Frequently Asked Questions About Drilling

Q
Can gun drills be used on conventional manual or CNC machines?

A
Yes – Gun drills can be used on conventional machinery with suitable coolant type and high pressure through tool coolant. Our Spraymist System is a low cost alternative to retrofit high pressure coolant delivery systems and is most practical on machinery that is not enclosed. The machine should have, or be made to have a workable, if not ideal, speed and feed capability for the hole diameter and material condition. A pilot hole to starts the gun drill instead of a guide bushing. Manual machines may require feed control modification.

Q
What materials can be drilled with gun drills?

A
Almost any materials can be gun drilled from ferrous and non-ferrous metals, powder metals, composites, woods, plastics, and more. Non-metals work better with gun drills ground specifically for that purpose but standard gun drill grinds will generally work bur may produce less desirable results.

Q
Why would I choose a gun drill over a coolant feed drill?

A
The main criteria is accuracy. The hole size, finish, straightness, are all far better with a gun drill than twist drilling. If there is any second, third, or more finishing operations to achieve the required accuracy, then gun drilling is preferred. If the hole is simply for clearance with speed more important than precision, the twist drill may still be the best.

Q
What is the maximum hardness for gun drills?

A
Gun drills will produce holes on materials up to around 50Rc. The highest ranges have to run at a considerably reduced RPM to less hard materials, so drilling time is extended and holes per grind are reduced. Tip coatings such as TiAlN can improve performance at elevated hardness ranges but if at all possible, material hardness in materials up to 40Rc provide best performance.

Q
What is the minimum and maximum hole depth?

A
The minimum hole depth can be any hole where a superior level of precision is required. High precision holes as shallow as 3 – 5 diameters deep could be gun drilled. At the other extreme, holes as much as 100 diameters deep or more can and should be gun drilled. At these depths, especially if a blind hole, there is usually no other choice.

Q
How do I define a gun drill?

A
Gun drills are most often described by their overall length. This should always be confirmed when someone asks? "I need a drill 16" long" Is this the drill depth or overall length? A 16" overall length gun drill with a standard 2 ¾" long driver will drill about 12" to 12.5" deep on manual or CNC conventional machinery. The extra ¾" to 1 ¼" allows for chip clearance and re-sharpening.

Q
What coolant should I use?

A
Gundrilling oil or its equivalent is the best for tool life and finish. If a water soluble is used, it should be high "EP", oil based, at 10% concentration. A synthetic coolant should never be used. Semi-synthetics are generally not acceptable and may lead to premature tool wear and failure. Vegetable based coolants that are reported to have very good lubricating qualities and may also be suitable in the certain applications.

Q
How do I start a gun drill?

A
On CNC or Manual machines, a pilot hole is usually easier to produce than to fixture for a bushing start similar to a gundrilling machine. The pilot hole should be sized to the gun drill diameter +.0005" to .001", but NEVER undersize. NOTE: On rotating drill applications, the gun drill should NEVER rotate outside of the hole or bushing. The carbide tip is much heavier than the tube and the unbalanced geometry of the assembly will cause the drill to whip and likely break off; extremely dangerous for personnel within range.

Q
What Speed & Feed?

A
Elsewhere on this site we have a downloadable Speed & Feed Chart. Speed & feed is dependent on coolant type, pressure, material type and condition, hole depth and drill style. Speed & feeds are provided as a conservative starting point and will be affected by all operating conditions.

Q
Do I need non-standard or special grinds for specific materials?

A
Generally NO, with exceptions for drilling conditions such as cross holes, angled entry and/or exit, or very deep holes. It is common for competitors to specify non-standard grinds for various materials when a standard grind will do. Special grinds by some manufacturers can be a method of making gun drill features unique so it adds complication to quotes from competitors. Or sometimes, special features are unnecessarily specified simply as a means to convince unsuspecting customers of technical ability. Most important is correct coolant type and pressure along with suitable speed & feed.

Q
What can I expect for tool life?

A
The hardest question to accurately answer. Tool life depends on every condition of the operation. The nearer to optimal conditions of oil type, pressure, speed & feed the better the tool life. Generally, tool life between re-grinds has to be determined by the drill in use, either by linear inches, minutes, parts drilled, or whatever tracking method one chooses and then sticks with it.

Q
How do I support a very long drill?

A
On gundrilling machines, intermediate supports with anti-whip guides handle this job. On manual and CNC conventional machinery, holes beyond 30-50 diameters deep should be supported with an intermediate flute support(s), or by use of intermediate length drill(s) enabling the drilled hole to act as a flute support for the next longer drill.

Q
How accurate is a gun drill?

A
.0005"/inch or better straightness is common with an accurate set-up and proper operating conditions. Best accuracy is achieved with counter rotating work and drill.

More information about Gun Drill Styles & their uses?