The latest issue of Machinist's Workshop (Vol. 35, No. 1, pg. 20) had an article that caught my interest. It suggested a solution to a problem I routinely face, aligning the drill with a punch mark. The solution is called a slip-on spud. It is essentially a point that slips over a spotting drill for spindle alignment.
The instructions in the article were quite detailed and focused on ensuring concentricity with the bore and the point. A 2" length of 3/8" steel hex was cut from a long bar. It was faced on both ends to a final length of 1.75" One end was chamfered and drilled. The hole was started with a spot drill and then drilled with a 15/64" drill to a depth of 1 1/8". It was drilled in 1/4" increments to ensure chip build-up did not push the drill off line. The hole was then reamed to 1/4". It was a nice slip fit on the spotting drill.
A mandrel was made to hold the spud. A 2" length of the same bar stock was reduced to 0.250" for 1 1/8". It was reduced in 1/4" increments to minimize flexing during cutting. This was going well until at some point the part came loose in the chuck! Alignment was not quite the same for the last 1/2". Miraculously the spud fit reasonably well over the mandrel. I tapped it with a hammer to make sure it would not come off.
The point was cut next. A quarter of an inch of the spud was reduced to 1/8" diameter. The headstock was rotated 10° and the point was cut in 0.005" increments. The headstock was returned to center and the next 0.25" was reduced to 0.125". The corner was chamfered.
One thing the article neglected to mention was how to remove the mandrel from the spud. I was unable to twist it off using a vise and a wrench. I also tried pulling with vise grips while twisting the spud back and forth. I was able to twist it about 1/4 turn total. A hammer and punch was also tried to separate the two parts to no avail. I decided to drill the mandrel out.
The part was returned to the chuck with the mandrel end sticking out. The mandrel end was faced and drilled with a 15/64" drill. I stopped short of full depth. The mandrel readily broke off at the attachment point. The drill was switched for the 1/4" reamer. After running the reamer in most of the waste readily came out. The reamer was not perfectly centered as judged by the waste. The last bit of the mandrel was carefully drilled and dropped out.
Remarkably, the spud was still a decent fit on the spotting drill. At least sufficient for my needs. The sides of the spud were sanded up to 600 grit to remove most of the dents left by the vise-grip pliers. The picture below shows the slip-on spud next to the spotting drill. The spud will be stored in a slip tube used for a 3/8" end mill.
