What is Picture Framing in CNC Machining?

Picture framing is a versatile workholding technique where a workpiece is machined while being held inside a larger piece of stock material. Instead of clamping on the part itself, this method uses the surrounding stock—the "picture frame"—to securely hold the part. 

This approach completely eliminates the need for traditional top clamps, which often get in the way of the cutting tool and limit access to the workpiece.

How is it Done? Common Methods

While the goal is the same, machinists use a few different methods to achieve a picture frame setup.


A comparison of three types of CNC workholding tabs: a small 0.010" x 0.010" tab, foil tabs in the middle of a profile, and foil tabs at the bottom of a part.

The Foil Tab Method

A common approach to picture framing involves leaving small tabs to hold the part to the outer stock. The placement and creation of these tabs are critical.

  • Tab Placement: Tabs are often created either in the middle of the profile wall or, more frequently, as a thin "foil" or "skin" at the very bottom of the part (typically .010" to .020" thick).

  • Creation: These tabs are usually formed during a full-width slotting operation around the part's profile. When tabs are at the bottom, the cutter must machine slightly into a sacrificial plate to ensure the part is fully cut.

 

The Challenge: Instability

While this method works, it introduces significant instability at the most critical moment of machining. Creating a thin, wide tab during a heavy slotting cut puts a lot of stress on a small amount of material. This can lead to:

  • Chatter and poor surface finish on the final profile.

  • Workpiece vibration, potentially compromising dimensional accuracy.

  • Tabs breaking prematurely, causing the part to shift or be thrown from the machine.

  • Difficult Tab Removal: The wider tabs left by a slotting cutter often require a die grinder and significant hand-blending to remove, adding time and potential for error to the finishing process.

Essentially, you are relying on the weakest possible connection at the moment of greatest cutting force, which can be an unreliable way to guarantee a high-quality finished part.

2. The Pocket & Release Method

This is a more advanced and stable approach to picture framing that is designed to maximize rigidity during the most critical finishing operations. The innovation lies in how the part is prepared in the first setup and released in the second.


Setup 1: Creating the Pocket and Tabs


An annotated diagram of the workpiece after Setup 1 of the Pocket Method, with callouts for the stock, inset tabs, and the pocket machined larger than the part profile.

Instead of leaving a thin foil across the entire bottom, a pocket is machined from the top of the stock down to the final bottom surface of the part.

  • This pocket is intentionally machined slightly larger (e.g., .010") than the final part profile.

  • To create the tabs, the pocket is strategically machined inside the final part profile in 2 to 4 small areas. These become the strong, well-defined tabs that will hold the part later.

 

Setup 2: The Two-Pass Release


A finished part after Setup 2 of the Pocket Method, still held to the stock frame by small tabs, with a visible gap of light showing the profile has been fully machined, except where the tabs are located.

After the part is fastened to a fixture plate, the CNC machine has complete, unobstructed access to the workpiece. This is where the bulk of the machining is performed:

  • Excess stock is roughed away from the top and sides of the part.

  • All other features, like holes, pockets, and chamfers, are machined to their final dimensions.

The process culminates in the final two passes that release the part's profile with maximum stability.

  1. The Finishing Pass: The finishing cutter machines the entire part profile, but it is raised slightly (e.g., .010") above the bottom of the part. During this critical pass, the part is still connected to the outer frame by a continuous, thin membrane of material all the way around. This provides maximum stability and rigidity, eliminating chatter and ensuring a perfect surface finish.

  2. The Release Pass: The cutter is then lowered to just below the bottom of the part (e.g., .010" deep). This second pass simply cuts through the thin membrane, separating the part from the frame. Because the cutter is only engaging a small amount of material (.020") at its very bottom, the cutting forces are extremely low.

 

The Result: Unmatched Stability

 

This method ensures that the part is held as rigidly as possible during the final, high-tolerance finishing cut. The release pass is a low-force operation that cleanly separates the part without vibration, solving the instability problems inherent in the traditional foil tab method.

Furthermore, because the tabs are small and well-defined, they are easily removed. A few simple strokes across a piece of sandpaper is all that's needed to blend them perfectly, eliminating the need for aggressive grinding and saving valuable time in the deburring stage.

The Key Advantages

 

Regardless of the method used, the benefits of picture framing are significant:

  • 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 or for full 5-axis machining.

  • 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.

A More Efficient Solution

While foil tabbing is a proven technique, the bottom fastening method offers greater rigidity and repeatability. The McChine Back Bolt Fixture Plate is a system engineered to perfect this approach, making it simple, fast, and incredibly secure.

Ready to see the most efficient method in action?  See Our Step-by-Step Guide on How to Use the Back Bolt Fixture Plate