Precision Drilling And Tapping Techniques

One of the most common tasks a gunsmith performs is drilling holes and tapping threads into those holes to keep some item in place on a firearm. This job can be performed in a casual manner or, by using a few simple techniques, it can be done with a degree of precision that will ensure these tapped holes hold with the maximum degree of strength. I’ll go over some of the preparatory set-up methods and then cover the whys and wherefores of what makes a strong thread and holding system.

The Setup is Everything The one piece of machinery that gets the most use in my shop is the drill press. Whether I’m working on a firearm or making some little gizmo to help me get a job done more efficiently, the drill press is called upon for many of the tasks involved. The one I have has 12 speeds that turn the chuck from 134 to 5,484 revolutions per minute by means of two belts and a three-pulley system that takes only a few minutes to set to the desired spindle speed.

To make sure that the job turns out in the best possible way, the drill press table needs to be as perfectly perpendicular to the drill chuck as we can possibly get it. This means that the table must be level from front to back and then side to side. This can be done with a level but for the utmost in precision, a dial indicator affixed to a shaft and then mounted in the drill chuck will help you sweep the table surface and then tweak flatness within thousandths of an inch.

When the drill press is used to drill holes to a specific depth, a dial indicator should be used to record the cutting depth of the drill, again, within thousandths of an inch. A decent vise that will hold your work firmly without damaging any finish is a good investment that should be considered. The vise will need to hold your work level if a drilling jig is mounted to the part or so that top-dead-center of the part being drilled can be found with a dial indicator.

Consider using drill bits that have a 135-degree angled cutting end rather than the common 118-degree type. When the job calls for drilling a hole to a specific depth, the 135-degree flatter face is less pointed and will allow at least two more full turns of thread over the 118-degree-type drill when fine threads (40 to 56 TPI) are tapped into place.

Drilling and Spacing Holes If you have access to a milling machine, the dials that move the table back and forth and in and out will help get the cutting tool over topdead-center of the barrel or receiver you need to put holes in. If you have a milling machine you already know how to do that. The tough part when using a drill press is to get the drill chuck over top-dead-center and maintain it when drilling holes with a specific spacing. This is where a jig

or fixture of some sort will help, as the precise hole spacing is built into the jig to match the hole spacing in the base or mount. I’ve made up several styles of hole-spacing jigs. When the job calls for installing a center bead along the top of a vent rib on a shotgun, it can be difficult to keep a free-spinning drill from "walking" off location because of the machined matte surface on the rib.

A simple little locating jig that I made up has a hole in the center of a 1-1/8-inch-diameter by 5/8-inchthick round stock that was drilled in the lathe to accept a #45 bushing. Knurling was formed on the outside diameter of the 1-1/8 inch stock to provide a secure grip on this diameter when in use. This round jig was then set up in an indexing fixture on the mill so that a 3/16 hole could be drilled 3/8 inch from the center.

The round stock is then indexed 180 degrees and another 3/16 inch hole then drilled across from the first hole. A 3/16-inch by 3/4-inch-long hardened dowel pin is pressed into each 3/16 inch hole, leaving 1/8 inch of the dowel protruding out the bottom face of the jig. A #45 drill bushing is then pressed into the center hole until the drill bushing shoulder meets the top face of the round stock.

In use, the jig is held on top of the vent rib in the location where the center bead is to be installed, and then rotated until each 3/16-inch dowel pin finds each side of the rib. The bushing hole is then centered in place and the #45 hole for the #3-56 tap is drilled to depth. Hole-spacing jigs are available from several different manufacturers in various configurations to help space sight-base holes accurately. These jigs eliminate the tough job of finding top-dead-center with an indicator and edge finder.

I combined several of the ideas used on these drill jigs and added a few of my own to come up with one that would make the outcome accurate using just one setup. The hole-locating jig I made has 1/4-inch reamed holes spaced at .562, .500, 1.000, and .800 inch that accept bushings made from 0-1 oil-hardening steel. The bushings area slip fit into any of the six reamed holes in the jig.

The sequence involves first drilling a starter hole with a #1 center drill which has a 1/8-inch body diameter with a 3/64-inch-diameter cutting tip through a 1/8-inch bushing installed in the jig. This drill will create a spot for the "pre-ream" drill and then slightly chamfer the edge of the holes at the top surface. This chamfering is important when we get to the tapping phase. This bushing is then removed and replaced with a bushing that will accept the pre-ream drill which drills the hole to depth.

This bushing is then replaced with a bushing that will accept the finishing reamer that will bring the hole to the tap-drill size. During each drilling step, cutting oil is used to lube the cutting action. After the appropriate holes have been reamed to size, the barrel is taken out of the vise and put in a padded vise on the bench without removing the jig. Tapping Threads When we work with firearms that have been drilled and tapped at the factory, we are not usually dealing with the best threads possible.

A drill is a roughing tool, so if we desire a more precise hole for tapping, the hole will need to be reamed. A reamer is a finishing tool that will remove irregularities caused by the drill and cut a more perfectly round hole with a much smoother surface and a more precise diameter. Reamers are available in the same wire sizes as the appropriate tap-drill diameters. You will find that with these small-diameter reamers there will be a male center on the cutting end used during the manufacture of the reamer.

This little center should be dressed off with a fine-grit sanding belt, taking care to avoid removing the temper of the reamer. Because these reamers will be cutting bottom holes, the cutting lead will only need to be around .030-inch long. When using a reamer, we need to drill the holes we want to tap to a diameter around

ten-thousandths of an inch smaller than the recommended drill diameter. For example: A #6-48 thread calls for a recommended tap-drill of #31 (.120 inch diameter). Instead of using this drill, we would use a #36 drill (.110 inch diameter) to rough out the hole, and then finish the hole to size with a #31-wire-size reamer. The most common size threads we use on firearms are #3-56, #6-48, and #8-40, along with the occasional #6-40 thread used by some sight-base manufacturers.

All of these taps are rather small in diameter and will be found in either a twoor three-flute configuration, meaning they have either two or three cutting edges. Cutting leads can be obtained in the plug style that has a 5-7 thread cutting end used to tap through holes, or the bottoming tap with a 1-1/2 thread-cutting lead used to cut threads very close to the bottom of a blind hole. I find it best to have two of each style on hand for each of the threads I use most often.

The #6-40 thread, being less common, doesn’t come across my bench all that often, so I only keep one bottom-cutting style and one plug style in stock. Buy the best taps you can afford. Taps made of high-speed tool steel are best and will out-last carbon steel taps better than four to one, without costing that much more. When they get dull, either have them sharpened by someone offering that service or toss them out. Trouble comes quickly when a dull tap is used in an attempt to cut threads.

Most common high-speed-steel taps are class GH-3 in the extra fine thread pitch we most often use. The "G" means that these taps have ground threads. The "H" means that this is a high-size tap, and the "3" represents the amount that the tap is oversize; in this case, around .001 inch. Some of the scopes we see these days can be quite large and can be a bit heavier than a normal hunting-type scope.

When these scopes are mounted to large caliber, Magnum rifles, usually .33 caliber on up, #6-48 screws may not be the best choice. During recoil, inertia always travels in a repetitive, linear direction. The stress is concentrated at the minor diameter of the screw, where the bottom surfaces of the scope bases meet the top face of the receiver.

Drilling, reaming, and then tapping the provided #6-48 holes to #8-40 will give added shear strength to the mounting system because of the larger minor diameter of the #8-40 screw. You will gain around 15 percent more shear strength per screw with the #8-40 screw diameter. Of course, the bases will then need to be countersunk to accept the larger mounting screws.

For open, screw-attached iron sights, there’s no reason why the #6- 48 screws wouldn’t be strong enough to overcome any inertia working on these lighter-weight sights. Fastener Choice Mounting screws are another one of those things we have little control over. You get what you get and hope for the best. There are several fastener styles in use on bases, both for scope mounts and iron sights. Most often seen are those with a 90-degree shoulder and those with an 82-degree shoulder.

Depending on which style you prefer, you will need to have an assortment of driver bits to install these screws. For holding bases of any sort in place, I prefer the slotted, 82-degree-angle head type. The 82-degree angled head has around 30-percent more bearing surface that bears against a mating surface of the same angle and will produce a bit more holding strength than the 90-degree-head fastener. My preference for the slottedhead-type screw comes only from my (continued on page 21)

experience and what I’ve observed with the different styles of fastener. The slotted-head screw can be tightened with reasonable care using a properly sized bit, and along with a dab of thread locker, will give consistent service. A close second would be the Torx-type socket screws. The bits for this type of head fit the sockets they were intended for very well, and there is less chance for cam-out. With the hex-socket-type fasteners, I’ve had several failures, with the head twisting off during installation.

This was no doubt caused by the hex socket in the screw being broached to maximum depth, leaving a thin wall at the bottom outside diameter of the head. The trouble is, how do we know if a socket has been cut too deep and a torque-overload condition has occurred during the tightening process, which could cause failure somewhere down the road? Conclusions Drilling and tapping holes should not be done in a hurry.

With a couple of extra steps, the holes you drill and tap will be of a better class than are now performed by most production processes. By setting up your equipment to be as accurate and sturdy as humanly possible, and then drilling and tapping through bushings perpendicular to the bore line, you can produce tapped holes as good as those done by any custom metalsmith around. ■