The following is an edited discussion of Picture Frame Machining found at the Mastercam Forum It takes a few moments to read the whole thing through, but if you aren't familiar with that type manufacturing process, it pretty much covers it. Skip to the bottom of the discussion for some helpful pictures.
posted
Because of high stress inherent in 7050/7075 and thin walls / floors in Aerospace industry parts. Boeing is pushing this method in their Residual Stress Management (RSM)program. Basically, it involves bolting plate to sub-table at 4 corners with stripper bolts, then concentrating only on inner pockets, without disturbing periphery, thus picture frame. This enables part to be flip/flopped as required using same 4 holes. Only when inner area is complete, do you attack outer areas. According to seminar I attended this procedure forces stresses outward away from thinner sections. Unfortunately, seminar was skimpy on examples.
posted
I have some files that use a style that I call Self-fixturing . I also put some tabs with shoulder-bolt holes thru them to hold the part on a subplate until the end of op-2 when we make a cut between the bolts and the part leaving .005 in z.Then remove part ,break off tabs, and send to deburr.
posted
Scott, I use similar method on most of my jobs, no luxury of vices and such. But this method uses whole block as one big tooling tab, after part is done you break-off or jig saw part out and entire frame is still intact. Thats the area im seeking info on, the approach to finishing outer walls and keeping part attached, sorta like runners? on injection mold parts.
posted
Picture framing a part has been around for a long time in the aerospace industry. It is typically used on more complex parts, but can be used for about anything. The idea is to trade off some additional stock material for not having to create an elaborate (expensive) fixturing setup. If you are using universal fixturing you don't even have to design a fixture at all. You use the same one for every part. Anyways, the process goes like this: The stock material serves both as a place to locate and hold the part through all phases of manufacturing. You layout the part in the stock material and then you layout "tabs" to connect the part to the sides of the frame. There are some cool ways to do this so that the whole setup remains extremely stable until the very last operation when the part is profiled and the tabs are revealed. The tabs are made to be pretty thin (I have seen as thin as .007") so that all you do is run a scribing tool along the tab to break it and very little (if any) handwork is required where the tab was. Shoulder bolts are nice because they hold and locate at the same time. Invert-a-bolt fasteners are nice because they are completely out of the way of the machining process, since they stick up out of the mill fixture plate just a short distance. I have seen the invert-a-bolt fasteners being used in high-speed machining since they are so stable and you really don't want to hit your shoulder bolts at 15,000 RPM and 300 IPM!
Regarding how to define the tabs: It could look like a runner that goes around some or all of the periphery of the part but it doesn't have to. When using over-the-top tabs you connect along the edge of the flange profile of the part. You leave the part fully connected to the outer frame until the very end. Example. Stage 1-Rough side 1 of the part. Stage 2, Rough & Finish side 2. In stage 2 you cut down below the flange of the other side. Then in stage 3, when you run along the profile with your cutter, you just skip some areas on the profile maybe an inch or so wide, then you come back and run along the inch long portion and cut the profile flange to leave maybe .030" wide. The part is unbolted and along that straight area of the top of the flangetop, where you left the tab, you just break the tab & deburr.
posted
I've been using this technique for a few years now, I've just never heard of it referred to as that. I've got some parts that I'll put pictures of up tonight. This was the only practical method of machining these things. Lukily my parts were only 6061-T6, if they were 7xxx series, YIKES!!!!
I'm programming a part right now that I'm using shoulder bolts to hang onto for it's last operation. Beginning weight - 58.7 lbs. Final weight - 4.5lbs!!! And it's 7075 T-651. These parts are fun, lot's of HSM. Should be about a 2 to 2 1/2 hour part. We'll see.
-------------------- James Meyette
MC Programmer ,MC X Beta Tester, and RABID University of Southern California Fan!!!
"The difference between death and taxes is death doesn't get worse every time congress meets." - Will Rogers
posted
The invert-a-bolt site is has some nice animation, but I don't understand how it applying downward force. It appears to me that the part is still able to move laterally by the amount of thread clearance and could potentially suspend the stock above the level of the table.
posted
Ah, I have used these. They don't do that. The inner thread hits a stop on the outer thread and friction is created at their intersection. It's the same way a nut and bolt work except the workpiece is t he nut.
posted
Scott, thanks on bolt tip. Will try to incorporate this in future jobs.
Julies-Man, great info you posted..but, a picture is worth a 1000 words.
posted
James, lets see a picture of your complete set up please. The part looks pretty cool. The first part I mean. The other part looks cool too, but the first one is what I am interested in.
--------------------
Trevor Bailey
San Diego CAD/CAM
Authorized Mastercam Distributor
Poway, California
posted
I uploaded an image (i hope). It didn't show in the preview but it is on the FTP server, so hopefully y'all will see it.
Here is an example of over the top tabs. Its not as pretty as James part pictures but hopefully explains the tabbing concept. As you can see when complete the part is held by just a small amount of material on the top of the flange. The tab is not reveled until the profile of the flange is cut in the last operation so the setup is stable throughout. I have also showed the outline of a invert-a-bolt in the fixture plate.
[img] ftp://www.ppcadcam.com/Mastercam_forum/All_pictures_Jpg,gif,bmp's/tabs.JPG [/img]
posted
You got to have enough tabs. But they are fully connected until the very last operation when the profile is cut and the tabs "show up." Until then the frame does all the work. Try it, you'll see.
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