boring 20 mm diameter hole in aluminium

I've bored accurate holes a bit larger than 25 mm with a tiny Emco round column C5 sized mill in steel plate and without a boring head since I didn't have one at the time. I just used a left hand lathe tool with a braised carbide tip that was specially sharpened for the correct clearances in one of the usual fly cutter tool holders, then set my offsets to the increasing sizes with a dial indicator against the end of the tool tip as it was moved in or out by hand. With even a mini mill and a boring head through aluminum, it should be much easier and faster. Mark out and chain drill with the holes almost touching a bit under the finished bore size, knock the slug of waste out ,and then bore to finished size. Rotabroaches, hole saws etc are costly and can only do a single fixed size. And with thicker plate, I've never found any hole saw to work at all well since there not really designed for that type of use. With a small 2" / 50 mm boring head and the proper range of boring bars, you can precision bore any hole size from about .080" on up to a couple of inches even with a mini mill if your careful. With a bit more power and enough torque at lower rpms, the cross hole most boring heads have can be used for even up to 4" - 5" diameter holes in a pinch. Although the depths of cut does have to be reduced at those diameters. I have a much larger mill, industrial level boring heads and bars now, But for anything quite a bit larger than my drills can provide and to speed up the job, I'd still use that chain drilling method. For shops with manual machine tools, it's still a commonly used industrial technique for large precision holes in plate today, and I've seen it done many times. Manual machining is about precision, for larger areas of bulk metal removal prior to that machining, drills and saw cutting will beat any end mill or boring bar out there both in more efficient metal removal rates and tooling costs.

Boring to an accurate size is fairly simple, bore until the hole fully cleans up but still under size, measure and for example lets say it was .030" under. I'd set a .005" offset on the head, bore through and accurately remeasure to see what the tool did for size. Lets say it actually took only .009". I'd then off set the head to take the next cut at .0055" and bore at that setting. Remeasure and lets the say it did take .010". Offset the head a further .0065" and take the last finish cut. Measuring and offsetting the head over or even under, compensates for any tool deflection at a known depth of cut in that particular metal type so it's closely repeatable for the next cut, or even poor inaccurate feed screws and dials. On critical sized holes and even though the feed screws and dials are very good on my BH's, I still prefer setting the last few cuts with an indicator. It's less the about the tool and more about the technique and methods used since tool and bar deflection are always present. Boring with a lathe still requires that measuring and compensation as well. And for high accuracy bearing fits using a lathe or mill, I don't know how else you would guarantee getting any bored hole to a known and close tolerance size without doing so.

I will try drilling some holes and then using a boring head. I may have time tomorrow, otherwise I will give it a go next weekend. Thanks for all the advice. Hopefully it will save me from spending more money and possibly still not succeeding.

I will should be able to access a lathe in a few weeks. So if I get totally stuck I will give that a try. I live on the London/Essex borders