Should I use a solid carbide or High Speed Steel (HSS) centre drill?

Centre drills are an essential tool for tailstock drilling on the lathe, especially for drilling longer holes on tougher materials where a hole not started with a centre drill is liable to wander off path.

While solid carbide centre drills are more expensive, harder wearing and generally machined to tighter tolerances, this does not automatically make them the best option for all jobs. High speed steel centre drills still have their uses.

Here is a break down of the pros and cons of both solid carbide and high speed steel centre drills, and which is best for which job.

Advantages of solid carbide centre drills

• Longevity - harder wearing, lasts much longer than HSS
• Rigidity - less likely to flex and wander off path
• Precision - made to and cuts to tighter tolerances
• Heat resistant - stays sharp at high temperatures

Disadvantages of solid carbide centre drills

• Higher cost
• More brittle, prone to breaking when jammed or dropped
• Less flexible, which can be a disadvantage when operating with lax workholding and relying on the drill to self-centre.

Advantages of High Speed Steel (HSS) centre drills

• Cheap outlay. Get a large selection of drill sizes at low cost. Cheap to replace broken/lost bits
• Tough. Resistant to breaking or chipping when dropped or jammed in a cut.
  
• Flexible. When used in a tailstock with some play, will naturally be driven towards the centre without breaking.

Disadvantages of High Speed Steel (HSS) centre drills

• Low hardness. Struggles to cut harder materials like tool steel.
• Low hot-hardness. Blunts faster at high temperatures.
• Less precise. Made to looser tolerances and more likely to wander off path when drilling.

When not to use solid carbide centre drills

When you can't guarantee low runout, or workholding is marginal

One of solid carbides greatest strength - its hardness, is also one of its biggest weaknesses. The downside of hardness and rigidity is brittleness. Being hard means the drill bit has much less ability to bend without breaking, so when drilling with a set up or work holding that can't limit radial forces on the bit, the drill is much more likely to bend than simply to deflect and wander off path.

When you just don't need to

Soft material, low tolerance requirements, cheap job/one offs.

When you need to use a solid carbide centre drill

Hard Materials

High speed steel can handle handle standard mild steels, stainless, cast iron, aluminium etc, but will struggle to cut hardened tool steels.

When you want to minimize tool cost over time

While solid carbide has a higher initial cost, the cost per hole drilled over the lifetime of the centre drill is much lower, albeit somewhat offset by the higher propensity to break. For one off jobs where even a HSS centre drill is likely to show any noticeable wear, this is less of an issue, but for repetitive production runs of many hundreds or thousands of operations, the increased longevity of solid carbide will soon pay for itself, so long as proper accommodation is made for the brittleness of solid carbide.

When you need to meet tight tolerances reliably

Solid carbide centre drills are both generally made to tighter tolerances, and will reliably cut to tighter tolerances due to the lower propensity of the drill bit to deflect. For certain jobs, solid carbide can remove the need for a separate reaming operation which can be a significant time and cost save during production.

When you want to maximize dimensional stability between cuts

When CNC machining, time must be taken to properly index the position of a tool. The superior wear characteristics of solid carbide mean that the tool bit retains the same dimensions for a much longer period of time than a HSS centre drill would, meaning less time needed to either re-index the drill position, or swap out a worn drill with a new one.