by Dan Murphy, Rem Sales
Long thin parts that are perfect for a Swiss type CNC often have a long thin hole through them that can present a machining challenge. Gun drilling, if properly configured, is a viable solution. However, there are a few pitfalls that you'll want to avoid. This article will cover machine setup, gun drilling's impact on cycle time, and walk you through the operations required to gun drill effectively.
Gun drilling machines have features that aren’t necessarily available on a CNC Swiss. Gun drills aren’t self starting and self supporting like an ordinary twist drill. The gun drilling machine uses a bushing to support the drill until it is in the work where the hole it is drilling will support the gun drill.
In order to gun drill, on a Swiss turn, you will need to have coolant plumbed through the drill and adequate pressure in the coolant pump. Usually this means a high pressure coolant pump and pressures between 1,000 psi to 3,000 psi with flow rates from 1 to 8 gallons per minute depending on the diameter of the drill, the material being drilled, and the depth of the hole. It is best to consult with a gun drill manufacturer on the exact requirements before starting. Gun drilling small holes generally requires low viscosity neat cutting oil.
If your Swiss machine has axes and motion that allow you to feed out the material from the bushing, park a gun drilling bushing in front of it, then feed into the bushing and work with the gun drill. You are all set. Very few CNC Swiss machines have all the room, axes, and motions required, so in the majority of cases you will need to create a pilot hole so that the gun drill is supported.
Gun drills are generally single flute tools that are half-round. The drill body is usually made from crimped tubing and the cutting edge is a piece of carbide that has been brazed in place on the tip. There are also solid carbide gun drills. They are usually used for holes under 2-3mm in diameter.
There are many misconceptions about gun drilling; the most common myth is that it’s a fast operation. Usually the SFM and as a result
the programmed rpm is high, but the inch per revolution feed rate is usually around 1/10 of that used for a twist drill. For example the recommended speed and feed for gun drilling ¼” hole in 303 Stainless Steel is 200 SFM and 0.0005” per revolution feed rate.
Another misconception is that gun drilling will automatically produce a perfectly straight hole. It won’t. The gun drill manufacturers will tell you what is feasible for your application. You can help the straightness along by indicating the drill in and using good practices.
The quality of your pilot hole will directly affect the straightness of your gun drilled hole. If your pilot hole is too big the gun drill will be more likely to walk. If your pilot hole is undersize then you will break the gun drill before you even begin. It’s best to use a pilot drill made to match the angle on the gun drill. If you can’t hold size with the pilot drill, then you should drill undersize, bore, then ream to final size. Aim for 0.0003” to 0.0005” over nominal. Boring will straighten out the pilot hole and reaming insures that the hole won’t go undersize on you.
The operation should look like this:
- Machine the pilot hole, make sure all chips are out of the hole.
- Stop spindle and shut off coolant. Some people prefer to run the spindle in the counter clock-wise direction at a low rpm in order to prevent the cutting edge of the gun drill from hitting the edge of the pilot hole and possibly chipping the edge.
- Insert the gun drill into the pilot hole.
- Turn on the high pressure coolant.
- Start spindle.
- Drill.
It is important to only turn on the high pressure coolant AFTER the gun drill is in the hole. 2,000 psi coolant exiting from the tip of the gun drill can cause the gun drill to bend or deflect and hit the face of the part which will break the drill before it ever gets into the hole. I learned this lesson after breaking a couple of expensive gun drills the first time I tried gun drilling on a Swiss. Doh!
I first used a gun drill on a CNC Swiss well over 20 years ago. Given the state of the art then versus today there is little doubt that almost any CNC Swiss can be adapted to gun drill.
Nowadays there are plenty of twist drill manufacturers offering 20x carbide coolant fed drills right down into miniature sizes making gun drilling less of a requirement than it was just a few years ago. Gun drilling is still tough to beat if you need good surface finishes and tight tolerances inside a small diameter deep hole.
Here are some links to gun drill manufacturers I have used:
Here are links to high pressure coolant pumps:
- MP Systems: Plug and play on Tsugami machines
- Cool Jet
- Chip Blaster
Custom coolant fed gun drill holder bushings can be ordered from:
Photos courtesy of Sterling Gun Drills and MP Systems
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