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Drilling and Tapping For Gunsmiths


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Drilling and Tapping in General and for Gunsmith’s in Particular​

Once in a while we find ourselves in need of securing to things together, and for some strange reason only the universe knows, Super Glue, JB Weld Epoxy, MIG/TIG/Stick/Oxy-Acetylene welding, explosion bonding or bailing wire and chewing gum just are not appropriate. Go figure.

One of the remaining options it to use a bolt, a machine screw, a nut and bolt, a bolt or machine screw and a threaded hole, etc. to get the job done.

Let’s start with drilling and tapping a hole, the operation a lot of gunsmiths are frequently asked to do to facilitate the installation of telescopic sights, iron sights, shell catchers, rat catchers, pig stabbers, etc.

First up, if you are a gunsmith and a customer asks you for a quote on a price to drill and tap a receiver or frame for a sight, or anything else, ALWAYS figure into your quote the cost of at least one each of the appropriately sized drill bits and the THREE taps you will need to complete the job.


Because if you are smarter than the average bear, on each gun that you drill and tap, you will use a brand-new drill bit of the appropriate size, and three brand new taps of the appropriate size as well.

Use them once, and put them aside for less risky jobs on things other than guns, you will incur a much lower rate of drill bit and or tap breakage, wandering holes, etc. if you do it this way. Trust me, in 65+ years of doing this, I learned this lesson early on and it has saved me a lot of time, money, frustration and worry.

Why 3 taps? Well, actually you might need 4, but you will probably have to make the 4th.

Taps come in three standard chamfers from most manufacturers.

They are the TAPER tap, with 7-10 of the first threads tapered, getting larger as they go back from the tip of the tap towards the shank.

This is the tap you use to start threading the hole, those tapered (reduced in diameter) first 7-10 threads actually do most of the cutting of the new internal thread, and having them tapered reduces the force necessary to start cutting the threads, thereby reducing tap breakage and the possibility of drunken threads because you didn’t start the tap straight because you didn’t use a tap fixture or a drill press or vertical mill.

Tapered taps are the only tap you need to use if you are tapping a hole that goes all the way through something, but if you are tapping a blind hole, one that does not go all the way through, then you need the next two taps at least.

Next is the PLUG tap, this one has 3-5 threads tapered at the tip, and it is used AFTER the taper tap to get the threads a bit closer to the bottom of a blind hole without excessive risk of breaking the tap, since the majority of the threads have been cut by the taper tap.

The last of the commonly found taps is the bottoming tap, this one has the first 1.0 to 2.0 threads tapered, and is used AFTER the plug tap to finish cutting the threads very close to the bottom of a blind hole. Not all the way to the bottom, but pretty darn close, close enough for most things on machinery, cars, and usually but not always, guns.

The 4th tap I mentioned that you may need, and will generally have to make for yourself, is the FLAT BOTTOMING tap, this can be made from any of the three taps I mentioned above, and I usually make them from a Taper tap that has been used on just one gun and set aside.

This one you snap the tapered section off (did I mention, taps are kind of brittle? Well, they are, whether made of carbon steel, High Speed Steel, M35 Cobalt (5% cobalt content) or M42 Cobalt (8% cobalt content) and then grind the broken end to a perfect 90 degrees perpendicular to the length of the tap, with ZERO tapered threads.

This one will get you to within one thread of the bottom of a flat-bottomed blind hole.

Some Tap manufactures offer a 4th type of tap, the Modified Bottoming with 2.5 to 3.0 threads tapered, and a 5th type of tap, a “Semi-Bottoming” tap that has 2 to 2.5 threads tapered at the start, but I’ve never seen the point to buying any of either of those.

That’s the types of taps as far as the thread profile / chamfer /taper goes.

There 3 basic STYLES of thread cutting taps in use today, all available in a multitude of metal alloys. You can get them in Carbide, Carbon Steel, High Speed Steel, M35 and M42 Cobalt, Powdered Metal, and with a wide variety of coatings, Titanium Nitride, Aluminum Titanium Nitride, Titanium Carbide, Diamond, Black Oxide, Steam Oxide, Hard Lube, Blue Oxide, and of course just left polished to a mirror bright finish.

They are the HAND TAP, designed to be driven by a tap wrench and turned by hand, although they can be mounted in a collet or chuck and driven in a Lathe, Mill, or Drill press if you are careful.

These have straight flutes, and the chips gather in the flutes, so you need to clear the flutes frequently to keep the tap from binding up and breaking in the hole. To do that use the following for hard steels.

Turn it in one turn, back it out half a turn, brush off the chips, turn it in a full turn, back it out a half, repeat until the hole is tapped, adding cutting oil frequently as well.

Then there are the SPIRAL POINT taps, often called a “Gun Tap” because when used in a thru hole, they shoot the chips out ahead of the tap.

These have a spiral flute cut into the nose that leads back at an angle to the straight flute, these push the chips ahead of the tap and are best used on through holes, and if using them in blind holes (in sequence, taper, plug, bottoming) the bottom of the hole will pack with chips and you need to fully withdraw the tap frequently, clean the flutes on it, clean out the chips from the hole, apply more cutting fluid and go back at it until the tapping is done.

Next is the SPIRAL FLUTE tap, sometimes called a “Draw Tap”, because they draw or pull the long, stringy chips out of the hole as they advance,

On this one the flutes spiral all the way back up the tap’s shank, like the flutes on a standard twist drill bit, and it pulls/ draws the chips up and out of the hole as it threads its way in.

They also can be had in the Taper, Plug and Bottoming configurations, but are best used for metals/ materials that produce stringy long chips, like aluminum, copper, brass, Delrin, Acetal, etc.

Last there is the THREAD FORMING tap, sometimes called a “Roll Tap”, this one does NOT cut threads, it forms them in the hole by DISPLACING metal, and therefore requires a larger hole than the other three types of taps which cut the threads in by removing metal.

Moving on to the other half of the process, the drilling and drill bits.

I have always favored the 135-degree split point style of drill bit for working on guns and metal in general, as it tends to start cutting sooner, and thus is less prone to walking.

And I use ones made of either M35 or M42 Cobalt, for their ability to stay sharp under heavy load in hard materials, and to resist losing their temper at temperatures of up to 1,000 degrees Fahrenheit, which if you are drilling in a really hard steel, the drill bit can come close to or even exceed, unless you are using copious amounts of cutting oil to keep it cool, and you should be!

Of course, you should always prick punch, then center punch the point to be drilled FIRST, this helps prevent the drill from wandering / walking, and the first drill used to start a hole should be a SPOTTING drill, for use with the 135-degree drill bits a 120-degree spotting drill of the appropriate diameter has always worked best for me.

A spotting drill is one that is very, very short, and stiff, and capable of drilling a hole to a depth of only 1 to 2 times the diameter of the drill.

Drill bits come in a variety of lengths, with the ones most useful in the gun shop being the Screw Machine length (sometimes called the Mechanics length) which is shorter and stiffer than the common Jobbers length found in most drill bit sets, but not as short or stiff as the Spotter drill bits.

Then we have the Aircraft length drill bits, rarely ever needed or used by a Gunsmith. These range in length from 6 inches up to 2 feet in all sizes.

For drill bits, taps and dies (dies make external threads, taps make internal threads) good brands to go with are Chicago-Latrobe, Greenlee, Greenfield, Celine, RNB, Pacific Twist Drill, and Cleveland.

Tapping a Hole​

So, you need to tap a hole for a particular sized screw or bolt, you know the screw size, but what size drill bit do you use?

Well, you can look up the proper sized drill bit on a tap and drill chart, those will give you the drill size for getting somewhere between 70% and 75% thread engagement, which is more than enough for nearly anything you do.

But what if you don’t have that drill bit on hand, or you don’t need that much thread engagement?

For many things gun and car related, 55% and up thread engagement will more than suffice as long as a human load will not be applied to it.

And with the lesser thread engagement (which requires a larger hole) you stress the tap less, and they tend to last a bit longer.

So how do you figure out what drill you have on hand that will work? Or if the thread you need to tap isn’t listed on the common tap and drill charts?

Easy, use the following formulas.

To find the Tap Drill (TD) diameter, you need to know the Major Diameter (MD) of the screw or bolt, the Threads Per Inch (TPI) of it, and the Percentage of thread engagement you desire.

Then plug that into this formula on your calculator, just as it is shown here, parenthesis and all:

TD = MD – ((.01299 x % desired) / TPI) and hit = button.

That reads Tap Drill Diameter Equals the Major Diameter minus the product of .01299 multiplied by the percentage of thread engagement required, then that result divided by the threads per inch.

Example, for the common 6-48 thread screw found on many scope mounts, the major diameter is .138”, ( all #6 screws, no matter what the TPI, have a major diameter of .138” by the way), and it has 48 Threads Per Inch and we will say we want 75% thread engagement.

So, the TD would be equal to .138” - ((.01299 x 75) / 48)

TD= .117”, there is no .117” drill bit in any of the common drill bit sets which are the Fractional set, which runs from 1/16” to ½” in increments of 1/64”) Alpha bit sets that run from A-Z, with the A being the smallest drill in the set, Z the largest, and Numbered Bit sets, which run from #1 through #60 (some sets go through #80) with #1 the largest and the drills get smaller as the number gets larger.

A 3.0 MM drill is .118”, that’s pretty close, and a 32 is .116”, the #32 would produce more than 75% thread engagement, and might break the tap or the screw, the 3mm would be fine, at about 73% engagement.

Most tap charts that show the 6-48 thread call out a #31 drill bit, which is .120” diameter, and will yield 66.5% engagement, more than enough, and far less likely to break this small, (and expensive!) Tap.

As another example, let’s take the common ¼-20 bolt.

TD=.250” -((.01299 x 75)/20)

TD= .201”, which just happens to be exactly the diameter of a #7 drill bit.

But what if we don’t have the needed bit(s) but we do have some close in size, and we need to know how much thread engagement they will provide?

Simple, the formula for that is:

Percentage of Thread Engagement (PTE)

PTE= TPI X ((MD-TD)/.01299)

So, let’s say we wanted to tap a hole for a 8-40 screw, which is another popular size for scope mounts on hard recoiling guns these days.

The tap and drill chart says use a #28 bit, which is .1405 inches in diameter. But we broke our last one yesterday, and Fed Ex hasn’t delivered the next dozen yet.

A number 8 screw, no matter what the TPI, has a major diameter of .164”.

So, plugging that in, we get

PTE = 40 x ((.164-.1405)/.01299)

PTE = 72% for the #28 drill we don’t have.

But we do have a 9/64 bit in our fractional set, that’s .1406” diameter, will that work?

PTE= 40 x ((.164-.1406)/.01299)

PTE = 72% bingo, we can do the job.

So lets actually drill and tap a receiver, the right way, so that nothing gets broken (drills, taps, screws) , the holes are straight and in the right spots.

We’ll go with the 8-40 screws because this .900 Megalasaurus Rex Bolt Action only weighs 3 pounds, fires a 18-ounce slug, and recoils like a bull elephant getting kicked in the ‘nads. And we want at least 70% thread engagement.

Fixture the receiver on the vertical mill (or drill press) table solidly so that it can’t move AT ALL. Get the top of the receiver level (a machinist’s level is great for this), now position the scope mount bases where they need to go, and clamp them in place so they don’t shift.

Using the proper transfer punch (a type of punch made in diameters increasing by .001 inch, with a sharp point on one end) in this case it would be a transfer punch of .164” diameter, which will just slide through the screw holes on the bases, we put punch marks on the receiver where the holes need to be drilled and tapped.

Remove the bases, and using a prick punch first, deepen the mark left by the transfer punch, then using a 60-degree center punch, make it wider and deeper still.

Now put in a #29, yes, I said #29, not #28, spotting drill, and drill down just enough to create the start of the hole, basically just the cone shape at the tip of the drill.

Replace the spotter drill with your #29 (yes, still #29!) 135-degree Cobalt Screw Machine length drill and drill the hole to the depth required.

Wait, what’s the required depth?

Well, for sufficient thread engagement length (not to be confused with Percentage of Thread Engagement), now we are talking about how far down into the hole the threads extend towards the holes bottom.

PTE is how much of the space in the V shaped threads is filled by the peaks and valleys (roots and crests) of the hole when a screw or bolt is screwed in, thread engagement length is, as I said, how far down into the hole we cut the threads, but I digress.

For sufficient length of engagement, the threads should extend into the hole by a dept that is 1.5 to 2 times the diameter of the screw or bolt, at a minimum. At least in hard steels, in soft metals 3 to 4 times the screw or bolt diameter is more acceptable.

So, for our 8-40 screw, with a diameter of .164” we need a hole that is at least .246” to .328” deep.

You may encounter a receiver where it is going to be tough to get the required depth, due to risk of penetrating into space (the barrel, the chamber, both baaad juju!) that you don’t want breached.

So, calculate the depth carefully, and using that #29 drill, drill down to that point.

That depth MUST include the distance from the tip of the drill back up the shank, we don’t want the pointy tip / cone busting through.

The depth of the hole where it is full diameter / round (well, mostly round, drills make holes, reamers make holes round, but we will get to that in a moment!) will be less than the depth you so carefully drilled down to, because the bottom of the hole will be a cone shape, matching the tip of the drill, and we have to account for that distance in our dept calculations.

The tip of a drill extends beyond the shoulders/ lips of the drill (the point where the drill is actually making nearly round hole) by an amount that is easily calculated.

For a 135-degree drill bit, (that’s 135-degrees at the point ) which would have an included angle of 67.5- degrees at the tip (the included angle is half the total angle of the tip) , and thus 22.5- degree angles (all triangles have 180 total degrees in them, subtracting the 135- degrees from 180 leaves 45- degrees, there are two remaining angles in the triangle here, and they are equal, so each is 22.5- degrees ) where the tip becomes the shoulder (the shoulders are the true diameter sides of the drill bit, also known as the Lips) , so we can take the Tangent of 67.5- degrees, and it will be equal to the opposite side of the triangle (formed by the tip, and the two shoulders) and multiply it by side of the triangle opposite the included angle to get the height of our right triangle.

Since the drill bit is .136 inches in diameter, half of that is .068”, (we divided the point angle of 135 degrees in half to get a right triangle to solve how far the tip extends beyond the lips/shoulder) we multiply the tangent of 67.5 degrees, which is 2.414 by .068 and we get .1641, which is how far the tip extends beyond the lips.

So, if we need a hole that is going to provide for threads extending into the hole by 1.5 to 2x the diameter of our 8-40 screw, we need to drill down a total depth of either 1.5 x .164 =.246” or 2x .164 = .328” PLUS the additional .1641 inches so that the round part of the hole ends up being either .246” deep or .328” deep, whichever you have chosen to go for.

There for the total dept of the hole at the center where the tip is would be .1641+.246 = .4101” or .1641 + .328 =.4921 inches, just shy of half an inch!

If you were using a digital read out (DRO) on your mill or drill press spindle to measure the depth of drilling, you would start with the tip of the drill just barely touching the surface of the part to be drilled, set the DRO to ZERO, and drill down until you reached either .4101” or .4921” in total depth, which would mean the round part where your bottoming tap will stop would be at the required .246” or .328” depth, but the total depth of the hole to the center would be .4101” or .4921”.

But what to do if you don’t have that much metal under you?

Simple, DO NOT DRILL DOWN to the TOTAL drill down dept, which in the above example would be either .4101” or .4921”. instead drill down only to the .246” or .328” depth, and then make the hole round all the way to the bottom of that depth.

The full diameter / round depth when you stop drilling would be either .246”-.1641” =.0819” or .328”-.1641” =.1639”, neither of which is deep enough to give you even 1.5 times the diameter of the screw as thread length.

Now take another #29 drill bit, and grind the tip to a flat 90 degrees, and grind in the cutting faces/edges so that you now have a 90-degree drill bit, that will drill a flat-bottomed hole.

Chuck it up and drill down in your hole, converting that last .1641” depth that is conical in shape to full diameter.

Now you can chuck up a straight flute Chucking Reamer of a .1405 (#28) diameter, they can be had in increments of .0005” increase in diameter) and REAM that hole perfectly round and to the depth of either .246” or .328”.

Now chuck up a #25 drill bit, and just lightly touch it down to the mouth of the hole you just reamed, to remove the ring burr that was raised there by the drilling and reaming, and this will also slightly chamfer the mouth of the hole so the tap will start easily.

Getting rid of that ring is important, it could prevent the base from seating squarely.

Now chuck up your 8-40 Taper tap, lube it well with plenty of cutting oil, and start it into the hole.

Turn it in one full turn, back it out half a turn, repeat, withdraw it completely, blow the hole clear of chips (EYE PROTECTTION MUST BE WORN DURING ALL THE ABOVE OPERATIONS!) clean the tap’s flutes, re oil, and repeat until you feel the tap bottom gently against the flat bottom of the hole.

With draw the tap, clean the hole, switch to the Plug tap, repeat the above, then again with the bottoming tap, and if you need all the thread length you can get, with the shop made Flat bottoming tap.

Clean the hole out one last time, and move on to the next hole location. Check the receiver in the fixture frequently to make sure it is still tightly held in place, no wobbles or vibrations.

Do the complete operation, drilling, reaming, taping on one location at a time, this guarantees as much as possible a straight hole, straight threads, no broken drills or taps.

Now you see why gunsmiths charge as much as they do for what some folks think are simple operations, it’s the set up time, planning, and execution, if done right, it takes time!

And remember, why is there always time to do it over, but never time to do it right the first time?

At a later date we can cover how to get around receivers that are super hard, or case hardened, without breaking drills or taps.

And if you are not asleep after reading all the above, I am sorry to have to tell you, you are suffering from TERMINAL INSOMNIA!


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To add….

An old, somewhat wise man once said “Universal torque spec for any size bolt or screw. Tighten it one half turn past where the threads strip out, then back it off a quarter turn.”
You quoted me correctly !