top of a set of clean parallels in your machine vise.

  2. Orient the plate so the steel stop pin is on the side of the solid vise jaw.

  3. Push the fixture plate firmly against the solid jaw, then slide it sideways until the stop pin makes contact.

  4. Critical Check for Proper Clamping: Before tightening, ensure the fixture plate is sitting low enough in the vise. The vise jaws must clamp on the solid outer section of the plate, not on the thin wall of the recessed pocket on the back.

    • Verification: A simple way to check this is to look at your stop pin. If it can nest properly against the side face of the solid jaw, the plate is deep enough for a secure clamp.

    • Warning: Take care to ensure the stop pin is against the side face of the jaw, not sitting inside the machinable area of the jaw itself, which could damage the plate when tightened.

  5. Tighten the vise.

  6. Using a dead blow hammer, firmly tap the fixture plate down onto the parallels—first near the solid jaw, then near the movable jaw.

  7. Verify the Seat: Try to push or wiggle the parallels. If they are tight and do not move, you have a perfect seat.

  8. Troubleshooting: If a parallel is loose, repeat the tapping. If it still won't seat, remove the fixture plate and thoroughly clean the vise jaws, the parallels, and the bottom and sides of the fixture plate. Feel for any burrs or high spots on the plate's nesting surfaces and carefully file them flat if necessary.

Step 4: Set Your Fixture Offset (G54)

This is the key to the system's repeatability. You will now tell the machine where the fixture plate is located.

A close-up, top-down view of the Back Bolt Fixture plate, with a label pointing to the Ø.250 inch datum hole used for setting X0 and Y0.

  1. Set X0 and Y0: Using an edge finder or probe, locate the center of the .250" diameter datum hole on the left side of the fixture plate. Set these coordinates as X0 and Y0 in your desired work offset (e.g., G54, G55).

  2. Set Z0: Using a probe or by carefully touching off on the top surface of the fixture plate, set this coordinate as your Z0.

The Beauty of the System: As long as your vise and parallels remain in the same position, this fixture offset is now set for ANY part you run on this plate. For your next job, you can skip this entire step. Simply load the new part program, change the bolt pattern on the plate, and press cycle start. This is a massive time-saver for high-mix, low-volume work.


Step 5: Rough & Finish Machining

With the part securely mounted, you can now perform the bulk of your machining operations.

  • Rough and finish the top surface of the part.

  • Rough away all excess material from the sides, leaving a small amount of stock (e.g., .010" or more) for the final profile pass.

  • Machine all other features like side holes, pockets, and chamfers to their final dimensions.

 

Step 6: Perform the Two-Pass Release

 

This is the final, critical step that guarantees a perfect profile while maintaining maximum rigidity.

  • Toolpath Strategy: For the cleanest possible finish, program your toolpaths for both the finishing and release passes to enter and exit the cut in the middle of one of your tab locations. This prevents tool marks on your final part profile.

  1. The Finishing Pass: Your CAM program will first machine the entire profile of the part to its final dimension, but with the cutter raised slightly (e.g., .010" above the bottom). During this pass, the part is held rigidly by a continuous membrane of material, ensuring a perfect surface finish free of chatter.

  2. The Release Pass: The cutter is then lowered to just below the bottom of the part. In a final, low-force pass, it cuts through the thin membrane, creating a clean gap between the part and the "picture frame" stock.

At the end of this process, the part is not free. It is now fully machined and held securely only by the strong, well-defined tabs you created in Setup 1, ready for removal.

Part 4: Removing the Part from the Frame

Video Demonstration: Picture Frame Removal

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Full Video Transcript

How to Remove Your Part from a Picture Frame

In this guide, we'll discuss two methods for removing a finished part from its picture frame after machining:

  • Standard Method: Breaking the tabs on a rigid part that is not prone to distortion.
  • Foil Tab Method: A gentler technique for fragile parts where "foil tab breaks" were machined into the frame.

Method 1: Standard Removal (For Rigid Parts)

This method should only be used on parts that will not easily distort or bend. This can be done with a machine vise or a pair of 1-2-3 blocks.

  1. Position the Part: Open the vise jaws just enough so the part fits loosely between them, with the picture frame resting on top of the jaws.
  2. Break the First Tab: Take a piece of soft aluminum bar and place it against one of the tabs. (Note: It's important to use aluminum on aluminum parts to avoid damage.) Lightly strike the bar with a hammer to break the tab.
  3. Remove the Frame: Once the part is loose and held only by the other tab, simply push the picture frame back and forth to wiggle it free.

Method 2: Foil Tab Removal (For Fragile Parts)

This is the recommended technique for delicate parts prone to bending, as it reduces stress and prevents distortion.

  1. Remove Island Stock: If your part has any internal scrap material ("island stock"), you can remove it first by pushing it back and forth until it falls out.
  2. Cut the Tabs: Use a pair of tin snips to carefully cut each foil tab in half. Be careful not to scratch your finished part with the snips.
  3. Break Away the Frame: Gently bend the frame material back and forth at each tab location to remove it from the part.

Final Deburring

After the frame is removed, you will be left with a few small tabs that are 0.005" to 0.010" tall.

  1. Place a sheet of 320-grit sandpaper on a flat surface.
  2. Gently sand the edge of the part until the tabs are gone, leaving a clean, finished edge.

And that's how you remove a part from a picture frame!

For more information on the McChine Back Bolt Fixture Plate, to purchase one, or to see more guides, please visit mcchine.com.

Like and subscribe for more videos!

Once your part is fully machined, the final step is to safely and cleanly remove it from the "picture frame" stock. The best method depends on the rigidity of your finished part.

Method 1: For Rigid Parts

This method is fast and effective for parts that are thick enough to resist bending.


  1. Position the part and its "picture frame" so the frame rests on top of your vise jaws or a set of 1-2-3 blocks.

  2. Close the vise jaws just enough so the part can still slide in and out easily—do not clamp the part itself.

  3. Take a soft bar (like aluminum or brass) and place it on the stock, right next to one of the tabs.

  4. Firmly tap the bar with a hammer. This will break the first tab.

  5. Once one tab is broken, you can often push the part back and forth by hand until the remaining tabs break loose.

Method 2: For Thin or Flexible Parts

For parts that could bend or warp, an additional machining operation is added at the end of Setup 2 to make removal safe and easy.

An annotated diagram showing a "Frame Brake" slot machined into the picture frame stock to allow for easy part removal.

Step 1: Machine "Frame Brake" Slots (in Setup 2)

  • The Goal: To isolate each tab so the frame breaks away without putting any stress on the finished part.

  • The Process: After all other machining is complete, use an endmill to machine slots into the picture frame material next to each tab.

    • Position: The slots should be placed to leave only one tab holding each small section of the frame. .

    • Dimensions: The "frame brake" cuts should be made .010" above the back bolt fixture plate and stop within .010" of the pocket wall that was machined in Setup 1.

    • Warning: Be sure your toolpaths avoid the 8-32 Back Bolt Assemblies.

 

Step 2: Snip and Wiggle

  1. Remove the entire assembly from the fixture plate.

  2. Using a pair of tin snips, carefully cut through the remaining material—both the bottom and the wall—at each "frame brake" slot.

    • Warning: Be careful to control the tip of the tin snips to avoid scratching or marring the surface of your finished part.

  3. You will now have several small, independent sections of the frame, each held by a single tab.

  4. Gently wiggle each small frame section back and forth by hand. It will quickly fatigue and break off cleanly at the tab, leaving your part completely free.

Part 5: Final Deburring

The final step is to remove the part and deburr the small tabs.

Because the tabs created by the Pocket & Release Method are so small and well-defined, this is a quick and easy process.

  1. Remove the "picture frame" from the fixture plate.

  2. Carefully break the finished part free from the frame at the tab locations.

  3. Place a piece of 320-grit sandpaper on a flat surface (like a surface plate or a flat piece of granite).

  4. Give the bottom of the part a few simple strokes across the sandpaper. The tabs will blend in perfectly, leaving a clean, finished edge with no need for grinding.