[Geof]

A few threads have had the query "will a spade drill work for drilling large holes in aluminum?". The answer is yes at least for a 1" spade bit going through 1/2" 6061-T6. The hole is shown in picture 1. The spade bit was completely standard not specially sharpened, a 1/4" pilot hole was drilled and then the spade bit run through dry at about 400 rpm taking around 3 minutes to finish the hole. In the process creating much vibration and noise. The bit wandered away from the intended location and the finished size was about 1-1/16" but this was not important because the goal was a 1.375" hole bored with only hand tools, a cheap drill press and a cheap grinder.

Picture 2 shows the start of a "line boring" setup on the drill press: A block of wood bolted to the table and drilled to take a 0.750" O.D. 0.500" I.D. 3/4" long brass bushing. (Bit of cheating here; this was turned on a lathe but identical bushings can be bought at a low cost.) The hole for the bushing was 3/4" deep with a 9/16" hole passing right through the wood and through the hole in the center of the drill table.

Picture 3 shows the complete line boring setup: a 1/2" diameter shank held in the chuck with a notch filed in the side at a 45 degree angle and a toolbit held in the notch with two 10-32 skt hd bolts passing through the shank and it into a small metal strap straddling the toolbit. This picture actually shows the finished hole which was reached in four cuts. The toolbit is adjusted by loosening the clamping screws and tapping it forward very gently, taking a small cut and checking the size.

Picture 5 shows the finished hole after deburring. The final size was just under 1.376" well within the range for using Loctite to secure a bearing.