Looking for strategy when machining window out of a plate
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  1. #1
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    Default Looking for strategy when machining window out of a plate

    So whenever we machine a window out of a plate, the inside part often gets in the way and can break the tool if you don't feedhold and physically remove the inside portion. If we are running "lights out" this is bad.

    Sometimes we will resort to machining the inside of a window area as a pocket so as to reduce inside portion to chips which are easily flushed out by flood coolant.

    Sometimes we will leave it attached by .01" or so and then just break it out and debur the edge to blend it in but that is not ideal and creates a lot of unnecessary work.

    Does anyone have a technique they would like to share?

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    We do a lot of this type of work. 90% of the time we treat it as a pocket and turn it into chips. The rest of the time we run a bolt thru the window into the fixture to hold it in place till the rest of machining is done. It really depends on your material, cycle time, and qty though.

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    ---^ exactly what is needed.

    If its aluminum, we just HSM the material away and get on with life.

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    I've found that using TWO bolts in the center will guarantee it from spinning when you break thru on the perimeter. One bolt will work most of the time, but not every time.

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    If it is steel, place a few strong magnets across the gap as the cutter goes around. Did this all the time, like how we did it with a wire EDM.

    Paul

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    I wonder how many times I wanted to take out of a programmers pay the costs of leaving loose slugs? I've seen broken tools, broken windows, damaged chip conveyors, etc. all from not restraining or cutting up slugs.

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    2 bolts through it has always been my strategy. Agreed that in aluminum most of the time it is easier/faster to just turn it into chips.

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    You have to decide 'what' time is more valuable.

    feed hold traps the operator watching the machine

    mill to chip spends spindle time

    bolts ads operator time and machine time

    most CAM programs have 'tab' milling so it will leave any number of thin tabs of variable width and thickness

    This saves machine time and limits operator time to either filing the remainder of the tab off or twisting the slug out and then pressing go to finish mill

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    Depending on if its worth keeping the inside portion, ill z level down to leave say .05", then machine hold tabs, manually cut hold tabs out, then machine inside of pocket.
    But its gotta be worth the time.

    For really thin sheet metal\lexan type stuff, ill lay a backing plate down and do the tape\tape trick using krazy glue to hold the assembly together. Pretty slick.

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    If the windows are small, or the slug is not worth salvage, make chips.

    Sometimes I am able to fixture plates that have big through features high enough off the table that the slugs fall safely to the table below (mostly aluminum, I don't want heavy steel slugs to do the same to the table)

    The only time I regularly don't do these is when running some large windows of tougher steel (4140, 304, and the like) when it is thicker (3/4+). In these cases, I often will rough out these kind of windows with 7/8 or 1" cobalt roughers. Yeah, the slow and steady type. With the kind of low RPM these go, I don't fear them getting ejected, so I just have an M0 afterward to pick it out.

    I generally only do the drill holes and bolts through method on parts that go on the horizontal. The chip conveyor doesn't appreciate 4lb chunks of sharp steel falling 3 feet down onto it.

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    I used to have a long running job out of 3/4" plate. I held down the slug with 6 1/2-13 Low Head Socket Screws... then I turned the slug into three Kurt style soft jaws. It was definitely slower than turning it into chips, but it made me happy every time I took a part off the mill.

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    If it's a big window such that the drop is worth keeping and meaningful volume, a vacuum fixture will keep it in place nicely. You'd still want to onion skin it and then do the final cut-out towards the end of the program.

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    I recall being asked by a supervisor why we didn't stop our Whitney punch press to remove a sheet metal corner slug. Whitney taught us to just nibble the whole thing out if it was ten hits or less.

    Sort of like milling the whole cutout into chips as described by the OP.

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    I don't think there is any one answer as it all depends on slug size and how critical the finish is, time allowed.

    A big window I'll let it drop away most of the time if not thick and heavy. Smaller windows...run a drill through to hog out and finish the pocket.

    If i want the drop, pretty large or parts I leave a small bit around to push out later.

    If not a big drop its all chips...

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    The options above are all valid and often it comes down to the time to design/make a fixture (vaccum) vs cost to turn it into chips vs cost to install a bolt.

    Let me toss a couple other ideas into the mix that might hit a sweet spot on the cost/time equation:

    1) Drill a through hole in slug and use the *machine* to put the screw in. Make a hex driver tool with a magnet, then pre-load the cap screw into a close-fit hole on an unused part of the fixture. Grab your tool, move to your hole and run a tapping cycle to pop the cap screw into a threaded hole in the fixture. Pain in the ass to make the tool (you'll need a drill/driver clutch on there too) but it works when you dial it in and is fast.

    2) Have a spring-loaded captive pin below the slug. You compress the pin down when you push the part on the fixture. The machine runs a drill down on the top, and on the way up the pin (now free from the covering material) extends up into the hole. It doesn't hold it super-securely, but definitely stops it falling out or contacting the cutter.

    3) Similar to #1 but install some plastic or metal push rivets like those that hold on automotive bumpers. Simple custom tool and easy program cycle and you're good.

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    Thanks all -

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    Quote Originally Posted by G00 Proto View Post
    I used to have a long running job out of 3/4" plate. I held down the slug with 6 1/2-13 Low Head Socket Screws... then I turned the slug into three Kurt style soft jaws. It was definitely slower than turning it into chips, but it made me happy every time I took a part off the mill.
    That is a great idea! Next time I have large enough cutout, I'll make the bolt pattern the same as standard jaws!

    That is kind of brilliant.

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    Just thinking out loud here.

    What about, in the case of machining a square window in a plate, first remove enough material on one side so the material to be chopped off can move, then chop it off using conventional milling?

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    Quote Originally Posted by Tichy View Post
    Just thinking out loud here.

    What about, in the case of machining a square window in a plate, first remove enough material on one side so the material to be chopped off can move, then chop it off using conventional milling?
    It'll still occasionally come back and get tangled up with the cutter. It happens fast, and the results can be pretty bad to machines, parts and people.

    Back in the scary old days, we used to wait until the slug was just about to break free, then we would reach in there and push the slug away from the cutter with the handle of a dead blow hammer. Then when the program moved to the next window, we'd crawl in there and grab the slug. Keep in mind this was a 10 foot by 30 foot table and the parts were coming out of a full sheet of aluminum. Ah the good old days before machining was dangerous.

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    Waterjet the plate out, leaving .05" all around.


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