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.

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Part 1: CAM & Design for Setup 1

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Part 2: Setup 1 - Machining the First Side

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.

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.

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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.

 

▶️ Watch the Video: How to load a Back Bolt Fixture Plate.

Step 1: Prepare the Fixture Plate

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

he 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.

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. 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. Secure the Part: Once both posts are loosely threaded, secure them first. Then, proceed to screw in the remaining Back Bolt Assemblies to fully secure the part to the fixture plate.

9. 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.

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.

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.

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Part 4: Removing the Part from the Frame

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.

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.

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

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

5. 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.

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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.