Gundrilling Oil– If you have a choice, oil is preferred over a water-soluble. Most conventional gundrilling machines use oil. It provides superior lubricity and in most if not all cases will yield better tool life and surface finish. The chemical makeup of most gundrilling oils includes high percentages of chlorine and active sulfur, which provide excellent anti-weld characteristics, combined with moderate viscosity which assists in effective chip flushing.

Oil is really a ‘friendly’ lubricant, although it’s gotten an environmental black eye over the years. Let’s face it, you don’t have to worry about concentration levels (unless it gets badly diluted with leaking hydraulic fluid); and oil doesn’t evaporate, it lasts forever (except for the carry-off on the workpieces.) Also, there’s no worry about ‘Monday morning aroma’ and periodically adding ‘bug killers’.

A few words of caution, though!

1. The high concentration of active sulfur in gundrilling oil has been known to dissolve the seals on some CNC machines. So, if you want to try oil in your CNC, before filling the sump, throw some spare seals into a beaker of the candidate oil and let them soak for a month or so before proceeding.

2. Have your supplier analyze your drilling oil on a regular basis to assure minimal contamination from hydraulic and machine lubrication oils. Otherwise, you could be faced with that age old problem of "The drilling operation was running fine until about three weeks ago when we started getting rough, oversize holes and poor tool life". The solution to the problem is probably Tramp oil strikes again!

3. Chlorine will attack certain materials, such as Stainless Steel. If you have a wash operation after drilling, assure that it effectively eliminates any chlorine residual. If you can't effectively remove the residual, you may subsequently encounter chemically induced stress failures.

Water Solubles– There is no ultimate coolant for gundrilling, and that’s probably as it should be. If you’re committed to one brand of coolant, that manufacturer probably has a grade that will meet your gundrilling requirements. Also, a coolant or oil that works well in the gundrilling portion of your process, with a few exceptions, will perform well on the remaining machining cuts. We don’t need to talk ‘brands’ here– only characteristics.


The successful candidate will probably be an emulsion or micro–emulsion with the following characteristics:

1. A high EP (extreme pressure) rating– ‘EP’ is the quality of a coolant which allows superior anti-weld characteristics. "Anti-weld" is the property which prevents rough and/or oversize holes. ('Welding' is also sometimes referred to as 'pickup' or 'built–up edge'.) This function is performed by the Chlorine and Sulfur.

2. Superior anti-foaming characteristics– this property is crucial in gundrilling applications due to the high coolant pressures involved. The additives which control foaming are usually proprietary compounds. Anti–foam products sometime have an identifying designation for this quality. (ie: Blaser-Swisslube uses a prefix of 'NF' [no-foam], Master Chemical prefixes with 'VHP', etc.)

3. High wetting and anti-corrosion capabilities– this is the quality which allows efficient carry-off of the chips coupled with chemical protection of the ferrous components of the machine tool.

4. Concentration is usually maintained at about 5 to 7%, although some manufacturers will recommend a ‘sweeter’ mix up around 10 to 12%. (If you don't have a refractometer in house to check and maintain concentration, buy one!)

Please note– if your supplier suggests putting an additive into your existing coolant supply to enhance anti–foam or anti–weld, it may mean he doesn’t have an effective gundrilling coolant to offer.

Additives may work the first or second time; but they won’t work every time. They may prove to be a quick fix; but they’re very sensitive to concentration levels, and what works today might not work two months from now. It’s enough of a problem just maintaining the correct balance between your concentrate and that ever-evaporating percentage of water in the sump.