Hi James,
I would suggest using a hole saw. Use the next size down from 2" / 50 mm which will probably be 44 mm then bore out to size. You will probably be able to avoid using the pilot drill if you do it in a lathe or mill. As a coincidence today I did this cutting a hole and a half hole in some 50 mm steel tube to make a T piece.

Les.

Edited By Les Jones 1 on 22/05/2013 16:44:56

Check how well the material machines first. If its a decently behaved alloy then trepanning or hole saw will work fine. If its pure aluminium or one of the ill mannered alloys that share the sticky, gummy, "build up on the tip as soon as look at you" characteristics of the pure stuff accepting the waste and mess of turning to swarf may be the sensible option. Even if you have a suitable rotary table and set-up stuff drilling a ring of holes is hardly worth the effort.

First steps in trepanning go a lot easier with something well behaved, OK true of everything but more so in this case. I frequently use the hole saw method but last week lost a tooth from a brand new Starrett holesaw, which promply wrecked the rest, due to assuming the scrapped component source meant well behaved material.

Whether hole saw or trepan Jasons advice to go from both sides is good. If hole sawing remember you can only go about half a tooth deep before having to withdraw and scrub out the chips. Need a top brand holesaw. El cheapos will melt thier teeth on this sort of deep job and, probably, be so far out of round that they jam.

I'd probably just drill 6 or 8 half inch holes in a suitable circle and whack a chisel through the left behind webs. Sometimes crude really is all you need.

Clive

I've used both hole saws and trepanning tools. The saw was easier, though it creates an annular channel which is hard to keep clear of swarf if done in a vertical miller/drill. The trepanning was done on the faceplate, with sacrificial backing, and the trick seemed to be to get almost there, leaving a thin web of material so the central disc could be broken/levered out while the job was still mounted, to preserve concentricity for further operations.

Andy

James,

Trepanning is fine provided you grind the correct front and side clearances on the tool because your are approaching a flat workpiece etc.

Yes use reduced speed but don't go overboard. I remember a set of about 20 apprentices (I was one of them) trepanning an 8" diameter hole in a 3/4" thick steel plate. Most of the ones that lost the tools were running too slowly.

You can usually tell when the cutting tool is not going to take it when you start the cutting. If it starts to rub something is usually not right with the tool setting.

PhilH