Spot drilling and center drilling aren’t the same, and they can’t always be used interchangeably. This article investigates why this is so and the appropriate application for each.
Difference Between Center Drill and Spot Drill
Spot and center drilling are often confused, and some use these methods interchangeably. These are not the same procedures, however, and using the wrong tool for the job can cause damage to your tools. This decreases their useful lifespan or can even snap off a drill bit.
The shape is the main difference between a center drill bit and a spot drill bit. A spotting drill bit looks like a regular twist drill bit, only far shorter and with a shorter flute. A center drill bit has two portions: a narrow, sharp-ended pilot portion, followed by a wider chamfering portion. This affects the application each is used for.
The primary purpose of a center drill is to create a seating area for the live centers of a lathe, milling machine, cylindrical grinders, etc. On the other hand, the sole use of a spot drill bit is to create an accurate start point for a drill bit to drill through. You could also use both these bits to do chamfering smaller holes and to do countersinking.
What Is Spot Drilling or Spotting?
Unlike a brad point drill bit used for woodworking, the metalworking twist drill bits have no sharp tip in the center. So, if you directly start drilling with a regular twist bit, it will start walking or wandering on the workpiece surface. A simple solution to this problem is to do a center punching at the spot to be drilled, but it is not an accurate method.
Spot drilling is a method used for precise borehole placement. Here, the spot drill created a shallow hole in the workpiece, placed precisely where you want the hole to be located. A spot drill bit is shaped to pre-chamfer the hole, easing subsequent drilling operations.
Once the spotting is completed, change the drill bit and complete the drilling operation knowing that the hole is accurately placed.
Spot drill bits are shorter and sturdier than longer, jobber length bits. This means they are less likely to wander, causing inaccurate hole placement, especially when drilling tougher materials or using drill bits that aren’t self-tapping.
What is Center Drill?
A center drill is used to center a hole for future operations. These drill bits have two defining features: a narrow pilot point and a wider countersink portion. The countersunk area is for the machine’s live center to seat and the small diameter hole is a relief for the sharp tip of the center.
Center Drill Angle
The drill point of the center drill has a drilling angle of 118°, while the countersink typically has one of three angles: 60°, 82°, or 90°.
The countersink angle should be chosen with the application in mind.
The standard 60° angle center drill bit is used for live centers and revolving centers on the tailstock of a lathe. Machining between centers is a common technique on the lathe and cylindrical grinding machines.
The 82° angle matches the head of flat head screws (Unified Thread Series) available in the United States. It is important to remember that the metric standard countersink screws have an included angle of 90° and the aviation industry standard is 100° for thin sections.
The 90° angle leaves a chamfer at the borehole’s lip, suitable for later operations.
How Do Center Drills Work?
Center drilling is usually done on a lathe machine and on a drill press or a milling machine.
Center drills have a small diameter pilot point, which contacts the material first. This creates a narrow, shallow hole that acts as a starting point for the hole. This pilot point is small, cutting easily into the material with a drill point angle of 118°.
Once engaged in the material, the countersink portion of the drill bit engages the material, creating a chamfer that widens the hole slightly. The deeper this countersink portion engages the material, the wider the hole becomes. The countersink angle matches subsequent operations, typically live centers or flat head screw installations.
A center hole is not a pilot hole for subsequent drilling operations.
A live center is used when a workpiece needs support from both ends, as on a lathe. Here, the spot drill creates a hole that matches the typical lathe centers, allowing the workpiece to be held securely while other operations are carried out.
Flathead screws require a slight chamfer to lie flush with the workpiece surface. Here, the center drill creates a chamfer that matches the typical head angle, allowing for a neat installation.
These examples show that the center drill isn’t meant for spot drilling operations. Using a center drill to do a spot drill’s job can cause damage to the drill bit cutting edges (especially in the case of carbide bits) used afterward.
Pros and Cons of Center Drill
Center drill bits are short and stubby, making them very sturdy and unlikely to wander. That’s why they tend to be highly accurate.
The angles at which center drill bits work make them unsuitable for spot drilling. Spot drills should provide a drilling angle at least as wide as the subsequent drill bit used. Center drills typically have a narrower angle (60° or 90°) than regular jobber drills, which usually has a point angle of 118° or 135°. If the spot drill angle is smaller than the jobber drill bit’s angle, the drill bit contacts the material on the drill tip’s corners, which are fragile. This could snap the corners off or create cracks or other forms of damage.
Here, the drill bit’s lifespan is shortened, or the drill bit is rendered useless. When the jobber drill pushes through the flange portion of the center drilled hole, the pilot portion contacts the drill bit point’s edges, creating notches on the surface. This further decreases tool life and negatively affects the drill bit’s accuracy.
What is Spot Drill?
Spot drilling creates a precise starting point for drill holes. Here, a short, wide-angled drill tip creates an accurately-placed starter hole for later drilling operations. Once the spot drilling is complete, a regular jobber-length drill bit, or other specialty drill bit, would be used to complete the process.
Spot drills typically have wide angles to ensure that the jobber drill bit contacts the workpiece at the center of the hole, on the drill bit’s cutting edge. Here, the drill bit engages as it is meant to, allowing it to drill accurately and without getting damaged.
What Angle Is a Spot Drill?
Spot drills come in various angles, and you should take care to match the spot drill’s angle to the drill bit you’ll use to complete the hole. Use a spot drill angle at least as wide as the jobber drill you plan to use, but a wider angle is better. So, use a 120° spot drill bit for drilling with general-purpose jobber drill bits.
If you plan to use a 135° carbide drill, use a 140° angle spot drill. A productive alternative is to use a split point bit which is self-centering and will save you the spotting time.
Can You Drill with a Spot Drill?
No. A spot drill isn’t meant to drill deep holes. It’s a short, stubby drill with a very short flute (that’s the helix portion that corkscrews along the length of a twist drill bit). It’s not meant to go deep into the material. Instead, it’s meant to create a pilot hole that allows accurate hole placement for subsequent drilling operations.
You could use a spot drill to drill holes through thin cross-sections. However, a Unibit or step drill bit is a better choice for drilling sheet metals.
Is Coolant Required for Spot Drilling?
Spot drilling creates small holes that are drilled quickly. So, even when drilling hard materials that cause lots of friction, the heat generated when spot drilling won’t be much. This means that coolants are generally not necessary for spot drilling. However, there are possible exceptions. When you’re using a high-speed, automated drill press and plan to undertake a large volume of spot drilling in a short time, you may need coolant.
- Difference Between Center Drill and Spot Drill
- Center Drill
- Spot Drill