Gunsmithing basics: Tapping a blind hole
If you work on guns, occasionally you’ll need to tap a blind hole (a hole that doesn’t go through the material you are working with). Tapping blind holes can be challenging, especially when they are shallow and require a small thread; both of which are typically the case with firearms. In this post, let’s take a look at how to tap a blind hole.
In the beginning of this post I’ll be drilling the holes on a CNC Mill, however there are many other simpler ways to drill the holes- if you don’t have access to a milling machine, don’t worry and keep reading, it gets pilgrim pretty quickly.
In this post I’m using the following tools I ordered from Brownells:
- Taprite tap guide
- #31 drill
- 6-48 high-speed steel plug tap
- 6-48 high-speed steel bottoming tap
- Tap handle
- Forster Universal Sight Mounting Fixture
Before we begin, let’s review the following disclaimer:
The contents of Rifleshooter.com are produced for informational purposes only and should be performed by competent gunsmiths only. Rifleshooter.com and its authors, do not assume any responsibility, directly or indirectly for the safety of the readers attempting to follow any instructions or perform any of the tasks shown, or the use or misuse of any information contained herein, on this website.
Any modifications made to a firearm should be made by a licensed gunsmith. Failure to do so may void warranties and result in an unsafe firearm and may cause injury or death.
Modifications to a firearm may result in personal injury or death, cause the firearm to not function properly, or malfunction, and cause the firearm to become unsafe.
I’m helping my friend Kevin build a clone of an old-school USMC Winchester 70. These guns use an Unertl scope with a mounting system that requires bases to be installed on the barrel. The bases are secured on the barrel with 6-48 screws. The screws and holes need to be shallow enough so that they don’t penetrate the bore, yet long enough to get a few threads into the metal to attach the scope.
In order to tap 6-48 screws I need a #31 (.120″) diameter drill, a spotting drill, hand taps and wax or some tapping paste. All of these items are readily available from Brownells.
The first thing I needed to do was to drill the holes for the bases. Since the bases sit along the surface of the barrel, and the barrel is tapered, I needed to drill the holes perpendicular to the taper, not the center of the bore. This requires some sort of fixture. In this case, I simply held the Winchester barreled action in the mill with a Forster Universal Sight Fixture. This fixture allows the use of a portable drill to install sights, but I’m just using it to level the barrel. Other methods could be used to hold the barrel in this position.
You’ll note in the image above a dial indicator is repeatedly run along the top taper of the rifle barrel until the dial no longer moves, this indicates the top of the barrel taper is parallel to the x-axis of the mill.
I then centered the mill over the diameter of the barrel and drilled the holes. To drill a hole on a curved surface you first need to spot the hole, this can be done with either a spotting drill or a center drill. The spotting process prevents the drill bit from walking when the hole is formed.
Deciding how deep to drill is an additional consideration. As a rule, it is bad practice and poor form to drill holes through barrels- especially rifle barrels. When I was planning out my holes I measured how thick the barrel was above the bore by the muzzle. This gave me an idea of how deep I could go. In this case the barrel had about .240″ above the bore. I decided to drill my holes .170″ deep. Since I used a 118 split point bit, that meant the full diameter of the #31 drill bit (.120″ diameter) could only be .134″ deep (.3x.120″=.036″, the point at the leading edge of the hole isn’t the full diameter).
Hand taps come in a couple of different types. A tapered tap (above, center) has a gradual taper with 8-10 tapered (or undersized) threads on the end. A plug tap (above, right) has 3-5 tapered threads, and a bottoming tap (above, left) has 1-2 tapered threads. Tapered taps are the easiest to start holes with and tend to feed straighter than plug and bottom taps since they cut the material at a more gradual pace; however, they are problematic in shallow holes. Each thread on a 6-48 thread is .0208″ apart (1″/48=0.0208″), so if you have 10 tapered threads, the last 0.208″ of your tap isn’t forming complete threads (10*.0208″=0.208″). That means the tap won’t cut in our .170″ deep hole, it is too long. We need a plug tap (you can’t start threading holes with a bottoming tap). I actually didn’t have a plug tap at the shop, so I simply ground the end off a tapered tap to make one (above, right).
When you buy taps, don’t be cheap. Buy good ones. Avoid carbon steel and buy quality high-speed steel taps. Gun parts cost a lot of money, trying to save a few bucks on a cheap tap can backfire big time if it breaks!
Now that we have a hole drilled and a plug tap to start cutting the threads, we need a way to guide it so that it remains straight. You can use alignment tools with a milling machine or drill press, or you can use a small guide like this Taprite guide from Brownells (above). It consists of a guide block with different sizes of guide bushings that can be changed depending on what size tap you use. In this case, I’ll be using the bushing for a #6 tap.
When cutting the threads, we need a way to remove the chips from the blind hole. I’m using a tapping paste that is packed into the holes. As the tap cuts, the paste will be displaced by the tap and forced out of the hole. It will take the chips with it and prevent the tap from breaking.
I secure the tap in a tap handle and use the guide to start cutting the threads in the first hole. This is a slow process and nothing is forced. You don’t want to break a tap off in a hole on a stainless steel barrel. You can break a tap either by turning it with too much force, or by bottoming it out and continuing to turn it. Remember, a broken tap causes massive problems. I turn the tap 1/2 a revolution, back it off to break the chip, then start turning it again. Once I feel it hit the bottom I back it out.
With the threads started, I head back with the plug tap. I repacked the holes with tapping paste. Since the threads are started, I didn’t use a guide for the second pass. This second pass will form threads deeper into the barrel. In the image above you can see the displaced tapping paste squeezing out through the flutes of the tap.
With the holes tapped I can test fit the base. Perfect!
If one of your projects requires you to drill and tap a blind hole don’t panic; just take your time, buy a quality tap and pay attention to what you are doing and you’ll be fine!