Need to make a big hole in Aluminum plate

Don't cut the guide slots for your fingers until the center hole is at least roughed out. That much material removal will cause the part to move.
Don't want that messing with your fits. If it benefits you, you can rough the slots first leaving .015 or .020 to clean up.
 
On a previous job about 30 yrs ago, I remember having to cut a 12" hole in a 3/4" steel plate.
I laid the hole out with blue dykem and drilled a 3/4" hole on the inside. Then proceeded to a 20" bandsaw. The bandsaw had a built in blade welder and grinder. I just broke the blade, threaded it through the work piece, and re -welded it. After I re-installed the blade and cut the hole out, I simply broke the blade and retrieved the work piece. It took less time to weld the blade twice, than to do it any other way.
 
I've been lurking this thread because I was getting ready to bore a 1 3/4" hole in a 3"x3"x2 1/2" block of aluminum. Not exactly the same but bigger than I'd done before. I also have an RF-30 and because I also have to mill some flats decided to do the whole thing on it (as opposed to the lathe) because I have a boring head. I also have a 1" drill bit I picked up out of a garage sale thrown in for free because it was messed up. Pushing the boundaries is always a big learning experience and for the first time I was not able to plan around having to raise and lower the head on the mill/drill in middle of the process. I remembered a tip about the laser pointer mounted to the mill shooting across the shop to keep index as I raise and lower the head. After mounting my plumb bob line on the opposite wall and making sure the mill was level I went for it. Works like a champ and the HF laser torpedo level was only $20 with a coupon! It's now staying with the mill and almost looks like part of it. I can't remember who specifically mentioned the laser but I had to send a shout out with my many thanks. And now we return to your usual programming........

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I would also suggest a hole saw, then clean it up on the rotary table or lathe. Chip evacuation holes are the key to getting holesaws to work - if the inside piece is going to be scrap like here, drill the holes just on the inside of the cut line. Four or 5 1/4" holes should work fine. Then run at a low speed (100-150rpm?) and use lots of WD40 and you find it goes really easy. Another tip is to pre drill the center hole and replace the drill in the arbor with the same size piece of rod, so it acts as a guide only, no cutting.

I've done 1" thick steel plate with a 2 1/2" holesaw. Took 20min or so but it worked fine. Got a nice chunk of steel round to play with too.
 
Based on the tools available to you, I'd recommend buying a boring head. Lots of good ideas above, but you have a lathe and a mill. You could get close to the right size with a hole saw, but the finish will not be close to what you could do with a boring head. Plus the boring head lets you make any sized hole within its range. Once you have it, you'll find other uses. The 2" import ones with 1/2" 9 pc. boring bars go for under $85 on eBay including shipping.

You have a good idea with the rotary table too. I've made holes and outside radii on pieces using an RT.

Bruce
 
derf said:
On a previous job about 30 yrs ago, I remember having to cut a 12" hole in a 3/4" steel plate.
I laid the hole out with blue dykem and drilled a 3/4" hole on the inside. Then proceeded to a 20" bandsaw. The bandsaw had a built in blade welder and grinder. I just broke the blade, threaded it through the work piece, and re -welded it. After I re-installed the blade and cut the hole out, I simply broke the blade and retrieved the work piece. It took less time to weld the blade twice, than to do it any other way.
Click to expand...

Many years ago I worked at a cutting die shop where we made credit card and label dies using that process on a DoAll saw. It was the fastest way to remove the stock on the manual machines we had.
 
There is no law that says hole has to be round. You could drill 4 holes and cut out a square hole if you want. Or look at post #40 and think of a rectangle shape opening. Cut the 3 slots before the clearance hole.
 
Don't pass up an opportunity to buy new tools. Get a boring head with boring bar set and a good hole saw. :laughing:
 
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