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Drill Bit Selection
Drilling and tapping horseshoes is an extremely specialized task. There's a huge variety of uses for drill bits and they come in a mind boggling variety of styles and sizes, but most farriers buy their drill bits off the shelf at Home Depot. These generic bits compromise their longevity and performance in metals to also be more versatile in woods and plastics. Drill bits are available in a variety of materials, lengths, configurations, diameters, helix angles, and tip designs.


Key Points:
  • Cobalt drill bits last longer than HSS but carbide bits last much longer than both but are more expensive
  • You should always use the shortest bit necessary to accomplish the job. Usually this means using screw machine length bits when drilling shoes
  • Chamfering or countersinking your holes is critical and this is best achieved with a step bit but you can use a second chamfer bit or collar as well.
  • 5/16” is the standard drill size for 3/8-16 but O diameter or 8 mm bits are stronger and make a hole that's easier to tap
  • low helix angle drill bits are stronger
  • split point or modified split points perform significantly better than regular points and hold an edge longer.



Materials:


Most people are familiar with “high speed steel” (HSS) drill bits and “cobalt” (HSS-Co) drill bits. They are commonly available and perform reasonably well in a variety of conditions without being too expensive. “HSS” doesn't actually refer to a specific type of steel, but to a general family of steels that have added tungsten, chromium, and molybdenum. They are so called because in the 1940s, toolmakers discovered that these types of steels are more heat resistant than normal high carbon steels, so they could be used at higher speeds. Modern HSS drill bits and taps are made of M2 tool steel. M42 tool steel is what is referred to as “cobalt” HSS. M42 is more heat and wear resistant than M2. In our testing, M42 bits outlasted M2 bits almost 2 to 1


In most industrial applications though, M2 and M42 have fallen out of favor in favor of carbide tooling. Carbide tooling is made from a metal matrix composite of tungsten carbide powder, cemented together with cobalt. Like HSS, the term carbide refers to a variety of actual alloys and not all carbide is of the same quality. High quality, modern carbide though is significantly harder, stiffer, and more wear and heat resistant than even M42. The downside to carbide is that since its harder material, its less impact resistant so its not suitable for use in hand drills or other unstable applications. Its also more expensive but in our testing lasted 9 times as long as HSS.

Lengths:

The drill bits generally sold in home improvement stores are jobber length drill bits. The standard effective length of a 5/16” jobber length drill bit is 3 3/16” which is a bit overkill when drilling shoes that are mostly 1/2” thick or less. A more appropriate sized drill bit is what's called a screw machine length drill bit which is less than half the length.


The issues with excessively long drill bits are cost, strength, and durability. You're paying for drill bit you'll never use. With that extra length between the spindle and the shoe, there's a lot more material to flex under the load making the bit more likely to walk, catch, bend or break. Every drill press spindle has some degree of run-out, meaning that it doesn't spin perfectly true, there's always some degree of wobble. The longer the bit, the more that wobble is magnified. The bit wobbling back and forth is very hard on the bit and the cutting edges, drastically shortening its life. In some cases its bad enough to cause chatter, which is when the bit bangs around in the hole. This can seriously damage a bit, very fast. The shortest bit that is long enough to accomplish the job is always the ideal choice.

Screw machine length (Left) and Jobber length (Right)

Configuration:

After drilling but before tapping, the hole should always be chamfered. Chamfering is cutting and angle in the top edge so that it creates a funnel shape. The chamfer should always finish slightly larger than the outside diameter of the threads it will be tapped to accept so in horseshoes tapped for 3/8-16 we want a chamfer slightly over 3/8”. Excessive chamfer in a shoe, especially thinner section shoes, will reduce the number of threads available and therefore the strength of the connection.

The terms chamfer and countersink are often mixed up or considered to be the same but they have different functions. A chamfer is a 45 degree treatment of an edge (90 degrees when measured across the whole which has 2 edges) whereas a countersink is an 82 degree treatment of a hole (41 degree of each edge) designed to recess screws with a tapered 

Chamfers have 3 purposes:
1.They make it easier to get the tap started
2. They make it easier to thread in studs
3. As the shoe wears, if the hole isn't chamfered, the ground surface edge can get folded over and block the hole


There's 3 ways to chamfer a hole after drilling, you can use separate chamfer bits and drill bits, add a chamfer collar to a drill bit, or use a step bit. Some farriers use a larger drill bit to countersink but the angle is too wide to create an effective chamfer.

Using separate chamfer or countersink bit is cost effective when purchasing and results in a good quality chamfer but it is slow because you have to change out bits in your drill press and go over each shoe twice. The cheap multi flute countersink bits are designed for soft materials and do not work well in metals. With so many cutting edges they are rarely well aligned and take inconsistent, shallow bites of material which causes them to jump around and create a very rough chamfer.

Adding a chamfer or countersink collar to a standard drill bit eliminates the need to switch bits but the only chamfer collar intended for use in metal is over $30 and only has a single cutting edge. This single cutting edge creates an off center cutting force that causes chatter which can damage your drill bit and the chamfer bit. They can't be used with carbide drill bits for this reason. These collars also require a longer drill bit in a lot of cases. There are countersink collars available that are less expensive and have 4 cutting edges but they are designed for wood and don't hold up in metal. Care must be taken in using any of these collars as they block the flutes of the drill bit. Chips being cut and pulled out of the hole by the bit become trapped by the collar and can damage both bits. To prevent this, its recommended to you have at least 5/8” clearance between the collar and the shoe when the bit breaks through the back side of the shoe.


The ideal solution is a step bit. They have a thicker, stronger shank, and continuous flutes for better chip clearing. The downside is that they can be expensive with step drills marketed to farriers costing nearly $70 for plain HSS versions.

Drill bit and countersink bit combo (left) Drill bit with countersink collar (Center) and Step bit (Right)

Diameters:

The diameter of the drill bit you use to tap for 3/8-16 seems fairly straight forward but in truth, a variety of diameters are used. A 5/16” (.3125) drill bit is commonly listed as the correct size to use but in truth the ANSI specification for 3/8-16 internal threads allow for a maximum hole size of .321”. 5/16”
is commonly used in the US as its easily available but overseas, 8 mm (.31496”) is more easily available and more commonly used. This slightly larger size is stronger and makes a hole that's easier to tap with minimal compromise to the strength of the threads. An O diameter drill bit is an ideal choice (.316”) as its the largest bit available that still under the .321” maximum size giving maximum bit strength and easiest tapping.
Helix angle

When drilling deep holes, a higher helix angle is necessary to pull the chips out but higher helix drill bits have narrower cores and are significantly weaker. Since we aren't drilling deep holes, a low helix angle is recommended.

Low Helix Angle Screw Machine length bit (Left) and High Helix Angle Jobber Length (Right)

Tip Design and Angle:

The angle of the tip of the drill bit is determined by material your drilling into. Softer materials use a smaller angle (90 degrees in woods and plastics) and harder materials use larger angle (135 degrees for aluminum and steel). General purpose drill bits are a compromise 118 degrees. Drill bits that are a smaller angle than ideal for the material being drilled are prone to wander, take more power to turn, more likely to chatter, and can result in over sized holes.

118 degree (Left) 135 (Right)

Drill bit points come in regular, split point and modified split point (also called 4 facet point). Regular drill bits have a small area in the center that has no cutting edge and is effectively stationary and useless when drilling. This dead zone makes them prone to wandering and dulling quickly, especially in hard materials. This is why drilling pilot holes is traditionally recommended. Split point tips address this problem by moving the cutting edges forward, “splitting” the point so they create overlapping cuts. The modified split point is like a split point but with a chisel cutting edge and a steeper relief angle ground in behind the cutting edge. This reduces cutting forces, improves chip clearing, and holds a better edge than regular split point. These 2 designs can be used without a pilot hole and are less prone to wandering when starting a hole.

Regular point (Left) Split point (Center) Modified Split Point (Right)

With a lack of good options on the market for farriers, we have designed a custom ground carbide high performance step drill. It features a 135 degree modified split point, low helix, stub length for optimal strength. Its made in the USA from premium carbide and features an advanced nano coating. Priced comparably to basic, uncoated HSS step drills commonly offered to farriers, it offers a large cost savings to farriers as one bit will outlast many HSS bits. It also eliminates the need for cutting oil and the associated cost and mess. 

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