SPY TECH
E
F
GH
Fig. E: Thread the shank of the bolt down 3 threads.
Fig. F: After threading, deburr and clean up the shank
with a file and sandpaper.
Fig. G: Drilling and tapping the bolt head is tricky,
because the head is shallow. Use the depth stop on the
drill press. Fig. H: Make a bottom drill from a twist drill.
3. THREAD THE SHANK.
Now the shank goes back to the bench vise for
threading. I chose a ½- 20 thread for this project.
When purchasing taps, dies, and twist drills, always
choose high-speed steel over carbon steel, and buy
the best you can afford.
Thread the shank of the bolt down 3 threads
(Figure E). Some dies have a taper on 1 end, and
some are tapered on both ends. If your die has
a single-sided taper, use that end first, and then flip
it around and clean up the threads with the non-tapered end. Deburr and clean up the shank with
a file and sandpaper, and it’s done (Figure F).
back of each cutting lip approximately 20° from flat
(Figures H and I). The idea is to produce a cutting
edge and to keep only the lips in contact with the
hole bottom.
After grinding the bottom drill and reinserting
it into the chuck, start the drill press on its lowest
speed, and don’t start the spindle until the bottom
drill is inside the hole. Use plenty of cutting oil, and
feed slowly. When finished, the hole will be at full
diameter throughout its entire length.
Without moving the bolt head or the drill press
vise, replace the drill in the chuck with a small
tapered center, which can be ground from a small
metal rod (Figure J). Its purpose is to allow the tap
4. DRILL AND TAP THE HEAD. to pivot on it, keeping it on center and square to the
Next, the head of the bolt requires drilling and bolt head.
tapping. Use a 29/32" drill bit for a ½- 20 tap. In this Using plenty of cutting oil, begin tapping the hole
step, it’s imperative to set the depth stop on the drill (Figure K). Hand taps come in 3 configurations: taper,
press; set it to leave around
5" of metal at the bot- plug, and bottom. A taper tap has several threads
tom of the hole. As the bolt’s head is already shallow tapered for a very gradual cut, and is used mainly for
to begin with, the twist drill will need to be modified through-holes. Plug taps have fewer threads tapered,
into a bottom drill after the initial drilling (Figure G). and bottom taps have a taper on only the first couple
To grind a bottom drill, start by grinding the end of threads. For the few threads needed here, a bot-of the drill flat and square. Continue by grinding the tom tap will suffice. As you advance the tap each
Photography by Brian Dereu (E, F, G, K) and Ed Troxell (H, I, J)
74 Make: Volume 16
