What is Picture Framing?

The "picture frame" or “window framing” method is a machining technique that uses the excess stock to hold the part instead of using dedicated tooling to hold the part for machining.

There are 3 ways to do a picture frame.

In the picture above, the 0.010 inch x 0.010 inch tabs are shown with the back bolt fixture plate. The foil tabs, a common method for workholding in this scenario, are shown with 0.500-inch hold-down bolts to work with a standard fixture plate with a 1 1/4-inch hold-down grid pattern.

0.010 inch x 0.010 inch Tabs Using the Back Bolt Fixture Plate

How Picture Frame method and Tabs

work with the back bolt fixture plate.

The method used with the Back Bolt Fixture Plate is a little different.

When the part is finished the tab profile will be .010 thick x .010 wide instead of .02 thick X .(cutter diameter).

When the part is fully machined in the second operation, it is removed from the picture frame using one of two methods.

If the part is substantial enough, lightly tap it out of the frame with a hammer and a small block of aluminum.
If the part is fragile, slots can be milled into the frame material after the part is finished. Then, tin snips can be used to cut the slots open, and the excess frame can be pushed back and forth to free it from the part.

Tabs are easily sanded off using 320 grit sandpaper on a flat surface.

The Back Bolt Fixture Plate is never machined into.

Foil tabs around the middle of profile walls.

Position 1

Machine back of part net.
Mill part profile net 1/2 way down
Bore 2 holes or machine 2 edge for locating in operating 2

Position 2

Mill the part complete then on the last operation mill down to past ½ way and in a few spots raise up to leave material of .02 to .05 thick in several areas.

Foil tabs around the bottom of profile.

Operation 1

Machine stock flat.
Drill holes for 1/2-in cap screws.
Bore 2 holes or machine 2 edge for locating in operating 2

Operation 2

Machine part complete.
Mill part profile net by dropping the cutter 0.005 to 0.010 inch into a sacrificial plate, raising the cutter 0.010 to 0.030 inch above the bottom of the part to leave thick tabs.

Issues with foil tabs and a conventional fixuter plate:

Hold-down bolts:

Bolt heads take up space with a head diameter of 3/4 inch, which will increase the material size of the raw stock.
Programmer has to be mindful about not running into bolt heads and take extra time to machine around them.
Machine clearance height must be higher to avoid running into bolts when moving to different areas on the part.

With the foil tab in the middle of the vertical wall, blending issues may occur between the 1st and 2nd positions.
Thick tabs must be cut off and then hand-blended with a die grinder.
With these tabs being the width of the cutter, this can lead to slower feeds and speeds around the profile to reduce chatter from the end mill.
To minimize deburring, the foil tab bottom method requires a sacrificial surface that must be periodically faced or replaced. This is because the cutter needs to go below the bottom of the part, which will machine into the fixture by 0.010 inch.

Here's how it works:

Step 1: First Position

The part is initially milled to create a flat nesting surface for the second position and locating posts.
A clocking mark is plunged on the part's edge for easy alignment during later stages.
Holes are drilled and tapped for hold-down screws.
A pockets is created to form tabs that secure the part to the fixture plate.
Bump-outs, also known as "mouse ears," are added to inside corners to ensure the cutter can reach the entire bottom surface of the part during machining.
The pocket walls are made slightly larger than the part's profile.
Tabs are positioned inside the part's profile.

Step 2: Second Position

The part is loaded onto the back bolt fixture plate using the clocking mark and locating posts.
The outside of the part is rough-milled, leaving a small amount of material.
All the part's features are machined to their final dimensions.
The tabs are exposed in two machining passes to ensure a clean cut.
An optional "frame brake" tool path can be added to make it easier to remove the part from the frame.

Parts with Through Pockets:

If the part has holes or pockets that go all the way through, extra material can be left as "islands" in the picture frame pocket.
These islands can provide additional support and hold-down points during machining.

Key Points:

The picture frame method creates a secure and stable setup for machining parts.
It's particularly useful for short-run parts and those with complex shapes.
The tabs provide a strong hold while being easily removable after machining.
The method can be adapted to parts with through holes or pockets.
Careful attention to pocket dimensions and tab placement is essential for success.

Overall, the picture frame method is a versatile machining technique that leverages the back bolt fixture plate for efficient and precise part holding.

Picture Frame Method: A Comparison of Traditional and Back Bolt Fixture Plate Techniques

The picture frame method, also known as window framing, is a machining technique used to secure a part for machining operations. This method utilizes excess stock material around the part to create a "frame" or "window" that holds the part in place, eliminating the need for dedicated tooling.

Traditional Picture Frame Methods

There are several ways to execute the traditional picture frame method:

Tabs Around Profile Walls: This involves machining tabs around the middle of the part's profile walls. The part is machined in two positions. In the first position, the back of the part is machined, and the part profile is milled halfway down. In the second position, the part profile is milled to just below where the profile was machined in the first position, leaving tabs midway up the part profile wall.
Thin Material Method: Suitable for materials 1.00 inches or less in thickness, this method involves a single operation. The part is machined completely, and then the cutter is dropped slightly into a sacrificial plate to create tabs.

Challenges with Traditional Methods:

Bleeding Issues: Machining in two positions can lead to bleeding issues between the cuts.
Thick Tabs: The tabs created are often thick and require additional finishing work, such as cutting and hand blending.
Slower Feeds and Speeds: The width of the tabs can necessitate slower feeds and speeds to prevent chatter.
Sacrificial Surface: The thin material method requires a sacrificial surface, which may need to be replaced over time.

Picture Frame Method with Back Bolt Fixture Plate

The back bolt fixture plate offers a different approach to the picture frame method.

Process:

First Position: The part is prepared with a flat nesting surface, clocking mark, holes for hold-down screws, locating posts, and pockets for tabs. Bump-outs are added to inside corners.
Second Position: The part is loaded onto the fixture plate using the clocking mark and locating posts. The outside is rough-milled, and then all features are machined to final dimensions. The tabs are exposed in two passes.

Advantages:

Thin Tabs: The tabs created are much thinner (0.010 x 0.010 inches), allowing for easy removal with sandpaper.
No Sacrificial Surface: The fixture plate itself is not machined into.
Improved Stability and Precision: The back bolt fixture plate offers a secure and stable setup for machining.

Key Points:

The picture frame method is a versatile technique suitable for various part shapes and sizes.
The back bolt fixture plate offers a refined approach with thinner tabs and improved efficiency.
Careful consideration of pocket dimensions and tab placement is crucial for success.

Conclusion

The picture frame method is a valuable machining technique that offers flexibility and efficiency in part holding. The back bolt fixture plate enhances this method by providing a precise and streamlined approach, particularly beneficial for short-run parts and complex geometries.