I presume all this is for a low speed 'toy'. If not, then consider very seriously what will happen when something breaks - and it will !
I guess that if you had access to a lathe you wouldn't be asking the question, because all you would do would be to chuck a piece of 3/4 and drill down the middle !
If the 1/2 inch hole is a bearing surface (on a bolt, for instance), then I have had success with forcing a piece of plastic (nylon for instance) down inside the larger tube (eg 3/4 erw), and then drilling for a fit on the axle bolt. You should do heavy welding first, but actually you get can get away with a 'light' weld afterwards, although the bore will need a touch up. These plain bearings are much tougher than any kind of ball-race or commercial bearing, and cheap too. You need to oil them a bit, unless you use one of the self-lubricating grades of nylon.
Incidentally, beware of bolts as axles, and especially beware of welding them in - the heat makes the bolt far more likely to break. Similarly, make sure that there are no threads or steps in any part of the bolt that is subject to bending stress, or you will get fatigue cracking.