Frequently Asked Questions

How deep is “deep hole” drilling?

Hole depth is defined by describing the depth to diameter ratio, and a deep hole has a very wide ratio. For example, a 1” diameter hole that is 4” deep is a 4:1 ratio, and not considered very deep. But a hole that’s 0.04” diameter and 4 inches deep is a 100:1 ratio – a very deep hole.

> Read more: What is Deep Hole Drilling.

At what point do you need specialized equipment for deep hole drilling?

When the hole depth-to-diameter ratio exceeds 40:1, a deep hole drilling machine can provide much better results in both drilling performance and accuracy. High-performance gundrilling machines can drill beyond 200:1 depth to diameter, and can perform secondary operations.

> Browse UNISIG’s deep hole drilling machines.

Can I drill deep holes on my CNC machining center?

Yes, a CNC machining center can drill deep holes; the limitations are with very deep holes requiring close tolerances and high process reliability. Large diameter holes can exceed the power, torque and thrust available from the machining center; and the coolant flow and chip removal can be over-taxed as well.

> Browse UNISIG deep hole drilling machines by application.

How deep can you drill with a deep hole drilling machine?

The depth to diameter ratio is the determining factor, but 40 ft [12 m] and longer is possible in larger diameters with a machine designed for such an application. A smaller example would be a 0.250″ [6.35 mm] diameter hole that’s more than 2 feet [0.6 m] deep.

> Find an appropriate UNISIG machine by hole depth and diameter on our website.

Is “gundrilling” the same as deep hole drilling?

Gundrilling is one method of deep hole drilling. A gundrill is a specific type of drill with a straight flute and single cutting edge, which uses high-pressure coolant delivered through the tool body to the cutting edge. The chips are evacuated from the hole outside of the tool via the drill’s v-shaped flute.

> Learn more here: What is Gun Drilling.

What keeps a gundrilled hole straight?

The tip of a gundrill is off-center, which creates and imbalanced load against guide pads, that burnishes the hole and creates a guiding bore as the hole is drilled.

> Read more here: What is Gun Drilling.

Do you need to spot drill before gundrilling?

A gundrill requires a pilot hole, slightly larger than the gundrilled hole diameter, or in the case of a gundrilling machine, a starter bushing to guide the drill. A conventional spot drill is not desirable when using a gundrill.

> Learn more here: What is Gun Drilling.

What is the smallest diameter hole you can gundrill?

Gundrills are available down to 0.5 mm (0.020 in).  Mainstream applications for small diameter drilling are between 1.5 mm and 3 mm (0.06 – 0.12 in).

> Learn more here: What is Gun Drilling.

What other methods of deep hole drilling can be used?

Another commonly used method is BTA drilling. High pressure coolant is forced around the outside of the drill, and chips are flushed up through the drill’s center to be evacuated through the drill tube that’s connected to the head.

> This is easier to visualize with the graphics on this page.

Which is better – gundrilling or BTA drilling?

Neither – it depends on the application. One of the first factors to consider is the diameter of the hole to be drilled. Gundrills are typically employed between 0.8 and 50 mm (0.03 – 2.0 in), while BTA drilling is typical for 13 – 300 mm holes (0.5 – 12.00 in).

> This reference page gives a nice overview of deep hole processes.