How can get value from drilling operations?

In this blog post, you’ll read:When it comes to twist drills, selecting the right one can be preponderant. From cheap no-name drills to high-end special helix, solid carbide through-tool drills, there’s an infinite number of choices you can take. We're focusing on getting value from your drilling operations. You may choose the most out of each of your drills, with the basic knowledge you need to select the right drill and some tips.

When it comes to twist drills, selecting the right one can be preponderant. From cheap no-name drills to high-end special helix, solid carbide through-tool drills, there’s an infinite number of choices you can take. We’re focusing on getting value from your drilling operations. You may choose the most out of each of your drills, with the basic knowledge you need to select the right drill and some tips.

So let’s get started. Three basic things are used to distinguish one drill from another.

Choose the right twist drill

Material

High-speed steel

High-speed steel is the most basic and least expensive general purpose drill material. It’s very forgiving in drill press and hand drilling operations. Moreover, they can be resharpened to extend their life.

  • Basic and inexpensive
  • Fogiving in drill presses and hand drilling
  • Easy to re-sharpen

High-speed steel with cobalt

The next is high-speed steel with cobalt added, which is more durable than generic high-speed steel. Cobalt gives high-speed steel more heat and wear resistance. And these drills can still be easily resharpened which is similar to high-speed steel. Carbide is the most expensive but most durable drill material. There are different grades. The most expensive drills usually give the heat and chip resistance.

  • Holdd up better than high-speed steel
  • More heat and wear resistance
  • Also easy to re-sharpen

Carbide

Carbide also allows for coolant through-holes to be added to the drill. These through-tool drills are primarily for deeper holes and tough materials, with a high-pressure coolant flowing to the tool, which flushes chips out much better, keeps the cutting zone cooler and provides extra lubrication to prevent wear. While all of these drills can cut a hole in most materials, the carbide drills can outlive cobalt by a factor of ten or 20 times in a rigid CNC machine. In other words, if a cobalt drill can cut 100 holes, the carbide drill can cut 1000 or 2000 holes before it needs to be resharpened. Here in the factory, with a properly dialed in drilling op, we have examples. We’re getting 5000 plus holes in cast iron from just one drill.

All that said, a carbide drill can easily cost ten times more than a cobalt drill. So the investment is a lot higher. Despite the high price, the cost per hole when using carbide will usually be the lowest, since it can produce so many more holes. Also because carbide is typically capable of running three to five times faster, the decreased cycle time to produce those holes goes right to your bottom line.

  • Usually give the best heat and chip resistance
  • Allows for coolant through-holes (T.S.C.)
  • Carbide can cost 10* more
  • Lower cost per hole

Coating

BRIGHT FINISH

Bright finish is the cheapest option and fares well in certain applications.

For example, usually low carbon steel and aluminum can both be drilled with a bright finish tool without problems.

  • Cheapest option
  • Can be used on low-carbon steel and aluminum

BLACK OXIDE

Black oxide provides an advantage over bright finish in that it has a bit more lubricity, offers resistance to oxidation, and additional heat treatment that can offer 50% longer life while still keeping your tooling costs low.

  • More lubricity tahn bright finish
  • Resistance to oxidation
  • Heat treatment offering 50% longer life

TITANIUM NITRIDE(TIN)

Titanium nitride, abbreviated TiN, is the most common coating. It is a great entry-level coating for applications where lots of heat won’t be transferred to the tool while cutting harder or tougher materials. You can tell titanium nitride by its bright gold color.

  • Great entry-level coating
  • Not meant for hard materials and high heat transfer
  • Bright colored

TITANIUM CARBONITRIDE(TiCN)

Titanium carbo-nitride coating, abbreviated TiCN, is a step up from TiN. It provides a higher surface temperature, Slightly harder and better wearing than TiN. It’s typically blueish or purple in color.

  • Step-up from Tin
  • Higher performing than TiN
  • Slight harder, better wearing than Tin
  • “Blueish” or Purple in color

TITANIUM ALUMINUM NITRIDE(TiALN)

Finally, titanium aluminum nitride, abbreviated TiAlN, has a much higher surface temperature rating than TiN or TiCN. This gray-colored coating is excellent for your high-temperature materials, and is still a good choice for steels and stainless steels. But it isn’t a good choice for drilling aluminum, since it contains aluminum.

  • Higher performing than TiN and TiCN
  • Excellent for high-temperature materials
  • Great for steels and stainless steels
  • Not great for aluminum

Beyond these common coatings, many manufacturers have proprietary ones of their own that tout features like high lubricity and extremely high surface temperature range.

Here at Bohrt we use TiN coated drills mostly when we’re working mild steel, for the increased hardness and heat resistance equals to long life. We move up to TiCN coatings for drills used on cast iron where it shows good toughness and resistance to chipping. And when we’re cutting high-strength harder steels, we’ll step up to the high-end TiAlN coatings to handle the heat and high stresses where the coating helps reflect the heat back into the chips, away from the tool and the workpiece.

Generally, unless you’re cutting difficult materials, a good quality cobalt drill with a TiN or TiCN coating is a relatively inexpensive way to get higher productivity. Since pricing varies a lot, you can find drills with high-end coatings at decent prices. So, we’ve talked about materials and coatings.

Geometry

The third key ingredient to choosing the right drill is geometry, which plays an equally important role in drill performance. 

Probably the most obvious aspect of drill geometry is the drill’s length. 

Drills come in two common lengths. Screw machine length, which is commonly referred to as stub length, and jobber length. When it comes to drilling on a CNC, stub length drills are the most common choice because they are more rigid. There are, of course, all kinds of custom lengths available for special applications. As with any cutting tool, you want to use the shortest drill length possible because the shorter the drill, the more rigid it is. Just make sure you have enough flute length to get the chips out of the hole.

The second geometry question showed up: How much flute length do I need for this hole?

Ideally, you want two times the drill diameter in flute length above the hole when the drill is at the deepest point in the hole. This allows for chip evacuation. If the length is less than this, the chips can pack up inside the flutes and cause poor surface finish, hole size, and straightness issues. Or even worse, they can break the drill. But you also don’t want a long jobber length drill with flutes all the way up if you’re just drilling shallow holes. This drill won’t be that rigid and will yield a less precise hole position.

DRILL POINT ANGLE

The drill point angle is probably another familiar aspect of drill geometry for most people. When you’re drilling metal on a CNC machine you’re generally choosing between 118 degrees and a wider 135 to 140 degrees.

118 point- MILD STEELS, ALUMINUM, OTHER SOFT METALS

For general purpose, 118 degrees is most common that used on high-speed steel drills, which is made for cutting mild steel, aluminum, and other soft metals. And it’s what you’ll usually find on regular length drills that the jobber provided.

135 point- HARD STEELS, HARDER, TOUGHER MATERIALS

The 135 degrees is more typical for stub length drills used on CNC machining to cut harder and tougher materials.

HELIX ANGLE

  • 30 point- GENERAL PURPOSE/ MOST MATERIAL
  • 10 point- SPECIAL APPLICATIONS/ HARDER STEELS/ ALUMINUM ALLOYS
  • 40 point- DIFFICULT MATERIALS/ STAINLESS STEEL

Generally speaking, high-speed steel drills aren’t usually self-centering. Since it’s more time-consuming and expensive to grind them with this feature, they tend to walk or wobble when they try to cut into a flat surface. More expensive cobalt and carbide drills are ground with this self-centering point allowing them to start cutting very easily with high pressure. This virtual self-centering means there’s no need for a spot drilled hole. And it’s another way these expensive drills can be more productive than their economical brothers, since not spot drilling every hole saves lots of cycle time.

APPLICATIONS

As we mentioned, drill manufacturers could put holes through the drill so the coolant can get delivered right to the cutting edge down in the hole. This keeps the cutting zone cool, lubricated, and greatly aids in chip evacuation.

Typically, steel drills without through tool coolant can only drill about two or three their diameter deep before requiring me to peck-drill to remove the chips and get more coolant down in the cutting zone.

Good carbide drills without through tool coolant can drill up to five times diameter deep in carbon steels and aluminum before needing to peck drill.

The problem with peck drilling is that most tool wear occurs when the drill is entering the material. Once the drill is in the cut, wear rates become very low. Peck drilling significantly increases tool wear because you’re restarting the cut multiple times per hole. Not to mention all the extra time spent pecking each hole where the TSC drill would do it in just one pass.

So, it is really necessary for tools more than five times depth and particularly when drilling tough or work hardy materials, TSC and through tool drills. If you need to drill very deep holes, let’s say eight times diameter or greater, you’ll usually need a pilot hole to start the drill. Typically, this is done by using a stub drill to cut the hole about one and a half times diameter deep. Then start the long drill with a spin let 300-500 RPM, and slowly feed it into the pilot hole. Once the drill main diameter is in the pilot hole you can crank up the RPM to full speed and finish drilling to full depth.

When drilling holes that break through the workpiece, you need to pay special attention to the material and cutting conditions. Drill manufacturers recommend slowing the feed rate before the drill point breaks through the material to prevent chipping and reduce heat in the cut. We want to reduce the heat because as the drill gets to the very bottom before it breaks through, the material is very thin and there is no place for the heat to go. So that the last bit of material can be hardened. In other words, it is literally heat treating the material. Breaking through this heat-treated layer can shorten the life of the drill. A 50% reduction in feed rate for the final two millimeters or 0.01″ before the drill point reaches the bottom usually eliminates this issue.

So, when does a drill need to be sharpened? Generally speaking, as long as your holes are intolerant, if wear and chipping are less than half a millimeter or 0.02″ it’s okay to continue using the drill. After that, it’s typically time to resharpen or regrind. Pay close attention to chipping on the drill margins. If the wear is even, it’s fine to regrind. But if it looks like this, the drill is no longer useful.

Now, you often buy tools and deciding what you should spend at a specific job. Is it a short one single lot? Or is it a large recurring job with thousands of parts?

Carbide might not be the best investment if you’ve got a short run and you can’t spend extra time dialing in your cutting parameters. High-speed steel or cobalt might make sense in this case. Keep in mind that you can always start with less expensive drills to get the job launched. Then if you end up making lots of those same parts down the road, you can work you’re your tooling supplier to find the best tool for the job whether it’s carbide or a high end cobalt drill.

So, let’s do a lightning-fast recap.

Carbide is much more expensive than the others. It’s also less forgiving if used incorrectly. High-speed steel and cobalt are easy to resharpen, but they are nowhere near the tool life of carbide. Typically, carbide can also run significantly faster.

When it comes to coatings, if your machining difficult materials or need max tool life for long part runs, then select the high-end coatings.

And for geometry, we’re just touching on some of the aspects but consider the material and your cycle time requirements when deciding which way to go with each of these elements.

If you have questions or comments about how drills have worked in your specific circumstances, let us know in the comments section. And don’t miss the opportunity to tap into the expertise of your local tooling rep. They’ve got the insider knowledge on using their tools best and will get you on the right track for your application.