Say you are standing at the hardware store and have found the perfect wheel.........
Then you take that perfect wheel over to the bar stock (if you don't know ask someone) and find the one that just fits through the wheel..........
Buy some washers, at least 4 that also just fit over the bar stock, and you will need 4 roll pins at least an inch long.
Put the axle on your cart. Drill a hole, put in a roll pin, place washer, slide on wheel, place washer, drill hole, put in roll pin.....Do the same for the other side....
Your axle will never bend, break and the wheels will never fall off but can be removed by driving out the outer roll pins......
But that's just one way to do it. I shy away from the easy "all thread" stock as it's inherently weaker than regular bar stock.