The 'Picture Framing' Workflow: A Guide to Reducing CNC Setups with Bottom-Up Workholding

Are you tired of wasting time, material, and money on single-use soft jaws? Frustrated with part warping caused by excessive vise pressure or inaccuracies creeping in from multiple, complex setups? Traditional CNC workholding often creates more problems than it solves, forcing you to machine around bulky clamps and re-fixture parts multiple times.

The "Picture Framing" workflow with the McChine Back Bolt Fixture Plate is the solution. This guide details a bottom-up workholding method that reverses the usual order of setups to give you unobstructed access to your part, drastically reducing setup time and eliminating common machining headaches. By securing the workpiece from below, you can finish more features in fewer setups with greater accuracy.

Video Demonstration

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Full Video Transcript

Are you tired of machining soft jaws or building dedicated fixtures for every new job? Managing and maintaining special fixtures can get overwhelming.

To solve this, we developed the McChine Back Bolt Fixture Plate for a standard machine vise. This system reduces setup time, eliminates part bowing from vise pressure, and lets you machine parts from billet material, from micro-sizes up to 5" x 11".

Hi, I’m J. McFarlane, owner of McChine, and today we're showcasing the Back Bolt Fixture Plate.

Features and Capabilities

Machined from durable 6061-T6 aluminum, our fixture plate is currently available in two sizes: 4" x 8" and 6" x 12". Each plate features a precise 0.250" grid pattern with rows of 3/16" counterbored holes spaced every inch for accurate locating.

It's the perfect solution for machining non-ferrous materials like aluminum, brass, and plastic, especially for job runs of up to 100 parts. While a dedicated fixture might be better for larger production runs, our plate offers a unique advantage: the flexibility to start production immediately while you finalize a permanent fixture design. The number of parts you can machine at once depends on their size, from multiple small components to a single large part.

The Back Bolt Advantage

Our unique workholding design uses small 8-32 cap screws that pull your part down securely to the fixture plate. This offers several key advantages:

  • Low-Profile Clamping: When tightened, the cap screws protrude only 0.100" from the plate's surface. Since the bottom of your part sits 0.150" above the plate, you can fully machine the entire profile without crashing into a clamp or hold-down bolt.
  • Secure Holding Force: Each screw provides 490 lbs of holding force, ensuring your parts remain secure throughout the entire machining process. This eliminates common issues like part bowing and shifting.
  • Zero-Point System: Here’s the real time-saver. Once you've established the fixture plate offset, you can machine a wide variety of parts without touching your machine setup. Simply reposition the 8-32 cap screws to match the new part and you’re ready to go. This dramatically reduces setup time and maximizes productivity.

The Machining Workflow

Let's outline the typical strategy for machining a part using this system.

Setup 1: Creating the Picture Frame

In this setup, we'll load raw stock into a standard vise on parallels and machine the back side of the part.

  1. Drill & Tap: Create the 8-32 threaded holes for the hold-down screws.
  2. Machine the Nesting Surface: Mill the surface that will sit on the fixture plate, leaving two 3/16" diameter posts that are 0.050" tall around two of the tapped holes. These posts are for precise locating.
  3. Mill the Pocket: Machine a pocket 0.150" deep around the part profile. This pocket should be 0.010" larger than the part, except in 2-4 small areas where you will leave tabs.
  4. Machine Other Features: Finish any other pockets or holes accessible from this side.

Setup 2: Finishing the Part

Now, the part is loaded onto the Back Bolt Fixture Plate in the vise.

  1. Rough Material: Rough out the excess material from the entire part, leaving about 0.010" of stock around the profile.
  2. Finish Features: Finish machining all the remaining features of the part.
  3. Finish Profile: The final operation is to machine the part's profile and expose the tabs. This typically involves two finish passes:
    • Pass 1: Machine the profile 0.010" above the bottom of the part.
    • Pass 2: Lower the cutter to 0.010" below the bottom of the part to cut through, starting and ending this pass in the middle of a tab.
  4. Remove Part: Finally, remove the finished part from its "picture frame," and you're done!

If you're eager to learn more or ready to purchase your own Back Bolt Fixture Plate, please visit our website at mcchine.com. We're confident this innovative tool will revolutionize your machining process.

 

The Workflow: Step-by-Step

This process requires a shift in thinking. Instead of machining your part and then figuring out how to hold it for the second setup, you will machine the holding feature first.

Phase 1: CAM Preparation (The Digital Setup)

In your CAM software, your model should be set up with the finished part positioned 0.200" deep within the raw stock. This specific depth is critical, accommodating:

  • Two 0.050" locating posts.

  • A 0.150" deep pocket for the "picture frame" and bolt clearance.

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.

Phase 2: Setup 1 (Machining the 'Picture Frame')

This first operation machines the back side of your part and creates the integrated workholding feature.

  1. Mount Stock: Secure your raw stock in a machine vise on parallels.

  2. Create Mounting Holes: Machine and tap the 8-32 hold-down holes in your stock.

    • CRITICAL NOTE: You must tap the holes before machining the Ø0.186" locating posts. Tapping after the posts are finished will deform them, preventing a proper fit in the fixture plate.

  3. Mill Nesting Surface: Mill the nesting surface 0.050" down from the top of the stock, leaving the two Ø0.186" locating posts proud.
  4. Machine the Pocket: Machine a 0.150" deep pocket that follows the profile of your part. This pocket creates the "picture frame" and provides clearance for the back bolts.
      • The pocket should be machined 0.010" larger than the part profile.
      • Leave 2 to 4 small tabs (0.050" to 0.150" long) by insetting the pocket profile 0.010" in those areas. These tabs will hold the finished part in the frame during the final profiling pass.
  5. Machine Features: Complete any other features on this side of the part.

Phase 3: Setup 2 (Finishing the Part)

Now, you'll flip the part over, mount it to the fixture plate, and finish it.

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.

  1. Mount Part on Plate: Secure your workpiece from Setup 1 onto the McChine Fixture Plate using the 8-32 Back Bolt Assemblies.

  2. Set Up Fixture: Place parallels in your vise and rest the fixture plate on them. Ensure the side pin on the plate is facing the solid vise jaw. Slide the plate until the pin makes contact and then tighten the vise. This provides a simple, repeatable XY zero position.

  3. Rough & Finish: Machine away all excess material around the part and finish all top-side features, staying 0.010" away from the final profile.

  4. Finish Profile: Mill the final part profile in two passes to break through the bottom of the part and expose the holding tabs.

      • 1st Pass: Machine the profile, staying 0.010" above the bottom of the part.

      • 2nd Pass: Machine the final profile, going 0.010" below the bottom of the part. Start and end this pass in the middle of a tab.

  5. Unload & Prep for Removal: Unload the fixture plate from the vise. Remove the 8-32 screws to release the part, which is still held in its picture frame.

Part Removal (Video Guide)

 

You have two methods for safely removing your finished part from its "picture frame."

Watch the Video: Both techniques described below are shown in detail in our "Picture Frame Removal" video. We highly recommend watching it to see exactly how to handle your part without causing damage.


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How to Remove Your Part from a PictureFrame

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

  1. Standard Method: Breaking the tabs on a rigid part that is not prone to distortion.
  2. 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. -
  3. 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.
  4. Break Away the Frame: Gently bend the frame material back and forth at each tab location to remove it from the part.
  5. Finish the Edge: You'll be left with a few small tabs that are 0.005" to 0.010" tall. Place a sheet of 320-grit sandpaper on a flat surface and sand the edge of the part until the tabs are gone.

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.

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Standard Method: For most parts, you can simply and carefully break the small tabs to free the component from the surrounding frame material.

Fragile Parts Method: If your part is thin or easily distorted, breaking the tabs can be risky. Use the "foil tab" technique for a gentler release.

  1. Before removing the part from the fixture plate, use a 0.250" end mill to machine slots into the picture frame, creating very thin "foil tabs."

  2. Once the part and frame are off the plate, use tin snips to cut the foil tabs.

  3. Gently wiggle the frame to break it away from the part with minimal stress.