Back Bolt Fixture Plate Workflow

The back bolt fixture plate requires a shift in how you think about machining operations. Typically, you'd machine the most complex features first, then use soft jaws with the part profile machined in the jaw to hold the part securely while you remove the excess material from the initial operation.

However, the back bolt fixture plate uses the picture frame concept for holding and machining the part, reversing the usual order of operations.

CAM MODEL:

Part Orientation: The part is set in the stock at a depth of 0.200 inches. This accommodates two 0.050-inch locating posts and a 0.150-inch deep pocket for the picture frame.

Machining begins with what is usually the second side.

Operation 1

Mount the stock in a machine vise on parallels.
8-32 hold-down holes. (Note: tap holes before machining Ø0.186 post. If holes are taped after maching post the tap will push post out of shape making the post to large to fit in the .1875 hole in the back bolt fixture plate.
Mill nesting surface .050 from top of stock leaving two .186 posts.
A .150 deep pocket is machined into the stock below the 2nd Operation nesting surface, reaching the bottom of the part. This pocket depth is necessary because the hold-down bolts protrude from the top of the fixture plate by .100. This pocket also give room for any drilled through holes with out having to worry about drilling into the back bolt fixture plate.

The resulting .150 stock thickness ensures the part can be profiled in the second operation without accidentally machining the hold-down bolts. The pocket is machined 0.010 inches larger than the part profile, except where the tabs are located. In these areas, the pocket profile is inset 0.010 inch from the part profile and the tabs are 0.050 to 0.150 long. This pocket is fully machined in the first operation, leaving provisions for 2 to 4 tabs. These tabs will be exposed when the final profile is machined in the second operation to hold the part.

Any other part features that are only accessible from this side are machined.

Operation 2

Part Mounting On Revers Fixture Plate: Refer to the CAD model for hold-down bolt placement on the back bolt fixture plate. Load the part onto the fixture plate and secure it by tightening the bolts.
Fixture Setup: Place parallels in the machine vise. Rest the back bolt fixture plate, with the part already mounted to it, on top of the parallels. Ensure the pin on the side of the fixture plate faces the solid vise jaw. Slide the fixture plate until the pin contacts the solid jaw, then tighten the vise.
Rough Machining: Machine away excess material from around the part, leaving at least 0.010 inches of material around the profile. Maintain a 0.010-inch offset above the part's
Finish Machine: all features except for the part profile.
Finish Profile Exposing Tabs: Mill part profile in 2 passes.

Start and end the final pass in the center of one of the tabs.
1st pass machine the profile of the part net stay .010 inches above the bottom of the part.
2nd pass machine the profile of the part net stay .010 inches below the bottom of the part.

Unload the back bolt fixture plate from the machine and blow off the chips.
Remove 8-32 cap screws from the part. Note: do not remove 8-32 cap screws from the back bolt fixture plat if there are more parts to be run.
Remove part from the picture frame.
Fragile Parts? If your part is thin and easily distorted then adding frame brakes will be necessary to remove the part from the picture frame.

Create foil tabs so the picture frame can be broken away from the part
Using a 0.250-inch end mill, slots into the picture frame staying off the fixture plate by 0.010-inch and milling up to the pocket profile within 0.010-inch.
After the part and frame are removed from the back bolt fixture plate, cut the foil tabs in half using tin snips.
Now gently wiggle the frame back and forth to break the part free from the frame material.