What is the difference counter bor and counter sink?

[Rolf]

What is the difference counter bor and counter sink?

Best regards
Rolf

[smylee grouch]

I wonder if counter bored means the same diameter hole all the way down and counter sink holes have a tappered hole , wider at the surface and tappering smaller in diameter as you go down.

[smart dog]

Hi Rolf,
In your previous thread on your tang bolt problem, Acer posted an excellent drawing that clearly shows the difference between a counter sunk screw and a counter bored one.  His number 3 shows a counter bored screw and hole.

dave

[okieboy]

Both of these terms relate to enlarging one end of a hole. For our purposes, most of these holes are for a screw to slip through. A counterbore enlarges the hole as a cylinder. This is common to install socket head cap screws below the surface and some lock "washers" like those used on Hawken rifles have a very shallow counterbore.
 A countersink enlages the hole in a cone shaped manner. This is how flat head screws, like tang screws, are made to fit flush. When countersinking there are various angles to choose from, but the most common are 82 degrees included for imperial flat heads, and 90 dergees included for metric flat heads.
 If you just flatten the surface at the top of a hole to provide a flat square surface, it is called spot facing.

[D. Taylor Sapergia]

I wasn't aware of the difference or the terminology either Rolf, but Acer explained it well in the thread pertaining to your countersink problem.  I can already see the advantage of the counterbore.  The screw, rather than having an 82 1/2 degree taper to the head to perfectly match the countersink you must cut so perfectly, has a 'filister' style head or what I have heard referred to as a 'cheese head' as it resembles a round of cheese.  In any event, the head is a short cylinder that mates exactly into the counterbore, no matter how out of alignment the hole is with the tang, and then the head is simply dressed off with the tang.  I don't know how HC that is, but it certainly would be efficient.

[D. Taylor Sapergia]

Good description Okieboy...I was a little slow on the keyboard.

[Acer Saccharum]

Countersinks:
]

[Jim Kibler]

As to historical correctness, I suspect you will see it infrequently on American stuff.  I have seen it fairly often on European stuff.  I typically use the practice of counter boring for the tang screw.  I just use a multi flute 5/16" countersink and then run it in past the corner a touch to create parallel vertical sides to the hole.  If you then take an off the shelf tang screw turn it down on the sides to 5/16" it will fit nicely in the hole.  Simple and works well. 

[Gaeckle]

From McMaster-Carr..........



About Counterbores

Counterboring— Enlarge the top portion of an existing hole, forming a cylindrical-shaped opening with a flat bottom that acts as a seat for the head of a screw or wood plug.

Shallow Counterboring— Bore a shallow spot (1/8" or less) around a hole so that the head of the screw, bolt, or nut is slightly raised above the surface of the material (also known as spot facing).

Pilots— A pilot guides the counterbore as it penetrates the material, keeping it centered over the drilled screw hole and ensuring a level seat for the screw head. Counterbores with built-in pilots have a counterbore and pilot made from one piece of steel. Changeable-pilot counterbores have a removeable pilot, allowing you to use various pilot diameters in a single tool.



About Countersinks
Countersinks form a cone-shaped opening at the top of a hole, which acts as a seat for the head of a screw or rivet. They can also be used for chamferring, deburring, and creating openings for holding material between centers.
Flutes




Fast Cut — Have one flute and won't vibrate at high speeds. Also great for cutting smaller holes.
General Purpose — Have two, three, or four flutes. Tool life increases with more flutes because the cutting load is distributed over more edges. However, fewer flutes provide better chip clearance, which is a consideration when machining stringy materials like plastic.
Smooth Finish — Have six flutes. These remove more material per revolution and have a longer tool life than other countersinks.
Body Diameter and Countersink Angle

The countersink body diameter must be equal to or larger than the head diameter of the screw, center, or rivet being countersunk. Use a pilot hole that's larger than 10% of the countersink body diameter.
60° Countersink Angle: For holding workpieces between the centers on a lathe.
82° Countersink Angle: For flat- and oval-head inch screws.
90° Countersink Angle: For flat- and oval-head metric screws. They're also great for chamfering.
100° Countersink Angle: For rivets.
120° Countersink Angle: For rivets.

............or in a game of Battleship, it is sinking the other guys ship.....

[Habu]

I typically use the practice of counter boring for the tang screw.  I just use a multi flute 5/16" countersink and then run it in past the corner a touch to create parallel vertical sides to the hole.  If you then take an off the shelf tang screw turn it down on the sides to 5/16" it will fit nicely in the hole.  Simple and works well. 

I did that--accidentally--on my first pistol build.  It worked well, so I continued doing it on pistols. . . I have no idea why I never thought of doing it on rifles.

[T*O*F]

Countersinks come in a variety of sizes, angles, and flutes.  Pictured are 1, 3, 6, and 12 flute ones.  The last one is almost universal and goes up to 2.5 inches.  The 6 flute one is 5/16ths and can be used as a counterbore.


These are combinations drill and countersinks.  You can run them down and create counterbores.  If you take a taper headed screw and round the edge off, then you can have a screw that sits in a tapered seat and has the head counterbored.


These are true counterbore drill bits.  They are usually sized for common fillister head screws.  You might find these used on the bridle screws of your lock, and they are commonly used in knifemaking for Corby rivets.

[Frenchy]

Jim,
That is an awesome way to do it!

[Acer Saccharum]

Rolf, when I plan out for my tang screw, I also plan where the screw is going to end in the trigger plate. This plate is sheet steel with a boss brazed on at the appropriate angle for the tang screw to drill in to. This kind of construction allows the tang screw to hit the trigger plate at just about any angle you will encounter in gun building.

[James Wilson Everett]

Guys,

Here is an original 19th c counterbore from the John Fry shop, it cuts a flat bottom hole for the head of a screw, like a lockbolt screw.



Here is an original 18th to 19th c countersink, looks just like a big drill bit.  You can't drill a hole in metal so big with just a hand brace, unless you are extremely strong, so larger bits such as this were used as countersinks for tapered head screws, like a modern wood screw.



Jim

[smylee grouch]

Interesting, thanks for sharring James.

[RAT]

Jim mentions counterboring for lock screws (lock bolts... lock nails...). This was apparently common on European guns. It allows the bolt head to appear flat to the side plate without worrying if the bolt is angled to account for the outward taper of the panels.