How to Read a Gun Drill Specification Sheet: Tip Diameter, Oversize, Shank, and Coolant Hole Explained

A gun drill specification sheet can look intimidating the first time you see one. There are diameters, angles, lengths, and tolerances — and each number affects how the drill performs.

I have ordered hundreds of gun drills. Here is how I read a spec sheet.

Tip Diameter: The Number That Determines Hole Size

The tip diameter is the most important number on the sheet. It determines the finished hole size. But the tip diameter is not exactly the hole size — there is an oversize allowance built in.

For a standard gun drill, the tip is ground 0.01-0.03mm larger than the nominal drill diameter. This oversize accounts for spindle runout, bushing clearance, and the burnishing action of the guide pads.

If the hole consistently comes out 0.03mm over the nominal drill diameter, the tip oversize is within spec. If it is 0.05mm over, the tip may be ground too large or there may be excessive runout in the setup.

I cover the relationship between drill diameter and hole size in Gun Drill Sizing and Selection.

Shank Diameter: How the Drill Is Held

The shank is the part of the drill that goes into the toolholder. It is smaller than the tip to create a step that seats against the collet or bushing.

Shank diameter tolerance is typically h6 (0 to -0.011mm for a 10mm shank). A shank that is too small will not grip properly. A shank that is too large will not fit the collet.

The clearance between the shank and the bore wall — called the step clearance — should be at least 0.5mm on each side for chip evacuation. For a 10mm drill with a 7mm shank, the step clearance is 1.5mm per side. That is adequate for most applications.

I have seen drills ordered with the wrong shank diameter for the machine’s collet. Always confirm the shank diameter matches your toolholder before ordering.

Overall Length and Flute Length

The overall length determines what depth the drill can reach. The flute length — the portion with the chip flute ground into it — determines how far the drill can cut in one pass.

The flute length must be longer than the hole depth plus the bushing length. If the hole is 400mm deep and the bushing extends 30mm from the face, the flute must be at least 430mm.

Longer drills deflect more. A 1000mm gun drill needs different support than a 500mm drill. I cover this in Steady Rest Setup.

Coolant Hole Diameter

The coolant hole runs through the center of the drill and delivers coolant to the cutting tip. The diameter determines the flow rate and pressure drop.

A smaller coolant hole creates higher pressure at the tip but restricts flow. A larger hole increases flow but reduces pressure. The manufacturer has optimized this for the drill diameter.

If you change the coolant hole diameter on a custom order, the cutting performance will change. I cover how coolant pressure and flow interact in Coolant Flow Rate vs Pressure.

Tip Geometry Angles

The spec sheet shows several angles at the drill tip. The three most important are:

Point angle. Typically 115-140 degrees. A wider angle (135-140) spreads the cutting force and produces a better surface finish. A narrower angle (115-120) penetrates faster and is used for harder materials.

Primary clearance. Typically 6-15 degrees behind the cutting edge. More clearance reduces friction but weakens the edge. Less clearance strengthens the edge but increases friction.

Secondary clearance. A steeper angle behind the primary clearance, typically 15-25 degrees. This provides chip clearance behind the cutting edge.

I cover tip geometry in detail in Gun Drill Tip Geometry Deep Dive.

Guide Pad Dimensions

The guide pads stabilize the drill in the bore and burnish the surface. The spec sheet shows the pad width, length, and the amount of pad relief.

Pad width is typically 3-6mm depending on drill diameter. Longer pads provide more stability but create more friction. The pad relief — how much the pad diameter is reduced relative to the tip diameter — is typically 0.005-0.015mm.

If the pad relief is too small, the pads rub and overheat. If it is too large, the drill does not self-guide properly and the hole may drift.

Coating Specification

The spec sheet lists the coating type and thickness. Common coatings for gun drills are TiAlN, AlCrN, TiN, and DLC.

Coating thickness is typically 2-6 microns. Thicker coatings last longer but can dull the cutting edge if applied too thickly on small drills. For drills under 6mm diameter, I specify coating thickness of 2-3 microns maximum.

I cover coating selection in Gun Drill Coatings.

Putting It Together: How I Read a Spec Sheet

When I get a new gun drill specification sheet, I check in this order:

1. Tip diameter — will it produce the hole size I need?
2. Overall length — is it long enough for the depth?
3. Flute length — is the cut length adequate?
4. Shank diameter — does it fit my toolholder?
5. Coolant hole — will my coolant system work with this?
6. Point angle — is it appropriate for the material?
7. Coating — is it right for the material?
8. Guide pads — are the dimensions standard?

This sequence catches mismatches before the drill arrives. I have caught three wrong-drill orders this way — saved weeks of lead time.

Key Takeaways

• Tip diameter includes an oversize allowance of 0.01-0.03mm. This is normal, not a defect.
• Shank diameter uses h6 tolerance. Confirm it matches your toolholder before ordering.
• Flute length must exceed hole depth plus bushing extension. A common mistake.
• Coolant hole diameter affects both pressure and flow. Changing it changes cutting performance.
• Point angle and clearance angles should match the material. Do not use a steel geometry for aluminum.
• Always verify the spec sheet against your machine setup before ordering. A mismatch discovered after delivery costs weeks of delay.