Cutting a 4" hole in a piece of .125 aluminum sheet

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Rcodenewf

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Hello. New to the site and certainly new to machining although i think i have the basics down. I recently purchased a mill, a Fuji which appears to be a copy of a bridgeport. Has an 8x30 table on it and dro and came with some tooling. My son is building an oil 'catch can' for a race car. He has a 6 inch aluminum pipe in which he will put a 4 inch pipe in the center. He's asked me if i could cut an accurate 4 inch diameter hole in the sheet of .125 aluminum. I do have boring bars etc. Is there a way i can go about cutting it accurately with my mill? thanks...John
 
Do you have a rotary table that you can fix the sheet to and cut the circle out?
 
Thanks for the prompt replies guys.
WhyW8: yes, i have a rotary table. Still in the box ..lol. But now is the time to get it out i guess.
Tq60: Once i rough cut it out with the smaller hole saw i should be able to use my boring bar i guess? Or a fly cutter?
Ulma Doctor: I don't have one of those.

He was hoping to have me cut the hole precise so the 4 inch pipe can slip perfectly through it so he can tig weld it without any gaps.

Much appreciated,
John
 
Hole saws tend to not run true. However they can be used to get you moderately close. I had a 2" hole saw with a 0.1" runout! Then use a boring head, or something like it to cut the true size hole. With some enginuity, you could make something like the circle cutter, if you need to.

If all you have is a rotary table, then use it. You will be challenged holding the work piece to the table, hopefully you can drill some small holes to hold down the piece.

Personally, I'd go for a boring head or circle cutter approach. Easier to setup than the rotary table.
 
I have cut literally hundreds of holes on 1/16" aluminum with a circle cutter like the one @Ulma Doctor hows. Half of them were 4" diameter. I drill the center hole and replace the pilot drill with a dowel pin. I also adjust the cutter so that it is just long enough to break through before the body of the hole cutter makes contact. It helps to grind the cutter for a smaller a smaller kerf a with a strong positive rake and relief on the sides and bottom. It cuts accurately positioned and sized holes and they have only a small burr on break through which is easily cleaned up with a file..
 
I would go with @tq60's suggestion, open up with undersize hole-saw and bore with boring head. It gives you the chance to sneak up on the size instead of finding out it's too big when it's too late.

Check the actual size of the "pipe". Real 4 in. pipe is something like 4.63 od. Tube may be 4 in., but likely isn't round. You might even want to bore a test hole in something unimportant before the real thing.
 
I have cut large diameter holes on aluminum ,Brass and copper sheets often for my projects and tried a few methods (nibbler, DIY circle cutter using a bead roller, HVAC disc cutter, ) the easiest tool I have used is the one Ulma Doctor(post#4) suggested, I liked it so much that I made one myself that is essentially the same design but can cut up to 6" circles, of course there are other more expensive options like Hydraulic knockout punch kit for up to 4.5" circles or jewelers disc cutter for smaller holes up to 2" but I wanted a tool that wasn't too expensive and easy to set up and accurate, so I went with diy circle cutter.
 
Since the subject of expensive alternative tools has been opened, I refer to the SPI trepanning tool, it comes with a circle cutter for running in a drill or mill and a lathe tool holder and uses propriatery tool bits that are set at an angle so as to drag cut, eliminating the tendency to chatter. Did I say EXPENSIVE? Well, they are Swiss made.
 
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